Htljata, New ^atk 
 
 BOUGHT WITH THE INCOME A V^E 
 
 SAGE ENDOWMENT FUND 
 
 THE GIFT OF 
 
 HENRY W. SAGE 
 
 1891 
 
Cornell University Library 
 arV17796 
 
 Chemistry for photographers, 
 
 3 1924 031 223 203 
 
Cornell University 
 Library 
 
 The original of tiiis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924031223203 
 
CHEMISTRY FOR 
 PHOTOGRAPHERS 
 
BOOKS ON PHOTOGRAPHY 
 
 PRACTICAL 
 PHOTOGRAPHY SERIES 
 
 Edited by 
 
 FRANK R. FRAPRIE, S.M., F.R.P.S. 
 
 Editor of American Photography 
 
 1 . The Secret of Exposure 
 
 2. Beginners' Troubles 
 
 3. How TO Choose and Use a Lens 
 
 4. How to Make Prints in Colors 
 
 5. How to Make Enlargements 
 
 Cloth, 50 cents; Paper, 25 cents 
 
 Pictorial Landscape 
 
 Photography 
 
 By John A. Hodges, F.R.P.S. Paper, 25c. 
 
 American Photography 
 Exposure Tables. Cloth, 25c. 
 Thermo Development Chart, 25c. 
 American Photography, a monthly 
 
 magazine, representing all that its name 
 implies. 15 cents a copy; $1.50 a year. 
 
 Published by 
 
 American Photographic Publishing 
 
 Company 
 
 221 Columbus Ave., Boston, Mass. 
 
CHEMISTRY 
 
 FOR 
 
 PHOTOGRAPHERS 
 
 By 
 WILLIAM RUTHVEN FLINT, PH.D. 
 
 American Photographic Publishing Co. 
 
 Boston, Mass. 
 
 1916 
 
Copyright, 1916, by 
 American Photographic Publishing Co. 
 
 BEVERLY PRINTING COMPANY 
 BEVERLY, MASS. 
 
TO MY WIFE 
 
 As a Souvenir of the Many Photographic Excursions 
 We Have Enjoyed Together 
 
 THIS BOOK 
 
 IS 
 
 AFFECTIONATELY DEDICATED 
 
PREFACE 
 
 The purpose of the following pages is two-fold. First, 
 the chemical principles whose application forms the 
 foundation of the photographic art are set forth in a 
 manner which it is thought will prove both intelligible 
 and interesting. In order to fix these principles in the 
 reader's mind and at the same time to aid him in the 
 acquisition of a better chemical technique, the subject 
 matter has been so arranged as to permit the intro- 
 duction of a series of illustrative experiments. 
 
 Second, without in any way interfering with the fore- 
 going intent, it has been possible to add very materially 
 to the practical value of the book by incorporating 
 much useful chemical and photographic information in 
 the way of solubilities, formulas, etc. This information 
 has been so simplified and tabulated as to make it 
 exceptionally convenient for reference purposes. 
 
 To make special acknowledgment of each of the 
 authorities, chemical and photographic, which must be 
 consulted in the preparation of such a volume as the 
 present one would be both cumbersome and superfluous. 
 The literature of the subject, technical as well as popu- 
 lar, has been freely utilized, and the author would here 
 express his indebtedness to all the writers and investi- 
 gators whose work has been of so much assistance, but 
 whose names it is impossible to enumerate. 
 
 The author's thanks are particularly due to Mr. 
 Lorenzo Whiting of Pasadena, California, who has care- 
 
 IX 
 
fully performed and checked most of the formal ex- 
 periments included in the text. 
 
 In committing this little book to the reader's hands 
 the author sincerely hopes that it will be found helpful, 
 not alone in the saving of time and materials which 
 must come from a better understanding of the proc- 
 esses of photography, but also in an augmenting of the 
 amateur's interest and pleasure in a delightful avoca- 
 tion. 
 
CONTENTS 
 
 Page 
 
 Preface ix 
 
 Chapter I. Introductory i 
 
 Chapter II. Chemical Reaction ... q 
 
 Chapter III. Light and Chemical Reaction . ^^ 
 
 Chapter IV. Applied Photo - Chemistry of 
 
 Silver Salts .... 43 
 
 Chapter V. Chemistry of Development . 61 
 
 Chapter VI. Chemistry of the Fixing Pro- 
 cess 79 
 
 Chapter VII. After-treatment of the Nega- 
 tive 93 
 
 Chapter VIII. Printing Processes with Silver 
 
 Salts 108 
 
 Chapter IX. Printing Processes with Iron 
 
 Salts 130 
 
 Chapter X. Printing Processes with Chro- 
 mium Salts . . . . 153 
 
 Chapter XI. The Chemicals of Photography i6q 
 
 Appendix 183 
 
 Index 200 
 
Chemistry 
 
 for Photographers 
 
 CHAPTER I 
 Introductory 
 
 THE two most marked tendencies in the photo- 
 graphic art of today are represented by two 
 different types of workers. One of the types includes 
 those who are the most advanced workers of the day, 
 the pictorialists, whose whole endeavors are being 
 earnestly directed toward the expression of ideas 
 through the medium of photographic processes suitably 
 controlled. The other type comprises the scientific 
 workers, interested primarily in the investigation of 
 the chemical, physical, and physico-chemical principles 
 upon which the science, if we may call it such, of 
 photography rests. Both of these classes have already 
 made their indelible impress upon the art, the former 
 largely in the extreme evolution of the coUoid-dichro- 
 mate methods by which the pictorialists are able 
 with the greatest freedom to give play to their artistic 
 feelings in their prints. The influence of the latter 
 has been felt, not only in the improvement of manu- 
 facturing processes and products, but in the revision 
 of our ideas upon the subject of development as a whole, 
 
2 CHEMISTRY FOR PHOTOGRAPHERS 
 
 and in the attempt at standardizati^ i of many methods. 
 
 These two types appear to be at the extremes of 
 photography, the pictorialists, on the one hand, sub- 
 ordinating the scientific and the mechanical as much as 
 possible and laying stress upon the purely artistic, and 
 the scientific workers giving practically the whole of 
 their attention to improving the quality and increasing 
 the accuracy of delineation of the photographic records 
 in which they are interested. Between these two rather 
 widely separated groups are the great body of amateurs 
 and a large proportion of professionals, most of whose 
 photographic operations are still carried on by rule-of- 
 thumb methods. Nevertheless, the numbers of en- 
 thusiastic amateurs who go snapshotting more or less 
 at random about the country testify amply to the 
 fascinating power of photography even when its 
 processes are empirically worked. Doubtless it is from 
 watching the wonderful succession of changes through 
 which their picture goes as it passes from one solution to 
 another, together with the certainty, greater or less 
 according to their skill, of being able to produce in the 
 event a particularly desired result, that for most ama- 
 teurs a great part of the pleasure in their craft comes. 
 It is with the purpose of helping to increase and extend 
 this fascination and pleasure, by adding a clearer 
 understanding of what goes on in these processes, that 
 we shall proceed, in the succeeding chapters, to discuss 
 the chemical principles which underlie them. 
 
 The endeavor will be made, with what success the 
 reader must judge for himself, to describe and explain 
 in an elementary way the manner in which chemical 
 reactions, delicate and, very many of them, exceedingly 
 intricate, are so controlled as to yield both the photo- 
 
INTRODUCTORY 3 
 
 graphs of the evfi?yday amateur and the works of art 
 of the skilled pictorialist. The author has been led to 
 make this attempt because of the conviction, impress- 
 ing itself more and more strongly upon his mind with 
 the growth of his own experience and the extension of 
 his reading and study in photographic matters, that 
 the admittedly low standards of most amateur and a 
 large proportion of professional work are due mainly 
 to two causes. The first of these is the failure to under- 
 stand the chemistry of photographic processes, and 
 the second, the lack of appreciation of the principles of 
 composition, perspective, and those qualities which 
 go to make a picture. Now, of course, not every photo- 
 graph needs to be a "picture," and no one can right- 
 fully quarrel with the amateur who prefers to make 
 photographically a purely literal record of his ex- 
 periences, whether everyday or vacation. However 
 much the advanced pictorialist may pity such an 
 amateur, there is a great deal of interest and charm in 
 the making of straight record photographs. But the 
 photographers who make them ought, for the sake of 
 their friends if not for their own, to try to make them 
 as perfect as possible. This is the reason why, of the 
 two causes just mentioned, the chemical was placed 
 before the artistic; for it must be from the teeming 
 ranks of the aspiring amateur "straight" photographers 
 that the majority of "advanced" and "pictorial" 
 workers are recruited. The first essential, therefore, 
 is to inculcate as widely as possible a more intelli- 
 gent conception of the chemical foundations upon 
 which photography rests, in order that the average 
 photographer may better understand his medium, 
 improve his practice, and thereby perfect his results. 
 
4 CHEMISTRY FOR PHOTOGRAPHERS 
 
 There has in the past been prevalent among more 
 or less advanced workers the fear that their art might 
 become too mechanical, and for this reason there seems 
 to have been almost a prejudice against putting the 
 formulas for developing and other solutions into reason- 
 able chemical shape. Such photographers, for example, 
 in recommending a recipe for developer, would suggest 
 adding "a pinch of pyro" to a specified mixture of 
 alkali and sulphite, or "as much pyro as will go on the 
 point of a penknife." At the present time there does 
 indeed appear to be a deal of the mechanical and 
 automatic in photography. It has come about, how- 
 ever, not from the standardizing of formulas and 
 manipulations, but from the output of numerous, and 
 it must be admitted extremely convenient, prepared 
 powders and solutions, most of which need the addition 
 of nothing but water. The user, therefore, is generally 
 ignorant not alone of the concentrations of the active 
 agents in his solutions, but often of the very identity 
 of their constituents. He performs certain manipula- 
 tions according to the printed directions, but naturally 
 can comprehend little of their real import. The author 
 hopes in the following chapters to aid in changing this 
 state of things, by helping the worker to understand, 
 to take an example, that when an exposed plate is 
 developed there is an equivalence between the quantity 
 of silver reduced and the amount of developing agent 
 oxidized; that it is this reaction which uses up the 
 developing agent, aside from its spontaneous oxidation 
 by the air; and that because by these oxidations the 
 concentration of the reducing agent is diminished, it is 
 better, for the sake of clean, uniform negatives, to use 
 a given small portion of the developer solution but 
 
INTRODUCTORY 5 
 
 once, and then to throw it away, rather than to use it 
 over and over again until it is exhausted. The quantities 
 of substance of which photographic images are com- 
 posed are so minute that the concentrations of the 
 active agents with which these images are treated have 
 to be controlled with considerable care. It is one of the 
 axioms of physical science that the same causes always 
 operate to produce the same effects. Since photog- 
 raphy depends so largely upon the operation of physical 
 and chemical laws, it ought to be plain that obedience 
 to them is the key to success^ represented by well- 
 executed, permanent, and attractive prints. 
 
 As all physical science rests upon an experimental 
 basis, and especially since a much better idea of the 
 laws and phenomena of a science can be had by actually 
 performing for oneself even the simplest of experiments 
 than by any amount of reading about them, the author 
 has incorporated with the text of this discussion a set of 
 selected experiments intended to illustrate the principles 
 which are developed. He emphatically recommends 
 that the reader perform every one of these experiments 
 in course. No elaborate apparatus is needed for them, 
 and the chemicals are those which are most commonly 
 used by photographers. Neither is any special equip- 
 ment necessary, but either the kitchen sink or the 
 amateur's ordinary darkroom will serve quite as well as 
 though it were the most fully equipped chemical 
 laboratory. In working them out it will be found help- 
 ful to determine first of all exactly what is intended to 
 be illustrated in each case, that is, the object of the ex- 
 periment. Then pay close attention to everything that 
 goes on as the reaction progresses. There is nothing 
 too insignificant to be noted down. A class of young 
 
6 CHEMISTRY FOR PHOTOGRAPHERS 
 
 chemistry students once discovered that in the very 
 simple operation of boiling a little water in a test tube 
 there are some twenty-five different phenomena which 
 can be observed. It is in this way that one's powers of 
 observation are cultivated, and that is an end that is 
 very well worth while in itself. And, finally, all the facts 
 which are observed ought to be written down, with such 
 inferences as can be drawn from them. By this means 
 they will be the better fixed in mind and can easily be 
 referred to later, if necessary, without the experiment 
 having to be repeated. The habitual use of a notebook 
 in which are recorded facts, observations, comments, 
 etc., is a good custom for anyone to adopt, and it is 
 particularly so for the photographer, who ought to 
 make a practice of recording the data of all his ex- 
 posures. While the author feels that his work will be of 
 the most help if his readers will conduct the experiments 
 as they are described and along the lines just suggested, 
 nevertheless he has so arranged the directions for doing 
 them that no one need be deterred from studying the 
 book by inability to perform some or all of them. 
 
 So much has been written upon the darkroom, per se, 
 and upon its plan and arrangement that we need spend 
 no time in discussing it here. Besides, almost every 
 photographer has his own special ideas about it, or has 
 his own special conditions to contend with in its location 
 and construction. It will be sufficient and quite satis- 
 factory to assume that he has devised an arrangement 
 which suits his convenience as well as possible. And 
 beyond this we need not go, since we have already re- 
 marked that the place in which the experimental work 
 is done is of little consequence beside the manner in 
 which it is done. But there are certain simple items of 
 
INTRODUCTORY 7 
 
 apparatus which, while often found in the photog- 
 rapher's workroom, we had best enumerate for the sake 
 of any one who may not have them at hand. The first, 
 and indispensable, is a balance and set of weights. The 
 balance may well be the regular photographic scales, or 
 either the very cheap hand scales with horn pans or the 
 slightly more expensive "coarse weighing" balance 
 supplied by the dealers in laboratory apparatus. It 
 need not cost more than $3.00 or $3.50 for the one last 
 mentioned, which must be capable of weighing to 0.05 
 gram at least and up to 100 grams. With the photo- 
 graphic scales there usually is furnished a set of grain 
 weights, but they will not be useful in connection with 
 the present work. Curiously enough, these sets of 
 weights are commonly a mixture of avoirdupois and 
 apothecary weights. This ought to be sufficient com- 
 ment upon the system. The set required runs from a 
 50-gram piece down to o.oy gram and costs not more 
 than a dollar. There is much satisfaction in the use of a 
 good, substantial balance which does not easily get out 
 of order, and it is excellent advice to get the best that 
 can be afforded. But there is no need to exceed the 
 specifications given for capacity and sensitiveness. In 
 lieu of means for weighing out the chemicals for the 
 experiments himself, the worker who has no balance 
 can probably persuade some druggist friend to weigh 
 them otjt for him. Besides the ordinary darkroom 
 equipment of trays, tanks for developing and fixing, 
 and glass-stoppered bottles for solutions, etc., it will 
 be most convenient to have a few items of glassware. 
 These are obtainable from any dealer in laboratory 
 apparatus and for a very moderate outlay. The list of 
 articles will be found in the appendix at the back of the 
 
8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 book, where is also included a list of the chemicals 
 needed in the experiments. 
 
 The subject of photography, as we have intimated, 
 is a very wide one. Viewed from whatever direction, 
 in its enormously varied applications, in its purely 
 scientific investigations, and in its adaptation to the 
 uses of the amateur, its development has been little 
 short of marvelous. What further progress is still to be 
 made only the future can tell. And yet, from the photo- 
 micrographs of the biologist to the stellar photographs 
 and the spectroheliograms of the astronomer, from the 
 record photography of the amateur to the best work of 
 the great pictorialists, the whole structure of the art is 
 based on the one simple fact of the remarkable action 
 of light upon certain chemical compounds. The succeed- 
 ing chapters will treat of this action and of these com- 
 pounds in a way that the author hopes may be found 
 not only intelligible and interesting but instructive and 
 helpful. 
 
CHAPTER II 
 Chemical Reaction 
 
 ONE of the most general observations which can 
 be made about the world in which we live is that 
 nothing remains unchanged for any considerable period 
 of time. The appearance of a landscape under bright 
 sunshine is continually subject to change as the shadows 
 shift with the altering position of the sun. The study 
 of these variations in the same view forms a most useful 
 exercise for the photographer who is interested in 
 landscape work. Beneath a clouded sky the view pre- 
 sents a totally different aspect from any of these 
 appearances. If we pass to a consideration of the sky 
 on such a cloudy, gray day, we shall see that even the 
 clouds are not uniform. There are lighter and darker 
 grays. By fixing the attention upon a single patch of 
 gray cloud, we find that the quality of grayness is 
 changing in both tone and texture. 
 
 Let us walk down beside the clear pool which, with 
 its elliptical outline and its reflections, forms such an 
 attractive feature in our imaginary landscape. Whether 
 the air is still or whether a breeze ripples the water; 
 whether we see in its mirror-like surface the clear blue 
 of the sky or the rolling masses of a dazzling white 
 thunderhead, surely, we may think, here is a body that 
 is not changing. Water is water; the observed changes 
 are external to the body itself and are only appearances. 
 The physicist comes along with a little delicate ap- 
 paratus and shows us that the vapor of water is con- 
 tinually rising from the surface of the pool. And more 
 
lo CHEMISTRY FOR PHOTOGRAPHERS 
 
 than this, by certain experimental and mathematical 
 considerations he can also prove to us that water vapor 
 is at the same time condensing from the air into the 
 pool. Here then it seems is a different sort of change 
 from the changes in the appearance of things of which 
 we have been speaking. And yet it is only seemingly 
 different because invisible to the eye. The physicist will 
 tell us that all these changes are physical changes 
 because the identity of the bodies concerned has not 
 in any case been really affected. For example, whether 
 water be liquid water or vapor of water, its identity is 
 the same; only its condition has suffered a change. 
 
 Let us now go around the pool to the big tree on the 
 other side. We know that its beautiful green leaves, 
 which came out of the winter buds early in spring, will 
 be changed into an autumn glory in the coming fall. 
 But for the time being they are just green leaves, and 
 cannot it be said that they are unchanged, temporarily 
 at least? The answer now comes from the chemist. By 
 the very simple device of putting a leaf into a tube full 
 of water and standing it in sunlight, he can show that 
 all the while the sun is shining upon it the leaf is busily 
 engaged in consuming one of the constituents of the 
 air, namely, carbon dioxide, retaining within itself the 
 carbon with which to feed the rest of the plant, and 
 breathing out the oxygen. That the bubble of gas 
 collected at the top of the tube really is oxygen can be 
 proved by bringing into it the end of a glowing splinter 
 of wood, whereupon the spark will burn much more 
 brightly than in the air. 
 
 So from the chemist we learn that in this operation 
 of the green leaf under the influence of light there are 
 three bodies concerned. First, there is carbon dioxide, 
 
CHEMICAL REACTION 1 1 
 
 a colorless gas in which a glowing splinter is extin- 
 guished; second, oxygen, also a colorless gas, but a 
 supporter of combustion; and third, the carbon. 
 Dismissing the leaf from further consideration with the 
 remark that it is able to assimilate this carbon in such 
 a way as to store it up in the woody structure of the 
 plant, we may note that carbon is ordinarily a black 
 solid substance, quite different in every way from 
 either of the other two. Stating the proposition again 
 and in another form, we may say that it is possible to 
 take the colorless gas, carbon dioxide, and to get from 
 it another colorless gas, oxygen, and black solid carbon. 
 This would be plainly a different kind of change from 
 the vaporization of liquid water into water vapor or the 
 condensation of water vapor into liquid water, which, as 
 we have seen, are physical changes because the identity 
 of the water is unaffected. It is, indeed, a chemical 
 change, for the identity of the carbon dioxide is lost, 
 and two new bodies whose identity is different are 
 obtained. Thus we may define as chemical changes all 
 those changes by which the identity of the bodies con- 
 cerned is altered. 
 
 By much patient investigation chemists have proved 
 that all the many thousands of different kinds of bodies 
 are various combinations made up from only about 
 eighty really different kinds of matter. These eighty 
 substances are known as the chemical elements, be- 
 cause none of them has so far ever been separated into 
 anything simpler. When any two or more of the 
 elements combine in such a manner as to form a definite 
 substance, a chemical change occurs because there is a 
 change of identity. This change is called a chemical 
 reaction, or simply a reaction. Thus the chemical 
 
II CHEMISTRY FOR PHOTOGRAPHERS 
 
 change already considered, that of the carbon dioxide 
 into carbon and oxygen, is a reaction. It is possible to 
 state reactions very simply in terms of the substances 
 involved and in the form of equations, and for the sake 
 of clearness, as well as for another reason which will 
 presently be shown, it is very useful to do so. For 
 example, we may write the reaction we have been dis- 
 cussing thus : 
 
 Carbon dioxide = Oxygen + Carbon 
 
 Now, in order to discover just how useful this way 
 of writing reactions may be, and especially to learn 
 what they really mean, let us perform and then con- 
 sider a carefully conducted chemical experiment. In 
 performing this experiment, keep the eyes and face 
 away, just as though you were setting off flash powder. 
 
 Experiment i. Upon a small piece of thin white 
 porcelain place a very little powdered magnesium taken 
 from a box of flashlight powder. Spread the magnesium 
 out in a thin layer, and, supporting the porcelain 
 suitably, cautiously apply heat to its lower side with 
 an alcohol lamp or a gas burner, until all action ceases. 
 Then remove the source of heat, let the porcelain cool, 
 and examine the substance which it now holds. 
 
 If we have made any adequate use of our powers of 
 observation, we shall have noted in the first place that 
 the magnesium, which by the way is one of the elements, 
 is a silvery white substance, noticeably light in weight, 
 or, as we should rather put it, of low density, and 
 metallic looking. In the second place, when it was 
 heated sufficiently it began to melt; but in the third, 
 before very much of it was able to fuse, it took fire and 
 gave out a bright light together with a puff of white 
 smoke. And, finally, after cooling it, we have found 
 
CHEMICAL REACTION 13 
 
 upon the porcelain, in place of the original magnesium, 
 a white powder which bears no resemblance at all to 
 the material with which we started. What is it that has 
 happened? Generally we shall find that restating such 
 a proposition will help to make it clearer. We have 
 taken the metallic element, magnesium, and added 
 heat to it in the preserice of the air ; and have thereby 
 obtained more heat, a bright light, and some white 
 powder. This white powder, a chemical analysis would 
 tell us, contains nothing but magnesium and oxygen, 
 and the chemist therefore calls it magnesium oxide. 
 No matter how many times we may repeat this ex- 
 periment, we shall always find at its conclusion the 
 same sort of residue, namely, magnesium oxide. And, 
 furthermore, if we had more experience and much more 
 elaborate apparatus, we might weigh the magnesium 
 beforehand and then, performing the experiment with 
 very great care, weigh the magnesium oxide produced. 
 If we were to do this a number of times with different 
 amounts of magnesium, we should find that, allowing 
 properly for what are called the experimental errors, 
 the weights of magnesium oxide produced would all 
 be in the same proportion to the weights of magnesium 
 taken to start with, and this ratio would be 40.32 of 
 magnesium oxide to 16 of oxygen. In other words, when 
 a definite weight of magnesium is heated in the air, it 
 takes from the air a definite weight of oxygen and forms 
 a definite weight of magnesium oxide. So, then, we may 
 write this reaction as an equation : 
 
 Magnesium (24.32) + Oxygen (i6) + Heat 
 
 = Magnesium oxide (40.32) + Heat + Light 
 
 By using other elements and substances than mag- 
 nesium and oxygen, it is found that the same rule of 
 
14 CHEMISTRY FOR PHOTOGRAPHERS 
 
 definite parts by weight holds true for all, although, of 
 course, the numerical ratios are different. Therefore by 
 our experiment, together with the considerations just 
 developed, it has been shown, first, that each definite 
 chemical compound always consists of the same 
 elements combined in the same proportions; and, 
 second, that when two substances combine chemically 
 they always combine according to definite proportions 
 by weight. These are two fundamental chemical laws, 
 the law of constancy of composition and the law of 
 definite proportions, and they are of great importance 
 because from these known ratios and proportions there 
 can always easily be calculated, not only the per- 
 centage composition of substances, but also either the 
 amounts of combining substances to use in order to 
 get a particular quantity of the compound, or what 
 quantity of a compound can be made from given 
 amounts of its constituents. Thus, from our equation 
 and the ratios given, the proportion of magnesium in 
 magnesium oxide is 24. 3 2-^40. 3 2 ; of oxygen is 1 6-7-40. 3 2 ; 
 and to make ten pounds of magnesium oxide will re- 
 quire the burning of 10 x 24.32-j-40.32 pounds of mag- 
 nesium. 
 
 We will next make an experiment by applying heat 
 to a compound. 
 
 Experiment 2. Crush to a fine powder a few small 
 crystals of silver nitrate, make a little pile of the powder 
 upon a clean piece of thin white porcelain, and heat it 
 carefully from below. Continue to heat until all fuming 
 has ceased and the residue is bright and silvery. Cool 
 and examine this residue. 
 
 Here we start with a white powder, and by heating 
 it convert it to a bright metal, evidently silver. In the 
 
CHEMICAL REACTION ly 
 
 course of the heating, brown fumes are given off, the 
 material is hot simply on account of the heat we are 
 applying to it, and no light is given out. So this re- 
 action appears to be in every way the reverse of that 
 in the first experiment. Before we can write out an 
 equation, it is necessary to know positively whether the 
 silver residue and the brown smoke, which is an oxide 
 of nitrogen, are the only products of the reaction. By 
 putting a little silver nitrate in the bottom of a glass 
 tube, closed at one end and open at the other, and 
 heating it, and presently applying a glowing splinter of 
 wood at the tube's mouth, we can show that oxygen is 
 also given off. Finally, by carefully preparing pure 
 silver nitrate in the first place and weighing it and then 
 weighing the residue of silver, we should find that, 
 just as there is a definite ratio between magnesium and 
 magnesium oxide, so there is a fixed proportion by 
 weight between the silver and the silver nitrate from 
 which it came. This ratio is i6q.8q silver nitrate to 
 107.88 silver. We are now in position to write the 
 reaction in the form of an equation if we add that it can 
 be shown by suitable means that fixed proportions of 
 oxygen and of nitrogen oxide are also produced : 
 
 Silver nitrate (169.89) + Heat 
 
 = Silver (107.88) + Oxygen + Nitrogen oxide 
 
 Leaving the further comparison of this reaction with 
 that of Experiment i for our later consideration, we will 
 pass at once to two others. 
 
 Experiment 3. Measure in a test tube with a small 
 glass graduate y cubic centimeters of water. Weigh 
 out upon the balance o.i gram of silver nitrate and 
 dissolve it in the water. In a similar manner make 
 another solution in a test tube of 0.3 gram of sodium 
 
1 6 CHEMISTRY FOR PHOTOGRAPHERS 
 
 chloride (common salt) in lo cubic centimeters of 
 water. Warm the silver nitrate solution, add to it a 
 little of the sodium chloride solution, and shake the 
 tube containing the mixture. Add more of the sodium 
 chloride and shake, and continue thus until it is seen 
 that no more white solid forms upon the addition of a 
 drop or two of sodium chloride solution. If the ex- 
 periment has been carefully done, the white solid, or 
 "precipitate" as it is called, will collect in little bunches 
 and settle quickly to the bottom of the tube. If not, 
 shake vigorously, and then let the tube stand until all 
 the solid has settled, which should take only a few 
 minutes. Now, remembering that a little of the sodium 
 chloride was added after the white powder ceased to 
 separate from the silver nitrate solution, consider how 
 to free the solid substance from this sodium chloride and 
 any other substance that may be in solution in the 
 water. Suppose that, taking care to disturb as little as 
 possible the solid in the bottom of the tube, we pour off 
 from it all of the liquid we can, and then put lo cubic 
 centimeters more of fresh water into the tube. Shake 
 the tube, let the powder settle again, and then pour 
 away the water. By sufficiently often repeating this 
 process, which is called "washing by decantation," it is 
 clear that each time we shall leave in the tube only a 
 portion of whatever substances may be in solution, and 
 thus may reduce the amount of them to as small a 
 quantity as we please. 
 
 We may test the efficiency of this washing process as 
 follows: Dissolve in 4 or 5 cubic centimeters of water 
 contained in a test tube a small crystal of silver nitrate. 
 Into another clean test tube pour the wash water 
 from the tube containing the original precipitate. Now 
 
CHEMICAL REACTION 17 
 
 add to this wash water a few drops only of the new 
 silver nitrate solution. If the wash water still contains 
 an appreciable amount of sodium chloride, a white 
 cloudiness will appear, just as the white precipitate 
 was formed in the first instance. As soon as a portion 
 of wash water shows no perceptible change upon test- 
 ing with silver nitrate in this way, we may consider the 
 precipitate thoroughly washed. 
 
 (Note. — It should be remarked here that other 
 soluble chlorides besides the chloride of sodium will 
 also form this white precipitate in a solution of silver 
 nitrate. Since in some localities the natural waters 
 contain appreciable amounts of chlorides in solution, the 
 addition to them of silver nitrate will produce a cloudi- 
 ness as above described. Thus with such waters it will 
 be impossible thoroughly to wash the precipitate. In 
 these cases distilled water should be employed for this 
 experiment.) 
 
 In performing this experiment we shall have ob- 
 served that in the case of both substances the solid 
 readily dissolved in the water without any other 
 perceptible change; but that a change occurred as soon 
 as the two solutions were mixed ; and lastly that after a 
 certain point the addition of sodium chloride failed to 
 show any further change. It might be supposed that 
 the sodium chloride caused the silver nitrate to come 
 out of the solution as silver nitrate, or, vice versa, that 
 the sodium chloride was forced out, in which cases the 
 change might be a physical change. But investigation 
 of the white precipitate would show that whereas it 
 contains silver it contains also chlorine; and that it 
 contains nothing besides these two elements. It is thus 
 silver chloride, and therefore the formation of the white 
 
1 8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 precipitate indicated a chemical change. We must next 
 consider what has become of the sodium from the 
 sodium chloride and the nitrate part of the original 
 silver nitrate. If we have in the first instance added 
 but a drop or two more of sodium chloride after the 
 silver chloride stopped precipitating, we could take 
 the solution decanted from this precipitate and boil it 
 down carefully to small bulk in a little dish and set it 
 at one side for a day or so. We should then find that a 
 white solid substance had crystallized out and examina- 
 tion, perhaps with a magnifier, would show that the 
 crystals have a different shape from either sodium 
 chloride or silver nitrate crystals. Also by conducting 
 the experiment quantitatively, adding to a known 
 amount of silver nitrate a definite quantity of sodium 
 chloride in excess, and weighing the silver chloride 
 precipitated, it can easily be proved that besides the 
 silver chloride the only other new substance formed is 
 sodium nitrate. Therefore it should be noted that we 
 have also indicated by the experiment that when 
 silver chloride is formed by interaction of silver nitrate 
 and sodium chloride, exactly the right amount of 
 sodium is involved to form sodium nitrate with the 
 nitrate from the silver compound. There is neither 
 nitrate nor sodium left over, so to speak. Looking at 
 the matter in this way, we may see that any definite 
 quantity of silver nitrate is perfectly matched by some 
 other definite quantity of sodium chloride, because, 
 while there is just enough and no more silver in the one 
 to combine with all the chloride in the other, and form 
 silver chloride, at the same time there are exactly 
 suitable amounts present of sodium and nitrate to form 
 sodium nitrate and have nothing left over. Such amounts 
 
CHEMICAL REACTION iq 
 
 of two interacting substances are called equivalent 
 
 amounts, or simply equivalents. We are now able to 
 
 write the reaction as an equation : 
 
 Silver nitrate (i6g.8q) + Sodium chloride (58.46) 
 = Silver chloride (143.34) + Sodium nitrate (85.01) 
 
 To keep the matter fresh in our minds we will note 
 again that by using the proportions of these numbers, 
 which we now will call the equivalents of the sub- 
 stances, it is possible to calculate from a given weight 
 of any one of the compounds the corresponding weights 
 of all the others. 
 
 Experiment 4. Dilute with water a few cubic centi- 
 meters of strong hydrochloric acid (muriatic acid) to 25 
 cubic centimeters, mix thoroughly, and put 20 cubic 
 centimeters of the solution into a small porcelain dish, 
 saving the remainder, 5 cubic centimeters, in a test 
 tube. To 5 cubic centimeters of water contained in 
 another test tube add two or three drops of strong 
 ammonium hydroxide (ammonia water) and mix by 
 shaking. Dip into the smaller portion of diluted 
 hydrochloric acid one end of a strip of red litmus paper, 
 and similarly a strip of blue litmus paper, and observe 
 the effects. In the same way try the action of the diluted 
 ammonium hydroxide upon red and upon blue litmus, 
 and compare these effects with the former. Evidently 
 the litmus can be used to indicate the presence of these 
 substances in solution, and for this reason it is called an 
 "indicator." 
 
 To the bulk of the hydrochloric acid solution in the 
 dish add cautiously strong ammonium hydroxide, little 
 by little, stirring after each addition to mix the liquids. 
 Continue this operation until the mixture smells very 
 faintly of ammonia. Try the effect of the solution now 
 
20 CHEMISTkY FOR PHOTOGRAPHERS 
 
 upon red litmus, and consider what has happened to the 
 hydrochloric acid in the operation. Leaving the strip 
 of litmus paper in the liquid, cautiously add, a drop or 
 two at a time, some of the diluted hydrochloric acid 
 from the test tube until, upon thorough stirring, the 
 litmus just turns red. Then add with still greater care 
 a very little diluted ammonium hydroxide, continuing 
 thus with one or the other of these dilute solutions until, 
 if possible, red litmus remains red and blue remains 
 blue, or at any rate the color change is very slow. After 
 the removal of all strips of paper, the dish is to be set 
 over a burner in such a way that all the water may be 
 evaporated slowly, the dish at no time being permitted 
 to become hotter than the boiling temperature. Ex- 
 amine the dish for any residue. Boil away a little of 
 each of the dilute solutions separately and see whether 
 any residue is left in either case. 
 
 (Note. — If, after all the water has been evaporated, 
 the dish is heated much above the boiling temperature, 
 the experiment will probably be spoiled, since the 
 residue also will be expelled. It is quite likely that both 
 the liquid and the residue may be slightly colored from 
 the litmus, if the strips of paper have been allowed to 
 soak in the dish for any length of time. Keep the litmus 
 out of the solution as much as possible, and the residue 
 will be nearly, if not quite, white at the end of the 
 experiment.) 
 
 Remarking that "hydrochloric acid" as supplied is 
 merely a solution of hydrogen chloride in water, and 
 that ammonium hydroxide is likewise a solution of 
 ammonia, both these compounds being gaseous, we 
 may, if we look at the equation which represents the 
 
CHEMICAL REACTION 21 
 
 reaction of Experiment 3, perhaps at once venture to 
 write an equation for this reaction, as follows : 
 Ammonium hydroxide + Hydrogen chloride 
 
 = Ammonium chloride + Hydrogen hydroxide 
 
 The white residue is therefore ammonium chloride, 
 since hydrogen hydroxide is only a chemical name for 
 water. This reaction is a very remarkable one, in spite 
 of its apparent simplicity. Two clear, colorless liquids, 
 neither one of which upon evaporation leaves any 
 residue, the one smelling strongly of ammonia and the 
 other producing a pungent, stinging sensation in the 
 nostrils, when suitably mixed form a solution which 
 has practically no odor at all, from which a white solid 
 substance is obtained by evaporating the water. It is 
 also to be noted that the two substances, ammonium 
 hydroxide and hydrogen chloride, interact spontane- 
 ously and without the intervention of heat, just as did 
 the silver nitrate and sodium chloride of the preceding 
 experiment; that heat is developed by reaction, as 
 could be shown by using stronger solutions of the 
 substances; and, as in the foregoing experiments, 
 that if the quantities of interacting substances and 
 products were measured they would be found in definite 
 proportions to each other. So we will write the equation 
 again in the following form ; 
 
 Ammonium hydroxide (35.05) + Hydrogen chloride (36.48) 
 = Ammonium chloride (53.502) + Water (18.016) -(-Heat 
 
 Since, as we have seen, the two odors mutually destroy 
 each other, it is but a step to the inference that the 
 ammonium hydroxide and hydrochloric acid have 
 neutralized each other. Indeed, there is a whole series 
 of substances that possess, like the acid, a sharp, sour 
 taste when sufficiently dilute, are generally powerful, 
 
22 CHEMISTRY FOR PHOTOGRAPHERS 
 
 corrosive liquids in concentrated form, and have 
 certain other important properties in common. And 
 there is another series, like the ammonium hydroxide, 
 possessing in dilute solution a soapy taste, generally 
 solids forming more or less caustic solutions. And the 
 members of the first series interact with those of the 
 second series in the same way as the hydrochloric acid 
 and ammonium hydroxide. The first are called acids 
 and the second, bases. The process of their interaction, 
 which is always accompanied by the development of 
 heat, is called neutralization, and the principal product 
 is a "salt" in each case, the other product always being 
 water. So when a given solution contains an excess of 
 acid, or free acid as it is called, if the excess is un- 
 desirable it may be readily destroyed by the addition of 
 a sufficient amount of a base, or vice versa. Am- 
 monium hydroxide is often used for getting rid of an 
 excess of acid because it is itself volatile, and if an excess 
 of alkali is also not wanted it can be expelled by heating. 
 In these four experiments we have learned that 
 elements will sometimes unite to form compounds if 
 sufficient heat energy be supplied to them; that com- 
 pounds may be caused by heat to break apart into the 
 elements of which they consist; and that often when 
 chemical reactions occur there is a development of 
 heat, and sometimes even of light. Sometimes, as in 
 the decomposition of silver nitrate in the second 
 experiment, the reaction does not yield heat, but on 
 the other hand heat disappears, being used up in the 
 progress of the reaction. Thus every chemical reaction 
 is accompanied by an energy change of some sort. 
 Even when, as in the neutralization of a base by an 
 acid, the reaction starts spontaneously, that there is 
 
CHEMICAL REACTION 23 
 
 actually a change in the energy conditions is proved by 
 the appearance of heat when the base and acid are 
 mixed. This fact of a relation between energy and 
 chemical reaction is of the greatest importance in very 
 many ways which need not here be discussed. But in 
 the chemistry of photography it is vital, for it is the 
 reaction produced in the photographic emulsion by the 
 energy of light which makes photography possible. 
 
 By a comparison of the experiments we may also 
 learn that there are several different types of reactions. 
 When two or more elements or substances are caused 
 to combine, as in the burning of magnesium with 
 oxygen to magnesium oxide, the elements are put 
 together, and the reaction is thus a reaction of synthesis. 
 On the other hand, it is possible, as in the second 
 experiment, to cause compounds to be broken up into 
 their constituents, and then we have reactions of 
 analysis. It is the use of this kind of reaction, whereby 
 the composition of substances is investigated and their 
 percentage composition determined, which constitutes 
 that branch of chemical science known as analytical 
 chemistry. In such reactions as that between silver 
 nitrate and sodium chloride and between hydrochloric 
 acid and ammonium hydroxide, we appear to have 
 compounds picked apart and put together again in a 
 different order. These reactions, in which there is a 
 change of partners so to speak, are reactions of me- 
 tathesis. But there are still other ways in which re- 
 actions may be classified, for example, we have already 
 shown that in Experiment 4 the process of neutraliza- 
 tion is illustrated. When the dry substances are mixed 
 together and reaction is brought about either without 
 or with the aid of heat, it is sometimes said that the 
 
24 CHEMISTRY FOR PHOTOGRAPHERS 
 
 substances react "in the dry way;" and the reaction 
 may be said to take place "in the wet way" when it 
 occurs between substances that are in solution. The 
 most important kinds of reaction for our discussion, 
 however, are typified in the first two experiments, since 
 it is by means of such reactions as these that the initial 
 work of light upon the photographic plate or film is 
 afterward made effective in the negative (or print) 
 and the result finally modified to suit the requirements 
 of the photographer. 
 
 When, as in Experiment i , oxygen is caused to com- 
 bine with a substance, that substance is said to be 
 oxidized, and the reaction is called oxidation. The 
 oxygen may come, as in the experiment, from the 
 - supply of free oxygen gas in the air, or it may be taken 
 from (or given up by, whichever way we choose to 
 look at it) some other compound, generally one that is 
 rich in oxygen. The substance which furnishes the 
 oxygen is called an oxidizing agent or simply an oxidizer. 
 If we now consider what has happened to the oxidizer, 
 namely that oxygen has been taken away from it, we 
 shall see that this is precisely what has occurred in 
 heating the silver nitrate, viz., oxygen has been re- 
 moved. This is evidently the reverse process to oxida- 
 tion, and it is called reduction. The substance, as for 
 instance silver nitrate, from which oxygen is taken 
 is said to be reduced ; and that which takes the oxygen 
 is called a reducer. It may now be remarked that a 
 reduction cannot occur without an accompanying and 
 equivalent oxidation, and vice versa. The substance 
 which is oxidized need not be an element, it may even 
 already be an oxide. As an example, there is an oxide, 
 of lead, litharge, which is capable of being oxidized, 
 
CHEMICAL REACTION 25 
 
 forming another oxide of lead, richer in oxygen, lead 
 dioxide, or, as it is sometimes called, lead peroxide. This 
 lead peroxide is a strong oxidizing agent, readily giving 
 up again exactly the amount of oxygen which was 
 required to change it from litharge to peroxide. After 
 the peroxide has been reduced to litharge, the latter 
 can be further reduced to metallic lead, by using still 
 more powerful reducers. Reduction, therefore, may 
 also be conducted in steps, a higher oxide being re- 
 duced to a lower, and the lower completely reduced, as 
 to a metal. By an extension of these principles, chemical 
 changes in which no oxygen at all is involved are 
 denominated oxidation and reduction. As an example 
 may be cited the action of a piece of iron, such as a 
 nail, on a boiling solution of the chloride of iron which 
 is called ferric chloride. The solution is originally 
 yellow in colpr, but the boiling over metallic iron 
 bleaches out the yellow and makes the solution color- 
 less. At the same time a little of the metallic iron is 
 dissolved. By suitable analytical tests it can be shown 
 that the solution now contains chloride of iron and 
 nothing else, but the iron chloride is a different one 
 from the original. If it were taken out of the solution 
 it would be found to be a white powder, whereas the 
 ferric chloride is yellow. The new compound is ferrous 
 chloride, and it is made from the ferric compound 
 by reduction, iron being the reducing agent. It will 
 perhaps be clear at once that the iron metal, from the 
 nail, which goes into solution in the .process is oxidized, 
 so that the quantity of ferrous chloride finally present 
 comes not only from the reduction of ferric chloride to 
 ferrous chloride, but partly also from the oxidation of 
 iron to ferrous chloride. Ferrous chloride can finally be 
 
26 CHEMISTRY FOR PHOTOGRAPHERS 
 
 oxidized back to ferric chloride by passing chlorine gas 
 into the solution. Thus in these oxidations and reduc- 
 tions we have nothing whatever to do with the element 
 oxygen, but have learned that chlorine is an effective 
 oxidizing agent, and that reduction may involve the re- 
 moval of other elements than oxygen. 
 
 (Note. — For a more complete account of this sub- 
 ject the reader is referred to any of the standard works 
 on chemistry. It will be, in fact, an excellent idea if he 
 will extend his reading in these chemistries to cover 
 such subjects as valency, the ionic theory, and molec- 
 ular and atomic weights, which are not within the 
 scope of the present work.) 
 
CHAPTER III 
 Light and Chemical Reaction 
 
 WE have learned that some chemical reactions 
 require, in order to start, nothing more than the 
 mere contact of the substances involved, at the ordinary 
 temperatures. On the other hand, of those reactions 
 which need to be initiated by the application of energy 
 to the reacting factors, there are two kinds : first, such 
 reactions as, when once started, develop greater or less 
 quantities of heat energy by means of which they are 
 enabled to proceed to a finish, one or other, or some- 
 times both, of the factors being used up; and, second, 
 those which continue only so long as sufficient energy 
 is applied, the energy being absorbed in the process. 
 The terms energy, and heat energy, are here employed 
 in the most general sense to include all those vibrations 
 of the ether, whereby radiant energy is transmitted 
 through space, and without reference to the wave- 
 length of the vibrations. In the present chapter we 
 shall limit ourselves to a consideration of the energy 
 of the shorter wave-lengths, and illustrate and discuss 
 some typical chemical reactions which are originated by 
 the energy of light, since it is these photo-chemical 
 reactions, as they are called, which are of primary con- 
 sequence in the making of pictures by photographic 
 methods. 
 
 That reactions of this kind are continually going on 
 in nature will readily be seen if we merely mention the 
 tanning of the skin by exposure to sunshine, the fading 
 of colors in bright light, and the process referred to in 
 
 27 
 
28 CHEMISTRY FOR PHOTOGRAPHERS 
 
 the preceding chapter by which chlorophyll of the plant 
 world, absorbing the energy of light and utilizing it, 
 decomposes the carbon dioxide of the air. Perhaps we 
 may get some idea of the amount of work involved in 
 this operation, if we stop to consider how great a 
 quantity of heat is given out when wood is burned, 
 remembering that the heat energy set free in the burn- 
 ing of each pound of carbon to form carbon dioxide 
 is exactly equivalent to the energy absorbed in the 
 separation of this amount of carbon. But the sub- 
 stances concerned in these photo-chemical reactions 
 are too complex and the changes too complicated for 
 our present consideration. It will be well, therefore, 
 for us to begin with something simpler and more easily 
 comprehended. 
 
 Certain of the elements are affected in a very peculiar 
 way by light. For example, the common form of sul- 
 phur, roll sulphur, or brimstone, occurs in rhombic crys- 
 tals and is very soluble in carbon disulphide. But up)on 
 exposure to light it is changed to an amorphous condi- 
 tion (non-crystalline) in which it is no longer soluble in 
 the solvent mentioned, and at the same time heat is 
 given out: 
 
 Sulphur (rhombic) + Light (Violet) 
 
 = Sulphur (amorphous) + Heat (Infra-red) 
 
 The element phosphorus goes through a similar change 
 in which the two varieties are even more differentiated 
 from each other. Phosphorus may be purchased either 
 in waxy, yellowish-white sticks or as a red powder. 
 Yellow phosphorus melts at 44.4 degrees centigrade, 
 is soluble in carbon disulphide, and is extremely poison- 
 ous. Matches were formerly manufactured with this 
 yellow form, and workmen in match factories were 
 
LIGHT AND CHEMICAL REACTION 2q 
 
 subject to a peculiarly dreadful disease. When the 
 yellow form is exposed to light, particularly the blue, 
 violet, and ultra-violet rays, it is thereby converted to 
 the red modification, with the liberation of heat. Red 
 phosphorus does not melt even at red heat, is insoluble 
 in carbon disulphide, and is not poisonous. After much 
 vigorous objection by the match manufacturers be- 
 cause of the greater cost of red over yellow phosphorus, 
 laws were passed several years ago prohibiting the. use 
 of the poisonous yellow form, and matches are now made 
 out of the harmless red variety : 
 
 Pliosphorus (yellow) + Light = Phosphorus (red) + Heat 
 Selenium is another element which is light sensitive. 
 It can be prepared as a bright red, amorphous powder 
 which is soluble in carbon disulphide, melts at 125 
 degrees centigrade, and is a non-conductor of electricity. 
 By the action of violet light, red selenium is changed to 
 a dark gray, crystalline, metallic form. This is insoluble 
 in carbon disulphide, melts at 2 1 7 degrees centigrade, 
 and is a variable conductor of the electric current 
 according to the intensity of light thrown upon it : 
 
 Selenium (amorphous) -|- Light 
 
 = Selenium (metallic) + Heat 
 
 Several interesting uses have been made of the peculiar 
 property of the metallic selenium, namely, its varia- 
 bility in conductivity produced by changes in the 
 illumination. The resistance of a "selenium cell" may 
 be as much as two hundred times as great in the dark 
 as in the light. By use of such cells in electrical circuits, 
 motors can be started and stopped by a beam of light, 
 or signal lights caused to operate automatically, the 
 gas or current being turned on by the coming of dark- 
 ness and turned off again with the daylight. Perhaps 
 
30 CHEMISTRY FOR PHOTOGRAPHERS 
 
 the most wonderful application of this light reaction 
 is in the transmission of photographs by telegraph. 
 
 The three photo-chemical reactions just described are 
 evidently quite different in their nature from such a 
 reaction, for example, as the reduction of ferric to fer- 
 rous chloride, which we shall presently consider. In 
 some so far unexplained way they appear to be read- 
 justments in the energy conditions prevailing in the 
 molecules of these light-sensitive elements. 
 
 If precisely equal volumes, measured under identical 
 conditions of temperature and pressure, of the two 
 gaseous elements hydrogen and chlorine are put into 
 a glass jar and kept in total darkness, the amount of 
 hydrogen chloride formed in a given time by the 
 chemical union of these elements will be inappreciable. 
 Practically we may say that in the absence of light no 
 reaction occurs. When, however, the jar is brought out 
 and set in a very weak light, the hydrogen and chlorine 
 begin slowly to combine, and after the lapse of sufficient 
 time there will be found in the jar only hydrogen 
 chloride gas. If the jar be put in a more brightly 
 illuminated place, the reaction becomes more rapid. 
 By exposing it to direct sunlight, indeed, it becomes so 
 much accelerated as to be violently explosive, and the 
 glass jar, unless very stout, will probably be shattered. 
 In performing this experiment it is usual to mix the 
 gases in the dark, wrap the jar completely in a black 
 cloth to which has been attached a long string, and set 
 the apparatus in the sunlight. The experimenter there- 
 upon retreats to the other end of the cord and jerks 
 away the protecting cloth. The explosion follows in- 
 stantly, and is occasioned by the fact that when 
 hydrogen and chlorine unite a tremendous evolution 
 
LIGHT AND CHEMICAL REACTION 31 
 
 of heat takes place. Since the product, hydrogen 
 
 chloride, is also gaseous, it expands with great pressure 
 
 under the influence of the liberated heat. It is needless 
 
 to say, perhaps, that this experiment is a very dangerous 
 
 one. The reaction is: 
 
 Hydrogen + Chlorine + Light 
 
 = Hydrogen chloride + Heat 
 
 Hydrogen iodide is a gas similar in many respects to 
 the hydrogen chloride we have just been discussing. 
 Its solution in water is colorless, and is commonly called 
 hydriodic acid. When such a colorless solution is ex- 
 posed to both the oxygen of the air and light, it turns 
 brown. Light causes decomposition of the hydrogen 
 iodide, setting free iodine, which in solution has a 
 brown color, if water is the solvent. The hydrogen 
 combines with oxygen from the air, forming water, 
 so the reaction may be expressed : 
 
 Hydrogen iodide + Oxygen + Light = Iodine + Water 
 
 Blue light is most effective in bringing this reaction 
 about. But we must note in this case that without 
 the presence of oxygen the reaction could not go on, as 
 hydrogen and iodine would recombine directly they 
 were separated, being both chemically active sub- 
 stances. It would probably be more correct to say that 
 under such circumstances both these actions would be 
 going on simultaneously, hydrogen and iodine re- 
 combining to form hydrogen iodide as fast as the latter 
 was being decomposed. That is, we might write the 
 reaction in the following way: 
 
 Hydrogen iodide "^^ Hydrogen + Iodine 
 
 The direction of the arrows signifies that both reac- 
 tions are proceeding at the same time. Such a reaction 
 
32 CHEMISTRY FOR PHOTOGRAPHERS 
 
 is said to be "reversible." But when oxygen is at hand, 
 the hydrogen unites with it in preference to iodine, and 
 thus the decomposition of hydrogen iodide is enabled to 
 go on, reaction progressing only from left to right in the 
 above equation. As we shall see, there are other photo- 
 chemical reactions in which the presence of some addi- 
 tional substance is required in order that light may 
 produce the effect. Photographicallyspeaking, we should 
 say that this extra substance is a "sensitizer," the de- 
 composable material, such, for example, as hydrogen 
 iodide, being "insensitive" in the absence of the "sen- 
 sitizer." 
 
 Another reaction of decomposition brou^t about by 
 light occurs with ferric oxalate, a compound which is 
 of considerable photographic importance. When dis- 
 cussing the subject of oxidation and reduction, we 
 showed how by suitable means a ferric salt is reduced to 
 ferrous salt, and the ferrous oxidized back to ferric. 
 Light is able to perform this reduction with ferric 
 oxalate, ferrous oxalate being produced, and carbon 
 dioxide escaping. 
 
 Experiment 5. Expose to strong sunlight for half or 
 three quarters of an hour a few of the brownish scales 
 of ferric oxalate, and at the expiration of this time look 
 for any change which has been produced. Save the 
 material for Experiment 7. It will be well for us here to 
 learn how to detect the presence of ferrous and ferric 
 iron in substances, since in the use of ferric oxalate for 
 photographic purposes it is very necessary to have no 
 ferrous salt present. If the ferric oxalate is from old 
 stock or has been handled much in the light, it is very 
 likely to have suffered reduction in part, and this will 
 render it useless. The brown color of the bottle in 
 
LIGHT AND CHEMICAL REACTION 33 
 which it is bought and must be kept is intended to pro- 
 tect it from the action of all light except that trans- 
 mitted by the brown glass. We will first make an 
 experiment to show how ferric salt is detected. 
 
 Experiment 6. Dissolve in 5 cubic centimeters of 
 water a small crystal of ferrous sulphate, add a few 
 drops of dilute sulphuric acid and shake up this solution 
 in the test tube for several minutes. Since there is air 
 in the test tube above the solution and ferrous salt is 
 very readily oxidized by air, this shaking will give some 
 of the ferrous sulphate in the solution an opportunity to 
 be oxidized to ferric sulphate. We might do the same 
 thing more quickly by adding to the liquid a drop or 
 two of nitric acid, which is a good oxidizer. In either 
 case we shall have the same result in effect, as it may be 
 represented : 
 
 Ferrous sulphate + Sulphuric acid + Oxygen 
 
 = Ferric sulphate + Water 
 
 In the presence of nitric acid there would be formed 
 also some ferric nitrate. 
 
 Next take a very small crystal of potassium ferro- 
 cyanide, or as the druggist would probably call it, 
 yellow prussiate of potash, break it into still smaller 
 bits and dissolve a minute piece in 5 cubic centimeters 
 of water. This will be a very dilute solution. In the 
 same way make 5 cubic centimeters of a dilute solution 
 of potassium ferricyanide, or red prussiate of potash. 
 Take about a cubic centimeter of the prepared solution 
 containing ferric sulphate in a clean test tube, dilute 
 it with water to about 5 cubic centimeters, and add a 
 few drops of the dilute potassium ferrocyanide. The 
 fine blue color, which is Prussian blue, is due to the 
 formation of fei-ric ferrocyanide, and is always ob- 
 
34 CHEMISTRY FOR PHOTOGRAPHERS 
 
 tained when solutions of potassium ferrocyanide and a 
 ferric salt are mixed. Thus the blue coloration is a 
 chemical test for ferric iron. Now boil 5 or 6 cubic 
 centimeters of water in a clean test tube, to expel all 
 oxygen which is in solution in the water, cool without 
 shaking (to prevent as much as possible the water from 
 absorbing more oxygen), and dissolve in it a very small, 
 clear crystal of ferrous sulphate. As quickly as may be, 
 add to it a few drops of potassium ferricyanide solution. 
 The blue color which appears in this case is Turnbull's 
 blue, and is always obtained when solutions of potas- 
 sium ferricyanide and a ferrous salt are mixed, and is 
 therefore a chemical test for ferrous iron. 
 
 So, to repeat, ferrocyanide gives blue with ferric iron 
 salts; and ferricyanide gives blue with ferrous com- 
 pounds. By writing out these reactions in equations we 
 shall see that they are reactions of metathesis, as follows : 
 
 (i) Ferric sulphate + Potassium ferrcx;yanide 
 
 = Ferric ferrocyanide + Potassium sulphate 
 (^) Ferrous sulphate + Potassium ferricyanide 
 
 = Ferrous ferricyanide + Potassium sulphate 
 
 If the experiments have been carefully done according 
 to directions, it will be apparent that these tests are 
 very delicate. 
 
 Experiment 7. Treat in a test tube with 5 or 6 cubic 
 centimeters of water a little of the ferric oxalate which 
 was exposed to light in Experiment 5. If any powder 
 remains undissolved, add a crystal or two of potassium 
 oxalate to get it into solution. Shake the tube, and to its 
 contents add a few drops of potassium ferricyanide 
 solution prepared in Experiment 6. Blue coloration 
 indicates the presence of ferrous iron. Compare with 
 this the action of ferricyanide upon a fresh, cold solution 
 
LIGHT AND CHEMICAL REACTION 35 
 
 of ferric oxalate. If the latter compound is pure, there 
 will be no blue color developed in salt which has not 
 been exposed to light. 
 
 The question whether ferric oxalate contains any 
 ferrous salt as an impurity must be answered upon the 
 basis of what takes place immediately when the mix- 
 ture of the two solutions is made and not upon what may 
 happen after this mixture has stood for some time : 
 
 Ferric oxalate + Light = Ferrous oxalate + Carbon dioxide 
 
 If the ferrous oxalate is put away in the dark in con- 
 tact with the air, ferric oxalate forms again by oxidation. 
 
 Experiment 8. Dissolve a crystal of uranium nitrate 
 in 5 cubic centimeters of water, spread about half of 
 this solution upon a piece of paper, using a sized paper 
 such as writing paper, and lay it aside in a dark place to 
 dry. To the remainder of the uranium nitrate solution 
 add a few drops of dilute potassium ferrocyanide, and 
 note the blood-red color produced. As soon as the 
 prepared paper is dry, expose it to strong sunlight for a 
 few minutes and then flow over it a little of the potas- 
 sium ferrocyanide and compare the result with the 
 preceding. 
 
 Uranium salts are extremely sensitive to light, 
 uranyl compounds being reduced to uranous. Potas- 
 sium ferrocyanide forms with uranyl nitrate red uranyl 
 ferrocyanide. When exposed to light, uranyl salts are 
 reduced, and then with ferrocyanide the. green uranous 
 ferrocyanide is obtained. 
 
 Experiment q. Make up a little potassium dichro- 
 mate solution, coat with it a piece of sized paper, and 
 dry it in the dark. When dry expose it to sunlight in 
 a printing frame under a negative for five minutes. On 
 
36 CHEMISTRY FOR PHOTOGRAPHERS 
 
 examination it will be seen that opposite the thin places 
 in the negative the orange color of the dichromate has 
 turned brown, but beneath the dense parts there is 
 little or no color change. 
 
 In order to investigate this reaction a little further 
 and to find out if possible what may have happened to 
 the chromium compound, let us take 4 or y cubic 
 centimeters of ferrous sulphate solution, add to it a little 
 of our solution of potassium dichromate and then make 
 the test with potassium ferrocyanide for ferric iron. 
 We shall discover by this means that potassium di- 
 chromate is a good oxidizer, since it quickly changes 
 ferrous sulphate to the ferric salt. Since the dichromate 
 oxidizes ferrous iron, it must itself be reduced. Now, 
 returning to the paper coated with dichromate, we 
 have here an oxidizing agent in contact with the 
 materials of which the paper and sizing consist. These 
 are organic substances, principally cellulose in the 
 paper and possibly starch in the sizing. Both these 
 substances are comparatively easily oxidized, and 
 consequently it becomes plain that in the coated paper 
 we have dichromate, a good oxidizer, in contact with 
 excellent reducing agents. So when light energy was 
 brought to bear upon this state of things, the organic 
 matter was oxidized and the dichromate reduced. With- 
 out the organic substances there would have been no 
 reduction of dichromate, and so the paper and sizing 
 act as sensitizers. There is reason to think that the 
 chemical reaction which takes place under the influence 
 of light may be expressed thus : 
 
 Organic matter + Potassium dichromate + Light 
 
 =■ Chromic oxide + Potassium hydroxide 
 + Organic matter (partly oxidized) 
 
LIGHT AND CHEMICAL REACTION 37 
 
 Another reaction with dichromate has interesting 
 applications in printing processes to be discussed later. 
 
 Experiment 10. Make up a solution of about o.y 
 gram of potassium dichromate in 5 cubic centimeters of 
 water. Soak in a very little cold water 0.5 gram of 
 gelatine ; when it has swelled, add more water so that the 
 gelatine may dissolve by gentle heating. Take about 5 
 cubic centimeters of the gelatine solution, add to it a 
 cubic centimeter of dichromate solution, mix thoroughly, 
 and coat some of the mixture on a piece of sized paper. 
 Expose the dried paper for two or three minutes under 
 a contrasty negative, and observe the effect. Wash the 
 paper in warm water and observe that all the gelatine 
 cannot be washed off the paper. 
 
 Gelatine is an organic substance, akin to the proteins 
 which are essential constituents of food-stuffs. It is 
 made from the bones and cartilages of animals. It is 
 soluble in cold water, ordinarily, and its solubility is 
 much increased by gentle heat; but by long continued 
 boiling it tends to become more insoluble. A solution 
 of gelatine differs in a marked way from one of such a 
 substance as sodium chloride or silver nitrate. When 
 solutions of these salts are allowed to stand, the solid 
 material is, by the gradual evaporation of the water, 
 caused to deposit in the form of crystals. Also if a 
 partition of parchment is placed in a dish so that a 
 salt solution can be poured in one side and pure water 
 in the other, the parchment serving to keep the two 
 liquids separate, after a little while it will be found that 
 the salt has passed through the parchment into the 
 water. This process is known as diffusion, and all 
 substances that form crystals have the power of 
 diffusing through such a membrane. Crystallizing sub- 
 
38 CHEMISTRY FOR PHOTOGRAPHERS 
 
 stances are often called crystalloids. But if these ex- 
 periments are repeated, using a solution of gelatine in- 
 stead of salt, in the first place no crystals will be secured 
 under any circumstances, but rather the gelatine 
 solution will "set," or, as the housewife calls it, "jelly." 
 In the second place, none of the gelatine will be able to 
 pass through the membrane; it cannot diffuse as a salt 
 is able to do. As distinguished from the crystalloids, 
 gelatine belongs to the class of substances called "col- 
 loids." Gum arable is another example of a colloid, and 
 one that also has photographic uses. 
 
 Referring now to the experiment just performed, we 
 learned that light possesses the power so to affect a 
 mixture of gelatine and potassium dichromate that the 
 gelatine, previously quite readily soluble in water, 
 becomes insoluble. One circumstance in the way this 
 reaction was conducted is worthy of special mention. 
 Since the gelatine-dichromate mixture was spread as a 
 coating upon the surface of paper, it is evident that 
 wherever the light was transmitted by the negative it, 
 so to speak, fell first upon the outer surface of the coat- 
 ing. Then, as it passed into the gelatine film, it was 
 more and more absorbed. Consequently its energy was 
 gradually diminished in proportion to the depth to 
 which it penetrated. Therefore the maximum effect of 
 this light reaction occurs at the outside surface of the 
 film, the greatest degree of insolubility being produced 
 there; and the insolubility diminishes as the surface 
 of the paper is approached. So we may have as the 
 result of the exposure of such a film to light an outside 
 layer of insolubilized gelatine protecting, as it were, 
 layers of much greater solubility which lie beneath it. 
 We may as well note, also, that the more soluble layers 
 
LIGHT AND CHEMICAL REACTION jq 
 
 are protected upon the other side by the paper. The 
 bearing of these observations we shall have occasion to 
 take up in a later chapter. 
 
 The photo-chemical reaction which historically was 
 the first to be discovered, we have reserved for the 
 conclusion of our theoretical discussion. It was in the 
 sixteenth century that the alchemists noted the change 
 produced in silver chloride by the action of light, al- 
 though they were unable to form any adequate idea 
 of the nature of this change. Not until the last quarter 
 of the eighteenth century, by the Swedish chemist 
 Scheele, was it shown that silver chloride when ex- 
 posed to light under water loses chlorine. In his ex- 
 periments Scheele found that after light had produced 
 its effect upon the compound, the water contained 
 chlorine in solution, which would form with silver 
 nitrate a precipitate of silver chloride again. By, treat- 
 ing with ammonium hydroxide the residue of darkened 
 silver chloride, Scheele also got what he supposed to be 
 metallic silver. So he concluded that light had reduced 
 the silver chloride to elementary silver, gaseous chlorine 
 escaping in the process. Perhaps it will be of interest 
 to repeat this experiment of Scheele's made in the year 
 1777. 
 
 Experiment 1 1 . Prepare a two per cent, solution of 
 silver nitrate by dissolving 0.2 gram of the solid in 10 
 cubic centimeters of water. Warm, and then com- 
 pletely precipitate the silver from this solution as silver 
 chloride by adding hydrochloric acid until no further 
 change is observed. There should now be present a 
 slight excess of hydrochloric acid. Wash the precipitate 
 thoroughly by decantation, keeping it away from strong 
 light as much as possibL. Prove, by testing as in Ex- 
 
40 CHEMISTRY FOR PHOTOGRAPHERS 
 
 periment 3, that all soluble chloride has been washed 
 from the precipitate. If kept away from the light, the 
 silver chloride will still be white. Treat a small portion 
 of it in another test tube with a little dilute ammonium 
 hydroxide, and observe that the solid is wholly dissolved. 
 Silver chloride is very soluble in ammonium hydroxide. 
 Cover the remainder of the precipitate with 3 or 4 
 cubic centimeters of chloride-free water and expose for 
 some little time to strong sunlight. After the exposure, 
 pour off the water into another clean test tube. Add a 
 drop of silver nitrate solution to this water, and look 
 for the formation of white silver chloride. If any is 
 formed, the chlorine must, if the work has been care- 
 fully done, have been separated by the light from the 
 original silver chloride. Cover again with a little water 
 the darkened residue and treat it with ammonium 
 hydroxide until there is no further solvent action. The 
 dark powder which is left is what Scheele took for 
 elementary silver. It is not the element, however, for 
 it is known to contain chlorine. It is now generally 
 considered to be either a sub-chloride of silver or a solid 
 solution of silver in silver chloride. Whichever may be 
 the fact, it will for our purposes be enough to call it 
 silver sub-chloride, since practically it is silver chloride 
 which has been robbed by the light of some of its 
 chlorine. Thus, when light is allowed to act upon silver 
 chloride, the compound is, as it were, given such a 
 shaking as to set free part of its chlorine. The shorter 
 wave-lengths are the most effective, and particularly 
 those in the region of the spectrum known as the ultra- 
 violet. Photographically speaking, these are the actinic 
 rays. The reaction may be expressed : 
 
 Silver chloride + Light = Silver sutxhloride + Chlorine 
 
LIGHT AND CHEMICAL REACTION 41 
 
 Experiment 12. Prepare silver bromide in an exactly 
 similar manner as the chloride was made in Experi- 
 ment 1 1 , using potassium bromide as the precipitant. 
 Try the solvent action of ammonium hydroxide upon 
 a small portion of the precipitate and note that silver 
 bromide dissolves, but not so readily as did the chloride. 
 Expose some of the salt to light and observe the darken- 
 ing. Reaction: 
 
 Silver bromide + Light 
 
 = Silver sub-bromide -|- Bromine 
 
 Experiment 13. Make another experiment similar to 
 the preceding, except that silver iodide is precipitated, 
 washed, and examined, using potassium iodide in- 
 stead of the bromide. Compare the solubility of silver 
 iodide in ammonium hydroxide with that of silver 
 bromide and silver chloride in the same reagent. Silver 
 iodide is practically insoluble in the ammonia. Ob- 
 serve that upon very carefully washed silver iodide 
 light produces little effect. Add a drop of silver nitrate 
 solution and note that darkening proceeds rapidly. 
 
 Silver nitrate is therefore a sensitizer for the iodide. 
 That it acts in the same capacity with the chloride and 
 bromide may be quickly determined by experiment. 
 The explanation of this action on the part of silver 
 nitrate is as follows : All three of these photo-chemical 
 reactions are essentially reversible, i.e., for example, 
 silver sub-bromide and liberated bromine tend to re- 
 combine as fast as they are formed. In the cases of 
 chloride and bromide, light energy is powerful enough 
 to cause decomposition unaided, although it proceeds 
 with much greater rapidity in the presence of silver 
 nitrate or any other substance which will take up 
 chlorine or bromine. With the iodide, on the other 
 
42 CHEMISTRY FOR PHOTOGRAPHERS 
 
 hand, there must be present some "sensitizer" in order 
 
 that the reaction may go on. Reaction : 
 
 Silver iodide (with sensitizer) + Light 
 
 = Silver ,sub-iodide + Iodine 
 
 In the types of chemical reactions described in 
 Chapter II, together with the photo-chemical ex- 
 periments performed and discussed in the foregoing 
 chapter, we have the basis of all the operations of the 
 photographic art. It is to the application of the princi- 
 ples here so briefly set forth that we now turn in the 
 succeeding chapters. 
 
CHAPTER IV 
 Applied Photo-Chemistry of Silver Salts 
 
 A COMPARISON of the action of light upon the 
 three silver salts, the chloride, bromide, and iodide, 
 shows that the visible effect is greatest with chloride 
 and least with iodide. But since, as we know, it is not 
 metallic silver which is the product of this reaction, 
 but only the relatively unstable sub-compounds, there 
 must be provided some means whereby the partial re- 
 duction initiated in this photo-chemical process may 
 be carried to a finish in completely reduced silver. A 
 little experiment upon the interaction of reducing 
 agents and silver salts will serve to throw considerable 
 light upon this phase of the problem. 
 
 Experiment 14. To a few cubic centimeters of silver 
 nitrate solution, add a little ferrous sulphate in solution 
 and observe the immediate separation of silver as a 
 black powder. Ferrous sulphate is a powerful reducing 
 agent. Next acidify a little ferrous sulphate solution 
 by adding a drop or two of dilute sulphuric acid, and 
 treat with this another portion of silver nitrate solu- 
 tion. The reduction of silver takes place very much 
 more slowly when acid is present. Next prepare small 
 quantities of thoroughly washed silver chloride, silver 
 bromide, and silver iodide, taking care to keep them 
 away from the light as much as possible. Treat a 
 portion of each salt with acidified ferrous sulphate. 
 There is little reduction when the silver salt has not 
 been subjected to the action of light. Spread out upon 
 glass plates more of the three salts and expose them for 
 
 43 
 
44 CHEMISTRY FOR PHOTOGRAPHERS 
 
 five or ten minutes to strong sunlight. Then put a few 
 drops of the acidified ferrous sulphate upon each salt 
 and note that reduction at once takes place. Finally 
 expose to light for a very brief interval, not over half a 
 minute, the rest of the silver salts and treat with acidi- 
 fied ferrous sulphate. In this case it is to be noted that, 
 whereas the chloride is the most visibly affected by 
 light and the iodide is the least affected, reduction 
 by ferrous sulphate of the exposed substances is most 
 marked with the iodide. Reactions : 
 
 (i) Silver nitrate + Ferrous sulphate 
 
 = Silver + Ferric sulphate + Ferric nitrate 
 
 (2) Silver bromide + Light 
 
 = Silver sub-bromide + Bromine 
 
 (3) Silver sub-bromide -|- Ferrous sulphate 
 
 = Silver -|- Ferric sulphate -|- Ferric bromide 
 
 From the experiment it might naturally be inferred 
 that silver iodide would be the most sensitive medium 
 with which to make a coating. In practice a mixture 
 of bromide and iodide gives the best results, a very 
 much greater proportion of bromide than of iodide 
 being used. In the past, various different methods have 
 been employed for supporting the light-sensitive film, 
 and for fastening it to its support. Since we are not here 
 concerned with any of these older methods, largely ob- 
 solete at the present time, the reader is referred for de- 
 tailed accounts of them to those works on photography 
 in which is given a historical survey of the subject. The 
 progress of the art has consistently been in the direction 
 of increased sensitiveness. Inasmuch as the general prin- 
 ciples underlying all the different procedures have been 
 the same, we shall not only save time but arrive at a 
 satisfactory understanding of the matter by confining 
 ourselves to the discussion of present-day practice. 
 
PHOTO-CHEMISTRY OF SILVER 45 
 
 As supports for the sensitive body there are now in 
 use principally three different materials, glass plates, 
 celluloid sheets, either cut in rectangles or in rolls, and 
 a considerable variety of papers. These may be 
 divided into two groups, the glass and celluloid on the 
 one hand and the papers on the other. The former are 
 employed in the first part of the photographic process, 
 namely, the making of the negative ; the latter chiefly 
 in the second part, the making of the print. This 
 distinction may reasonably be made for our conven- 
 ience, but cannot be considered a hard and fast rule, for 
 paper negatives are often made, and the use of glass and 
 celluloid for lantern-slides and transparencies and of 
 celluloid for motion-picture reels must be taken into 
 account. But by this classification we are enabled to 
 treat separately the principles involved in the making 
 of negatives, leaving the numerous printing processes 
 for later chapters. Of the relative advantages of glass 
 plates and celluloid sheets as supporting media, it is 
 needless here to say more than a word. Glass is heavy 
 and celluloid is light when it comes to carrying large 
 quantities, and the question is easily answered by the 
 factor of weight, taking into account also the con- 
 venience of the roll form in which celluloid film is 
 supplied. For the amateur who gets beyond the "snap- 
 shot" stage and begins really to be initiated into the 
 photographic "mysteries," glass plates are almost cer- 
 tain to be found most desirable, partly on account of 
 their lower price, but chiefly because they can be had 
 of different qualities to suit different purposes. 
 
 So far, we have considered the light-sensitive sub- 
 stance and the foundation upon which it is laid down. 
 Now it remains to bring these two factors togeither. 
 
46 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Whatever be the nature of this foundation, the link that 
 binds them together is of the same general character. 
 Confining ourselves to glass as a basis for simplicity's 
 sake, we find that the ordinary "dry plate," or glass 
 plate, consists of a coating of gelatine containing the sil- 
 ver salt and sometimes other materials, spread upon one 
 surface of a rectangle of clear, uniform glass. TJie next 
 experiment illustrates the nature of this gelatine coating. 
 Experiment ly. Soak in 25 cubic centimeters of 
 cold water contained in a small conical flask 1.5 gram 
 of gelatine. As soon as the gelatine has swelled, dissolve 
 it by gentle warming. Next dissolve 3.5 grams of 
 silver nitrate in 5 cubic centimeters of water and mix 
 thoroughly with the former solution. In 10 cubic 
 centimeters of water dissolve together o.i gram of 
 potassium iodide and 3.0 grams of potassium bromide. 
 Warm the gelatine mixture and, by the red light of the 
 darkroom, add to it the solution of bromide and iodide 
 drop by drop, shaking it up thoroughly after each 
 addition. By this means there are formed everywhere 
 throughout the mass of gelatine minute solid particles 
 consisting of silver bromide and silver iodide. Such a • 
 mixture of liquid with finely divided solid particles 
 suspended in it is called an emulsion. Set the little 
 flask in a dish containing boiling hot water and keep it 
 at that temperature for half an hour, protecting it 
 from all white light during this operation. Transfer 
 the emulsion to a small wide-mouthed bottle such as a 
 vaseline jar, for example; stand the jar in cold water, 
 and allow the emulsion to set to a firm consistency. 
 It will now be in order to consider just what the 
 emulsion contains. According to the proportions given 
 for silver nitrate and potassium bromide and iodide, 
 
PHOTO-CHEMISTRY OF SILVER 47 
 
 there is an excess of the potassium salts present above 
 the amount required to form silver bromide and silver 
 iodide with the given quantity of silver nitrate. Con- 
 sequently there are now present in the gelatine emulsion 
 the unused potassium salt and the potassium nitrate 
 formed in the metathesis. It is necessary to free the 
 emulsion from these soluble salts. As it is very sensitive 
 to light, all these operations must be performed in the 
 darkroom. When the emulsion has set thoroughly, 
 take it from the jar and put it into the middle of a 
 small square of white mosquito netting. Gather up the 
 corners, twist them together, and squeeze the jelly 
 through the cloth into a dish of cold water, holding the 
 cloth under the surface, which will prevent the shredded 
 emulsion from running together again. Put the shreds 
 back in the cloth, pour cold water liberally over them, 
 and let them soak in cold water for a quarter of an 
 hour. Repeat the process of squeezing through the 
 cloth and washing, after which allow the water to drain 
 off, and then by gentle heat melt up the emulsion and 
 pour a portion of it upon a small, clean, dust-free glass 
 plate, tilting the plate in such a way as to allow the 
 emulsion to run completely over its surface. Any ex- 
 cess of gelatine may be run from one corner back into 
 the jar. Put the plate down level to permit the emulsion 
 to set and dry. When thoroughly dried this "dry plate" 
 can be exposed under a negative, developed, fixed, and 
 washed in the usual way. Coat several more small 
 squares of glass with the rest of the emulsion which has 
 been prepared; when dry they may be stored in a light- 
 proof box for further experiments. 
 ^ All the reactions which take place in the operations 
 of making and exposing this plate have already been 
 
48 CHEMISTRY FOR PHOTOGRAPHERS 
 
 described. The reason for using an excess of bromide 
 and iodide in the manufacture of photographic emul- 
 sions is that silver nitrate and gelatine interact chemi- 
 cally. The compound formed by their reaction will also 
 react with the bromide and iodide salts to form silver 
 bromide and silver iodide. But, were there an excess of 
 silver nitrate used, this excess could not be washed out, 
 since the silver would be chemically combined with 
 the gelatine. On the other hand, when the excess of 
 bromide and iodide is added, all the silver present in 
 the gelatine solution reacts by "metathesis with them. 
 Then the excess of soluble potassium salts can readily 
 be removed by washing. If the solutions out of which 
 the emulsion is made are comparatively cold when 
 mixed together, suitable examination will show that 
 the particles of insoluble silver salt are very minute. 
 A plate coated with such an emulsion will be relatively 
 "slow," i.e., it will require a long exposure to light to 
 get a satisfactory image. In the manufacture of dry 
 plates it was early discovered, first, that they could be 
 made more rapid by allowing the emulsion to stand at a 
 temperature of about 30 degrees Centigrade for some 
 days, a process called ripening; and second, that a 
 similar ripening could be secured by keeping the 
 emulsion at the boiling temperature for only a few 
 miniutes; and finally, that simply the addition of 
 ammonium hydroxide produced the same result with- 
 out any heating at all. The reason for the physical 
 difference between the unripened and the ripened 
 emulsion can very easily be shown by a simple ex- 
 periment. 
 
 Experiment 16. Treat a little cold dilute solution of 
 silver nitrate with a few dfopfs of dilute hydrochloric 
 
PHOTO-CHEMISTRY OF SILVER 49 
 
 acid without shaking. The precipitated silver chloride 
 is seen to be in a very finely divided state. Now add 
 more of the acid until precipitation is complete and 
 warm the mixture to boiling. Shake it vigorously. 
 
 The precipitate now is gathered in larger bunches, 
 or, as the chemist says, is flocculent. So silver chloride 
 may be formed in a very fine state of division, but the 
 minute particles, when heated and shaken tend to 
 collect together into larger aggregations. The same 
 thing is true of silver bromide and silver iodide. In 
 general, the more rapid the emulsion the larger the 
 particles of silver salt in it, or, as the photographer 
 says, the coarser the grain of the plate. Thus the slower 
 plates have the advantage over fast emulsions of fine- 
 ness of grain, which is a point that it is sometimes 
 useful to remember. It is not now considered that the 
 rapidity of an emulsion is dependent upon the size of 
 its particles, but that the increase in size is an accessory 
 circumstance to the ripening. 
 
 We may now review briefly the reactions which take 
 place when one of our home-made dry plates is sub- 
 mitted to the action of light for a short interval of 
 time. Examination after the exposure discloses no 
 perceptible change in its appearance, but from our 
 previous experiments we may be sure that, wherever 
 the light has fallen upon the sensitive emulsion, silver 
 bromide has been altered to silver sub-bromide and 
 silver iodide, to sub-iodide. In the case of the iodide 
 we have learned that the condition necessary for this 
 action is that there should be present some substance 
 which is capable of reaction with the liberated iodine. 
 And with the bromide also the action is very much 
 facilitated by the presence of a bromine-absorbent. 
 
50 CHEMISTRY FOR PHOTOGRAPHERS 
 
 in order to prevent the reverse reaction from occurring. 
 Gelatine, as it fortunately happens, is such an ab- 
 sorbing substance. Thus when the light energy is 
 received in the emulsion, it is transformed into the 
 chemical energy of separation of bromine and iodine 
 from bromide and iodide respectively, and the free 
 bromine and iodine immediately interact with the 
 gelatine. In this way they are safely removed from the 
 sphere of chemical action. It is chiefly due to this fact 
 that the present day photographic emulsion is so 
 extremely rapid. That this partial reduction has been 
 effected by the exposure to light may finally be dem- 
 onstrated by flowing over the plate, in the darkroom, a 
 little ferrous sulphate solution acidified with sulphuric 
 acid. Any portions of the plate which were in the ex- 
 posure protected from the light will be unchanged, but 
 wherever the light reached the emulsion an immediate 
 reduction of the sub-salt to metallic silver will occur. 
 
 If in the exposure the whole surface of the plate is 
 not uniformly illuminated, but different portions are 
 subjected to light of different intensities, in the various 
 regions different amounts of sub-salt are formed. This 
 follows from the fact that the quantity of silver salt 
 reduced to sub-salt is directly proportional to the 
 amount of light absorption. But since the whole of the 
 plate was exposed for the same length of time, the 
 quantity of light absorption is directly proportional 
 to the intensity. So, when the plate is developed by 
 ferrous sulphate, in those portions affected by more 
 intense illumination there is a proportionately greater 
 amount of silver reduced than in the parts affected 
 by less illumination. At the upper limit, or maximum 
 effect, all the silver in the part of the emulsion con- 
 
PHOTO-CHEMISTRY OF SILVER 51 
 
 cerned will be reduced to metallic silver. In the mini- 
 mum, no reduction whatever will take place. And 
 between these two limits there is possible theoretically 
 an infinite number of steps. We have here an ex- 
 planation of the exquisite gradation of tones which 
 forms one of the pre-eminent characteristics of the 
 photographic print. Practically the number of grades is 
 limited on account of the fact that the particles of 
 silver salt have appreciable size, and therefore the 
 number of particles in a given area varies with the size 
 of the individual particles. Thus the depth of a given 
 tone depends upon the number of particles of silver 
 salt reduced to silver, and the least possible difference 
 in tone is that due to a difference of one in the number 
 of particles. Therefore a coarse-grained emulsion, or 
 other things being equal, a rapid one, necessarily gives 
 less gradation than a fine-grained, or slow, emulsion. 
 
 So far we have considered only the effect of white 
 light. But, as we know, white light consists of a 
 mixture of an infinite number of different wave-lengths 
 in the luminiferous ether, and we have previously 
 stated that some photo-chemical reactions are especially 
 promoted by particular sets of wave-lengths, such as, 
 for example, the change of yellow phosphorus to its red 
 modification, in which the blue, violet, and ultra-violet 
 rays are effective. If we should expose some of our 
 home-made dry plates to the different sets of wave- 
 lengths, or colored rays, of the spectrum, which is pro- 
 duced by passing a beam of white light through a 
 prism, a very long exposure indeed would be necessary 
 to produce upon development a perceptible effect with 
 the infra-red rays, which are invisible of course because 
 their wave-lengths are too great. With the red rays 
 
52 CHEMISTRY FOR PHOTOGRAPHERS 
 
 hardly less exposure would be needed to give an 
 appreciable image. The yellow would require some- 
 what less. But even in the green the exposure would yet 
 have to be markedly long, although very much shorter 
 than with the red rays. When we came to the blue and 
 violet, however, we should find the sensitiveness of 
 the plate as notable as was its lack of sensitiveness at 
 the other end of the spectrum. And in the ultra-violet, 
 invisible because of the shortness of wave-length, the 
 sensitiveness is but a little diminished. To put the 
 matter in another way, if a strip of dry plate is ex- 
 posed to the whole spectrum at once, little or no effect 
 is produced by the infra-red and red rays, and very 
 little by the orange and yellow even with a long ex- 
 posure. A more pronounced effect begins in the green, 
 the maximum occurs in the blue-violet, and there is a 
 falling off in the ultra-violet. The sensitiveness of the 
 eye to the various portions of the spectrum is quite 
 different from this. The infra-red, it is true, makes no 
 impression upon the sense of sight, but beginning with 
 the red its sensitiveness rapidly increases to a maximum 
 in the yellow. From here the brightness diminishes 
 again until it reaches zero once more in the ultra-violet. 
 To the eye the brightest part of the spectrum is the 
 region from orange through yellow to green, parts that 
 are practically dark to ordinary photographic emulsion; 
 whereas to the emulsion the brightest portion lies in 
 the blue-violet, which is comparatively dark to the eye, 
 and the brightness extends beyond into the ultra-violet, 
 which is totally black to the eye. Since daylight 
 ordinarily contains a preponderance of blue, violet, and 
 ultra-violet rays, and red, orange, and yellow rays act 
 to no appreciable extent upon the ordinary emulsion, a 
 
PHOTO-CHEMISTRY OF SILVER 53 
 
 plate coated with it and exposed in the camera to a 
 landscape, for example, registers the view as it would 
 appear to an eye whose sensitiveness instead of having 
 its maximum in the yellow has this maximum in the 
 blue-violet, the same as the plate. The landscape, too, 
 commonly has an abundance of green foliage, to the 
 rays from which the emulsion is but little sensitive. 
 Using such emulsion, it is a matter of no small difficulty 
 photographically to make a landscape which will show 
 the verdure in foreground, the distance, and the sky 
 as these factors appear to a discriminating eye. To 
 represent the blue sky and even the billowy clouds by a 
 practically uniform expanse of white paper, and the 
 varied greens of grass and trees by dense blackness is 
 to give a very untruthful rendering. 
 
 The position of the maximum sensitiveness of the 
 photographic emulsion cannot be moved from the blue- 
 violet region, but by incorporating with it certain 
 dyes, it is possible to produce secondary maxima in 
 other regions of the spectrum. Thus, by treating one 
 of our home-made plates with a solution of the blue 
 dye cyanin, and allowing it to dry again, we shall have 
 a plate which is sensitive far into the red. A purple 
 solution of pinacyanol also gives red-sensitiveness and 
 great rapidity to the plate, but one so prepared must 
 be handled and developed by a green light, for it is 
 quite insensitive to the green rays. With pinaverdol 
 there is a great increase in the sensitiveness to the 
 orange, yellow, and green. If these dyes are added in 
 the process of manufacturing the emulsion, the plates 
 so prepared have fair keeping qualities. Plates thus 
 made keep better, but are not quite so color-sensitive 
 as those which are dyed just before using. The action 
 
J4 CHEMISTRY FOR PHOTOGRAPHERS 
 
 of these dyes in producing red, yellow, and green 
 sensitiveness is not wholly clear. Possibly they form 
 very complex silver compounds which are specially 
 sensitive to the respective rays. Inasmuch as it is the 
 light absorbed which performs the chemical work, 
 the suggestion has been made also that perhaps the 
 dye, after absorbing its particular rays, is able to give 
 out again the energy so stored up. Where these rays 
 act upon the plate, therefore, an increased reduction 
 of silver salt occurs. Color-sensitized plates are sold 
 under the names isochromatic, orthochromatic, pan- 
 chromatic, etc. But as already stated, the maximum 
 sensitiveness to the violet end of the spectrum cannot 
 be changed by any means that has so far been dis- 
 covered, and even when red-sensitiveness has been 
 increased as much as possible, the blue-sensitiveness 
 still overbalances it. For this reason it is necessary in 
 order to secure the full benefit of color correction to cut 
 out most of the blue, violet, and ultra-violet rays from 
 the light which is transmitted to the plate. This is 
 done by putting over the lens a piece of yellow glass, 
 called a ray filter or ray screen. Being, as we say, 
 colored yellow, this glass has the property of absorbing 
 the rays from the blue end of the spectrum and so 
 protects the plate from their action. A disadvantage 
 in the use of ray filters lies in the fact that the artificial 
 sensitiveness of the plate to the red cannot be made as 
 great as its natural sensitiveness to the blue. With the 
 ray filter the plate is therefore much less rapid than 
 without it. 
 
 Thus far, in speaking of the exposure of our emulsion 
 to light, we have been concerned merely with that 
 duration of exposure which would give an appreciable 
 
PHOTO-CHEMISTRY OF SILVER 55 
 
 image by use of such a reducing agent as ferrous 
 sulphate, for example, or as we should say, photo- 
 graphically, upon development. If we now inquire 
 what will be the effect of submitting a plate to ex- 
 posures of various durations, we shall obtain some very 
 interesting results. Let us suppose that with the con- 
 ditions of illumination, the size of stop it is desired to 
 use in the lens, and the sensitiveness of the plate 
 employed, an exposure of one second would give us 
 what the photographer is wont to call a "perfectly 
 timed" negative. We recognize at once that, if we 
 allow but a very small fraction of a second for the 
 exposure, there will not be time enough, or much 
 better, not energy enough in the light which reaches 
 the plate, to affect more than a very small proportion 
 of the sensitive silver salt. Thus, we would get an 
 "under-exposure," a sort of negative generally char- 
 acterized by a few dense patches corresponding to the 
 highest lights in the object, and clear glass, giving none 
 of the details corresponding to the deepest shadows. 
 By making a series of exposures, each one, say, double 
 the preceding, we find as we progress towards the 
 standard exposure of one second that finally a point is 
 reached where a negative can be developed which com- 
 pares quite favorably with one perfectly timed, and yet 
 the duration of the exposure which produced it was 
 considerably less than the normal. Furthermore, by 
 carrying our series of exposures up above one second, 
 we can discover an upper limit at which a passable 
 negative will also be secured. This range of expos- 
 ures within which acceptable negatives can be made 
 is commonly called the "latitude" of the emulsion, 
 and varies somewhat with different brands of plates 
 
56 CHEMISTRY FOR PHOTOGRAPHERS 
 
 and with different speeds of emulsion. It is the safe- 
 guard of the amateur. In general, slower plates have 
 greater latitude than the faster ones. But above this 
 upper limit, the negatives which result become more 
 and more disappointing. They are indeed "flat, stale, 
 and unprofitable." The best cure for overexposure, 
 as it is also for underexposure, is to throw away the 
 plates and try again, using an exposure-meter to 
 determine the correct time. 
 
 But suppose we allow light to act for periods succes- 
 sively very much greater than the normal one second. 
 Presently upon development one of them will come up a 
 positive, instead of a negative, highlights in the plate 
 now corresponding to highlights in the object, and 
 shadows to shadows. This being the reverse of what 
 usually happens, is known as the "reversal" of the 
 image. A further increase in the length of exposure 
 will give a negative again; still more time, a positive; 
 and yet more, a negative. 
 
 Experiment 17. Under such conditions as would 
 normally demand an exposure of about one second, 
 submit an ordinary plate or film in a camera to the 
 action of light from some object for about one minute, 
 and develop as usual. Give to another plate an ap- 
 proximately normal exposure; in the darkroom place 
 it for two minutes in a dilute solution of potassium 
 dichromate, rinse off the dichromate, and then try to 
 develop it into a negative. Consider that the silver 
 salt which has been partially reduced by light has first 
 been subjected to the action of a good oxidizer (di- 
 chromate) before the application of the developer. 
 Give to a third plate an exposure approximately normal. 
 This will naturally develop into a negative. When it 
 
PHOTO-CHEMISTRY OF SILVER 57 
 
 has been developed, rinse, and put it into a dilute 
 solution of dichromate, and carry it out, in the tray, 
 into daylight. As soon as the plate has bleached out, 
 wash it in running water for half a minute and develop 
 it again, in daylight, with the same developer as before 
 used. The image should be reversed. 
 
 Any other soluble oxidizing agent, such as potassium 
 permanganate, hydrogen dioxide, etc., would do just 
 as well as the dichromate. In explaining this curious 
 behavior of the light-sensitive emulsion, we must take 
 several things into account. The silver sub-salt formed 
 by the reducing action of light is not only readily 
 capable of being further reduced by suitable reducing 
 agents, but, since the reaction is essentially a reversible 
 one, in the absence of a sensitizer (gelatine in this case), 
 the sub-salt is quite as readily oxidized again. Thus, 
 silver bromide, sealed up in a glass tube from which 
 the air has been exhausted, when submitted to light is 
 separated into bromine and silver sub-bromide. But as 
 soon as the tube is taken away from the light, bromine 
 and sub-bromide commence to recombine, forming sil- 
 ver bromide again. By experiment with the different 
 regions of the spectrum, it has been shown that the 
 reversing effect is produced by the rays of greater 
 wave-length {i.e. from the lower, or red; end of the 
 spectrum). Taking this fact in connection with our 
 experiment in which potassium dichromate prevented 
 the negative image from being formed after a normal 
 exposure, we see that when a plate is exposed for a 
 sufficient length of time, the silver sub-salt formed, so 
 to speak, by the short wave-lengths in the normal 
 part of this exposure is by the action of the longer 
 wave-lengths in the balance of the exposure time 
 
58 CHEMISTRY FOR PHOTOGRAPHERS 
 
 caused to unite with oxygen from the air, that is, it is 
 reoxidized. But in the meantime those portions of the 
 silver salt which, after normal exposure, would have 
 remained unreduced to sub-salt, by the excessive ex- 
 posure have been so reduced. The condition of the 
 plate, consequently, now is such that the portions of 
 silver salt which normally would be reducible by the 
 developer, having been reoxidized, are incapable of 
 reduction; whereas those portions which normally 
 would not have been reducible are present as light- 
 reduced sub-salts and therefore are reduced to silver 
 by the developer. So a reversed image must be formed. 
 By incorporating in the emulsion a suitable reducing 
 agent, which not only prevents the reoxidation of sub- 
 salt, but also serves as absorbent of the greater amounts 
 of bromine set free in prolonged exposures, reversal 
 can be entirely prevented. Plates thus prepared are 
 incapable of forming reversed images even with ex- 
 posures many hundred times the normal, and thus 
 cannot be overexposed. By this means sufficient ex- 
 posure can be given to secure details in the shadows 
 without at the same time completely blocking up the 
 highlights. Hydrazine salts are the compounds em- 
 ployed for the purpose, and the plates are called 
 Hydrazine, or Hydra, plates. At first thought it might 
 seem that here is a solution of the whole problem of 
 exposure. Use Hydrazine plates and give plenty of time, 
 thus avoiding the Scylla of underexposure, there being 
 no Charybdis. But the matter is not quite as simple as 
 this. There is such a thing as getting too much detail 
 even in the shadows. As in all over-exposed plates, 
 there is a tendency toward flatness in the Hydrazine 
 plates, so that they must be used with discrimination. 
 
PHOTO-CHEMISTRY OF SILVER 59 
 
 There is one defect inherent in glass plates which we 
 must mention before concluding this chapter. It is 
 present very little in celluloid films, and this fact 
 forms another point of advantage of films over plates. 
 When an interior is to be photographed in which 
 brightly illuminated windows are included in the view, 
 the glass negative always shows much blurring about 
 these highlights. There is so much reduced silver all 
 around the images of the windows as sometimes to 
 obscure their outlines. The same effect is also often 
 produced in out-of-door work when patches of sky 
 show through branches of foliage; and it can even be 
 found in portraiture where the subject is dressed in 
 white. The effect is known as "halation," and has a 
 very simple cause, and an equally simple cure. In the 
 thin coating of emulsion upon the glass plate it is 
 impossible that all of the impinging light should be 
 absorbed. The part that is absorbed by this film per- 
 forms, as we have seen, its chemical work and pro- 
 duces the latent photographic image. That portion of 
 the light which, failing to be absorbed, is transmitted 
 through the coating passes into the glass behind it. 
 Here again a small part is absorbed by the glass, while 
 the remainder finally reaches the under surface of the 
 plate. At this surface another division occurs, part of 
 the light passing through and into the space behind the 
 plate, and the rest suffering reflection. It is the re- 
 flected part of the light, which has come through the 
 sensitive coating and the glass, which produces the 
 untoward effect of halation, for it now is directed back 
 toward the under side of the gelatine film, at an angle 
 such as to strike the film at a different place from where 
 it came through. Passing into the emulsion it, of course, 
 
6o CHEMISTRY FOR PHOTOGRAPHERS 
 
 forms sub-salt of silver all around the latent image of 
 the highlight in question. To prevent this reflection at 
 the back surface of the glass is to do away with hala- 
 tion, and there are two methods in use. One is to apply 
 a coat of slow emulsion upon the glass, over which a 
 second rapid film is spread. Such double-coated plates 
 are a great improvement over the single-coated variety 
 in several ways, but while halation is very much 
 diminished by this means, it is not in fact entirely 
 eliminated. The second method is to coat the back 
 of the plate with a substance which will absorb all of 
 the light which reaches that surface of the glass. None 
 can then be reflected to the under side of the emulsion. 
 Such a "backed" plate can be had at a slight extra 
 cost over the unbacked kind, and once used will prob- 
 ably mean always used. 
 
CHAPTER V 
 Chemistry of Development 
 
 CHEMICALLY speaking, the development of a 
 negative consists in the complete reduction, to 
 metallic silver, of the light-affected silver salt. In 
 order thus to get a negative, it is necessary, of course, 
 that the unaffected silver salt should escape such reduc- 
 tion. This fact, then, establishes the prime character- 
 istic of a photographic developer, namely, that its 
 reducing power with silver salts must be so adjusted as 
 to enable it to act selectively upon these light-exposed 
 compounds in the emulsion, reducing the sub-salts and 
 leaving unreduced the remaining silver bromide and 
 silver iodide. In our experiments with the strong 
 reducing agent, ferrous sulphate, we learned that its 
 simple solution in water is capable of instantly reducing 
 silver nitrate to silver. But by acidifying the ferrous 
 sulphate, we were able to control and modify its re- 
 ducing power, even a few drops of acid greatly retarding 
 the reaction, although not preventing it entirely from 
 taking place. Since all chemical action involves energy, 
 i.e., work, it is plain that a certain amount of chemical 
 work must be performed when silver is reduced from 
 such a compound as silver nitrate, or silver bromide. 
 Now, in consequence of the law of conservation of 
 energy, the amount of work done when a given quantity 
 of silver bromide, for example, is completely reduced to 
 silver is the same as the sum of the amounts con- 
 cerned in the partial reduction of this quantity to sub- 
 bromide and the final reduction of sub-bromide to 
 
 6i 
 
62 CHEMISTRY FOR PHOTOGRAPHERS 
 
 silver. In other words, it makes no difference in the total 
 quantity of chemical work done whether the reduction 
 be all at once to silver or whether it be carried out in the 
 two steps. Therefore, evidently, since the energy of light 
 has been used in the first step, that is, in the reduction 
 to sub-salt in the exposure, there is less chemical work 
 for the reducing agent in the developer to do in reduc- 
 ing the sub-bromide than in reducing the unaffected sil- 
 ver bromide. This explains why a suitably regulated 
 reducing agent (the photographic developer) is able to 
 bring out upon the plate in metallic silver the latent 
 image produced in the exposure to light. Because le» 
 chemical work is required to be done in the reduction 
 of silver sub-bromide to silver than in reducing silver 
 bromide to the metal, the developer proceeds to act first 
 upon the former. That it will in time act also upon the 
 silver bromide we shall later see. 
 
 Experiment i8. Make up a o.y% solution of pyro- 
 gallic acid by dissolving o.i gram of the solid in 20 
 cubic centimeters of water, and mix by stirring with a 
 glass rod. Avoid shaking the tube. Pour a few cubic 
 centimeters into a test tube, shake well, and note that 
 there is no darkening of the solution. Add a drop or 
 two of dilute sulphuric acid to the bulk of the solution 
 and mix by stirring. Shake up a little of this solution in 
 a test tube. The color change is slight. In half of the 
 remaining liquid dissolve 0.2 to 0.3 gram of dry sodium 
 sulphite, and in the other half about the same quantity 
 of anhydrous sodium carbonate. Shake up each of 
 these mixtures and observe how rapidly the second 
 turns yellow, whereas the first shows no change. 
 
 Pyrogallic acid is an organic compound obtained by 
 heating gallic acid, which is a constituent of gall-nuts 
 
CHEMISTRY OF DEVELOPMENT 63 
 
 and many other vegetable substances such as, for ex- 
 ample, tea. It is a strong reducing agent, being itself 
 oxidized in the process to oxalic and acetic acids. Thus, 
 when exposed to the air, its solutions absorb oxygen, 
 just as do solutions of ferrous sulphate, and oxidation 
 of the pyrogallic acid occurs. But this oxidation does 
 not take place so rapidly in a plain solution of pyro- 
 gallic acid, or in one containing free sulphuric acid, as it 
 does when the solution is made alkaline as with sodium 
 carbonate. Also when sodium sulphite, which is like- 
 wise a red ucing age nt and oxygenabsorber, is present 
 in the solution, the oxidation of pyrogallic acid is 
 hindered, since the absorbed oxygen is used up first in 
 oxidizing the sulphite to sulphate. 
 
 Experiment iq. Dissolve o.y gram of sodium car- 
 bonate (anhydrous) in 10 cubic centimeters of water, 
 add to this o. i gram of pyrogallic acid and observe the 
 color immediately developing. Now add to the mix- 
 ture a few cubic centimeters of a strong solution of 
 sodium sulphite and note that the yellow color is at 
 once bleached, but that after the lapse of sufficient time 
 discoloration again occurs. 
 
 In alkaline solution the brown substance formed by 
 oxidation is soluble. It is in the nature of a dye, and is 
 fairly insoluble in water in the absence of alkali. Its 
 dilute solutions are yellow, turning brown when the 
 concentration increases. Plates and films developed 
 with "pyro" sometimes are stained a yellowish brown 
 because after the free alkali has been washed out this 
 brown dye is insoluble in the water. Also, it seems to 
 have a tendency to become a "fast" color in the 
 gelatine similar to other dyes in fabrics. 
 
 Experiment zo. Treat with a few drops of sodium 
 
64 CHEMISTRY FOR PHOTOGRAPHERS 
 
 sulphite solution a little silver nitrate dissolved in 5 
 cubic centimeters of water, and note that white silver 
 sulphite is precipitated. No reduction occurs. Next 
 prepare a small quantity of washed silver bromide 
 unexposed to white light. Dissolve a little of this un- 
 exposed salt in a test tube with enough ammonium 
 hydroxide, add to it a little pyrogallic acid solution, 
 and observe the immediate blackening of the liquid. 
 This is due to the reduction of silver and to the dark- 
 colored oxidation products of the alkaline pyrogallate. 
 Thus in alkaline solution, pyrogallic acid is able to ef- 
 fect the complete reduction of even unexposed silver bro- 
 mide. The essential condition for this reaction is that the 
 silver bromide should be in solution. In the operation 
 the pyrogallic acid is oxidized, and the bromine com- 
 bines with the alkali to form alkali bromide, in the case 
 mentioned, ammonium bromide. Reactions : 
 
 (i) Silver nitrate + Sodium sulphite 
 
 = Silver sulphite + Sodium nitrate 
 
 (2) Silver bromide + Ammonium hydroxide 
 
 = Silver-ammonia bromide + Water 
 
 (3) Silver-ammonia bromide 4- Pyrogallic acid 
 
 = Silver -|- Ammonium bromide -t- Oxidation 
 products of pyrogallic acid 
 
 Experiment 21. Make up a little "pyro" developer, 
 in two solutions, as follows : 
 
 I . Sodium sulphite, dry 10 grams 
 
 Pyrogallic acid 3 grams 
 
 Sulphuric acid (concentrated) 2 drops — 
 
 Water, to 100 cubic centimeters 
 
 1. Sodium carbonate, anhydrous 5 grams 
 
 Water, to 100 cubic centimeters 
 
 When ready to use this developer, mix the two solu- 
 tions and dilute to 300 cubic centimeters with water. 
 For carrying out this experiment take three small- 
 
CHEMISTRY OF DEVELOPMENT 6y 
 
 sized dry plates. The first plate is to be given a normal 
 exposure in the camera or under a negative, but should 
 be taken, like the other two, from the package in total 
 darkness when loading it in the plateholder. After ex- 
 posing this plate, mix the prepared developer solutions 
 and put about equal portions in each of three trays, 
 adding i cubic centimeter of a ten per cent, solution of 
 potassium bromide to the second tray. Turn off the 
 red light, and in the dark take out the other two un- 
 exposed plates from the box and the one which has 
 been exposed from the plateholder and, as quickly as 
 possible, slip the three plates into the trays, the un- 
 exposed two in the first two, the exposed in the third 
 tray. Cover the trays, after which the red light may be 
 turned on again. At intervals for ten to twelve minutes 
 the covers may be lifted and the plates examined. 
 
 The unexposed plate in the first tray will begin to 
 show reduction of silver, that is, "fog," in about two 
 minutes, whereas the exposed plate in the third tray 
 will develop without fogging for as much as ten minutes. 
 In the second tray to which has been added the potas- 
 sium bromide, the unexposed plate remains free from 
 fog for at least as long as the exposed plate. If a fourth 
 plate were to be exposed normally and put into a 
 solution containing pyrogallic acid, sodium sulphite, 
 and acid, without the addition of any of the alkali, 
 we should see that development, otherwise reduction 
 of silver, would be extremely slow. This effect might be 
 expected from the discussion of Experiment i8, re- 
 calling that a solution of pyrogallic acid containing 
 mineral acid possesses far less reducing power than one 
 containing free alkali. 
 
 In order to make clear what is indicated by the ex- 
 
66 CHEMISTRY FOR PHOTOGRAPHERS 
 
 periment just described, let us consider that in the 
 gelatine emulsion with which the dry plate is coated 
 we have a layer of gelatine, of definite and measurable 
 thickness, in which are embedded minute lumps, 
 spaced with some irregularity, of light-sensitive silver 
 salt. For convenience we will disregard the silver 
 iodide and consider only the bromide, sincic the action 
 is essentially the same in the case of both. Thus the 
 particles of silver bromide in the body of the gelatine 
 may be likened to plums in a pudding. The structure 
 of the gelatine itself may be compared somewhat to 
 that of a honeycomb if it be imagined first that the 
 comb consists of the same material precisely as the 
 honey, and second, that, instead of the cells of the 
 comb fitting tightly together all round, there are 
 spaces between in places, making in effect tubes or 
 tunnels into and through the mass. It is by means of 
 these infinitesimal passageways that the reducing solu- 
 tion which we call the developer makes its way into 
 the body of the gelatine and thus comes into contact 
 with the silver bromide particles. If we now limit the 
 discussion to a single one of these particles of bromide, 
 we shall see that its microscopic mass is surrounded on 
 all sides by comparatively transparent gelatine, but 
 that it can be reached by water and by substances dis- 
 solved in water through the pores in the gelatine. That 
 is, the particle is accessible to both the light and the 
 developer, but is somewhat protected, after all, from 
 the latter. Taking the first plate which was put, un- 
 exposed, into developer containing no potassium 
 bromide, let us see what happens to our particle of 
 silver salt. First, let it be noted that, microscopic 
 though it be in size, nevertheless the particle contains 
 
CHEMISTRY OF DEVELOPMENT 67 
 
 a very large number of molecules of silver bromide. 
 As soon as the alkaline pyrogallate solution penetrates 
 through the gelatine envelope and touches the surface 
 of the particle, silver bromide commences to dissolve 
 in the liquid. That is, molecules of silver bromide 
 begin to pass from the solid particle into the liquid. 
 But we have already learned by an experiment that 
 alkaline pyrogallate reduces silver from a solution of 
 silver salt, and therefore it is not surprising to find here 
 that reduction occurs, if we can be assured that any 
 silver bromide, which we have thought of as an insolu- 
 ble compound, really goes into solution. As a matter 
 of fact, no substance is really insoluble in the absolute 
 sense. Silver bromide, indeed, is soluble to the extent 
 of 0.002 gram in i ,000 cubic centimeters of water, but 
 its solubility can be varied, as we shall presently see, 
 by having other substances too in the solution. So, as 
 the molecules of silver really do go into the liquid one 
 after another, and at once are reduced there, the silver 
 from them is deposited in solid form practically in the 
 spot formerly occupied by the bromide, the bromine 
 is taken up by the liquid, forming sodium bromide in 
 solution, and a corresponding amount of pyrogallic 
 acid is oxidized. In the experiment with the plate we 
 actually found that silver was reduced, and we called 
 it "fogging" of the plate. This chemical "fog," there- 
 fore, is simply the reduction^ silver" frorri'silver salt 
 without any mediation of light, and is bound to take 
 place so soon as silver bromide and alkaline pyrogallate 
 come together in solution. 
 
 But, as appeared from the experiment with the 
 second unexposed plate, such reduction can be very 
 much hindered. This plate was put into developer 
 
68 CHEMISTRY FOR PHOTOGRAPHERS 
 
 which differed from that in the first tray by containing 
 a small quantity of potassiijm bromide. Since this was 
 the only difference between the two experiments, the 
 delay in the reduction of silver can have been caused 
 only by the presence of this potassium bromide. Let 
 us see how this reagent acts towards silver bromide in 
 solution. 
 
 Experiment 22. Dissolve a little silver bromide with 
 a few cubic centimeters of dilute ammonium hydroxide. 
 Add to this solution 2 or 3 cubic centimeters of a fairly 
 strong solution of potassium bromide in water and 
 observe that silver bromide is reprecipitated. 
 
 The solubility of silver bromide is therefore less when 
 potassium bromide is present in the solution than in 
 the absence of the latter salt. Thus, when the developer 
 solution containing potassium bromide came in con- 
 tact with the gelatine-embedded silver bromide particle 
 in the second plate, on account of the diminished 
 solubility of the silver bromide the molecules of this 
 compound were very much hindered in their passing 
 into the solution by the potassium bromide already 
 contained in it. Since the rate at which the silver 
 bromide was going into solution was in this way 
 diminished and since also the rate of its reduction 
 depends upon this rate of solution, it was inevitable, 
 now that we understand the situation, that the re- 
 duction, or otherwise the "fogging," should be retarded 
 by the addition of potassium bromide. In photo- 
 graphic parlance this action is called "restraining" 
 action, and potassium bromide, a "restrainer." 
 
 Passing finally to the third plate, which was normally 
 exposed, but developed in an unrestrained solution, 
 comparably with the second plate which has just been 
 
CHEMISTRY OF DEVELOPMENT be, 
 
 under discussion, we may perhaps see at once that 
 bromide must have been somehow furnished by the 
 plate, since none was beforehand added to the developer. 
 To explain this circumstance, let us consider again our 
 single bromide of silver particle, enveloped in gelatine, 
 and note what, has happened to it during the exposure 
 to light. Let us suppose that the particle was so situated 
 that it received a very intense illumination. Because 
 the energy of the light affecting the particle was great, 
 a correspondingly large number of molecules received 
 such a shaking as to jar loose one atom of bromine from 
 each molecule, and this photo-chemical action was able 
 to penetrate into the body of the particle to a pro- 
 portionate depth. In the maximum, all the silver 
 bromide molecules of the particle would be in this way 
 reduced to sub-bromide molecules by the light. But 
 as fast as the atoms of bromine were set free, they were 
 immediately taken up by the gelatine, so that, prac- 
 tically speaking, none escaped. Therefore, when the 
 developer solution reached the particle, it found, 
 temporarily combined with the gelatine, a supply of 
 bromine ready at hand with which the sodium carbon- 
 ate could at once form sodium bromide. This com- 
 pound, produced right "on the ground," so to speak, 
 where it was most needed, by its restraining effect upon 
 the solubility of any silver bromide in the particle 
 which happened to escape reduction to sub-salt by the 
 light, delayed the reduction of such silver bromide by 
 the alkaline pyrogallate. The action of sodium bro- 
 mide is exactly similar to that of potassium or any 
 other soluble bromide as a restrainer. 
 
 In an area of the plate affected by a weak illumination 
 a much smaller number of bromide of silver molecules 
 
70 CHEMISTRY FOR PHOTOGRAPHERS 
 
 will be broken down to sub-bromide, and correspond- 
 ingly less bromine will be set free, and thus throughout 
 the total extent of the exposed body of emulsion the 
 amount of reduction to sub-bromide will be directly 
 proportional to the intensity of the light. Furthermore, 
 it is important to observe here that the depth in the 
 film of emulsion to which the photo-chemical effect 
 penetrates is dependent directly upon the light intensity. 
 That is to say, in the highlights there is reduction to 
 sub-bromide deeper down in the emulsion than occurs 
 in the shadows. Clearly, in some areas the intensity 
 of the light may be so low that its feeble energy is 
 wholly absorbed in the surface layer of silver bromide 
 particles, while in other regions there will be an excess 
 of energy over the amount absorbable by the whole 
 thickness of the film and consequently some light will 
 be transmitted. Here halation will occur unless the 
 plate is suitably backed. But the silver salt reducible 
 to form an image by the developer is not limited to the 
 light-reduced sub-bromide, for a secondary reaction 
 occurs between the initially reduced silver and the 
 neighboring silver bromide that has escaped partial 
 reduction by light. When a molecule of silver sub- 
 bromide is decomposed into its constituents by the 
 developer, the silver is for the instant left in the atomic 
 condition, the so-called nascent state, in which it is ex- 
 cessively reactive. Nascent silver and silver bromide 
 react together immediately, forming silver sub-bromide, 
 which is reduced by the developer, and in this manner 
 the particle of metallic silver resulting may be much 
 larger than would have been produced merely from the 
 light-reduced salt. So, oftentimes, when it is desired 
 to bring up details in the shadows without at the same 
 
CHEMISTRY OF DEVELOPMENT 71 
 
 time carrying the development of the highlights too 
 far, it is recommended to take the plate from the 
 developer and to allow the action to continue with the 
 amount of solution which has soaked into the emulsion. 
 In the highlights where there is abundance of sub- 
 bromide, the reducing agent is quickly used up and the 
 action there stops, but in the shadows there is pro- 
 portionally as much reducer present and fewer sub- 
 bromide molecules, so that by the secondary reaction 
 just described a considerably greater quantity of silver 
 can be reduced to form an image than would be given 
 merely by the light-reduced salt. Too great stress must 
 not be laid upon this idea of modifying in the develop- 
 ment the gradation established by the exposure. It was 
 formerly believed that gradation could be considerably 
 altered by suitably adjusting the developing solution. 
 But it is now known that with a given plate the re- 
 lations between the densities of the grades are practic- 
 ally determined by the exposure, and can be little 
 affected by the development process. If the ratios of 
 the intensities of three tones as given in the exposure 
 are as 1:2:3, then in the negative produced the 
 densities will be in the same ratios without regard to 
 the constitution of the developer or to the time of 
 development. By the term "density" is here meant the 
 quantity of silver reduced under definite conditions by 
 the developer. It is possible to stop development at 
 such a point that all three tones are, so to speak, 
 printable, that is, are sufficiently transparent to print; 
 or development can be carried so far, for example, as 
 to make density "3" so opaque that the ratio of trans- 
 parency between "2" and "3" is quite different from 
 the density ratio of 2 : 3. In such a case the gradation 
 
72 CHEMISTRY FOR PHOTOGRAPHERS 
 
 obtained in the print will be quite different from the 
 original intensities of tones. This example will suffi- 
 ciently indicate, perhaps, that the principal means of 
 modifying or controlling gradation lies in changing 
 the duration of development, and not in varying the 
 composition of the developer. It is upon this principle 
 of Constant Density Ratios that all the various factorial, 
 time, and tank methods of development are based. 
 
 So far, in speaking of developing agents, we have 
 referred to them in general terms as organic compounds. 
 Let us examine them a little more closely, in order to 
 find out what these interesting substances are and how 
 they are related to one another. When coal is sub- 
 jected to distillation for the purpose of making coke and 
 illuminating gas, there is separated as one of the by- 
 products a quantity of tarry material known as coal- 
 tar. The substance, once a waste product, but now the 
 basis of important industries, is fractionally distilled and 
 by this means there is obtained, among other valuable 
 things, the compound benzene (or benzol) . When pure, 
 benzene is a colorless liquid, lighter than water, and 
 boiling at 80. y degrees C. A great many substances 
 which are insoluble in water, such as fats, resins, etc., 
 are soluble in benzene, so that it is much used as a 
 solvent, but still more extensively for the manufacture 
 of the so-called benzene derivatives, among which, in 
 fact, are most of our developing agents. Benzene is 
 an extremely stable compound, consisting only of the 
 two elements carbon and hydrogen. It is with difficulty 
 decomposed, suffering no change when boiled with 
 strong alkali, and only slowly oxidized by such vigorous 
 oxidizers as chromic acid and potassium permanganate. 
 But when suitably treated, benzene very readily yields 
 
CHEMISTRY OF DEVELOPMENT 73 
 
 a large number of what are called substitution products, 
 or derivatives — substances, that is, derived from 
 benzene by substituting a great variety of different 
 elements and combinations of elements in place of 
 some or all of the hydrogen. The percentage com- 
 position of benzene is such that, taking into account 
 also the density of its vapor, we know it to consist of 
 six atoms of carbon united with the same number of 
 atoms of hydrogen. Employing the usual chemical 
 symbols, the formula of this compound is consequently 
 written CgHa. With so many atoms of two elements 
 there is possible a number of different ways in which 
 these atoms might be joined together, but it is fairly 
 easy to show that one method of representing their 
 arrangement best explains the chemical behavior of the 
 compound. The well-known substance acetylene gas 
 is also a compound of hydrogen and carbon, and with a 
 percentage composition identical with that of benzene. 
 Its vapor density is but one-third as much as the vapor 
 density of benzene, and so its formula is written C2H2. 
 But also it can be proved that in acetylene each hydro- 
 gen atom is combined to a carbon atom and that the 
 two carbons are tied together, so that this compound 
 is graphically represented thus: H-C^C-H. Now, if 
 acetylene gas be passed into a tube which is kept at dull 
 red heat, there will emerge from the other end of the 
 tube, not acetylene gas, but the liquid benzene. So 
 three molecules of acetylene are in this way combined 
 together to form one molecule of benzene, 3 C2H2 = 
 CeHg, and such a reaction is known chemically as 
 polymerization. Since in acetylene each hydrogen atom 
 is joined to a carbon atom, it is reasonable to suppose 
 that in benzene, the polymer of acetylene, the same 
 
H 
 -C C 
 
 H 
 C-H 
 
 V 
 
 I 
 
 H 
 Benzene 
 
 H 
 
 I 
 
 O 
 
 74 CHEMISTRY FOR PHOTOGRAPHERS 
 
 arrangement holds true. Therefore the arrangement of 
 the atoms in benzene, or its "constitution," to use the 
 chemical term, is considered to be representable 
 graphically as shown in the margins. 
 
 The six carbons are all joined to- 
 gether in a ring (this forms the so- 
 called "benzene ring") and each has a 
 hydrogen atom attached. If we sup- 
 pose that the ring is difficult to break 
 apart chemically, but, on the other 
 hand, that the hydrogen atoms can 
 easily be taken off and replaced by 
 other elements and groups of elements, 
 we shall see that this ring formula very 
 well represents the chemical facts 
 which we have previously stated 
 about the compound; 
 
 When one hydrogen is taken off and 
 in its place is put -0-H, we have the 
 compound phenol, or carbolic acid, a 
 crystalline solid. Such' an action il- 
 lustrates the formation of "substitu- 
 tion products" or benzene "deriva- 
 tives." If we also make a similar sub- 
 stitution of -0-H on the opposite side 
 of the ring, we get the well known 
 developing agent hydrochinon, or para- 
 dihydroxybenzene. By substituting 
 three -0-H's on three adjacent carbon 
 atoms in the ring, there is formed 1,2, 
 3, trihydroxybenzene, the photog- 
 rapher's good old stand-by, pyrogalUc 
 acid, "pyro" for short. 
 
 i 
 
 H-C C-H 
 
 H 
 
 .& i-H 
 
 Y 
 
 Phenol 
 
 H 
 
 6 
 
 / % 
 
 H-C C-H 
 
 H-i i-H 
 
 H 
 Hydrodiinon 
 
CHEMISTRY OF DEVELOPMENT 
 
 7J 
 
 With proper treajtment chemically, H 
 
 hydrochinon, which as a developer 
 gives excellent density but has a 
 tendency to "hardness" and lack of 
 details in the shadows, can be con- 
 verted into adurol, a more rapid de- 
 veloper, giving better detail, and one 
 that keeps better in solution than the 
 hydrochinon from which it may be 
 derived. Adurol is monochlorhydro- 
 chinon, or, as made by another manu- 
 facturer, monobromhydrochinon. 
 
 As examples in another series of 
 substitution products may be given the 
 developers amidol and metol. These 
 are both substituted phenols. Metol 
 chemically is monomethylparamido- 
 phenol. Amidol is diamidophenol, and 
 ortol is hydrochinon combined with 
 monomethylorthoamidophenol. 
 
 If we particularly note these seem- 
 ingly awkward chemical names of 
 photographic developers, we find in 
 many of them the prefix "para," and 
 in others "ortho." These prefixes, to- 
 gether with a third, "meta," are used 
 to signify which of the hydrogen atoms, 
 as related to a given hydrogen, have 
 been replaced. Thus, if two adjacent 
 hydrogens are substituted, as by -0-H 
 and -NH2, orthoamidophenol is pro- 
 duced, but if instead the first and third H - N - 
 hydrogens are replaced, the compound Metol 
 
 6 
 
 / \ 
 
 H-C C-O-H 
 W-h i-O-H 
 
 A 
 
 Pyro 
 
 O-H 
 
 H-C C-Cl 
 
 H-t i-H 
 
 i-H 
 Adurol 
 
 H 
 h 
 
 (b 
 H-C C-H 
 
 H-(! d-H 
 
 Y 
 
 CH3 
 
76 CHEMISTRY FOR PHOTOGRAPHERS 
 
 H is metamidophenol, and if the sub- 
 
 stitutions are on the first and fourth 
 d; carbon atoms, paramidophenol results. 
 
 j_j _ „^ '^ _ J.JJ, These three compounds can actually 
 
 i 1 be prepared, and that they are differ- 
 
 ~ \ ^ ~ ent substances, although possessing the 
 
 9 same composition, is attested by the 
 
 NHi fact they have different properties. 
 
 Amidol JY^Q difference is in their constitution. 
 
 A very peculiar circumstance about the 
 
 series of benzene derivatives is that, 
 
 whereas the ortho- and para-com- 
 
 H pounds possess the power of develop- 
 
 d) ing the latent photographic image, the 
 
 JL „ corresponding meta-compounds do not 
 
 / % I have this power. Also, in general, de- 
 
 H-C C-N-CH3 , . ^ . . J u • 
 
 II I veloping power is increased by in- 
 
 ^~\ Y'^ creased substitution. As an example of 
 
 C this is the case of adurol already 
 
 H mentioned, which is a more active 
 
 Ortol developer than hydrochinon, and yet 
 
 differs from it in composition and con- 
 stitution only in having another hydro- 
 gen atom replaced by an atom of 
 j^ chlorine. Recalling that in amidol de- 
 
 A veloping solutions no alkali is in- 
 
 1 eluded and referring to the graphic 
 / % formulas for metol and amidol just 
 
 ^~9 C-NHi given, we see that it is the third sub- 
 
 H-C C-H stj,tution on the ring which gives to 
 
 C amidol so much greater developing 
 
 j!j power than is possessed by metol. 
 
 Orthoamidophenol Amidol is so vigorous a reducer that 
 
CHEMISTRY OF DEVELOPMENT 77 
 
 its solution is used for development H 
 
 simply with the addition of sodium o 
 
 sulphite. If even a few drops of alkali (L 
 
 solution are added, fog is likely to u _ p V _ u 
 supervene, on account of the general \\ \ 
 
 reduction of silver bromide all over the \,!^~ 
 
 plate. C 
 
 A comparison of all the developing H 
 
 agents in the market will show advan- Metamidophenol 
 tages and disadvantages for the ama- 
 teur. Some produce excellent negatives, H 
 but their solutions do not keep well; 6 
 others have poisonous properties be- c 
 sides, as for example, metol. Still H-c' C-H 
 others, like pyro, are prone to stain \js_\' /-_« 
 the gelatine. Some are rapid ; and some \ // 
 work slowly. Perhaps it would be . 
 difficult to say whether pyro or metol- ^^-t 
 hydrochinon is the more used devel- Paramidophenol 
 oper, but one or the other of these is probably the most 
 widely used of all. It would be about equally difficult 
 to explain why these two developers should be so popu- 
 lar, since neither keeps well in solution, one stains badly 
 and the other is poisonous. Of course, the merits and 
 demerits of developers, like so many other things, are 
 largely matters of opinion. But, nevertheless, there are 
 developing agents which keep well in solution, do not 
 stain the film, and are not especially poisonous ; which 
 also, in reality, produce quite as excellent gradation 
 as pyro, for example, work with as much rapidity as 
 metol, and are no more expensive. One such developer 
 is the duratol-hydrochinon, formulas for which have 
 been extensively published. It is particularly to be 
 
78 CHEMISTRY FOR PHOTOGRAPHERS 
 
 recommended to the amateur since it keeps well, almost 
 indefinitely, in fact; and the same solution can be 
 used for plates, films, papers, and lantern slides, by 
 merely varying the dilution. Standard recipes for 
 developers will be found in the Appendix. 
 
CHAPTER VI 
 Chemistry of the Fixing Process 
 
 WHEN the operation of transforming the latent 
 photographic image into reduced metallic silver 
 has been completed, according to the judgment of the 
 photographer, the gelatine film upon the plate con- 
 tains a considerable number of different substances. 
 To enumerate them, there is first the silver which is 
 to form the negative; besides this, there is also the un- 
 reduced remainder of the unexposed silver bromide and 
 silver iodide, and in addition to these, the gelatine is, 
 like a sponge, saturated with the solution of all the 
 soluble materials which the developer now holds. 
 These last include the unoxidized reducer, the alkali, 
 the sulphite, sodium bromide, and sodium iodide, and 
 the oxidation products of the developing agent used up, 
 some of which may possess the character of dyes. In 
 order to make a permanent negative out of this com- 
 bination, it is necessary that everything be removed so 
 far as it is possible except the metallic silver and the 
 gelatine. Upon taking the plate from the developer 
 solution, the greater part of the soluble materials 
 clinging to the gelatine should be washed away by 
 rinsing liberally with water. But primarily it is re- 
 quired to get rid of the silver salt which is still light- 
 sensitive, and the means for doing this must now be 
 discussed. By recalling experiments already performed, 
 we remember that silver chloride and silver bromide 
 are soluble in ammonium hydroxide. Silver iodide is 
 very insoluble, but this reagent . has another disad- 
 
 7Q 
 
8o CHEMISTRY FOR PHOTOGRAPHERS 
 
 vantage which is serious, namely, that it has a bad 
 effect upon the gelatine. This is the reason why 
 ammonium hydroxide is no longer much used as the 
 alkali in developer solutions. Another compound whose 
 solution in water is capable of dissolving all three of the 
 silver halogen salts is potassium cyanide. But, al- 
 though very extensively used in metallurgical industry 
 for the extraction of gold and silver from their ores, 
 potassium cyanide, on account of its extremely poison- 
 ous character, is not a reagent that can safely be 
 recommended to the photographer. 
 
 Experiment 23. Prepare small amounts of silver chlo- 
 ride, silver bromide, and silver iodide and treat each of 
 them with a few cubic centimeters of a strong solution 
 of sodium thiosulphate (the "hypo" of the photogra- 
 pher), noting the solvent effect in each case. Also to 
 a little cold, dilute sodium thiosulphate solution add a 
 drop or two of silver nitrate in solution and note that 
 the silver thiosulphate at first precipitated is white. 
 
 The color changes are due to the fact that silver 
 thiosulphate decomposes and ultimately forms black 
 silver sulphide. This can be shown by heating the 
 mixture. If an excess of the thiosulphate is added 
 quickly to a solution of silver nitrate, the precipitate 
 of silver thiosulphate at first formed is at once dissolved, 
 producing silver-sodium thiosulphate, which is the 
 same soluble compound that is formed by treating 
 silver chloride with thiosulphate. Reactions: 
 
 ( 1 ) Silver bromide + Sodium thiosulphate 
 
 = Silver-sodium thiosulphate + Sodium bromide 
 
 (2) Silver nitrate + Sodium thiosulphate 
 
 = Silver thiosulphate + Sodium nitrate 
 
 (3) Silver thiosulphate 
 
 = Silver sulphide + Oxygen -|- Sulphur dioxide 
 
 (4) Silver thiosulphate + Sodium thiosulphate 
 
 = Silver-sodium thiosulphate 
 
CHEMISTRY OF FIXING PROCESS St-. 
 
 Although the unreduced sliver salt remaining after 
 development has been completed is still sensitive to 
 light, it is by no means as sensitive as it was before 
 development. If, when it is judged that enough silver 
 has been reduced, the plate is well rinsed in two or three 
 changes of water, the unaffected silver salt is so little 
 sensitive that it can be exposed for a short time to the 
 weak illvimination of an ordinary room without damage. 
 Of course it must not be handled in a strong light. This 
 is why in the tank development of roll film it is possible 
 to remove the film from the tank and place it in the 
 fixing bath without resorting to a darkroom. But 
 although the sensitiveness is so much lessened, never- 
 theless to make the negative permanent it is necessary, 
 as has been remarked, to take away from the gelatine 
 everything possible but the silver. Thus the "fixing" 
 process in reality is a twofold operation, since it con- 
 sists in making the insoluble silver bromide and silver 
 iodide over into soluble silver-sodium thiosulphate by 
 means of the action of the "hypo" of the so-called 
 "fixing bath," and afterwards washing out of the 
 gelatine all the soluble salts which it contains. Ideally, 
 a negative should consist solely of pure silver particles 
 embedded in clean gelatine. The more nearly this 
 condition is approached the better the chemical 
 quality of the negative. In practice it is not so simple 
 a matter to effect this desirable state of things. Some 
 of the soluble substances contained in the gelatine 
 appear to "fix" themselves there somewhat as dyes are 
 fixed in fabrics, in part, perhaps, adhering to the silver 
 image. Certainly, at least, the staining matter from 
 the developer solution is in some way retained by this 
 image, as can be ascertained by treating such a dis- 
 
82 CHEMISTRY FOR PHOTOGRAPHERS 
 
 colored negative with nitric acid. In this manner the 
 silver will be dissolved out, and the stain, being in- 
 soluble in acid (although it forms soluble products 
 with alkali), remains in the film, in fact forming, as it 
 were, an image made of this dye-like substance. It has 
 been claimed that the reduced silver holds fast some 
 of the unreduced silver bromide, refusing to give it up 
 even to the thiosulphate. Whether or not this be true, 
 if the decantation process of washing precipitates is 
 recalled to mind, it will be evident that here also the 
 soluble materials can be washed away from the in- 
 soluble to any desired extent, but never quite entirely 
 removed. It is possible and often desirable to make 
 chemical tests of the wash-water in order to determine 
 the degree to which the soluble thiosulphate has been 
 eliminated from the plate. The reaction between silver 
 nitrate and sodium thiosulphate already mentioned can 
 be utilized for this purpose. 
 
 Experiment 24. Make up a i.o per cent, sodium 
 thiosulphate solution by dissolving 1.57 gram of the 
 solid in a little water and diluting up to 100 cubic 
 centimeters with water. Mix this solution thoroughly. 
 Make a o. i per cent, solution by diluting lo.o cubic 
 centimeters of the i.o per cent, solution to 100 cubic 
 centimeters; and lastly a 0.0 1 per cent, solution by 
 diluting 1.0 cubic centimeter of 1.0 per cent, to 100 
 cubic centimeters. Take 10 cubic centimeters of each 
 of these three solutions, warm, and add to each one 
 drop of silver nitrate in dilute solution. Even the third 
 solution, containing so little thiosulphate as one part 
 in ten thousand, gives a perceptible black precipitate 
 of silver sulphide. If the most dilute of these solutions 
 be still further diluted, the precipitate will be yellowish 
 
CHEMISTRY OF FIXING PROCESS 83 
 
 instead of black, but will be distinct. Thus this re- 
 action furnishes a method of detecting very small 
 amounts of thiosulphate. Reaction: 
 
 Sodium thiosulphate (heated) + Silver nitrate 
 = Silver sulphide + Silver sulphate + Sodium nitrate 
 + Sulphur dioxide + Sulphur 
 
 In Utilizing the reaction of the preceding experiment 
 for testing the thoroughness of the washing, a plate may 
 be taken out of the washing tank and the water allowed 
 to drain from one corner into a test tube until 5 to lo 
 cubic centimeters have been collected. This water is 
 next warmed, and then a drop of silver nitrate solution 
 is added. If even the faintest darkening occurs, wash- 
 ing should be continued. However, we may remark 
 that if the water used for washing contains more than 
 a very small amount of soluble chlorides, the addition 
 of silver nitrate will precipitate white silver chloride, 
 which will interfere somewhat with the delicacy of 
 this test. In such a case use may be made of another 
 reaction, as in the following experiment. 
 
 Experiment 25. Prepare a solution as follows: In 
 
 100 to 150 cubic centimeters of distilled water dissolve 
 
 1 .0 gram of potassium carbonate (anhydrous) and o. i 
 
 gram of potassium permanganate. Dilute to 1,000 
 
 cubic centimeters with distilled water. Treat 10 cubic 
 
 centimeters of each of the three solutions of thiosulphate 
 
 made up in the previous experiment with 1 or 3 drops 
 
 of this alkaline permanganate solution. The mixture 
 
 will have a green color, or blue, if the thiosulphate is 
 
 very dilute. Reaction : 
 
 Potassium permanganate (Pink) 
 
 -f- Potassium carbonate + Sodium thiosulphate 
 = Potassium manganate (Green) + Sodium sulphate 
 + Potassium sulphate + Carbon dioxide 
 
84 CHEMISTRY FOR PHOTOGRAPHERS 
 
 This solution may be kept in a bottle for the purpose 
 of testing the wash-water from plates, films, and papers. 
 Although it may seem an unnecessary complication 
 to take the time to test the completeness of washing, 
 when by merely allowing the water to run long enough 
 negatives or prints may be considered free from soluble 
 substances, there are in reality sufficient benefits 
 derived from making this test to warrant the photog- 
 rapher in adopting it as a part of his regular procedure. 
 If he knows from actual experiment that the quantity 
 of thiosulphate ("hypo") remaining in his negatives 
 is less than a definite very minute amount (and so for 
 practical purposes a harmless amount), he will never 
 have the unhappy experience of beholding the crystalli- 
 zation of salts upon his plates, which not infrequently 
 befalls the amateur who takes for granted the thorough- 
 ness of his work. And besides this, there is nothing to 
 be gained, leaving out of account the waste of water, 
 and very often considerable damage, especially to 
 papers, by unnecessarily prolonged soaking. As we 
 have repeatedly observed, the essentials of photo- 
 graphic processes are chemical in their nature, and since 
 one is never safe in taking things for granted in regard 
 to the results of chemical operations, it may be laid 
 down as axiomatic that the particular worker, the 
 photographer who makes a practice of taking pains 
 ./ 'and precautions in his chemical work, will produce the 
 |most technically successful negatives and prints. In 
 this connection there is a point which should not be 
 lost sight of, namely, that no amount of washing can 
 be expected to take away insoluble substances. It is 
 necessary in the first instance to allow the "hypo" to 
 act upon the silver bromide and silver iodide for a 
 
CHEMISTRY OF FIXING PROCESS 85 
 
 sufficient time so that tl'ie conversion of these water- 
 insoluble substances into the water-soluble double 
 thiosulphate of silver and sodium may be effected 
 completely. By allowing the plate to stay in the fixing 
 bath for at least ten minutes after the creamy silver 
 halogen salts have disappeared, one can always be 
 assured of thorough "fixing," provided the thiosulphate 
 solution is reasonably fresh, and not approaching ex- 
 haustion. Much of the amateur's trouble in the way 
 of poor negatives and prints is doubtless traceable to 
 the overworking of the fixing solution, not to say also 
 of the developer. It may be that a supply of develop- 
 ing solution and a stock of "hypo" will last for a greater 
 length of time measured in months, and will develop 
 and fix more negatives, if used until the last vestiges 
 of chemical activity have been exhausted, but such a 
 proceeding is not conducive to excellence and uni- 
 formity and permanence of results. Neither is such a 
 method really economical, for the saving in developer 
 and "hypo" is far overbalanced by waste of the much 
 more expensive plates, films, and papers that are 
 spoiled. The best practice for the amateur to adopt is 
 first never to use over again a solution which has once 
 developed a plate or film, and second to mix enough 
 fresh, plain thiosulphate "fixing bath" for each batch 
 of plates or films, throwing it away when they are 
 done. There is no better fixing solution for plates and 
 films than plain "hypo," if one of the non-staining 
 modem developing agents has been used. Besides this, 
 if a tendency to discoloration appears, the stain can be 
 bleached by adding a little sodium sulphite, or, even 
 better, the potassium disulphite which is commonly 
 called "metabisulphite." 
 
86 CHEMISTRY FOR PHOTOGRAPHERS 
 
 The "acid" fixing bath is simply a solution of "hypo" 
 to which is added sulphite and enough acid to liberate 
 some sulphurous acid from the sulphite, but not suffi- 
 cient to decompose the thiosulphate. Recalling the use 
 of sodium sulphite and potassium metabisulphite in de- 
 veloping solutions where they are commonly called "pre- 
 servatives," we may remember that their function is 
 equally toprevent the formation of dyes which would pro- 
 duce discoloration. So in this sense they are ' 'bleachers. ' ' 
 In the fixing bath they perform the same service, pre- 
 venting the gelatine from becoming discolored by bleach- 
 ing the dye which has been formed in the developer. It 
 is in fact the sulphur dioxide, or sulphurous acid, as the 
 solution of this gas in water is called, which is the effec- 
 tive bleaching agent. The acid used to set free the sul- 
 phurous acid from sodium sulphite is ordinarily citric 
 or tartaric acid, but sometimes very dilute sulphuric 
 acid is employed. The reactions are as follows : 
 
 Sodium sulphite + Citric acid 
 
 = Sulphurous acid + Sodium citrate 
 Sodium sulphite + Sulphuric acid 
 
 = Sulphurous acid + Sodium sulphate 
 
 Sulphurous acid is a very unstable compound and is 
 known as such only in solution. Even there it is con- 
 tinually breaking down into its constituents, sulphur 
 dioxide gas and water, and this is the reason why its 
 solutions have such a pungent odor. Like its sodium 
 salt it is a reducing agent, and it is upon this fact that 
 its efficiency in preventing stain depends. All dyes, on 
 becoming reduced, form colorless products, and it is 
 this action which is called bleaching. Potassium metabi- 
 sulphite, or "disulphite" as it really should be named, 
 produces similar results by reason of the fact that its 
 
CHEMISTRY OF FIXING PROCESS 87 
 
 solution is, in effect, the same as a sulphite solution to 
 which acid has been added. This compound is, as it 
 were, potassium sulphite containing extra sulphur 
 dioxide which it easily gives up. Therefore, to re- 
 capitulate, the addition of acid to sodium or potassium 
 sulphite, or to disulphite, produces the sodium or 
 potassium salt of that acid and sulphurous acid, if the 
 action goes on in solution. But, as we have said, 
 sulphurous acid spontaneously decomposes into water 
 and sulphur dioxide, so that the reaction given above 
 might be written quite as well : 
 
 Sodium sulphite + Sulphuric acid 
 
 = Sulphur dioxide + Water + Sodium sulphate 
 
 Next we must inquire into the effect of acid upon 
 sodium thiosulphate. 
 
 Experiment 26. Add a little dilute sulphuric acid to 
 5 cubic centimeters of a fairly concentrated solution of 
 sodium thiosulphate, note the yellowish-white precipi- 
 tate, which is sulphur, and cautiously smell the odor of 
 the sulphur dioxide gas which is given off. Try the effect 
 of the same acid on more dilute thiosulphate solutions, 
 and observe that similar results follow. Dissolve a crys- 
 tal or two of citric acid in water and try the effect of its 
 dilute solution upon thiosulphate. Similarly experiment 
 with dilute acetic acid, observing that with these so- 
 called organic acids also the separation of sulphur and 
 evolution of sulphur dioxide occur. Reactions : 
 
 1 . Sodium thiosulphate + Sulphuric acid 
 
 = Sodium sulphate + Sulphur + Sulphur dioxide 
 
 + Water 
 
 2. Sodium thiosulphate + Citric acid 
 
 = Sodium citrate + Sulphur + Sulphur dioxide 
 
 + Water 
 
 3 . Sodium thiosulphate + Acetic acid 
 
 = Sodium acetate + Sulphur + Sulphur dioxide 
 
 + Water 
 
88 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Next dissolve 0.2 or 0.3 gram of sodium sulphite in 
 about 5 cubic centimeters of water and add several 
 drops of dilute sulphuric acid, avoiding an excess of 
 the acid. This solution will now contain, referring to 
 the reaction above, sodium sulphate and sulphurous 
 acid, besides sodium sulphite, since the last-named 
 compound is in excess. Treat a little thiosulphate in 
 solution with some of this sulphurous acid and note that 
 there is no deposition of sulphur. 
 
 Since the sulphur set free, when thiosulphate is 
 acidified, comes from the spontaneous decomposition 
 of the thiosulphuric acid formed, it is plain that sul- 
 phurous acid is unable to react with sodium thiosul- 
 phate to liberate thiosulphuric acid. Therefore if we 
 make a mixture of sodium sulphite and thiosulphate 
 in the same solution and add to it a less amount of acid 
 than would be equivalent to the sulphite, we should 
 expect the acid to interact with the sulphite, forming 
 sulphurous acid in the solution, and, since in this way 
 the danger of forming thiosulphuric acid is removed, 
 there should be no separation of sulphur. An experi- 
 ment will show that this is the case, and it is in such a 
 manner that the "acid" fixing baths are made up. 
 Whenever sulphur is precipitated from such a bath, it is 
 evidence of the presence in the solution of some free 
 acid other than sulphurous acid. It is this separation 
 of sulphur from thiosulphate which is one of the causes 
 of lack of permanency in negatives and prints that 
 have been imperfectly washed, so that any agency 
 which is capable of decomposing thiosulphate should 
 be used in photographic solutions with care and in- 
 telligence. For the making of negatives, at least, except 
 possibly in very hot weather or in the tropics, there is 
 
CHEMISTRY OF FIXING PROCESS 89 
 
 no fixing bath better than one of plain "hypo," to) 
 reiterate a statement already made. At most ther^ 
 need be added to this nothing more than a little potas- 
 sium metabisulphite. If there is real danger that the 
 gelatine will become too much softened, then a harden- 
 ing agent should be employed in an acid "hypo" bath. 
 One of the best of these is the "acid fixing and hardening 
 bath" which has been recommended for a long time 
 past with Cramer plates. The hardening consists im 
 rendering the gelatine more insoluble in water and for j 
 this purpose a chromium salt is used, generally "chrome / 
 alum," or, chemically, potassium chromium sulphate. 
 By treatment with chrome alum the solubility of gela- 
 tine is very greatly diminished, so much so that after a 
 sufficiently prolonged application the plate can be 
 washed even with hot water without danger of melting. 
 There is no special advantage in hardening the film to 
 such an extent, but on the other hand it will be dis- 
 tinctly disadvantageous if any after-treatment is to be 
 used. Another disadvantage of the acid fixing bath 
 is that it is often liable to be used after its efficiency has 
 become too much impaired. For a plain "hypo" fixing 
 bath for general use, dissolve 100 grams of the crys- 
 tallized salt in enough warm water and dilute to 500 
 cubic centimeters. Since sodium thiosulphate crys- 
 tallizes with 5 molecules of water, this will make a 
 solution a little stronger than 12 per cent, in sodium 
 thiosulphate. This is for fixing plates and films; for 
 prints, use 75 grams, instead of 100 grams, of "hypo," 
 and 12.5 grams of potassium disulphite (metabisulphite) . 
 The same amount of the latter compound can be added 
 to the bath for plates also. 
 There has at various times been suggested a number 
 
qo CHEMISTRY FOR PHOTOGRAPHERS 
 
 of "hypo eliminators," but none of them is to be recom- 
 mended on account of the lack of permanency in the 
 negatives so prepared. They all work upon the same 
 principle, the oxidation of thiosulphate either to 
 tetrathionate or to sulphate. Among these oxidizing 
 agents are hypochlorites, hydrogen dioxide, potassium 
 permanganate, percarbonates and persulphates. It 
 is quite as easy to wash soluble thiosulphate out of 
 the gelatine as any other soluble products, and no 
 "hypo eliminator" can eliminate the process of washing, 
 so that there seems little to be gained by adding an 
 extra operation to the chemical processes of the photo- 
 graphic art. 
 
 After the removal of all dissolved materials from the 
 gelatine film, the last step in the making of a negative, 
 unless some modifications are to be made upon it, is the 
 drying out of the water. The plates may be set in a 
 drying rack and put in a cool, airy place until com- 
 pletely dried. The drying-room should be as free as 
 may be from dust, and there ought to be no consid- 
 erable change in temperature during the drying. The 
 more rapidly drying takes place, therefore, the safer 
 for the negatives. When a plate dries slowly until 
 partly done and then, with a wet patch in the middle, is 
 finished at a different temperature, there may be a 
 marked difference between the density of the portion 
 that dried at the one temperature and that which 
 dried at the other. In this way the printing quality of 
 the negative will be spoiled. A mistake which is 
 common in many plate-racks is that the grooves are too 
 close together. Plates are brought so near to each 
 other that free circulation of air between them is 
 impeded and uniform drying is rendered difficult. The 
 
CHEMISTRY OF FIXING PROCESS qi 
 
 least distance which it is advisable to have between 
 the plates is about one inch. A rapid method of drying 
 which has been devised is to immerse the plate in a 
 very concentrated solution of sodium or potassium car- 
 bonate. Such a solution has a strong affinity for water, 
 so that the water in the gelatine is at once seized by 
 the sodium or potassium carbonate, and in a short 
 time the plate can be removed from the bath quite dry. 
 The process must be used with care, however, as the 
 film sometimes leaves the glass. It must not be used 
 with films, which will be ruined by distortion. When it 
 is desired to dry a negative rapidly, it may, after drain- 
 ing for a few minutes, be placed in a tray containing 
 ordinary alcohol. An immersion of y to lo minutes 
 allows the water in the gelatine to become mixed by 
 diffusion with alcohol, and thus largely replaced by 
 the alcohol, after which the negative will dry very 
 quickly owing to the rapid evaporation. 
 
 Upon the skill with which the principles we have so far 
 considered are applied depends the success of the pho- 
 tographer, whether he be specially interested in straight 
 record photography or in pictorial work. Although it 
 is at the same time true that the negative is merely 
 the means to an end, viz., the print, nevertheless it is in 
 the wonderfully modulated silver deposits of the nega- 
 tive that are recorded the gradations of light and shade 
 of which the object depicted, speaking photographic- 
 ally, consists. No matter what modifications may be 
 later introduced in the print to suit the desires and 
 taste of the worker, the basis of all good photography 
 of whatever sort is careful attention to the psychological, 
 optical, and chemical laws, which, when understandingly 
 obeyed, make the negative a truthful and yet flexible 
 
qz CHEMISTRY FOR PHOTOGRAPHERS 
 
 medium between the object photographed and the 
 picture which is finally obtained. For this reason the 
 reader is urged, if he wishes to secure the greatest 
 pleasure and benefit from his excursions in the photo- 
 graphic realm, to pay particular heed to his chemical 
 work. The cultivation of accurate and intelligent 
 methods in field, studio, and darkroom will lead even 
 the tyro farther into the "mysteries" than he may 
 anticipate. 
 
CHAPTER VII 
 After-treatment of the Negative 
 
 WHEN the exposure of the plate, upon an object 
 whose range of gradation is not too great, has 
 been confined within the limits of the latitude of the 
 emulsion, and its development has been carried out 
 to the proper degree of opacity, nothing remains but to 
 select such a printing medium as will give the effects 
 desired in the finished picture. But more or less 
 frequently it will happen that these ideal conditions will 
 not all be fulfilled. Either the original contrasts of the 
 object photographed were too great, or under- or over- 
 exposure has been given, or sometimes the duration 
 of development is not properly regulated. If the di- 
 vergence from these normal conditions is more than a 
 comparatively moderate amount, there is no better 
 advice to give the photographer than to make another 
 trial and use greater care. Not always, of course, can 
 the exposure be repeated, and when this is the case, if a 
 print must be had, it is possible to modify the negative 
 to a considerable extent by the processes of intensifica- 
 tion and (as it is called) reduction. In general, both 
 kinds of operation consist in the beginning of the same 
 type of reaction, namely, the oxidation of a part of the 
 silver of which the image is composed. 
 
 Some intensification methods do not follow this rule, 
 
 consisting, instead, of the suitably-controlled reduction 
 
 of silver from the solution applied to the plate. Let 
 
 us consider first the mercury intensifiers. 
 
 Experiment 27. Give a brief exposure to each of 
 
 Q3 
 
Q4 CHEMISTRY FOR PHOTOGRAPHERS 
 
 three small plates and develop, fix, and wash them as 
 usual. Make up loo cubic centimeters of a two per 
 cent, or three per cent, solution of mercuric chloride 
 ("corrosive sublimate," a violent poison and anti- 
 septic); Immerse one end of each prepared negative in 
 this solution until completely bleached, observing the 
 change from the back by transmitted light. Wash the 
 three plates thoroughly for ten to fifteen minutes and 
 then treat them severally as follows. Take 50 cubic 
 centimeters of water in a small tray, add a few drops 
 of ammonium hydroxide, and put one of the plates into 
 this solution. An irrjmediate blackening of the bleached 
 image will be noted. Dissolve y grams of sodium 
 thiosulphate in 25 to 30 cubic centimeters of water, 
 dilute to 50 and treat the second bleached plate with 
 this solution. Here again will occur a darkening of 
 the whitened portion. To blacken the third plate, use 
 a regular developer solution. The three plates may now 
 be well washed and dried, and the opacities of the 
 intensified and unintensified parts compared. 
 
 In explaining the chemical reactions which take 
 place in this operation, we may first observe that there 
 are two classes of mercury compounds representing 
 two conditions of oxidation, namely, the mercuric salts, 
 the higher condition, and the mercurous, the lower. 
 Mercuric salts can be changed, by the action of re- 
 ducing agents, to mercurous; and vice versa, the mer- 
 curous are oxidizable to mercuric compounds. Further, 
 mercurous compounds are completely reducible to 
 elementary mercury, and, when this reduction takes 
 place from a solution of the mercury salt, the resulting 
 mercury is in a very finely divided condition, and, 
 since its absorption of all wave-lengths of light is very 
 
AFTER-TREATMENT OF NEGATIVE qy 
 
 complete, its color is an intense black. Now, as we saw 
 
 in the chapter on the photo-chemistry of silver salts, 
 
 the silver of which the negative image consists is also 
 
 in a fine state of division, and in this condition is a 
 
 good reducing agent. Consequently, when mercuric 
 
 chloride solution is applied to the negative, the silver 
 
 reduces an equivalent amount of the mercuric chloride 
 
 to mercurous chloride and is at the same time itself 
 
 oxidized to silver chloride. Thus the bleached image 
 
 , consists of a mixture of equivalent quantities of silver 
 
 chloride and mercurous chloride, both being insoluble 
 
 white substances. There may also be some unchanged 
 
 silver. Reaction: 
 
 Silver + Mercuric chloride 
 
 = Silver chloride -|- Mercurous chloride 
 
 The character of the mercury compound will be seen 
 from 
 
 Experiment 28. Prepare a few cubic centimeters of 
 sodium sulphite solution, acidify with a drop or two 
 of dilute sulphuric acid, and add to it about 10 cubic 
 centimeters of two per cent, mercuric chloride solution, 
 observing the white precipitate of mercurous chloride, 
 which is formed by the reduction of the mercuric salt. 
 (If the mixture were now to be heated, mercurous 
 chloride would be further reduced to elementary 
 mercury, but this is not desirable in the present ex- 
 periment). Allow the mercurous chloride to settle, wash 
 it by decantation, and keep it for use in a subsequent 
 experiment. Reactions: 
 
 (i) Sodium sulphite + Sulphuric acid 
 
 = Sulphurous acid -|- Sodium sulphate 
 (2) Mercuric chloride + Sulphurous acid -f- Water 
 = Mercurous chloride -|- Sulphuric acid + Hydrochloric 
 
 acid 
 
qb CHEMISTRY FOR PHOTOGRAPHERS 
 
 Now, of the three bleached plates the first was treated 
 with diluted ammonium hydroxide. That is to say, the 
 mixture of silver and mercurous chlorides was acted 
 upon by the ammonium hydroxide in such a way as 
 to form a black product. By reference to the behavior 
 of silver chloride with ammonium hydroxide, we re- 
 member that a soluble product is formed, the reaction 
 being thus : 
 
 Silver chloride + Ammonium hydroxide 
 
 = Silver-ammonia chloride + Water 
 
 The action of ammonium hydroxide upon mercurous 
 chloride may be shown as follows : 
 
 Experiment zq. Take a portion of the mercurous 
 
 chloride prepared in Experiment 27 and treat it with a 
 
 few drops of dilute ammonium hydroxide, observing 
 
 the instant blackening which occurs. There are formed 
 
 as insoluble products in this reaction elementary 
 
 mercury, which is black in a fine state of division, and 
 
 mercury-ammonium chloride, which is white. Reaction : 
 
 Mercurous chloride + Ammonium hydroxide 
 = Mercury + Mercury-ammonium chloride + Ammo- 
 nium chloride -|- Water 
 
 The mixture of these black and white insoluble sub- 
 stances is black. Thus in this first process of intensifica- 
 tion, the silver image is changed initially to one com- 
 posed of silver, silver chloride, and mercurous chloride. 
 By the ammonium hydroxide the silver chloride is 
 rendered soluble and the mercurous chloride is converted 
 to an insoluble black mixture of mercury and a complex 
 mercury compound. So, when the first plate is finally 
 washed, the image remaining consists of silver and 
 mercury, plus the small amount of white complex 
 substance. The color of the intensified image formed in 
 
AFTER-TREATMENT OF NEGATIVE q; 
 
 this way is generally a dark brown, which makes the 
 process, when successfully carried out, suitable for 
 securing good prints from the negative, but the method 
 has the disadvantage of being somewhat uncertain in its 
 results. 
 
 In the case of the second plate with which we have 
 experimented, the mixture of silver and mercurous 
 chlorides was acted upon by sodium thiosulphate. 
 Here again it becomes immediately clear that silver 
 will be removed from the image, since silver chloride 
 and sodium thiosulphate form soluble silver-sodium 
 thiosulphate, carried away in the wash-water. Re- 
 action : 
 
 Silver chloride -f- Sodium thiosulphate 
 
 = Silver-sodium thiosulphate 
 
 As for the action of the sodium thiosulphate upon 
 mercurous chloride, we already know that not only is 
 thiosulphate a mild reducing agent, but that mercurous 
 chloride is an oxidizer, and therefore the result of their 
 interaction will be to form elementary mercury by the 
 reduction of the one and an oxidation product of the 
 other, thus: 
 
 Mercurous chloride -|- Sodium thiosulphate 
 = Mercury -|- Sodium chloride -f Sodium tetrathionate 
 
 Therefore, in the second method also, by use of 
 ' 'hypo, ' ' a portion of the silver image is removed, but is at 
 the same time replaced by a larger quantity of black 
 mercury. This removal of silver from the image is 
 avoided in the third method in which developer solution 
 is used, as the organic reducer acts upon both the silver 
 chloride and the mercurous chloride, reducing them to 
 elementary silver and elementary mercury respectively. 
 Another method of intensification, whereby mercury 
 
q8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 is added to the silver, consists in treating the plate with 
 
 a solution containing mercuric iodide and concentrated 
 
 sodium sulphite. Here silver is oxidized to silver iodide, 
 
 while mercuric iodide is reduced to the mercurous salt, 
 
 but immediately the last is reduced by the sodium 
 
 sulphite to mercury. The plate is then treated with a 
 
 developer, by which silver iodide is reduced to silver. 
 
 If this last step is omitted, the intensified image is 
 
 liable to fade. Reactions : 
 
 (i) Silver + Mercuric iodide 
 
 = Silver iodide + Mercurous iodide 
 
 (2) Mercurous iodide + Sodium sulphite + Water 
 
 = Mercury + Sodium sulphate + Hydriodic acid 
 
 (3) Silver iodide + Organic reducer 
 
 = Silver + Oxidation products of organic reducer 
 
 An intensification process in which metallic silver is 
 added to the silver image is one which is known as the 
 "Wellington intensifier." In this a solution containing 
 silver, an organic reducing agent, and an alkali is so 
 adjusted that it is just on the point of precipitating 
 elementary silver. Under these circumstances, when 
 the solution is applied to a negative, the particles of 
 silver already composing the image act as nuclei upon 
 which the silver reducing from the solution can deposit. 
 Thus the image can be built up to any extent desired. 
 This action can be illustrated by taking a little satu- 
 rated solution, for example, of copper sulphate ("blue 
 vitriol"), putting into it a small crystal of the same 
 substance, and allowing crystallization to take place 
 slowly. The introduced crystal will gradually increase 
 in size, and possibly no other crystals will form, at least 
 for a time. It is easier, so to say, for the copper sulphate 
 in the solution to separate out upon an already formed 
 crystal than to start new ones. In a similar way, it is 
 
AFTER-TREATMENT OF NEGATIVE qq 
 
 also easier for the silver separating by reduction from 
 the intensifying solution to deposit upon the silver 
 particles already present in the gelatine than to com- 
 mence the formation of new particles. To experiment 
 with this process, make up the following solutions : 
 
 1 . Ammonium sulphocyanide 4.0 grams 
 Sodium thiosulphate 4.0 grams 
 
 Water, to 25.0 cubic centimeters 
 
 2. Silver nitrate 2.5 grams 
 
 Water, to 25.0 cubic centimeters 
 
 Take 1 5 cubic centimeters of Solution i and add to it 
 slowly and stirring constantly 15 cubic centimeters of 
 the second. The action of a silver solution upon am- 
 monium sulphocyanide is, as may be quickly ascertained 
 by experiment, to precipitate white silver sulpho- 
 cyanide. But in the presence of sodium thiosulphate 
 this precipitation is prevented, owing to the formation 
 of the soluble silver-sodium thiosulphate, until the 
 quantity of thiosulphate present has been used up, 
 when immediately silver sulphocyanide will commence 
 to come down. Reaction: 
 
 Silver nitrate -|- Ammonium sulphocyanide 
 
 = Silver sulphocyanide -|- Ammonium nitrate 
 
 If a permanent precipitate is formed by the mixture of 
 the two solutions, add cautiously, a few drops at a time, 
 more of the first until the precipitate is just dissolved. 
 If, on the other hand, no precipitate has been formed, 
 add more silver nitrate carefully until a slight per- 
 manent precipitate appears and then barely dissl^^e 
 it with a drop or two of Number i . Next dissolve^in 2 
 cubic centimeters of water 0.4 gram of sodium sulphite 
 and 0.2 gram of pyrogallic acid, add 2 cubic centimeters 
 of a ten per cent, solution of ammonium hydroxide. 
 
loo CHEMISTRY FOR PHOTOGRAPHERS 
 
 and pour this mixture into the liquid already prepared. 
 Put a dry negative that needs intensification into a 
 clean tray and cover it with this solution, rocking the 
 tray until sufficient density has been obtained. Then 
 put the plate into a fresh fixing bath for five minutes, 
 wash thoroughly, and dry it. In explaining the chemical 
 reaction of this process, we may assume that we have 
 silver sulphocyanide in solution mixed with the organic 
 reducing agent and in contact with the metallic silver of 
 the image. Under these circumstances the silver, which 
 is gradually reduced, deposits upon the metal in the gela- 
 tine and not elsewhere, and so there results a building 
 up of the image by an accumulation of silver. Reaction : 
 
 Silver sulphocyanide + Pyrogallic acid + Water 
 
 = Silver + Sulphocyanic acid + Acetic acid 
 + Oxalic acid 
 
 There are two other methods of increasing the opacity 
 of negatives which are much employed, one using 
 uranium and the other, chromium salts. The uranium 
 intensifier has the advantage that the operation can 
 be carried on in a single solution, after which the 
 plate must be thoroughly washed, but the negative 
 must in the first instance have been very completely 
 freed from thiosulphate, else the intensification will not 
 be uniform and stains will appear. 
 
 Experiment 30. Mix a few cubic centimeters of very 
 
 dilute solutions of potassium ferrocyanide and uranium 
 
 nitrate and observe the blood-red coloration that is 
 
 developed. Uranium ferrocyanide is formed. Compare 
 
 Experiment 8. Reaction : 
 
 Uranium nitrate -|- Potassium ferrocyanide 
 
 = Uranium ferrocyanide + Potassium nitrate 
 
 The solution used in this process contains, in addition 
 
AFTER-TREATMENT OF NEGATIVE loi 
 
 to the uranium nitrate, potassium ferricyanide acidified 
 with acetic acid. The latter salt, in the presence of the 
 acid, is an oxidizing agent, and thus, when applied to 
 the silver image, oxidizes silver to silver oxide, which 
 with the excess of acetic acid forms silver acetate. But 
 the ferricyanide is simultaneously reduced to ferro- 
 cyanide, whereupon there occurs the reaction just 
 mentioned in which insoluble red uranium ferrocyanide 
 is produced. Therefore some of the silver, in being 
 oxidized, is rendered soluble and taken away, but is 
 replaced by the very non-actinic and insoluble uranium 
 compound. Reactions: 
 
 (i) Potassium ferricyanide + Acetic acid 
 
 = Ferricyanic acid + Potassium acetate 
 
 (2) Silver + Ferricyanic acid -f- Water 
 
 = Silver oxide -|- Ferrocyanic acid 
 
 (3) Silver oxide + Acetic acid 
 
 = Silver acetate + Water 
 
 (4) Uranium nitrate + Ferrocyanic acid 
 
 = Uranium ferrocyanide -|- Nitric acid 
 
 To prepare the uranium intensifier, make equal quan- 
 tities, say 100 cubic centimeters each, of a one per 
 cent, solution of uranium nitrate, and of one per cent, 
 potassium ferricyanide solution containing twenty per 
 cent, of acetic acid (glacial). These two solutions are 
 to be united and may then be used as desired. If by 
 leaving the plate too long in this intensifier the opacity 
 is too much increased, it may be diminished somewhat 
 by long washing in running water, especially if the 
 water be hard. Ordinarily the intensified negative 
 should be washed for about 10 minutes. 
 
 The chromium intensifier is one of the simplest to 
 use, in addition to which the operation can be repeated 
 as often as desired. For these reasons it is perhaps more 
 
I02 CHEMISTRY FOR PHOTOGRAPHERS 
 
 satisfactory than any other method, particularly for 
 the beginner, if he uses the "tabloid" chromium in- 
 tensifier, which is especially convenient. The plate is 
 bleached in a solution of one of the tabloids, it is washed 
 for a quarter of an hour, and it is then redeveloped in the 
 usual developing solution, the whole operation proceed- 
 ing in daylight. Solutions for this method may be 
 prepared as follows: 
 
 1. Potassium dichromate 5.5 grams 
 
 Water, to 100 cubic centimeters 
 
 2. Hydrochloric acid (concen- 
 
 trated) y.j cubic centimeters 
 
 Water, to loo cubic centimeters 
 
 For use take one part of Number i , one part of Num- 
 ber 2, and two parts of water. This mixture does not 
 keep well, and should be made only as it is to be used. If 
 the plate is dry, it must be soaked in water for an hour. 
 Leave it in the chromium solution until bleaching is 
 completed, wash it free from all chromic acid, expose 
 it to daylight for a few minutes, and then redevelop it, 
 best in a developing solution suited to bromide paper. 
 Finally wash thoroughly. By this process the printing 
 value of a negative may be increased to one and one- 
 half times its original printing value. 
 
 When a dichromate solution is acidified, chromic 
 acid is liberated, an unstable compound which is a 
 good oxidizer. With hydrochloric acid in concentrated 
 solution chromic acid interacts, producing chlorine 
 and, by the consequent reduction of the chromic acid, 
 chromic chloride. Thus, even in dilute solution, there 
 is a tendency for this reaction to go on, which explains 
 the poor keeping qualities of the mixed solution. The 
 action of the intensifying solution upon the plate may, 
 
AFTER-TREATMENT OF NEGATIVE 103 
 
 therefore, be considered to consist first in an oxidation 
 of silver to silver oxide. This, in presence of hydro- 
 chloric and chromic acids, will be, at least superficially, 
 converted to silver chloride and silver chromate, since 
 both are insoluble substances. But in the oxidation of 
 the silver an equivalent amount of chromic acid is 
 reduced, possibly forming an approximation to chromic 
 oxide because of the presence of an abundance of water 
 and the small concentration of the hydrochloric acid. 
 The exposure to light reduces the silver salts to sub- 
 salts, after which the reducing agent of the developer 
 causes the redeposition of elementary silver. But along 
 with this action there has been formed a corresponding 
 quantity of chromic oxide, more or less hydrated, and 
 quite insoluble, the addition of which to the original 
 silver causes a considerable increase in the opacity of 
 the negative. 
 
 Occasionally, even the most expert photographer will 
 find himself possessed of a negative which is altogether 
 too dense throughout. The matter may be remedied by 
 submitting the plate to the process which, photographic- 
 ally, is known as "reduction," but which consists, 
 chemically, simply in the oxidation of some of the silver 
 of the image and the conversion of the oxidized silver 
 to a soluble compound, which is then removed in 
 solution. There is a considerable number of oxidizing 
 agents which can be utilized for this purpose. It has 
 been found possible so to regulate the action of some of 
 these as to produce somewhat different results with the 
 different oxidizers. In the case just mentioned, where 
 there is a general opacity all over the plate, it may be 
 desirable to diminish this opaqueness with a fair 
 degree of uniformity throughout the scale of gradation. 
 
I04 CHEMISTRY FOR PHOTOGRAPHERS 
 
 If so, the "permanganate reducer" may be employed, 
 since it gives a nearly uniform reduction, acting only 
 a little more strongly upon the highlights {i.e. the 
 densest deposits of silver) than upon the shadow 
 details. To prepare the solution, dissolve 0.5 gram of 
 potassium permanganate in 200 cubic centimeters of 
 water and add 2 cubic centimeters of concentrated 
 sulphuric acid. This stock solution is four times too 
 concentrated, so that in using it for the reduction of a 
 plate one part of the permanganate is to be diluted with 
 three parts of water. The negative, if dry, is well 
 soaked and then immersed in the solution until suffi- 
 ciently reduced in density. The dish must be con- 
 stantly rocked to secure uniformity of action. The plate 
 is next quickly rinsed in two or three changes of water 
 and transferred for five minutes to a previously unused 
 acid thiosulphate fixing bath, and finally washed and 
 dried. Reaction: 
 
 Silver + Potassium permanganate + Sulphuric acid 
 
 = Silver sulphate + Potassium sulphate 
 + Manganese sulphate + Water 
 
 The products of this reaction are all soluble in water 
 and therefore removable from the gelatine by washing. 
 The function of the thiosulphate bath is to stop in- 
 stantly the action of the small amount of acidified 
 permanganate which is retained by the gelatine after 
 the plate has been rinsed, since it would continue to 
 dissolve away a little more silver. Reaction: 
 
 Sodium thiosulphate + Potassium permanganate 
 
 + Sulphuric acid 
 = Sodium sulphate + Potassium sulphate 
 + Manganese sulphate + Water 
 
 A reduction method which is very useful for over- 
 
AFTER-TREATMENT OF NEGATIVE loj 
 
 exposed, or foggy, or veiled, negatives is the one which 
 
 for a long time has been known as "Farmer's reducer." 
 
 The solution consists of a mixture of ten per cent. 
 
 potassium ferricyanide with ten per cent, "hypo," and 
 
 acts more energetically upon the details in the shadows 
 
 than upon the highlights. The result is therefore an 
 
 increase of contrast, desirable for the class of negatives 
 
 noted. This reducing bath, which must be made up 
 
 just before it is needed, can be conveniently prepared 
 
 by dissolving lo grams of sodium thiosulphate in water 
 
 and diluting to lOo cubic centimeters, and lastly 
 
 adding 5 to lo cubic centimeters of a ten per cent. 
 
 ferricyanide solution. Evenness of action is secured, as 
 
 usual, by rocking the tray in which the operation is 
 
 conducted. Wash the plate well afterwards. Reaction: 
 
 Silver + Potassium ferricyanide + Sodium thiosulphate 
 = Silver-sodium thiosulphate -|- Potassium-sodium 
 
 ferrocyanide 
 
 Two excellent processes, whereby the contrast of a 
 plate may be diminished, are found in the eerie sulphate 
 and the ammonium persulphate reducers, the latter 
 being known as "Bennett's reducer." In the former, 
 advantage is taken of the powerful oxidizing action of 
 the cerium compounds in which cerium is in the 
 higher condition of oxidation, having present also an 
 acid, such as sulphuric acid, with which the oxidized 
 silver forms a soluble product. A stock bath may be 
 made up by dissolving 10 grams of eerie sulphate 
 crystals in water, acidifying with 4 cubic centimeters 
 of concentrated sulphuric acid, and diluting this 
 mixture with water to 100 cubic centimeters. Take for 
 use one part of the stock solution and one part of water. 
 It will be found to attack the highlights more strongly 
 
lob CHEMISTRY FOR PHOTOGRAPHERS 
 
 than the details in the shadows. After reduction, wash 
 well and dry the plate. Reaction : 
 
 Silver + Ceric sulphate + Sulphuric acid 
 
 = Silver sulphate + Cerous sulphate + Water 
 
 For "Bennett's reducer" make the following solution: 
 Dissolve in water 1 2 grams of ammonium persulphate, 
 add 2 grams of sodium sulphite and i cubic centimeter 
 of concentrated sulphuric acid, and dilute to 100 cubic 
 centimeters. Use this solution in the proportion of one 
 part to four to eight parts of water, and apply it to the 
 wet negative, rocking the tray while the action is going 
 on. Rinse the plate quickly and immerse it for six 
 minutes in a ten per cent, acid "hypo" fixing bath, 
 which will stop the action in the same way as with the 
 permanganate reducer. In order to get the full advan- 
 tage of this method in lessening the contrasts of a neg- 
 ative, it is important not to allow the persulphate to 
 act too long upon the plate. For a limited time the so- 
 lution will dissolve away the densest portions of the de- 
 posited silver without appreciably attacking either the 
 middle tones or the finest details in the shadows. But 
 if the plate is left for a longer time in the bath, the 
 shadow details will be materially affected. Therefore, 
 it is necessary to watch the operation closely, removing 
 the negative from the persulphate solution the instant 
 that a solvent effect appears in the thinner parts. 
 Reaction : 
 
 Silver + Ammonium persulphate 
 
 = Silver sulphate + Ammonium sulphate 
 
 It is, perhaps, more frequently the case that both 
 these kinds of processes, of intensification and reduction, 
 will be utilized, than that either one of them will be 
 applied alone. For example, when the range of contrast 
 
AFTER-TREATMENT OF NEGATIVE 107 
 
 of a given subject that must be photographed is too 
 great to be expressed by the emulsion, it is quite 
 feasible to adjust the exposure and the development to 
 the shadow details, and, after fixing and washing the 
 plate, first reduce the highlights with persulphate and 
 then, if required, intensify the whole negative, as with 
 the uranium bath. By this means a pleasing picture 
 can oftentimes be made, under unfavorable conditions 
 of lighting, showing the subject which is photographed 
 as it appears when advantageously lighted. These 
 purely chemical methods of modifying the negative 
 will be found among the most useful helps to the 
 photographer in saving numbers of negatives which 
 would otherwise be wasted. 
 
CHAPTER VIII 
 Printing Processes with Silver Salts 
 
 THE object of all the solicitude which has been 
 expended in exposing the plate, in carrying out its 
 development, and in any subsequent treatment applied 
 to it is merely to provide a means for expressing in the 
 final print the varied tones of light and shade which, 
 photographically speaking, constitute the subject 
 portrayed. Whether it be the photographer's purpose 
 to make a picture which shall record facts as ac- 
 curately as possible or whether it be his intention to 
 represent a mood or a phase of the subject, giving 
 expression in the print to his own artistic feeling, the 
 quality of the negative determines very largely the 
 quality of the finished picture. Successful photography 
 depends in the first instance upon the worker's ability 
 to make properly exposed and developed negatives. 
 But in the wide range of printing media at his com- 
 mand, and especially in view of the variety of qualities 
 and surfaces in which the popular brands of papers 
 may be obtained, there is the possibility of a consider- 
 able degree of control. It becomes, therefore, a matter 
 of no small importance to select a medium which is 
 suitable for the purpose in hand, whatever it may be, 
 and to determine the precise character of negative 
 best adapted for printing in that medium. All printing 
 processes depend, for their effect, upon the use of a 
 substratum or support, which is, nearly universally, 
 white paper, or paper that is at most but slightly 
 
 tinted. The highest light is thus the plain, unshaded 
 1 08 
 
PRINTING WITH SILVER loq 
 
 paper, the deepest shadow is formed by deposits heavy 
 enough to obscure the paper entirely, and the inter- 
 mediate tones are produced by the different densities 
 of deposit, which, veiling the substratum to different 
 degrees, allow the transmission of more or less light by 
 the deposit and its reflection by the substratum. Since 
 not only are most of the printing processes which 
 depend upon silver salts the simplest to use, but also, 
 largely for that reason, of course, are they undoubtedly 
 the most extensively employed, we shall begin with 
 them our discussion of the different methods of making 
 prints. 
 
 The action of light upon silver compounds, and in 
 particular its effect upon the silver halogen salts, 
 whereby, according to the reaction: 
 Silver chloride + Light 
 
 = Silver sub-chloride + Chlorine, 
 
 the so-called latent image is formed, has already been 
 sufficiently described in previous chapters. But in the 
 case of photographic papers, as compared with plates or 
 films, we must note that there are certain differences 
 produced by the use of the different kinds of support. 
 In the plates and films we find a comparatively thick 
 layer of photographic emulsion spread upon glass and 
 celluloid, respectively, both these substrata possessing 
 no appreciable chemical activity towards the emulsion. 
 Paper, however, is an organic substance, which, con- 
 sisting largely of cellulose, is readily capable of inter- 
 acting chemically, especially with silver compounds. 
 It is, moreover, necessary, in order that the deposited 
 silver should be kept strictly upon the surface, that the 
 paper support be thoroughly sized. And this sizing, 
 generally arrowroot, starch, or gelatine, is also organic 
 
no CHEMISTRY FOR PHOTOGRAPHERS 
 
 and chemically active in the same way as the paper. 
 In the sunlight printing paper, P.O.P., as it is called, 
 there is the further difference that the light is allowed 
 to produce a visible effect, whereupon, instead of its 
 being treated with a reducing agent, the image is 
 acted upon by metallic salts. By these, in conjunction 
 with the fixing bath, a part of the silver salt is removed 
 and the final image consists of silver and some other 
 metal or metallic compound added to it. For making 
 this printing-out paper there is required a binding 
 material in which the photo-sensitive silver salt may 
 be formed. Albumen, collodion, and gelatine are three 
 such binders which have been much used, the last 
 probably being most employed at the present time. A 
 salt bath is prepared containing soluble chloride and the 
 binder, and the sheets of paper are given a coating, 
 which can very simply be done by floating the paper 
 upon the solution. After it has been dried, the "plain 
 salted paper" is again floated, upon a silver nitrate 
 solution of suitable concentration, and dried in non- 
 actinic light. It is then ready for use, but thus pre- 
 pared has the disadvantage of not keeping well for 
 any considerable length of time. For the salt bath, 
 make up a solution consisting of: 
 
 Ammoniiim chloride 6 to 8 grams 
 
 Sodium citrate lo grams 
 
 Sodium chloride 2 to 3 grams 
 
 Gelatine i gram 
 
 Distilled water, to 440 cubic centimeters 
 
 First, dissolve the gelatine in hot water and then add 
 
 the other compounds and filter the resulting solution. 
 
 Pour the solution into a tray of sufficient size. Grasp 
 
 a sheet of the paper to be salted by two diagonally 
 
 opposite corners and bend the sheet upon the other 
 
PRINTING WITH SILVER 1 1 1 
 
 diagonal as an axis, by bringing the hands together. 
 
 Lower the paper until it touches the surface of the liquid 
 
 along the axis, and then by separating and lowering the 
 
 hands allow the full area of the sheet to lie, without any 
 
 air bubbles, upon the solution. Float the paper thus 
 
 for three minutes, after which it is to be removed slowly 
 
 and hung up by one corner to dry. It is then ready to be 
 
 sensitized by similar flotation upon a solution of silver 
 
 nitrate in the proportion of one part of the solid to 
 
 nine parts of water. In hot weather the salted paper 
 
 may be floated for three minutes, but in cold, for five 
 
 minutes. Again hang up the sheet by one corner to 
 
 dry, but as it is now light-sensitive, drying must take 
 
 place away from actinic light. The keeping qualities 
 
 of this paper, as previously stated, are poor, as it will 
 
 be good for about two days in warm, but for a week in 
 
 cold, weather. The paper, however, can be "salted" 
 
 in quantity and sensitized only as desired. 
 
 It is now in order to consider the changes which occur 
 
 in the printing, toning, and fixing of such paper. First 
 
 it will be apparent that there is present in the very thin 
 
 coating of the unprinted paper a considerable number 
 
 of substances, since, from the nature of the operations 
 
 just described, it is impossible for all of even the 
 
 soluble products of the reactions to escape. When the 
 
 salted paper is sensitized upon the silver nitrate, the 
 
 following reactions must occur : 
 
 ( I ) Ammonium chloride + Silver nitrate 
 
 = Silver chloride + Ammonium nitrate 
 (z) Sodium chloride + Silver nitrate 
 
 = Silver chloride + Sodium nitrate 
 
 (3) Sodium citrate + Silver nitrate 
 
 = Silver citrate + Sodium nitrate 
 
 (4) Organic compounds in sizing and paper + Silver nitrate 
 
 = Complex organo-silver compound + Nitric acid 
 
112 CHEMISTRY FOR PHOTOGRAPHERS 
 
 All of these insoluble silver compounds and greater 
 or less amounts of the soluble products of the reactions, 
 besides a portion of the silver nitrate from the sensitiz- 
 ing bath, will be present in the coating and soaked up 
 in the body of the paper support. The silver-organic 
 compound is an exceedingly complex substance whose 
 nature is not at all well understood. It is said to be 
 somewhat less easily formed, that is, is rather more 
 soluble, than silver chloride, and therefore the sensitized 
 paper will in general have more of the chloride than of 
 the organic compound formed upon it, the relative 
 proportions of the two depending directly upon the 
 concentrations of silver nitrate in the sensitizing bath 
 and chloride in the salted paper. If the sensitizing 
 solution is, for example, sufficiently dilute, practically 
 none of the organic silver compound will form, since all 
 the silver from the solution in contact with the paper will 
 go to form chloride. This organic compound is, before 
 exposure to light, decomposed by sodium thiosulphate, 
 and by the action of light is reduced to an organic 
 silver oxide possessing a reddish tint. When the photo- 
 chemical reaction: 
 
 Silver chloride + Light 
 
 = Silver sub-chloride + Chlorine 
 
 takes place on exposure of the paper to light, the 
 liberated chlorine, by a secondary reaction, forms with 
 the excess of silver nitrate present more silver chloride : 
 Silver nitrate -f Chlorine -1- Organic matter 
 
 = Silver chloride + Nitrated organic matter 
 
 This new silver chloride is also photo-sensitive, and 
 thus the quantity of sub-chloride is increased. If the 
 printed paper is at once put into sodium thiosulphate 
 solution to remove unreduced silver salt, as must ulti- 
 
PRINTING WITH SILVER 113 
 
 mately be done to get a permanent print, the silver 
 
 chloride remaining is rendered soluble as silver-sodium 
 
 thiosulphate, and the silver sub-chloride is so acted upon 
 
 as to form this same soluble compound and elementary 
 
 silver. The reddish reduction product from the complex 
 
 silver-organic substance is practically unaffected by 
 
 thiosulphate. Reactions : 
 
 (i) Silver chloride -|- Sodium thiosulphate 
 
 = Silver-sodium thiosulphate -|- Sodium chloride 
 (2) Silver sub-chloride + Sodium thiosulphate 
 
 = Silver + Silver-sodium thiosulphate + Sodium 
 
 chloride 
 
 So the image left after fixing and washing, of a rather 
 
 unpleasant red color, consists of metallic silver and 
 
 complex silver-organic compounds. Papers sized with 
 
 gelatine give redder tones than albumen papers, and 
 
 those sized with starch, bluer tones. If the prints on 
 
 these papers are merely fixed after being exposed to 
 
 the action of light, therefore, the resulting pictures, 
 
 being very unattractive, are totally unsatisfactory, 
 
 except for use as proofs. By treating them, before fixing 
 
 in thiosulphate, with a suitable solution of gold chloride, 
 
 the resultant color of the mixture of silver and gold may 
 
 be made very pleasing. If a gold tri-chloride solution 
 
 be used upon paper from which the excess of silver 
 
 nitrate has been washed before toning, the reduction 
 
 of gold is accompanied by a simultaneous oxidation of 
 
 the sub-chloride of silver, and the latter compound is 
 
 finally removed as soluble thiosulphate. In this way 
 
 one atom of gold will replace three atoms of silver, and 
 
 the outcome of the reaction will be a feeble blue deposit 
 
 of gold, as 
 
 3 Silver sub-chloride -f Gold trichloride 
 
 = Gold + 3 Silver chloride 
 
114 CHEMISTRY FOR PHOTOGRAPHERS 
 
 But if the gold solution is so regulated that the tri- 
 chloride is reduced to monochloride, that is, auric 
 chloride is converted by reduction to aurous chloride, 
 then, in the reaction between the aurous chloride of the 
 toning bath and the silver sub-chloride of the print, 
 there will be a replacement, atom for atom, of gold for 
 silver, and a stronger image will result. Reaction : 
 
 Silver sub-chloride -|- Aurous chloride 
 
 = Gold + Silver chloride 
 
 In making up gold toning solutions auric chloride is 
 
 the form of gold compound used, but it is necessary 
 
 either to use hot water for dissolving the salts, and then 
 
 wait for the solutions to become cool, or to allow the 
 
 cold solutions to stand for a day or two before using 
 
 them. In this manner the auric salt is partially reduced 
 
 to aurous chloride in the solution, after which action 
 
 the toning baths work properly. Such a toning bath 
 
 is the following recommended by Abney : 
 
 Gold trichloride i part 
 
 Calcium hypochlorite ("Chloride of lime") i part 
 
 Calcium carbonate ("Chalk") i teaspoonful 
 
 Water 3 500 parts 
 
 If this bath is made up with hot water it can be used 
 
 as soon as it has cooled. Gold trichloride as usually 
 
 prepared contains an excess of free hydrochloric acid, 
 
 and it is partly for the purpose of neutralizing the 
 
 acid that the calcium carbonate is added to the bath. 
 
 Reaction ; 
 
 Calcium carbonate -f- Hydrochloric acid 
 
 = Calcium chloride -|- Carbon dioxide + Water 
 
 Some salts have the property of interacting to a 
 greater or less extent with water in which they are dis- 
 solved, the reaction being called ' 'hydrolysis. ' ' 1 1 is prac- 
 
PRINTING WITH SILVER 115 
 
 tically the reverse of a reaction of neutralization in 
 which base and acid unite to form a salt and water, 
 since in hydrolytic action a salt and water interact to 
 form a base and an acid. Calcium hypochlorite hydro- 
 lyzes in this manner to some degree, and hypochlorous 
 acid and calcium hydroxide tend to form in the solution. 
 Likewise, when auric chloride is contained in the same 
 solution with the calcium hydroxide, it may be con- 
 sidered to react with the latter to produce aureus 
 chloride, calcium hypochlorite, and water, thus: 
 
 Auric chloride + Calcium hydroxide 
 
 = Aurous chloride + Calcium hypochlorite 
 
 + Water 
 
 A similar reaction may occur with the solid calcium 
 carbonate placed in the toning bath : 
 
 Auric chloride + Calcium carbonate 
 
 = Aurous chloride + Calcium hypochlorite 
 + Carbon dioxide 
 
 The gold salt in such a solution may be said to be 
 chemically in a state of neutral equilibrium ; the least 
 disturbance renders the equilibrium unstable, and 
 metallic gold begins to precipitate. If into this bath 
 there is put a print, owing to the presence of the sub- 
 chloride of silver and the light-reduced, silver-organic 
 oxide, the chemical balance of the solution is upset, 
 gold commences to separate, and the sub-compounds 
 of silver are, by chlorine from the aurous chloride, 
 oxidized back to silver chloride, according to the re- 
 action previously written. Before toning with the gold 
 solution it is usual to wash the printed paper in several 
 changes of water in order to free it as much as possible 
 from silver nitrate. Any soluble silver salt not washed 
 out of the paper is at once precipitated in the toning 
 
ii6 CHEMISTRY FOR PHOTOGRAPHERS 
 
 bath as silver chloride by action of the gold chloride. 
 The hypochlorite acts as a restrainer in the bath, re- 
 tarding the reduction of gold. Perhaps it does this in 
 two ways, since not only does hypochlorous acid de- 
 compose spontaneously, forming hydrochloric acid and 
 oxygen, by the reaction : 
 
 Hypochlorous acid = Hydrochloric acid + Oxygen 
 but it also reacts with hydrochloric acid directly, 
 liberating chlorine : 
 
 Hypochlorous acid + Hydrochloric acid 
 
 = Chlorine -f- Water 
 
 The oxygen evolved must tend to reoxidize the gold, 
 thus diminishing its rate of reduction; and the free 
 chlorine also tends to retard this reduction by oxidizing 
 the sub-chloride of silver directly. Evidently, from these 
 considerations, the bath should be expected to become 
 acid in time, and this is in fact the case. 
 
 Another toning solution may be prepared from 
 sodium acetate and gold trichloride, but it must be 
 made up the day before it is to be used, to allow the 
 partial reduction of auric to aurous chloride to occur. 
 Afterwards it keeps indefinitely, and, like the preceding 
 bath, it may be strengthened by the addition of more 
 gold trichloride as it becomes worked out. 
 
 Sodium acetate 3o parts 
 
 Gold trichloride i part 
 
 Water 4,400 parts 
 
 This formula also is given by Abney. Aurous chloride 
 
 may be formed by the following reaction, in the ripening 
 
 of the bath : 
 
 Auric chloride + Sodium acetate 
 
 = Aurous chloride -|- Sodium trichloracetate 
 -|- Hydrochloric acid 
 
PRINTING WITH SILVER 117 
 
 In using the solution the print must first be well 
 
 washed to free it from the greater portion of silver 
 
 nitrate. This bath also becomes after a time too acid 
 
 to work well. The reaction, by which metallic gold is 
 
 substituted for silver, is the same as previously given : 
 
 Silver sub-chloride + Aureus chloride 
 
 = Gold + Silver chloride 
 
 The unfixed image, therefore, after toning, is com- 
 posed of metallic gold and the chloride of silver, to- 
 gether with any sub-chloride of the latter metal which 
 has not suffered oxidation. 
 
 To fix prints after toning, they should be well washed 
 
 in water and then put for ten to fifteen minutes into 
 
 a plain solution of sodium thiosulphate, one part, to 
 
 water, five parts; after this they may be soaked for 
 
 fifteen minutes and finally washed in running water 
 
 for twelve hours. Reaction : 
 
 Silver chloride + Silver sub-chloride 
 
 + Sodium thiosulphate 
 = Silver -(- Silver-sodium thiosulphate 
 + Sodium chloride 
 
 The tones of the print are, in part, caused by the vari- 
 ous sizes of the particles of gold deposited, the color by 
 reflected light of finely divided gold ranging from purple 
 to a dirty green, according to the fineness of the sub- 
 division. An over-toned print is blue on account of the 
 excess of gold deposited. The final tone of the fixed 
 print is due to the mixture of colors, by reflected light, 
 of the silver and gold of which the image is composed. 
 Before fixing, the print can be further washed and put 
 into a solution of a platinum salt in which the gold 
 will be partially replaced by platinum, in a similar 
 way as the gold itself was plated upon the silver. 
 
ii8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Formulas for the platinum toning of printing out paper 
 
 are the following: 
 
 Potassium chloroplatinite . . i grams 
 
 Phosphoric acid (Sp.G. i . 1 2) 28 cubic centimeters 
 Water (distilled), to 1000 cubic centimeters 
 
 After toning, the print is to be washed and then fixed 
 in thiosulphate and washed as usual. Or; 
 
 Potassium chloroplatinite . . 0.25 gram 
 
 Sodium chloride 2.5 grams 
 
 Citric acid 2.5 grams 
 
 Water (distilled), to 500 cubic centimeters 
 
 To use this solution, tone first in an ammonium sul- 
 phocyanide bath, wash, and put into the platinum bath, 
 and finish as usual. The sulphocyanide bath will be 
 given presently. 
 
 A gelatine emulsion, originated by Abney, which 
 can be coated upon paper for prints or upon glass for 
 transparencies, is prepared in much the same way as 
 the gelatino-bromide emulsion for plates : 
 
 1. Sodium chloride 3.7 grams 
 
 Potassium citrate i .8 grams 
 
 Water, to 50 cubic centimeters 
 
 2. Silver nitrate 11 grams 
 
 Water, to 50 cubic centimeters 
 
 3. Gelatine 15 grams 
 
 Water, to 175 cubic centimeters 
 
 After 2 and 3 have been mixed, i is to be added in 
 small quantities successively with shaking. When the 
 emulsion has set, squeeze it through cloth into cold 
 water, and allow it to soak for ten or fifteen minutes. 
 Pour off the water, drain the emulsion, and dissolve 
 it, with the aid of heat, with ly cubic centimeters 
 of alcohol. Paper is best coated with this mixture by 
 flotation. After printing, the paper can be either fixed 
 
PRINTING WITH SILVER iiq 
 
 at once, which will give a brown color, or it can be toned 
 
 in a bath made up of ammonium sulphocyanide and 
 
 gold trichloride : 
 
 Ammonium sulphocyanide ... 7 grams 
 
 Gold trichloride 0.12 gram 
 
 Water, to 1000 cubic centimeters 
 
 The gold salt and ammonium sulphocyanide interact 
 to form a soluble double compound, according to the 
 equation : 
 
 Gold trichloride + Ammonium sulphocyanide 
 
 = Ammonium-gold sulphocyanide 
 + Ammonium chloride 
 
 Gold is reduced from the double salt by the action of 
 silver sub-chloride, perhaps as follows : 
 
 Silver sub-chloride + Ammonium-gold sulphocyanide 
 = Gold + Silver chloride -|- Ammonium 
 sulphocyanide -|- Silver sulphocyanide 
 
 The toned print is rinsed, fixed in thiosulphate solu- 
 tion, and washed. Chloride printing out papers are man- 
 ufactured in which the toning salts are incorporated in 
 the emulsion, so that after printing it is only required 
 to immerse the dry print in a thiosulphate bath, in 
 which the toning proceeds simultaneously with the 
 fixing. These are the "self-toning" papers. But it is at 
 least sometimes true that the attempt to secure too 
 great simplicity in photographic manipulations may 
 lead to results less permanent and inferior in beauty. 
 
 The foregoing criticism does not necessarily apply to 
 the second great order of silver papers, namely, the 
 developing out papers. For certainly, if they are worked 
 with sufficient care, there appears to be no reason why 
 they should not be as permanent as a print in silver 
 can be made, or, allowing for the difference between the 
 paper and glass supports, why they may not be practic- 
 
I20 CHEMISTRY FOR PHOTOGRAPHERS 
 
 ally as permanent as the negative itself. The chief 
 cause of the lack of permanency in silver images seems 
 to be the incomplete removal of thiosulphate, which 
 in the course of time breaks down, being in reality a 
 none too stable substance, and liberates sulphur. This 
 produces a yellowing of the paper. Besides this, if 
 any intensification or toning processes are applied to an 
 image which still retains thiosulphate, there will in- 
 evitably occur in time complicated reactions by the 
 products of which the plate or print will be stained or 
 bleached. In the making of photographs, by whatever 
 method, there is no "dodge" nor "trick" which can 
 possibly surpass an intelligent grasp of the chemical 
 principles involved, if coupled with a strict application 
 of good laboratory practice. 
 
 The developing papers are subdivided into two 
 classes, those in which silver chloride is the sensitive 
 salt and those in which silver bromide is used. In the 
 former, or "gaslight" papers, the light sensitiveness is 
 not too great to permit of handling the paper in the 
 weaker illumination of an ordinary room lighted by 
 lamps, so that the darkroom is eliminated. But in the 
 case of the bromide paper the sensitiveness, approxi- 
 mately one twentieth of that of the photographic 
 plate, is so great that the paper must be carefully 
 protected from white light. The action of light upon 
 these salts, in the formation of the latent image, and 
 the development of the image have already been dis- 
 cussed in their respective chapters, and nothing further 
 need be added here upon these points. The principal 
 precautions to be taken in working these papers are 
 to use a developing agent which gives a good black 
 deposit of silver, to adjust the solution so that staining 
 
PRINTING WITH SILVER 121 
 
 is prevented, and to fix thoroughly and in a fresh 
 "hypo" solution, finally freeing the paper as completely 
 as possible from all removable substances by sufficient 
 washing. 
 
 If it is desired to alter the ordinary black deposit of 
 silver to some color, there is available a considerable 
 number of processes for the purpose. The change most 
 commonly made is from black to sepia, and this is 
 effected by transforming the silver image into one 
 composed of silver sulphide. There are two methods of 
 doing this, as shown in the experiments. 
 
 Experiment 3 1 . Make up 500 cubic centimeters of a 
 solution containing 35 grams of sodium thiosulphate 
 and 7 grams of powdered potassium alum, heating the 
 water to boiling for the purpose. Put into this (un- 
 filtered) bath a well-washed print and observe the 
 alteration of the black image to brown. 
 
 In the process the thiosulphate is decomposed and 
 the sulphur combines with the silver, forming silver 
 sulphide. The slow breaking down of the thiosulphate 
 is due to the fact that aluminum salts hydrolyze, 
 tending to go over to aluminum hydroxide, thus 
 liberating acid in the solution. This acid causes the 
 decomposition of the thiosulphate. The action may 
 be represented as follows by equations in steps : 
 
 I . Potassium aluminum sulphate + Water 
 
 = Potassium aluminum hydroxy-sulphate 
 + Sulphuric acid 
 
 ^. Sodium thiosulphate + Sulphuric acid 
 
 = Sodium sulphate + Sulphur + Sulphur dioxide 
 3. Silver + Sulphur = Silver .sulphide 
 
 A better way of changing the silver to silver sulphide 
 consists in first oxidizing the image, as to silver bromide. 
 
122 CHEMISTRY FOR PHOTOGRAPHERS 
 
 and then treating it with a solution of a sulphide, 
 whereby silver sulphide is formed by metathesis. 
 
 Experiment 32. Dissolve 5 grams each of potassium 
 ferricyanide and potassium bromide in enough water 
 and dilute to 250 cubic centimeters. Put a dry print 
 into this solution until completely bleached. Rinse the 
 print thoroughly and transfer to a solution of sodium 
 sulphide, in which the formation of silver sulphide 
 gives the sepia color. 
 
 The stock sulphide solution is prepared by dis- 
 solving 25 grams of pure sodium sulphide in 250 cubic 
 centimeters of water. If impure sodium sulphide is 
 used, dissolve in 150 cubic centimeters of water, boil, 
 and dilute to 250 cubic centimeters. The impurity is 
 precipitated and will settle to the bottom, where it 
 may be left, the liquid being decanted as desired. 
 For use, take 10 cubic centimeters of the stock solution 
 and dilute to 200. Leave the print in this diluted 
 solution until it is certain that the action is complete, 
 wash it in running water for about 15 minutes, and 
 dry it. The reactions which take place may be repre- 
 sented as follows: 
 
 1. Silver 4- Potassium ferricyanide + Potassium 
 
 bromide 
 = Silver bromide + Potassium ferrocyanide 
 
 2. Silver bromide + Sodium sulphide 
 
 = Silver sulphide + Sodium bromide 
 
 Another process of sulphide toning, by which it is 
 possible to control the depth of the tone, is one in 
 which mercuric chloride is added to the bleaching 
 solution. By varying the proportions of the mercury 
 salt, a range of browns from sepia to a pure black can 
 be secured. This process is naturally not quite so simple 
 
PRINTING WITH SILVER 123 
 
 as the preceding. Two stock solutions which keep well 
 are required for bleaching: 
 
 1 . Potassium ferricyanide .... 30 grams 
 Potassium bromide 45 grams 
 
 Water, to 250 cubic centimeters 
 
 2. Mercuric chloride 7 grams 
 
 Potassium bromide 7 grams 
 
 Water, to 250 cubic centimeters 
 
 The prints to be toned should each have been fully 
 developed in fresh developer, and dried after fixing 
 and washing, before toning. For sepia, take i part of 
 No. I and dilute with 1 1 parts of water. One part of 
 No. I, I of No. 2, and 10 of water give a colder 
 brown, and as the relative proportion of the second 
 solution is increased, the tone approaches nearer to 
 black. For deep brown, take ^ part of No. i , i part 
 of No. 2, and loX parts of water; brown-black, i 
 of No. I, 2 of No. 2, and 13 of water; and for a pure 
 black, yi of No. i , 2 of No. 2, and q>^ parts of water. 
 In every case the bleaching should be thorough, and 
 if any of No. 2 solution is used in the bath the bleached 
 prints after being well washed must be transferred for 
 two or three minutes to a dilute solution of hydro- 
 chloric acid, 3 cubic centimeters of the concentrated 
 acid in 200 cubic centimeters of water, in order to 
 prevent the mercury salts from combining with the 
 gelatine coating. Move the prints on into a second and 
 third similar acid bath, and finally wash for twenty 
 minutes. After this the sulphide toning is carried out 
 in a bath made by diluting to 200 cubic centimeters 
 15 cubic centimeters of the stock solution of sodium 
 sulphide above described. The diluted sulphide toning 
 solution should be used but once. After toning, the 
 print is to be thoroughly washed, say for a half hour, 
 
124 CHEMISTRY FOR PHOTOGRAPHERS 
 
 and then dried. Printing-out paper cannot be success- 
 fully sulphide-toned. The reactions may be outlined 
 thus: 
 
 I. Silver + Potassium ferricyanide 
 
 + Potassium bromide 
 = Silver bromide -|- Potassium ferrocyanide 
 I. Silver -|- Mercuric chloride 
 
 = Silver chloride + Mercurous chloride 
 ■}. Silver bromide -|- Sodium sulphide 
 
 = Silver sulphide -|- Sodium bromide 
 
 4. Silver chloride -|- Sodium sulphide 
 
 = Silver sulphide -|- Sodium chloride 
 
 5. Mercurous chloride + Sodium sulphide 
 
 = Mercuric sulphide + Mercury -|- Sodium chloride 
 
 The toned image is therefore composed of silver and 
 mercuric sulphides with elementary mercury, if any 
 of the No. 2 bleaching solution is included in the bath. 
 The character of the tone is determined by the pro- 
 portion of black mercury and black mercuric sulphide 
 added to the brown silver sulphide image. 
 
 For producing a variety of colors, some ranging 
 from a warm black through brown to red, and for blues 
 and greens, salts of copper, uranium, iron, and 
 vanadium are used. So far as concerns the permanency 
 of the results, it is well to observe at the start that the 
 pure silver image and the image in silver sulphide may 
 be considered sufficiently durable if the chemical work 
 is well done. But when the simple metallic silver image 
 is converted into complexes containing silver salts with 
 copper, uranium, iron, or vanadium compounds re- 
 spectively, the way is opened not only for the gradual 
 decomposition of these complicated bodies, but more 
 especially for the attack upon them of an atmosphere 
 containing moisture and other impurities. Conse- 
 quently the experimenter should, knowing the dangers, 
 
PRINTING WITH SILVER 125 
 
 pay all the more attention to cleanliness in manipulation 
 and in particular to the thoroughness with which soluble 
 products are washed out of the prints ; and even then 
 should not be unprepared for alterations in them in the 
 lapse of time. 
 
 It will be most convenient to make up a number of 
 stock solutions from which the different toning baths 
 can be quickly and easily prepared whenever wanted: 
 
 SOLUTION A 
 
 Potassium citrate loo grams 
 
 Water, to 1000 cubic centimeters 
 
 SOLUTION B 
 
 Potassium ferricyanide 2y grams 
 
 Water, to zyo cubic centimeters 
 
 SOLUTION c 
 
 Copper sulphate 25 grams 
 
 Water, to 250 cubic centimeters 
 
 SOLUTION D 
 
 Uranium nitrate 5 grams 
 
 Water, to yo cubic centimeters 
 
 SOLUTION E 
 
 Ferric ammonium citrate iz.y grams 
 
 Water, to 250 cubic centimeters 
 
 Toning with copper gives a range of colors from a 
 
 warm black through brown, purple, many shades of 
 
 red, to the so-called "red chalk." The image which is 
 
 finally obtained consists of a mixture of silver ferro- 
 
 cyanide, copper ferrocyanide, and silver. Take of the 
 
 solutions : 
 
 Solution A 100 cubic centimeters 
 
 Solution B 10 cubic centimeters 
 
 Solution C 10 cubic centimeters 
 
 and place the wet print in the mixture until it has 
 altered to the desired color. The action may be stopped 
 at any point by washing for ten minutes, after which 
 
126 CHEMISTRY FOR PHOTOGRAPHERS 
 
 the print is to be dried. The following reactions may 
 be supposed : 
 
 1 . Silver + Potassium ferricyanide 
 
 = Silver ferrocyanide + Potassium ferrocyanide 
 
 2. Copper sulphate + Potassium ferrocyanide 
 
 = Copper ferrocyanide + Potassium sulphate 
 
 To get red tones more quickly, an alkaline bath may 
 be made by dissolving 2 grams of copper sulphate and 5 
 grams of potassium ferricyanide in 1,000 cubic centi- 
 meters of saturated ammonium carbonate solution. 
 
 By referring to the preceding chapter, it will be re- 
 called that uranium salt is used in a process for in- 
 tensifying the silver image. For this reason the uranium 
 toning solution is not adapted to dense prints, since 
 they are intensified by it. And, further, we remember 
 that prolonged washing of the uranium-intensified 
 image results in weakening it. This action is, of course, 
 more pronounced in the case of a print, and therefore 
 the final washing cannot be too long continued if the 
 print is to hold its tone. This not only indicates a 
 quite appreciable solubility of the uranium ferrocya- 
 nide, but also does not tend towards permanency of the 
 image. Take of the stock solutions as follows : 
 
 Solution B 10 cubic centimeters 
 
 Solution D 5 cubic centimeters 
 
 Glacial acetic acid 10 cubic centimeters 
 
 Water 400 cubic centimeters 
 
 and, after the tone wished for is reached, wash the print 
 in two or three changes of water acidified with acetic 
 acid (one part acid to 100 parts of water), and wash 
 not longer than i y minutes. The colors given by this 
 formula run from black through brown to a red that 
 has more yellow in it than the reds given by the copper 
 bath. The reactions are the same here as those written 
 
PRINTING WITH SILVER 127 
 
 for uranium intensification, and therefore need not be 
 
 repeated. If not enough acetic acid is used, permanent 
 
 yellow stains will appear. 
 
 In order to color the print blue, take : 
 
 Water 300 cubic centimeters 
 
 Solution B 30 cubic centimeters 
 
 Solution E 30 cubic centimeters 
 
 Nitric acid (concentrated) .... i cubic centimeters 
 
 In the acidified solution the oxidation of metallic sil- 
 ver is accompanied by the reduction of an equivalent 
 amount of ferricyanide to ferrocyanide, when not only 
 is silver ferrocyanide formed, but there is produced, also, 
 by interaction of the ferric salt present with the ferro- 
 cyanide, ferric ferrocyanide, which, as we saw in mak- 
 ing the tests for ferric and ferrous iron, is the substance 
 known as "Prussian blue." It is quite insoluble, and 
 therefore adds itself to the image. Too long washing 
 after the print has been toned tends to bleach it out, 
 since the blue ferric ferrocyanide is after all slightly 
 soluble in water. Reactions : 
 
 I. Silver + Potassium ferricyanide 
 
 = Silver ferrocyanide + Potassium ferrocyanide 
 1. Ferric ammonium citrate -j- Potassium ferrocyanide 
 = Ferric ferrocyanide + Potassium citrate 
 + Ammonium citrate 
 
 Green tones, which become lighter with longer action 
 of the bath, can be secured by a mixture of iron and vana- 
 dium chlorides. The solution is a rather complicated one : 
 
 NO. I 
 
 Ferric chloride 23 grams 
 
 Oxalic acid, saturated solution 125 cubic centimeters 
 
 Vanadium chloride 4.6 grams 
 
 Nitric acid (concentrated) .... 10 cubic centimeters 
 
 Water, to 500 cubic centimeters 
 
 NO. 2 
 
 Potassium ferricyanide 23 grams 
 
 Water, to yoo cubic centimeters 
 
128 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Add No. 2 to No. i with constant stirring and use the 
 mixture without further dilution. Prints should tone in i 
 to 2 minutes, and are then to be fixed in a solution of: 
 
 Sodium thiosulphate 6o grams 
 
 Boric acid 12 grams 
 
 Water, to 300 cubic centimeters 
 
 After fixing, wash for 10 minutes. The reduction of 
 the vanadium salt may be accomplished by both the 
 finely divided silver of the image and the ferrocyanide 
 produced in the reduction of potassium ferricyanide. 
 Thus vanadium compounds in which the vanadium is 
 present in several lower stages of oxidation are probably 
 added to the image. Mixtures of such vanadium com- 
 pounds are green. If the reduction of vanadium proceeds 
 too far, as by allowing the print to remain in the 
 bath too long, the resulting color is lighter owing to 
 changes in the proportions of the coloring constituents. 
 
 Bromide paper may also be toned with a platinum 
 
 solution, in which toning for about 20 minutes gives a 
 
 good black: 
 
 Hydrochloric acid (concentrated) i cubic centimeter 
 
 Potassium chloroplatinite i gram 
 
 Water (distilled), to 1000 cubic centimeters 
 
 The prints are washed thoroughly after toning, to get 
 rid of the acid, are fixed in sodium thiosulphate to re- 
 move the silver chloride formed, and given a final 
 washing. Reactions: 
 
 I . Silver + Potassium chloroplatinite 
 
 = Platinum + Silver chloride + Potassium chloride 
 z. Silver chloride + Sodium thiosulphate 
 
 = Silver-sodium thiosulphate 
 
 Asimilar formula maybe used for toning lantern slides : 
 
 Hydrochloric acid (concentrated) o . 2 cubic centimeter 
 Chloroplatinic acid, 5 % solution 4 cubic centimeters 
 Water (distilled), to 1000 cubic centimeters 
 
PRINTING WITH SILVER izq 
 
 This bath gives rapid toning with some reduction in 
 density. The reactions are similar to those lust given. 
 
 A great deal of study has been put by "he manu- 
 facturers of photographic materials into the preparation 
 of ready-to-use powders and solutions for performing 
 practically all of the different chemical operations in- 
 volved in the making of photographs. Doubtless these 
 same manufacturers have spent considerable sums of 
 money in advertising their preparations. While it is 
 quite true that these may be on occasion extremely 
 convenient to use, nevertheless it is equally true that 
 it is both more costly in actual cash outlay and much 
 less enlightening to the earnest amateur worker to 
 make use of them. It would be idle to argue that the 
 photographer should now, as he was once obliged to do, 
 make his own plates, when they can be so much better 
 and more uniformly turned out by the factories. Some- 
 thing can be said on both sides when it comes to the 
 preparation of papers, for, as we shall see presently, 
 there are several interesting and beautiful processes 
 for homemade prints. But not only can the amateur 
 save a little money by purchasing the necessary 
 chemicals in bulk and making up his own solutions, he 
 can, if he will allow himself to become interested, 
 derive a great deal of pleasure and satisfaction from the 
 study of the wonderful chemical processes which go 
 to make up this fascinating art of photography, benefits 
 that are largely lost to those who work only with ready- 
 made materials, and entirely so to the kind that "pushes 
 the button" and lets the professional "do the rest." 
 
CHAPTER IX 
 PiuNTiNG Processes with Iron Salts 
 
 IN an earlier chapter reference was made to the two 
 classes of iron salts, ferric and ferrous, and several 
 experiments were described in illustration of the readi- 
 ness with which they can be reduced and oxidized 
 respectively. The light-sensitiveness of ferric com- 
 pounds was investigated by Sir John Herschel, and he 
 indeed invented, in 1840, one of the simplest photo- 
 graphic printing processes, using salts of iron, namely, 
 the "blue print" process. 
 
 Experiment 33. Coat a piece of sized paper with a 
 little ten per cent, solution of ferric ammonium citrate, 
 either by floating the paper upon the liquid or by 
 applying the solution with a brush or a wad of absorb- 
 ent cotton. When the coated paper has become dry, 
 expose it for one or two minutes to sunlight under a 
 negative. Develop the print in a little dilute potassium 
 ferricyanide solution and wash it thoroughly. Note 
 that the shadows are blue and the highest lights white. 
 Coat another piece of paper with the same ferric 
 ammonium citrate solution, expose it similarly under a 
 positive transparency, such as an unmounted lantern 
 slide, and develop with dilute potassium ferrocyanide 
 and wash. Observe in this case also that a blue positive 
 has been obtained, but from a positive instead of from 
 a negative. 
 
 By the action of light the ferric citrate, in contact 
 
 with the organic matter of the sizing, has been reduced 
 
 to ferrous citrate. This same reaction of course takes 
 130 
 
PRINTING WITH IRON 131 
 
 place in both parts of the experiment. Since ferrous 
 salts interact with ferricyanide to form an insoluble 
 blue compound, "TurnbuU's blue," the application of 
 potassium ferricyanide to the first print, from the 
 negative, forms blue ferrous ferricyanide wherever any 
 ferric salt has been reduced, viz., in the shadows and 
 half tones. The interaction of ferric salts with ferri- 
 cyanide produces ferric ferricyanide, but this is a 
 soluble compotind, and therefore washing the paper 
 sufficiently removes all the unreduced ferric salt and 
 whatever ferricyanide is not combined in the insoluble 
 dark blue deposit. Thus a blue and white positive is 
 obtained by printing under a negative. Reactions : 
 
 1 . Ferric ammonium citrate + Light -(- Organic matter 
 
 = Ferrous ammonium citrate + Organic 
 oxidation products 
 
 2. Ferrous ammonium citrate + Potassium ferricyanide 
 
 = Ferrous ferricyanide + Ammonium citrate 
 + Potassium citrate 
 
 In the second print, from the positive, the unreduced 
 ferric compound by interaction with potassium ferro- 
 cyanide forms ferric ferrocyanide, an insoluble pre- 
 cipitate of dark blue "Prussian blue," in the shadows. 
 In the highlights where the reduction of the iron salt has 
 occurred, ferrous citrate reacts with the ferrocyanide 
 and an insoluble precipitate results, which is originally 
 ferrous potassium ferrocyanide and, like most ferrous 
 salts, white. But it instantly begins to oxidize, absorb- 
 ing oxygen from the air, and rapidly turns blue on 
 account of the oxidation to ferric ferrocyanide. Thus 
 the highlights are unavoidably stained a light blue by 
 the mixture of white ferrous potassium ferrocyanide 
 with dark blue ferric ferrocyanide. Reactions : 
 
I3Z CHEMISTRY FOR PHOTOGRAPHERS 
 
 3. Ferric ammontum citrate + Potassium ferrocyanide 
 
 = Ferric ferrocyanide + Ammonium citrate 
 + Potassium citrate 
 
 4. Ferrous ammonium citrate + Potassium ferrocyanide 
 
 = Ferrous potassium ferrocyanide + Ammonium 
 citrate + Potassium citrate 
 y. Ferrous potassium ferrocyanide + Oxygen + Water 
 = Ferric ferrocyanide + Potassium ferrocyanide 
 + Potassium hydroxide 
 
 Blue print paper as commonly prepared is coated with 
 a mixture of ferric ammonium citrate and potassium 
 ferricyanide and dried in the dark. It does not keep 
 well, but this disadvantage is somewhat counterbal- 
 anced by the fact that it is so easily made. When it 
 has been fully exposed to the action of light, as under a 
 negative or other design, it requires but a quarter of an 
 hour's washing and subsequent drying to be completed. 
 As soon as the printed paper is put into water, the re- 
 duced ferrous salt and the potassium ferricyanide begin 
 to go into solution and instantly react, giving the in- 
 soluble blue ferrous ferricyanide wherever the light 
 has acted. At the same time all the other substances 
 dissolve and are washed away, and the print is there- 
 fore developed and fixed by the one operation. The 
 reactions are the same as in (i) and (2) above. Washing 
 must not be too long continued, since the blue pre- 
 cipitate has an appreciable solubility and the pro- 
 longed application of water degrades the image. Water 
 containing lime (i.e., hard water) has a greater solvent 
 effect than soft water. The prints can be made some- 
 what brighter if citric acid be added to the first wash 
 water, about 2 grams to each liter. If it is desired to 
 use others than the strictly photographic papers, they 
 should be sized. This may be done by mixing 1 5 grams 
 of arrowroot to a paste with a little cold water, making 
 
PRINTING WITH IRON 133 
 
 it up to 600 cubic centimeters with water and boiling 
 the mixture gently until it becomes clear. The thinner 
 papers may be immersed in sizing them, but for thick 
 ones it is better to brush the size on as uniformly as 
 possible, brushing the paper one way and then across, 
 and finally to smooth out streaks and any unevenness 
 with a soft sponge or tuft of absorbent cotton. The 
 sheet may then be hung up with clips until dry. As the 
 sizing has a tendency to drain away from the upper edge 
 and accumulate along the lower edge of a sheet so hung 
 up, it is sometimes advisable to resize the paper and in 
 drying hang it the other side up. But if it is permitted 
 to become nearly or quite surface dry before hanging up, 
 there will be no need of the second sizing. For coating 
 the paper a mixture of two solutions is prepared: 
 
 NO. I 
 
 Ferric ammonium citrate 1 60 grams 
 
 Water, to 1000 cubic centimeters 
 
 NO. 2 
 
 Potassium ferricyanide 1 20 grams 
 
 Water, to 1000 cubic centimeters 
 
 Fresh, clean crystals of the ferricyanide are to be used. 
 If the crystals are covered with a powder, they should 
 be quickly washed in a little water before making the 
 solution, and in this case a few grams more than the 
 amount specified may be weighed out. These solutions 
 are mixed and should be kept in a brown bottle in the 
 dark. The mixture will keep better, and for a longer 
 time, if potassium dichromate be added in the propor- 
 tion of one gram per liter. When ready to make up 
 some paper, filter as much of the solution as may be 
 required and apply it evenly with a brush and tuft 
 of absorbent cotton. The coated paper should be dried 
 as quickly as possible in the dark. 
 
134 CHEMISTRY FOR PHOTOGRAPHERS 
 
 A blue print process which is useful for making 
 positive copies from positives is based upon the second 
 part of Experiment 33. It is employed particularly for 
 copying tracings, and these must in printing be put into 
 the frame with the back in contact with the surface of 
 the blue print paper. The coating is a mixture of ferric 
 salt solution with a little gum arable. After printing, 
 it is developed in a mixture of ferrocyanide and ferri- 
 cyanide, in which both the reduced and the unreduced 
 iron salts form insoluble blue precipitates, but where 
 the light acted the gum washes off, carrying away the 
 blue pigment with it, so that the image consists of 
 a blue deposit upon a white ground. 
 
 In the blue print processes the photographic image 
 is formed directly from the iron salt itself. Ferrous 
 compounds, as we remember, are excellent reducing 
 agents, and upon this fact are based two printing 
 methods which are indirect. That is, the image is de- 
 posited, of another substance from that reduced by the 
 light, by a secondary reaction. In the first of these, 
 the platinotype process, the final photographic image 
 is formed in elementary platinum by the modification 
 known as "platinum black." This finely divided dense 
 black variety of the element is conveniently prepared 
 by the action of suitable reducing agents upon acid 
 solutions of platinum salts. When, as in the platinum 
 print, it is deposited upon the surface of a very pure 
 and durable paper, there may be produced not only an 
 effect of extreme beauty, but probably the most per- 
 manent form in which a photograph can be made. 
 Practically the only alteration which affects a properly 
 made, pure paper upon which a platinum image has 
 been superposed is a slight yellowing with age, due to 
 
PRINTING WITH IRON 135 
 
 iron salt which was imperfectly removed in the fixing 
 process. The other constituent of the platinum print, 
 namely, the elementary platinum, is one of the most 
 resistant substances with which the chemist has to 
 deal. In any of its forms, the element is only slightly 
 affected by even high concentrations of the single acids, 
 requiring aqua regia, a mixture of hydrochloric and 
 nitric acids (generally in the proportions of three to 
 one) to dissolve it. It is fusible only by use of the elec- 
 tric current or by the oxyhydrogen blowpipe, melting 
 at over 1,700 degrees Centigrade, is at no temperature 
 attacked by either water or oxygen, and, except in 
 the presence of alkali, is not acted upon by sulphur. 
 Under the conditions in which photographs are usually 
 kept, platinum prints may therefore be considered to 
 be perfectly permanent. 
 
 Not including the preparation of the coated paper, 
 platinum printing consists of the same three distinct 
 operations which have been described for the making 
 of silver prints, viz., the exposure to light, development 
 of the image, and the fixing of this image by removal 
 of soluble materials. But, as has just been intimated, 
 platinotype is somewhat different from the processes 
 of the silver print, in that the platinum is not primarily 
 the image-forming material, but is, by a supplementary 
 reaction, substituted for the original image, which is, in 
 fact, formed in photo-chemically reduced iron salt. 
 When the thin film of mixed ferric and platinous salts 
 spread upon paper is subjected to the action of light, 
 the reduction to ferrous salt produces a very pale and 
 unpromising appearance, the picture showing in a faint 
 brown upon a yellow ground. Reaction : 
 
 Ferric oxalate -|- Light = Ferrous oxalate -f- Carbon dioxide 
 
136 CHEMISTRY FOR PHOTOGRAPHERS 
 
 The platinum salt, during and after the exposure, 
 itself practically unaffected, is in the presence of a fer- 
 rous compound, but the two are inactive towards each 
 other in the dry state. Thus at this stage no elementary 
 platinum is in the image, unless by the access of moist- 
 ure from the air the substances come into chemical con- 
 tact by commencing to dissolve. In order that the re- 
 ducing power of ferrous iron may have full play upon the 
 platinum salt, it is necessary to bring the iron into solu- 
 tion together with the platinum. This, with the water- 
 insoluble ferrous oxalate, which is the form of ferrous 
 salt used, can be done by treating it with a solution, 
 which is used warm (not below 15.5 degrees Centigrade 
 or 60 degrees Fahrenheit), of potassium oxalate, 26 
 grams of the salt in each 100 cubic centimeters of 
 solution. By this means the insoluble ferrous oxalate 
 image is altered to the soluble double ferrous potassium 
 oxalate. This at once reacts with the dissolved potas- 
 sium chloroplatinite, and platinum black is deposited 
 by reduction in place of the iron salt. As the quantity 
 of platinum reduced is equivalent to the amount of 
 ferrous salt, and this last is proportional to the exposure, 
 the varying densities of the silver deposit of the negative 
 are inversely repeated in varying densities of platinum 
 black upon the paper. Reactions : 
 
 1 . Ferrous oxalate + Potassium oxalate 
 
 = Ferrous-potassium oxalate 
 z. Ferrous-potassium oxalate 
 
 + Potassium chloroplatinite 
 = Platinum + Potassium chloride 
 -f- Ferric-potassium oxalate 
 
 In this condition the print is saturated with a solution 
 containing whatever unreduced potassium chloroplat- 
 inite is left, ferric-potassium oxalate, and potassium 
 
PRINTING WITH IRON 137 
 
 chloride. All of these, but the iron salt especially, must 
 be washed out of the paper in order that no changes 
 may later take place in the picture. If the print were 
 simply washed with plain water, as the concentration 
 of the iron compound diminished the tendency of 
 ferric salts to hydrolyze in dilute solutions would be- 
 come apparent in the formation of an insoluble basic 
 (hydroxy-) ferric compound. Reaction : 
 
 Ferric oxalate -|- Water 
 
 = Ferric hydroxy-oxalate + Oxalic acid 
 
 No amount of water would serve to effect its complete 
 elimination from the paper. For these reasons, the 
 first wash waters into which the developed platinum 
 prints are put must contain small quantities of hydro- 
 chloric acid, about lo cubic centimeters of the con- 
 centrated acid to each 600 cubic centimeters of water. 
 Since the basic ferric compounds are soluble in hydro- 
 chloric acid, presence of the acid prevents their forma- 
 tion. Reaction : 
 
 Ferric hydroxy-oxalate -|- Hydrochloric acid 
 
 = Ferric chloride H- Ferric oxalate 
 
 Several of these acid iron-clearing baths must be used 
 and the paper passed along from one to the next, 
 draining each print as thoroughly as possible from one 
 solution before putting it into the succeeding bath. 
 At least four or five of these clearing-solutions should 
 be used, prints being allowed to stay in the first one for 
 ten minutes and in each of the others for five. The 
 last one, while in use, must show no trace of yellow 
 color, indicating that there is in it only an inappreciable 
 amount of iron salt. After this, a fifteen minutes' wash- 
 ing in water will be ample, or half an hour if the paper 
 is very thick. The amount of ferric salt left in the 
 
138 CHEMISTRY FOR PHOTOGRAPHERS 
 
 print as it comes out of the last acid clearing-bath may 
 be indicated by catching the drainings from the paper in 
 a test tube and adding a drop of ferrocyanide solution. 
 
 Most of the soluble platinum salt will be retained 
 in the first washing tray, and, if much work in platinum 
 is done, it will be worth while to save not only this first 
 clearing-bath, but also all old developing solutions and 
 scraps of paper. The paper is burned and the ashes 
 saved. Solutions are best treated by pouring them all 
 together in a jar, adding a little hydrochloric acid, if 
 the mixture does not already contain an excess, and 
 suspending in it, a little above the bottom, some 
 metallic zinc. The zinc goes into solution, while the 
 platinum separates out. Reactions, using platinic 
 chloride as a type : 
 
 1 . Platinic chloride -f Zinc 
 
 = Platinum + Zinc chloride 
 
 2. Zinc + Hydrochloric acid 
 
 = Hydrogen + Zinc chloride 
 
 3. Platinic chloride + Hydrogen 
 
 = Platinum + Hydrochloric acid 
 
 After sufificient standing, the solution will have become 
 decolorized, owing to the deposition of all the platinum 
 and the reduction to the ferrous condition of all the iron 
 salt. When this is the case, most of the liquid can be 
 carefully decanted, or siphoned, from the precipitate 
 which has settled to the bottom, after which more waste 
 liquids may be introduced into the jar and the recovery 
 process continued. When enough solid material has 
 accumulated, it is separated on a filter, washed with 
 hot water, and, combined with all the solid residues on 
 hand containing platinum, boiled in a porcelain dish 
 with aqua regia until complete solution of the platinum 
 is effected. Evaporate the liquid down on a water- 
 
PRINTING WITH IRON 139 
 
 bath adding a little hydrochloric acid before it comes to 
 dryness, and evaporate again, to boil out the nitric 
 acid. Repeat the evaporation to the crystallizing point 
 with hydrochloric acid to make sure that all nitric acid 
 is expelled; dilute up with enough water and filter the 
 solution. Add an excess of sodium carbonate and a 
 little alcohol and boil the mixture, which will precipitate 
 the platinum as a black powder. Wash it with hot 
 water and afterwards with hot hydrochloric acid and 
 dry it thoroughly and weigh it. Dissolve in aqua 
 regia, evaporate down with hydrochloric acid several 
 times as before to free it from nitric acid, add one gram 
 of potassium chloride for each gram of platinum present, 
 and dilute with just enough distilled water to get the 
 potassium chloroplatinate all in solution. Each gram 
 of platinum will make about 2.5 grams of potassium 
 chloroplatinate. The solution is next to be so ad- 
 justed that for each two grams of the chloroplatinate 
 there will be 25 cubic centimeters of water (distilled) 
 and one gram each of acid potassium sulphite and 
 potassium chloride. This mixture is boiled for twenty- 
 five minutes, counting the time from the actual com- 
 mencement of ebullition, is then cooled, put in a shal- 
 low dish, such as a thoroughly cleaned tray, and left 
 to crystallize. Red crystals of potassium chloroplat- 
 inite will separate out; they should be recrystallized 
 several times to purify them, and when carefully dried 
 the salt is ready for use in making up homemade 
 platinotype papers. 
 
 The following formula, suitable for the so-called "cold 
 bath development," is recommended by Abney, not 
 only on account of the keeping qualities of the paper 
 prepared with it, but also because, by variations in the 
 
140 CHEMISTRY FOR PHOTOGRAPHERS 
 
 developer, the prints are susceptible of considerable 
 modification : 
 
 NO. I 
 
 Ferric ammonium oxalate 50 grams 
 
 Water, distilled 50 cubic centimeters 
 
 Oxalic acid, 10 per cent, solution 15 cubic centimeters 
 NO. 2 
 
 Potassium chloroplatinite i part 
 
 Water, distilled 5 parts 
 
 NO. 3 
 
 Ammonium dichromate 4 grams 
 
 Water, distilled, to 100 cubic centimeters 
 
 For each sheet of paper 20 inches by 26 inches, take 8 
 
 cubic centimeters of No. 2, mix with 4 cubic centimeters 
 
 of No. I , and 8 cubic centimeters of No. 3 . Unsized 
 
 papers must be given a good sizing with gelatine (or 
 
 arrowroot may be used) : 
 
 Gelatine i gram 
 
 Water 100 cubic centimeters 
 
 Potassium alum 0.3 gram 
 
 Methyl alcohol 25 cubic centimeters 
 
 Swell the gelatine thoroughly in the water and after- 
 wards dissolve it by heating, then add the powdered 
 alum and methyl alcohol. Filter while hot through mus- 
 lin, taking care to avoid bubbles. Paper may be treated 
 with the sizing by immersion or by coating with a brush, 
 in the manner previously described in the present 
 chapter. In applying the sensitive coating to the paper, 
 it must be remembered that we are dealing with a very 
 unstable mixture. For this reason an ordinary bristle 
 brush will not serve, as it will soon become saturated 
 with the decomposition products of the solution, and 
 these will be transferred to other sheets, with very 
 disappointing results when made into prints. A form 
 of brush, figured and described by Abney, is made out 
 
PRINTING PROCESSES 141 
 
 oi a ■}}4 inch by 4^ inch piece of photographic film, 
 (i.e. an old negative)^ strips of cotton flannel to fit it, 
 and a film clip such as is used for tray development of 
 roll films. A strip of the flannel is laid, fleece out, upon 
 the celluloid, and the ends of both are then doubled 
 back, without creasing the film, with the cloth on the 
 outside, until they can be firmly held together with the 
 clip. The clip becomes a convenient handle, and the 
 resiliency of the celluloid gives plenty of stiffness without 
 the rigidity which would abrade the paper. For each 
 large sheet a fresh cloth covering must be used, but 
 when smaller sheets of paper are coated, several may be 
 done before renewing the flannel. In any event it is not 
 advisable to use the same piece longer than 10 to 12 
 minutes. The sheet of paper to be coated may be 
 pinned, the sized side up, at one end upon a board and 
 grasped at the other by the fingers of the left hand. 
 This will allow for the expansion which results from 
 wetting it with the solution. A measured amount of 
 the sensitizing liquid, proportioned to the dimensions 
 of the sheet as given above, is poured upon the middle of 
 the paper and as rapidly as possible brushed with the 
 spreader in circular sweeps out into the corners and 
 over the entire surface. After this the strokes are criss- 
 crossed and all uneven places blended out as much as 
 may be. The next part of the operation consists in the 
 surface-drying of the coated sheet, which is allowed to 
 occur spontaneously, but should not be achieved in 
 under five nor over ten minutes. If dried too quickly 
 the sensitizer will be liable to come off in the developer, 
 but if drying is too slow it may soak into the body of 
 the paper, which produces flat and foggy pictures. 
 When surface-dry, the paper is completely dessicated 
 
142 CHEMISTRY FOR PHOTOGRAPHERS 
 
 over a stove at a temperature not too great for the hand 
 to bear (not to exceed 40 degrees Centigrade), this 
 change being indicated by a deepening of the color. 
 The paper, when dry, must be kept dry, which means 
 very dry indeed, since if any moisture is absorbed 
 traces of the iron and platinum salts immediately go 
 into solution. In this condition not only is the ferric 
 oxalate more unstable and therefore liable to spon- 
 taneous decomposition, but also as soon as the sheet is 
 exposed to light and the ferric salt begins to be reduced, 
 the solubility of the resulting ferrous salt, although 
 very small, enables the reaction between it and the 
 chloroplatinite to proceed at once and to a more or less 
 indefinite extent, depending upon the actual quantity of 
 water present. For these reasons the prints made 
 from improperly preserved papers are liable to be 
 granular and flat. Platinum papers are therefore 
 commonly stored in tin cylinders with caps which can 
 be made air-tight by wrapping adhesive tape or a wide 
 rubber band around the joint. In the cylinder there 
 is also put a small package of anhydrous calcium 
 chloride, so packed that none of the lime can sift out 
 upon the paper in the cylinder. Calcium chloride, as it 
 crystallizes from water solution, contains six molecules 
 of "water of crystallization." When the crystallized 
 salt is heated, at 200 degrees Centigrade it loses two- 
 thirds of this water of crystallization, and upon further 
 heating, until the dehydrated salt is melted, the re- 
 maining third is driven off, and the product is "an- 
 hydrous" calcium chloride. In this condition it has a 
 very great affinity for water, so great that, when a 
 little of it is exposed upon a saucer to the air, in a very 
 short time by the absorption of moisture the salt be- 
 
PRINTING WITH IRON 143 
 
 comes sticky and soon dissolves in the accumulated 
 water. These reactions may be written: 
 
 I . Calcium chloride crystals 
 
 = Calcium chloride (anhydrous) + Water 
 1. Calcium chloride (anhydrous) + Water 
 
 = Calcium chloride crystals 
 
 It is by the application of the second reaction that 
 platinum papers are preserved, since the lime salt is so 
 much more hygroscopic than the iron and platinum 
 salts of the paper that all moisture is seized and held 
 by the calcium chloride. But there is naturally a limit 
 to the quantity of water which can be absorbed by a 
 given amount of calcium chloride. When this limit 
 is approached, its efficiency as a dehydrating agent is 
 much lessened, and of course when the limit is reached 
 the effectiveness is gone. Therefore in storing platino- 
 type paper it is necessary to consider the condition of 
 the calcium chloride. If it has already absorbed much 
 water, it can quickly be dehydrated by heating in an 
 iron dish. Otherwise fresh anhydrous salt must be 
 taken. 
 
 For producing sepia prints in platinotype, mercuric 
 chloride is included in the sensitizing solution. The 
 brown color of the image is due in some way to the 
 mixture of platinum and mercury reduced by ferrous 
 salt. There are both "cold bath" and "hot bath" 
 sepia papers, the latter being developed in potassium 
 oxalate solution at a temperature between 70 degrees 
 Centigrade and 80 degrees Centigrade (160 degrees 
 Fahrenheit to 180 degrees Fahrenheit). "Hot bath" 
 sepias appear to be rather more permanent than the 
 others, but both are naturally less so than the black 
 papers which contain nothing else than pure platinum. 
 
144 CHEMISTRY FOR PHOTOGRAPHERS 
 
 A sensitizer for "hot bath" sepia paper is prepared 
 thus: 
 
 NO. I 
 
 Ferric oxalate 40 grams 
 
 Oxalic acid 24 grams 
 
 Water, distilled, to 200 cubic centimeters 
 
 NO. 2 
 
 Potassium chlorate 0.4 gram 
 
 No. I solution, to 100 cubic centimeters 
 
 NO. J 
 
 Potassium chloroplatinite 10 grams 
 
 Water, distilled, to 60 cubic centimeters 
 
 NO. 4 
 
 Mercuric chloride, saturated solu- 
 tion in distilled water $0 cubic centimeters 
 
 To coat a sheet of 20 inch by 26 inch paper, take 5 
 cubic centimeters of No. i , 4.7 cubic centimeters of No. 
 2, q.y of No. 3, and i .8 of No. 4. The paper, after print- 
 ing, is developed in potassium oxalate solution of the 
 same concentration as that used for the ordinary black 
 process, 26 grams per 100 cubic centimeters, the temper- 
 ature being maintained at 70 degrees Centigrade to 80 
 degrees Centigrade, as above noted, over a gas stove, 
 or better on an electric hot-plate. The acid clearing- 
 baths are used at half the concentration given for black 
 prints, that is, j cubic centimeters of concentrated 
 hydrochloric acid in each 600 cubic centimeters of the 
 solution. 
 
 The many modifications to which platinotype is 
 susceptible not only render the process one of much 
 flexibility when used as a straight photographic method, 
 but also adapt it especially to the use of the artist 
 photographer. Besides the control which is given by 
 variations in the constituents of the sensitizing solution, 
 there are at least three forms of control which can be 
 exercised over the print. In ordinary development the 
 
PRINTING WITH IRON 145 
 
 image appears instantly upon application of the oxalate 
 solution. If the surface of the exposed sheet is first 
 smeared all over with glycerine and the print is then 
 brush-developed, as the worker may choose, with a 
 mixture of equal parts of glycerine and developer or 
 with the full-strength developing solution, the normal 
 gradation of light and shade may be altered to any 
 extent desired. The effect of the glycerine is a purely 
 physical one, dependent upon the property of viscosity, 
 in retarding the rate of development. Another me- 
 chanical process, for treating the completed print if it 
 has been over-developed, consists in abrading the 
 image, or such portions of the image as may require 
 local reduction, with sawdust and water. A non- 
 resinous sawdust is needed, and that from box-wood is 
 the best. It is shaken up in water and the mixture is 
 poured in a thin stream upon the gently inclined print, 
 the latter being supported by pinning to a board. In 
 this way the shadows may be lightened up and the 
 darkened highlights cleared. The resultant effect has 
 the quality of an engraving. The third method of 
 control is chemical and is the same in principle as the 
 toning processes which were discussed under silver 
 printing. Although, as we have said, the platinum 
 image is essentially a very permanent thing owing to 
 the refractoriness of the element, yet it can be affected 
 chemically by suitable reagents, and for the purpose 
 in hand the same oxidizing agent can be used as is 
 effective in toning the silver image, viz., potassium 
 ferricyanide. Just as in the case of silver, the platinum 
 is oxidized and the ferricyanide is reduced to ferro- 
 cyanide. The toning solution contains also uranium 
 nitrate and consequently insoluble red uranium ferro- 
 
146 CHEMISTRY FOR PHOTOGRAPHERS 
 
 cyanide is precipitated upon the image. Since this is an 
 intensifying operation, the print to be toned should in 
 the first instance be printed light, and over-development 
 is to be avoided. Brilliant red tones are secured by tak- 
 ing special care to free the paper from iron salts and to 
 this end an extra clearing-bath or two is advisable : 
 
 NO. I 
 
 Uranium nitrate 0.5 gram 
 
 Acetic acid (glacial) 3.0 cubic centimeters 
 
 Water, to 1 25 cubic centimeters 
 
 NO. 2 
 
 Potassium ferricyanide 0.5 gram 
 
 Acetic acid (glacial) 3.0 cubic centimeters 
 
 Water, to 125 cubic centimeters 
 
 Solution No. i may be made up in quantity as it keeps 
 well, but No. 2 is unstable and must be prepared just 
 before using. Mix the quantity of No. 2 given with an 
 equal volume of No. i and dissolve in the mixture o. i 
 gram of sodium sulphite. Leave the print in this 
 solution until the tone is satisfactory, take it out 
 quickly and sponge it, both front and back, with 
 moistened absorbent cotton, and transfer it to an acid 
 clearing-bath containing 5 cubic centimeters of con- 
 centrated hydrochloric acid in 600 of water. Pass the 
 print on to a second acid bath, and wash for ten minutes, 
 and, lastly, lay it upon clean muslin to dry. Blue tones 
 may be had, from the same toning bath, merely by 
 purposely failing to clear all the iron salt from the 
 print immediately after development. Printing should 
 in the first place be carried a little further than would 
 give a normal black and white print. After develop- 
 ment, the print is put for but ten seconds into the 
 normal clearing solution and is then placed immediately 
 in the toning bath until colored as desired. Clearing 
 
PRINTING WITH IRON 147 
 
 takes place in a solution of half the concentration used 
 for red tones, i.e., 2.5 cubic centimeters of hydro- 
 chloric acid in 600 of water. After clearing and washing 
 the blue print is dried upon muslin, but on no account 
 should the same pieces of cloth be used for both red 
 and blue prints, as the quality of the red will be de- 
 graded. It is plain that the blue color is due to ferric 
 ferrocyanide produced by metathesis between the ferric 
 salt which is on the paper and the potassium ferro- 
 cyanide reduced from ferricyanide by the platinum. 
 Success in the use of these toning methods depends 
 wholly upon scrupulous cleanliness and the keeping of 
 blue-toned paper away from the red. 
 
 A process practically identical in principle with plati- 
 notype is known as kallitype. Instead of platinum, the 
 image is silver, produced by secondary reduction with 
 ferrous oxalate, the primary image being formed in the 
 iron salt, as is the case in platinum papers. It has 
 been much complained of by photographers and in the 
 photographic journals on account of its alleged lack 
 of permanency. There is no possibility of doubt but 
 that a silver print by the kallitype process may be 
 made equally permanent with silver prints by any other 
 method. When properly worked the process leaves an 
 image in pure silver upon a substratum of pure paper, 
 which is the condition essential to durability. Further- 
 more, the testimony of time may be advanced, for a 
 certain kallitype print has hung upon the wall of an 
 ordinarily illuminated room for upwards of seven years 
 without appreciable alteration. Any worker who does 
 not get both permanent- results and exceptionally 
 pleasing pictures with kallitype ought to test his 
 chemicals thoroughly and, especially, examine carefully 
 
148 CHEMISTRY FOR PHOTOGRAPHERS 
 
 his methods of working, for somewhere in these he is 
 sure to discover the cause. There is one characteristic 
 of this printing medium which ought to make it interest- 
 ing to a great many amateurs, its exceeding inexpensive- 
 ness. It is a fact that, including the cost of the chem- 
 icals, paper, developer and fixing salt, the completed 
 prints need not cost more than ten cents per dozen for 
 the 5 inch by 7 inch size. But the quality which it is 
 possible to obtain in kallitype is not at all measured by 
 this cheapness, for successful pictures can scarcely be 
 told from platinum prints. Another advantage lies in the 
 fact that negatives which no othersilver process will pririt 
 satisfactorily will yield excellent pictures in kallitype. 
 
 Any kind of paper may be used, with the same 
 proviso as in platinotype, that the unsized varieties 
 must be given a coat of sizing beforehand. The high 
 grade drawing papers are particularly desirable; but 
 for the beginner there is nothing more satisfactory than 
 a good, heavy ledger paper, such as Weston's ledger, 
 since it is already well sized, is very durable, and has 
 surface which gives good quality in prints. As the 
 operations of sizing the paper and coating it with the 
 sensitizing solution are the same as in platinotype, and 
 have just been described, we may pass at once to the 
 preparation of the necessary solutions: 
 
 NO. I 
 
 Ferric oxalate 30 grams 
 
 Water, distilled, to 1 50 cubic centimeters 
 
 Gum arabic, selected 3 grams 
 
 NO. 2 
 
 Ferric potassium oxalate 15 grams 
 
 Water, distilled, to 240 cubic centimeters 
 
 NO. 3 
 
 Oxalic acid i y grams 
 
 Water, distilled, to 120 cubic centimeters 
 
 Ammonium hydroxide, concentrated 6 cubic centimeters 
 
PRINTING WITH IRON uq 
 
 NO. 4 
 
 Potassium dichromate 7.5 grams 
 
 Water, distilled, to lao cubic centimeters 
 
 All these solutions are to be made up and kept in 
 
 chemically clean, dark brown bottles with tight-fitting 
 
 glass stoppers. Besides this precaution, they should 
 
 be put away in the dark. Number one is prepared by 
 
 putting the measured quantity of ferric oxalate in the 
 
 bottle, adding enough water, and shaking from time 
 
 to time for half an hour. After it has stood in the dark 
 
 for a day it will generally be found completely dissolved, 
 
 and when this is the case the gum arable is added and 
 
 left to dissolve. The solution is then diluted up to the 
 
 specified volume with water. It will keep for several 
 
 months. The other three solutions need no comments. 
 
 The sensitizing mixture for normal negatives consists of: 
 
 No. I solution 30 cubic centimeters 
 
 No. 2 solution 15 cubic centimeters 
 
 No. 3 solution 1.8 cubic centimeters 
 
 No. 4 solution 4 drops 
 
 Silver nitrate, pure 2.2 grams 
 
 About 2 cubic centimeters of the mixture are required 
 to coat a sheet 1 1 }4 inches by 1 8 inches. Free nitric 
 acid in the silver nitrate is detrimental and should be 
 tested for with litmus paper beforehand. The salt may 
 be purified by recrystallization, if necessary. Special 
 sensitizers can be prepared for different kinds of nega- 
 tives. For example, for very contrasty negatives. No. 4 
 may be omitted and No. 2 and No. 3 increased, and 
 more silver nitrate added. Too great increase in the 
 proportion of No. 3 induces fog. For negatives that 
 are very soft and thin No. 4 may be increased, diminish- 
 ing the quantity of No. 3, However, it is generally 
 better to use a uniform sensitizer and exercise the 
 
I50 CHEMISTRY FOR PHOTOGRAPHERS 
 
 necessary control by modifications in printing and 
 development. The coated paper will keep in good 
 condition for a month without any special precautions 
 except against exposure to light. If it is desired to 
 preserve it longer, it may be stored in the same way 
 as platinum paper with anhydrous calcium chloride as 
 a dehydrating agent. 
 
 After printing, which is similar to platinum, develop- 
 ment is brought about in a solution of sodium acetate, 
 variously modified according to the tone which is 
 wanted. The acetate solution, 125 grams of the salt 
 dissolved and diluted to one liter, can be made up in 
 bulk, as it works best when a day or two old. For 
 warm blacks, take: 
 
 Acetate solution 140 cubic centimeters 
 
 Tartaric acid 0.75 gram 
 
 No. 4 solution 0.6 to 6.0 cubic centimeters 
 
 The longer the exposure or the flatter the negative, the 
 greater should be the proportion of No. 4 solution used, 
 as potassium dichromate appears to act somewhat as a 
 restrainer. For platinum black tones take: 
 
 Acetate solution 240 cubic centimeters 
 
 Tartaric acid 1.5 gram 
 
 Phosphoric acid (jopercent.) i .a cubic centimeters 
 
 No. 4 solution 0.6 to 3 .0 cubic centimeters 
 
 To make sepia prints with kallitype, the developer 
 contains borax and Rochelle salts : 
 
 NO. 5 
 
 Water, distilled, hot, to 1000 cubic centimeters 
 
 Borax, powdered 37.5 grams 
 
 Sodium-potassium tartrate . . . 75 grams 
 
 NO. 6 
 
 Sodium-potassium tartrate. . . 75 grams 
 
 Water, distilled, to 1000 cubic centimeters 
 
PRINTING WITH IRON iji 
 
 In making up No. y, dissolve the borax in the hot 
 water, let it cool, and finally add the tartrate, and dilute 
 to one liter. Sepia tones are obtained by mixing these so- 
 lutions in the proportion of one part of No. 5 to three 
 parts of No. 6, and adding No. 4 solution as required. 
 
 Kallitype prints must be both cleared of iron salts 
 and fixed in thiosulphate solution, and it is in the last 
 operation that the process differs from platinotype. 
 The kind of clearing-bath to use depends upon the 
 developer. Following immersion in sodium acetate 
 solution, prints are cleared in a bath, made fresh each 
 
 time: 
 
 Sodium citrate 7.$ grams 
 
 Citric acid 1.5 gram 
 
 Water, to 250 cubic centimeters 
 
 After the borax and Rochelle salts developer, use a 
 solution of Rochelle salts, 7$ grams to the liter of water, 
 to which are added 7.^ cubic centimeters of No. 4 solu- 
 tion. Leave the prints in this bath for ten minutes. 
 After coming out of the clearing-bath, they must be 
 thoroughly washed to free them from all traces of acid, 
 since this would cause decomposition of the thiosul- 
 phate fixing bath which follows. They are then fixed for 
 ten minutes in : 
 
 Sodium thiosulphate 25 grams 
 
 Water jco cubic centimeters 
 
 Concentrated ammonium hydroxide 5 cubic centimeters 
 
 and given a final thorough washing until all thiosulphate 
 has been removed. After this the prints are dried, 
 best between lintless photographic blotters, to keep 
 them free from dust. 
 
 In this chapter the attempt has been made principally 
 to give an outline of the processes with ferric salts, 
 showing as simply as possible the rationale of the 
 
152 CHEMISTRY FOR PHOTOGRAPHERS 
 
 methods and describing in an elementary way the 
 chemical reactions upon which they depend for making 
 technically successful prints. It is possible that some 
 readers, at least, may become sufificiently interested 
 to wish to pursue the subject further. Such may find 
 in the accompanying bibliography full and practical 
 accounts for handling these fascinating and beautiful 
 photographic media. 
 
CHAPTER X 
 Printing Processes with Chromium Salts 
 
 ORGANIC colloids, such as gelatine, gum arable, 
 etc., have the property of combining with chro- 
 mium salts (in which the element is positive, or basic), 
 with mercurous chloride, tannic acid, and various 
 resins. The reaction is apparently not a photo- 
 chemical one, and its product is the colloid so altered 
 by the chemical admixture of chromium salt, to take 
 an example, as no longer to be soluble in water. That 
 this insoluble compound has photo-chemical properties 
 is shown by the fact that, if it is not exposed to light, 
 it is able to combine with water, becoming hydrated and 
 swelling, as the water is absorbed, in a similar manner 
 to the original, uncombined colloid; whereas after it 
 has been exposed, not only is it still insoluble, but it 
 refuses to absorb any water, that is, does not become 
 hydrated. When the colloid, however, is mixed with a 
 chromate (usually a dichromate)and kept in the dark, 
 no insolubilizing action takes place. If the solution is 
 coated upon paper, and not exposed to light, it will 
 quickly be dissolved and washed off by warm water. 
 But by the action of light upon the mixture, reduction 
 of the chromium, on the one hand, and partial oxidation 
 of the colloid, on the other, occur; and these reaction 
 products are chemically combined, forming the in- 
 soluble, non-absorbing end-product of colloid-chromium 
 salt. The oxidizing action of chromates upon organic 
 compounds is well illustrated by the accompanying 
 experiment. 
 
154 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Experiment 34. Acidify with hydrochloric acid 5 or 
 10 cubic centimeters of dilute potassium dichromate 
 solution, heat the liquid to boiling, add a little ethyl 
 alcohol, and continue the boiling. The gradual change 
 in the color of the solution from orange to green in- 
 dicates the reduction of the chromate to chromic 
 chloride; and at the same time the pungent odor of 
 aldehyde shows the oxidation of alcohol. Reaction: 
 
 Potassium dichromate + Ethyl alcohol 
 
 + Hydrochloric acid 
 = Chromic chloride + Potassium chloride + Ethyl 
 
 aldehyde + Water 
 
 In chromic acid and its salts, such as the dichromate 
 for example, the chromium atom forms part of the 
 acidic or negative portion of the molecule; but in such 
 compounds as chromic chloride, chrome alum, etc., 
 chromium is in the basic or positive part. Reduction of 
 a chromate, therefore, as illustrated above, consists in 
 changing the chromium atom from the negative part 
 of the compound to the positive part, as well as in 
 removing oxygen from it. We are now ready to ex- 
 periment with the action of light-reduced chromium 
 salt upon gelatine. 
 
 Experiment 35. Soak up in enough cold water 20 
 grams of gelatine and get it into solution by warming. 
 In a weak light dissolve in this 6 grams of potassium 
 dichromate and dilute up to 100 cubic centimeters. 
 Coat some of this sensitive mixture upon several small 
 pieces of paper and dry them. Expose three of these 
 under a contrasty negative, giving full exposure, and 
 treat them severally as follows : 
 
 (i) Place one of the prints in hot water. The 
 gelatine dissolves off from the paper in proportion as it 
 
PRINTING WITH CHROMIUM ijj 
 
 has been protected from the light, but, wherever light 
 has acted, more or less gelatine has become insoluble 
 and non-absorbent, according to the amount of the 
 light-effect. 
 
 (2) Put a second print into cold water and observe 
 that the areas which were protected from light by the 
 dense parts of the plate swell, by the absorption of 
 water, the light-affected portions being unchanged. 
 
 (3) Spread upon the third exposed paper some thin 
 printers' ink, soak it for a time in cold water, and then 
 gently wipe the surface with a wet sponge or wet 
 absorbent cotton. Upon those areas which were af- 
 fected by the light, since they have been rendered non- 
 absorbing, the ink adheres. From the other parts it is 
 removed according to the amount of water absorption, 
 which is proportional to the photo-chemical effect. 
 
 In these simple experiments we have the basis of all 
 the processes which depend upon the dichromatizing of 
 colloids, and we must now try to make clear not only 
 what happens in the photo-chemical reaction, but ex- 
 actly what conditions prevail at the end of the reaction 
 in the thin layer of medium spread upon the surface 
 of the paper. When gelatine and a chromic compound 
 are mixed they combine, as we have previously noted, 
 and an insoluble product results which is water-ab- 
 sorbing, if imexposed, but non-absorbing after ex- 
 posure. This reaction may perhaps be expressed in 
 the following way, in the absence of light : 
 
 Gelatine 4- Potassium-chromic sulphate 
 
 = Gelatino-chromic sulphate -|- Potassium sulphate 
 
 A mixture of gelatine and a salt of chromic acid, al- 
 most insensitive in solution, or before the paper is dry, 
 when dehydrated appears to form a photo-sensitive 
 
156 CHEMISTRY FOR PHOTOGRAPHERS 
 
 compound. It is certain that the dried mixture, as we 
 learned in the experiment, is sensitive. The combina- 
 tion may perhaps be represented thus : 
 
 Gelatine + Potassium dichromate 
 
 = Gelatino-potassium dichromate 
 
 By light, this complex compound is so altered that 
 hydrogen is abstracted from the gelatine portion 
 (oxidation) and oxygen from the chromium part (re- 
 duction), and the final product is both insoluble and a 
 non-absorbent of water: 
 
 Gelatino-potassium dichromate + Light 
 
 = Oxidized-gelatino-chromic oxide + Potassium 
 
 hydroxide 
 
 At all events, it is practically certain that the par- 
 tially oxidized gelatine and the reduced chromium com- 
 pound are in chemical combination in the end. The con- 
 ditions prevailing in the sensitive gelatino-dichromate 
 film during the exposure can. now be considered. Let 
 us in imagination isolate one ray of the white light and 
 allow it to fall upon different portions of the negative 
 at will. When it is directed upon an area of clear 
 glass, the instant it reaches the gelatine surface, ab- 
 sorption of its energy commences and the photo- 
 chemical change is initiated. Consequently, as the 
 layer of gelatine immediately beneath the surface is 
 met, there is a diminished amount of light-energy 
 applied to it, and so on, through layer after layer, until 
 either the film is entirely passed through or the light- 
 energy is wholly absorbed. Of course, it must be under- 
 stood that it is the energy of the photo-chemical, or 
 actinic, wave-lengths of which we are speaking. In 
 the case of the yellow dichromated gelatine the energy 
 of the actinic wave-lengths is rapidly used up, that is, 
 
PRINTING WITH CHROMIUM 157 
 
 the light is able to penetrate but a few layers before 
 being completely absorbed. In areas partially shaded 
 by thin deposits of silver, much of the light has already 
 been absorbed by the negative before arriving at the 
 gelatine surface of the paper, therefore the depth to 
 which its influence extends in the film is less, in pro- 
 portion to the amount of absorption by the negative, 
 than in the unshaded parts. If in any regions the silver 
 deposit of the plate is sufficiently dense to absorb all 
 actinic wave-lengths, no effect upon the gelatine is 
 produced. Dichromated gelatine is sensitive to longer 
 wave-lengths than are the silver salts, and its maximum 
 occurs in the blue-green. Since most of the photo- 
 chemical action is caused by the blue rays, it will be 
 clear that only a very thin layer of film can be affected 
 under the best conditions of exposure and that but 
 little preliminary absorption by the silver image makes 
 marked differences in the depth to which the light- 
 effect penetrates. Usually the whole of the surface 
 layer of gelatine-coated paper is insolubilized and the 
 still soluble portions are consequently enclosed between 
 this layer and the paper support, where they are 
 protected from the water in which the prints are de- 
 veloped, as was remarked in Chapter III. 
 
 This brings us naturally to the subject of carbon 
 printing, for the situation just described is the cause of 
 the marked difference between the manipulation of 
 carbon and that of silver and platinum prints. The 
 principles which have been outlined in the preceding 
 chapters are applied, in the carbon process, as well as 
 in the other colloid-dichromate methods, by incorporat- 
 ing with the gelatine-dichromate mixture a permanent 
 pigment in suitable proportion. This coloring matter 
 
iy8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 may be the form of carbon known as "lamp-black," or 
 it may be almost any water-color pigment desired. 
 The photographic image is formed by the finely divided 
 particles of this pigment embedded in insolubilized 
 gelatine, since in all parts of the sensitive film where 
 the gelatine remains soluble, owing to its not having 
 been acted upon by light, the washing away of the 
 gelatine removes also the pigment, but wherever the 
 gelatine is insoluble it firmly retains the coloring 
 material. In working the carbon process it is scarcely 
 to be recommended that the amateur attempt the 
 preparation of "carbon tissue," as it is called. A much 
 better and more uniform, and therefore more satis- 
 factory, product can be purchased from the dealers. 
 These tissues come provided with the gelatine-pigment 
 film in a very considerable range of colors and either 
 cut to size or in rolls. Before printing they are sen- 
 sitized by immersion of the tissue in a solution of di- 
 chromate, and dried in the dark : 
 
 Potassium dichromate i j to 30 grams 
 
 Ammonium carbonate 1.5 grams 
 
 Water, to 1000 cubic centimeters 
 
 The proportion of dichromate is varied to suit climatic 
 conditions, less being used in summer, or in a warm 
 climate, than is needed in winter, or in a cold climate. 
 The function of ammonium carbonate is to aid in pre- 
 serving the gelatine from becoming spontaneously in- 
 soluble, and it is slightly variable in amount. Less 
 may be used in cool, dry weather, and more if it is 
 damp. Alcohol, or a mixture of alcohol and ether, is 
 sometimes recommended to be added to the sensitizing 
 solution to secure rapid drying in hot weather when the 
 humidity is high. Such a formula is the following: 
 
PRINTING WITH CHROMIUM ijq 
 
 Potassium dichromate. ... 30 grams 
 
 Ammonium hydroxide, con- 0.75 cubic centimeters 
 
 centrated (about ii drops) 
 
 Water 700 to 800 cubic centimeters 
 
 To this add a mixture of ethyl ether, 30 cubic centi- 
 meters, and ethyl alcohol, 15 cubic centimeters, and 
 make up the solution to 1,000 cubic centimeters with 
 water. Keep all sensitizing solutions away from the 
 light. 
 
 When sensitized, care has to be taken that no actinic 
 light reaches the tissue, as it has been found that once 
 the reaction is started by light it will proceed slowly 
 even in the dark; and for this reason when carbon 
 tissue is exposed it is at once developed. If it be 
 under-exposed, however, and put away in the dark for 
 a suitable length of time, on account of this continuing 
 action a normal print will be developed up. No visible 
 image is formed in printing, so that the exposure is 
 regulated by actinometer. In order to develop the 
 printed tissue by washing away the soluble portions 
 of pigmented gelatine, since these soluble layers lie 
 between the insoluble upper surface and the paper 
 backing, it is necessary to remove the backing. This 
 operation is performed by putting into cold water the 
 exposed tissue and with it either a sheet of "single 
 transfer" paper or waxed "temporary support" accord- 
 ing as the picture is to come out finally reversed as to 
 right and left, or unreversed. The transfer paper has 
 a gelatine coating upon one side and this is placed face 
 to face with the tissue. Both are drawn together from 
 the water and are firmly squeegeed into contact, all air 
 and water being expelled from between them. The 
 united sheets are next put between blotting papers 
 under pressure, and left from a quarter of an hour to over 
 
i6o CHEMISTRY FOR PHOTOGRAPHERS 
 
 an hour, depending upon the pressure used and the 
 character of the paper's surface, in order that the tissue 
 and transfer paper may adhere together uniformly. 
 As soon as the dichromate in the tissue penetrates 
 to the back of the transfer paper, staining it yellow, 
 development may be undertaken. This part of the 
 process consists merely in a preliminary soaking of the 
 combined papers in water at a temperature between 
 32 degrees Centigrade and 38 degrees Centigrade. The 
 gelatine around the edges presently begins to dissolve. 
 After a few minutes longer, put the papers into a second 
 tray of water, arranged to be kept at a temperature 
 from 38 degrees Centigrade to 48 degrees Centigrade, 
 and in a minute or two, beginning at one corner, strip 
 off the tissue. The print, which is now to be left upon 
 the transfer paper, must be held under the surface of 
 the water during this process of stripping. By rocking 
 the tray, the solvent action of the warm water is caused 
 to begin removing the soluble gelatine, and details 
 commence to appear in the picture. Development is 
 completed from this point by laying the print face up on 
 a plate of glass inclined over the tray and gently pouring 
 the warm water over it with a cup. The print dries 
 slightly darker than it looks when wet, so it is developed 
 to a little lighter tint than is desired in the final 
 picture, and is placed in cold water as a stop-bath. 
 After this it is allowed to soak in a five percent, solu- 
 tion of potassium aluminum sulphate ("alum") for ten 
 minutes up to an hour, according to the thinness or 
 thickness of the paper, and is lastly washed in running 
 water for half an hour. If right-and-left reversal is 
 not objectionable, as in pictorial work and some 
 portraiture, and the "single transfer" paper has been 
 
PRINTING WITH CHROMIUM i6i 
 
 used, the picture is finished by drying. Otherwise the 
 "double transfer" method will have been employed, 
 and the thoroughly washed print has finally to be 
 transferred again, from the "temporary support," to 
 the final support on "double transfer" paper. This 
 paper, also gelatine-coated, is given a preliminary soak- 
 ing in water at 32 degrees Centigrade to 38 degrees 
 Centigrade to soften up the gelatine, and is afterwards 
 put, together with the print, into cold water. Face 
 together, the two are withdrawn from the tray, avoiding 
 air-bubbles, then they are squeegeed into contact and 
 hung up to dry. When thoroughly dried, in six hours 
 or so, the two papers are separated at one corner and 
 stripped apart. The gelatine image leaves the waxed 
 surface of the "temporary support" and sticks perma- 
 nently to the final support, upon which it is dried. In 
 this way a non-reversed picture is obtained. Waxing of 
 the "temporary support" is accomplished by spreading 
 with a tuft of cotton upon the gelatine surface a solution 
 of 20 grams of yellow resin and 7 grams of beeswax 
 in 400 cubic centimeters of turpentine. Afterwards it 
 has to be polished, with a larger wad of cotton, and 
 when all the turpentine has evaporated the paper is 
 ready for use. If carbon prints are made from enlarged 
 negatives, the process can be simplified by making the 
 enlargement a reversed negative, when the "single 
 transfer" method will give a non-reversed picture. 
 The published accounts and descriptions of carbon 
 printing all sound, in the reading, much more com- 
 plicated than the process actually is. This fact, 
 together, perhaps, with the seemingly unorthodox 
 bandying about of the photographic image, turning it 
 upside down for development and putting it back 
 
i62 CHEMISTRY FOR PHOTOGRAPHERS 
 
 right-side up again, has probably deterred a great 
 many amateurs from experimenting with it. This is 
 unfortunate, for, even if we admit the charge of greater 
 complication, one really well-executed print in carbon, 
 or in gum, is worth a whole handful of the usual de- 
 veloping-out prints which so many amateurs seem to 
 be fond of turning out wholesale. The point to be 
 established here is not that beautiful pictures are 
 impossible in silver, whether on printing-out or on 
 developing-out papers, for they are not impossible, but 
 that by reason of the widely advertised simplification 
 of photography for the amateur, the art has become 
 too automatic and stereotyped. All his thinking and 
 study have been performed for him by the manu- 
 facturers. The real photographic education of the 
 amateur generally begins only when he takes up one or 
 other of the individualistic printing methods, platinum, 
 carbon, gum, etc., and endeavors to express something 
 in his pictures. 
 
 Printing in gum, or gum-bichromate as it has been 
 called, is founded on the same principles as carbon, 
 since it is, in fact, a variation of the carbon process. 
 A mixture of gum arable, potassium dichromate, and 
 pigment is coated upon paper, dried, exposed to light, 
 and washed in water and dried. The result, if success- 
 ful, has a distinctiveness which has commended the 
 method to the best pictorialists the world over. In 
 working gum, the photographer theoretically can ac- 
 quire absolute command over the process, since the 
 choice of paper and pigment is entirely under his 
 control, the proportions in which they may be put 
 together with the colloid can be varied to suit his 
 requirements, and the development process may be 
 
PRINTING WITH CHROMIUM 163 
 
 such as to allow unlimited scope to his artistic abilities. 
 It is not a complicated process, at least in single print- 
 ing; requires no transfer of the colloid-pigment image; 
 is capable of giving as fine prints as any other method 
 (some think finer) ; and withal is so exceedingly inex- 
 pensive that the amateur can experiment with it with- 
 out stint. If he is so unfortunate as to fail to get satis- 
 factory pictures in the end, he can console himself with 
 the thought that he is out little besides the time spent. 
 But there is no need for anyone to fail, for various 
 methods have been published for the gum print, and it 
 is necessary only to study out carefully the reasons for 
 the different manipulations and then apply patience and 
 pains in performing them for oneself in order to make 
 prints that at the very least will bear inspection. 
 
 The essential materials for working in gum are four 
 in number, paper, which may be of different weights, 
 surfaces, and textures, but must be sufficiently tough 
 to withstand soaking in water ; gum arable, suspended 
 in a muslin bag in cold water in the proportion of 40 
 grams of the gum to 60 cubic centimeters of water, 
 and to which is added formaldehyde 10 drops for each 
 100 cubic centimeters, or 0.02 gram of mercuric chloride; 
 a saturated solution (at ordinary temperatures) of 
 potassium dichromate kept in a brown bottle; and, 
 for the beginner, two or three pigments such as lamp- 
 black, burnt sienna, and burnt umber, or, for the 
 advanced worker, in addition to the foregoing, ivory 
 black, Indian red, Venetian red, Indian yellow, per- 
 manent blue, cobalt blue, madder lake, and emerald 
 oxide of chromium, etc. Only permanent colors 
 should be selected, and on this account the aniline dyes 
 are discarded. It is immaterial whether the pigments 
 
i64 CHEMISTRY FOR PHOTOGRAPHERS 
 
 are in water-color tubes or in powder form, the ad- 
 vantage of cheapness being with the latter. Of this 
 list of colors, lamp-black, ivory black, burnt sienna, 
 Indian yellow, cobalt blue, madder lake, and emerald 
 oxide of chromium are transparent; burnt umber, 
 Venetian red, and permanent blue are semi-trans- 
 parent; and Indian red is opaque. Besides these four 
 essentials there are several accessories needed. For 
 coating the emulsion upon paper, the best method is 
 by use of the "air-brush." It gives the most uniform 
 coating, without streaks or brush-marks, and by it 
 the whole sheet is coated, without being cut up. In 
 lieu of this desirable implement, ordinary brush-coat- 
 ing will do. Use a flat brush that is soft and thick and 
 about four inches in width, or narrower for smaller 
 sizes of paper, and even up streaks, etc., by blending 
 with a three- or four-inch flat badger blender. The 
 coating mixture is made by grinding the ingredients 
 together in a mortar with a capacity of about 200 cubic 
 centimeters. To make this mixture, put about equal 
 quantities (by volume) of pigment and gum solution 
 into the mortar and grind them together with the pestle. 
 If any of the pigment fails to be taken up by the gum, 
 add just enough more of the latter to take up in sus- 
 pension the rest of the former. In this way the gum 
 and pigment are to be balanced against each other, 
 there being an excess of neither. Complete the mixture 
 by the addition of approximately four to six times the 
 bulk of gum-pigment of the saturated dichromate, mix 
 thoroughly in the mortar, and strain through muslin. 
 Coat and dry the paper, and, if it is to be kept any 
 length of time, avoid exposing it to any actinic light 
 after it becomes dry. So long as it is wet, it remains 
 
PRINTING WITH CHROMIUM i6y 
 
 insensitive, but it becomes about as sensitive as plat- 
 inum paper as soon as the moisture is removed, and, 
 on account of the continuing action that has previously 
 been described, even short exposure to actinic light 
 in this condition produces after a time general fog 
 when development takes place. The sensitized paper 
 will keep for some days in good condition if unexposed 
 and stored where it will be thoroughly dry. The ex- 
 posure of paper, coated as described, is a simple matter if 
 pigments other than browns and blacks are used, since 
 the paper prints out. Give a full printing to bring out 
 all detail and gradation. When using browns and 
 blacks, of course there will be no visible image, and the 
 printing time has to be determined by actinometer. 
 Development is performed by means of wet blotting- 
 paper of the photographic quality. First soak the 
 cut-to-size blotters thoroughly so that all bubbles of 
 air may be expelled. Lay down a clean sheet of glass 
 the same dimensions as blotters and prints, and upon 
 it put one of the wetted blotters. Soak the prints for 
 a few minutes in cold water, separately, and put one 
 face down upon the wet blotter, the second one face 
 up upon the first, then another wet blotter and two 
 more prints back to back, and so on until all the prints 
 are in the pile, with a blotter upon the top, and finished 
 off with a second plate of glass. Cover the entire pile 
 with water in a sufficiently deep dish and allow the 
 developing process to go on automatically. The 
 length of time required will depend upon depth of 
 printing, prints that have been most fully exposed 
 developing slowest. Lightly printed paper may be 
 completely developed in an hour or so. Consequently, 
 until one has acquired the knack of judging before- 
 
1 66 CHEMISTRY FOR PHOTOGRAPHERS 
 
 hand about when his prints will be ready, it will be 
 well to examine them after about an hour. This task 
 is done by taking the whole pile of glass, prints, and 
 blotters in one body from the water and, resting it on 
 one corner, letting it drain for several minutes until the 
 superfluous water has run away. Take down the pile, 
 setting it up again in the reverse order as fast as it 
 comes down, but removing and drying any prints that 
 appear to be fully developed. They will dry a trifle 
 darker than they appear when wet. Then put the 
 unfinished ones, in the pile, back to soak, and examine 
 again in an hour, or longer if they were thought to 
 need more time than that. Prints will sometimes be 
 put to soak without being examined, needing no atten- 
 tion in the interim, over night, or for twenty-four 
 hours. The blotting-paper can be used over again, 
 up to three times, if it is not permitted to dry until 
 the absorbed gum has been washed out, and it is then 
 kept from the light. The method here briefly outlined 
 is due to Mr. Walter Zimmerman, to whom photog- 
 raphers both professional and amateur are indebted 
 for many helpful suggestions besides his excellent 
 gum process. A much more complete and detailed 
 account of it is given in the Photo-Miniature, Number 
 113, "Gum-Bichromate Printing," which the reader 
 is earnestly advised to study. 
 
 There are several interesting modifications of colloid- 
 dichromate methods, two of which are the inventions 
 of Mr. Thomas Manly. In the ozotype process a gela- 
 tine film impregnated with dichromate, manganous 
 salts, alum, and boric acid is prepared upon paper and 
 dried in the dark. The mixture of alum and boric 
 acid acts as a preservative. By the effect of light in 
 
PRINTING WITH CHROMIUM 167 
 
 the reduction of the dichromate, manganous salt is 
 oxidized to the manganic state and the image, which is 
 visible and of a brown color, is considered to be com- 
 posed essentially of manganic oxide. This image is 
 merely intermediate and does not form the completed 
 picture. The final print is made by putting a sheet of 
 carbon tissue of the desired color into an acetic acid 
 solution containing an organic reducing agent, mag- 
 nesium and ferrous salts, and bringing the face of this 
 tissue into contact with the paper carrying the man- 
 ganic oxide image. In the presence of acid manganic 
 oxide is reduced, by the ferrous salt and the developer 
 (hydrochinon), to the manganous condition, and the 
 gelatine of those portions of the carbon tissue which are 
 in contact with the manganese compound, of which 
 the image consists, becomes insoluble. The papers 
 are subsequently separated in water at 43 degrees 
 Centigrade and development carried out in the ordinary 
 way by dissolving the soluble parts of the gelatine. 
 Since the pigment-carrying gelatine of the carbon tissue 
 is finally left upon the surface of the printed sheet, 
 there is no right and left reversal of the image, and 
 this is one of the advantages of the process. Others are 
 found in the visibility of the image during printing, and 
 the brush control of development which is common to 
 the pigmented gelatine methods. In iqoy Mr. Manly 
 brought out his ozobrome process, using bromide prints, 
 instead of light shining through negatives, for making 
 the exposure.' By this means the worker is made inde- 
 pendent of daylight and enjoys the advantage of not 
 requiring enlarged negatives. The image either can 
 be formed upon the bromide enlargement directly or is 
 transferable to another support and is non-reversed in 
 
1 68 CHEMISTRY FOR PHOTOGRAPHERS 
 
 both cases. The second method is generally preferable 
 because then the bromide can be used over again to 
 make other ozobrome prints. If carefully handled also 
 it may be entirely uninjured in the end. The peculiar 
 feature of the process is that the pigmented gelatine 
 film is so treated chemically that a contact exposure of 
 it to the silver image of a bromide print for twenty 
 minutes produces an insoluble gelatine-pigment image 
 upon the ozobrome tissue, which is adaptable to the 
 usual forms of development with hot water. 
 
 In the bromoil process photography seems to have 
 been carried to its extreme in the way of control, 
 for the pigment is applied with a brush. A vigorous 
 bromide enlargement is prepared, the silver image is 
 oxidized by ferricyanide and removed by solution in 
 thiosulphate and washing. At the same time the 
 portions of the gelatine film originally occupied by the 
 metallic image are insolubilized by reduction of di- 
 chromate so that there is substituted what may be 
 called a latent image in insoluble gelatine which is 
 ink-absorbing. The surrounding gelatine is water- 
 absorbing and therefore refuses to "take up" ink. 
 The latter is put upon the print by gently dabbing with 
 a soft brush, and obviously its amount and its dis- 
 tribution are wholly at the command of the worker. 
 
 These last pigment processes are distinctly ultra- 
 advanced methods suited to the worker of more than 
 usual artistic gifts and attainments. They are indeed 
 photographically based, but their chief dependence for 
 success is, after all, not so much upon the control of 
 delicate chemical and physical reactions as upon the 
 worker's technical skill with the brush and his personal 
 powers of expression. 
 
CHAPTER XI 
 The Chemicals of Photography 
 
 WE have employed the metric system of measures 
 in this book up to the present point without any 
 attempt at its justification. It does not, in fact, seem 
 likely that it will need to be justified in the minds of 
 the majority of readers, even though it be true that most 
 photographic formulas are still given in English units 
 for English and American workers, and even though 
 this majority may still weigh out its photographic 
 chemicals by grains, scruples, and ounces. But if 
 justification be required in the minds of some and in the 
 practice of many more, it ought to be sufficient to say 
 that since the use of the metric system has been found 
 to simplify things materially for the scientific investi- 
 gator, it will certainly be to the advantage of the 
 photographer to adopt it. On this account all the 
 experiments and all the formulas given in this book 
 have been written in metric units exclusively, since 
 undoubtedly these units will be familiar enough to the 
 more advanced readers, and any readers who have not 
 previously been acquainted with them will quickly 
 become so in the course of the experiments. Very 
 fortunately it is coming to be the practice to express 
 formulas in both English and metric units in many of 
 the journals and other publications, so that we may 
 expect soon to find the former measures left out alto- 
 gether. Nevertheless, if any one insists upon making 
 use of his apothecary and avoirdupois weights, he can 
 
 convert the metric amounts into the old system by 
 
 i6c) 
 
no CHEMISTRY FOR PHOTOGRAPHERS 
 
 use of the conversion tables in the appendix. Let us 
 hope, however, that the simplest way out has been 
 adopted by every reader, namely, by the purchase of a 
 set of metric weights at the start. 
 
 Most photographic operations depend upon having 
 the necessary substances in solution in water, as, for 
 example, the developer solution, which acts upon the 
 latent image by first dissolving it piecemeal and im- 
 mediately reducing and depositing the silver before it 
 has an opportunity to get away; or the intensifying 
 mixture, which dissolves out a little of the silver image 
 and puts in its place some more non-actinic material. 
 So there are two things at least which it is needful to 
 know about every solution, first precisely what are its 
 components, and second what are the concentrations 
 of these constituents. Frequently also the temperature 
 of the solution is important and this is told by the 
 thermometer. There are two ways in which the con- 
 centration of a dissolved substance can be expressed, 
 and in both cases the matter is exceedingly simple if 
 metric units are employed, but not nearly so simple 
 with the English units. As we have shown in some of 
 our experiments, solutions may be made up in per- 
 centages, lo grams of sodium chloride dissolved in 
 water and diluted to loo cubic centimeters forming a 
 lo percent sodium chloride solution. It is well to 
 note that by dissolving lo grams of the solid in exactly 
 loo cubic centimeters of water there is not made a lo 
 per cent solution for the reason that the resulting 
 volume will in such a case be a little different from 
 I GO cubic centimeters. Taking a suitable volume of the 
 lo per cent solution and diluting, it is a very easy matter 
 to prepare solutions of any desired percentages less than 
 
CHEMICALS OF PHOTOGRAPHY 171 
 
 ten. The simplicity of this proposition comes from the 
 established relation between the gram weight and the 
 cubic centimeter of water, one cubic centimeter of liquid 
 water, at all ternperatures, weighing very nearly (al- 
 though not quite exactly, except at + 4 degrees Cen- 
 tigrade) one gram. There is no such simple relation 
 found in the English system. The second method of 
 expressing the concentration of a substance in solution 
 is based upon either the molecular weight of the sub- 
 stance or upon its equivalent weight. In this way 
 solutions are prepared which, volume for volume, are 
 chemically equivalent to one another, and this be- 
 comes very convenient for the chemical experimenter. 
 Thus, the molecular weight of sodium chloride is 58.46; 
 by dissolving 58.46 grams of pure sodium chloride in 
 distilled water and diluting as exactly as possible to one 
 liter, we should have a normal solution of sodium chlo- 
 ride, as it is called. In a similar way, i6q.8q (the molec- 
 ular weight) grams of silver nitrate in one liter of solution 
 will make a normal solution of silver nitrate ; and by mix- 
 ing together one liter of normal silver nitrate with one 
 liter of normal sodium chloride there would be formed 
 143.34 (the molecular weight) grams of silver chloride. 
 So also a thiosulphate fixing solution at 20 per cent, con- 
 centration is very closely 0.8 normal, since the molecular 
 weight of sodium thiosulphate is 248.22, and a normal 
 solution would contain that number of grams per liter. 
 To illustrate the concentration of the substances pres- 
 ent in a developer, we may take a hydrochinon formula 
 essentially as given by Schering, thus : 
 
 Sodium sulphite, dry zo.s grams 
 
 Hydrochinon 4-7 grams 
 
 Potassium carbonate, dry 65.7 grams 
 
 Water, to 1000 cubic centimeters 
 
172 CHEMISTRY FOR PHOTOGRAPHERS 
 
 A normal sodium sulphite solution contains 63.04 
 
 grams per liter ; and normal potassium carbonate, 6q. 10 
 
 grams of the salt per liter. At present it is impossible to 
 
 express the concentrations of most developing agents 
 
 in terms of normal solutions because chemical research 
 
 has not so far determined quantitatively the reactions 
 
 between the developing agents and the silver salt of 
 
 the plate. Until these quantities have been measured, 
 
 the concentrations can very well be expressed in terms 
 
 of the gram-molecular weights. Thus a solution of 
 
 hydrochinon containing 110.02 (the molecular weight) 
 
 grams per liter may be called a "gram-molecular 
 
 solution" (G.-M.S.). Therefore the Schering formula 
 
 when translated into molecular weight concentrations 
 
 becomes practically (N meaning normal) : 
 
 Hydrochinon 0.043 G.-M.S. 
 
 Sodium sulphite 0.32 N. 
 
 Potassium carbonate o.qy N. 
 
 Another developer frequently used is the so-called 
 Wellington Pyro-Soda in two solutions : 
 
 NO. I 
 
 Pyrogallic acid 100 grams 
 
 Sodium sulphite, dry 100 grams 
 
 Citric acid 8.3 grams 
 
 Water, to 1000 cubic centimeters 
 
 NO. 2 
 
 Sodium carbonate, dry 40 . 4 grams 
 
 Sodium sulphite, dry 54 . 5 grams 
 
 Water, to 1000 cubic centimeters 
 
 For use, 4 cubic centimeters of No. i, 30 cubic centi- 
 meters of No. 2, and 30 cubic centimeters of water are 
 mixed. When diluted for use in developing, the solution 
 has the following concentrations : 
 
 Pyrogallic acid 0.05 G.-M.S. 
 
 Sodium sulphite o. 50 N. 
 
 Citric acid o.oi N. 
 
 Sodium carbonate 0.35 N. 
 
CHEMICALS OF PHOTOGRAPHY 173% 
 
 Since normal solutions are all equivalent to one 
 another, and the same is true of o. i normal solutions, 
 etc., we can at once compare the strengths of solutions 
 when they are expressed in the fractional normal form. 
 It is seen at a glance that the concentration of sodium 
 sulphite in the Wellington formula is nearly double that 
 of the other; whereas the concentration of carbonate is 
 only about one third. The reason for these differences 
 in concentration will be plain if we recall the density- 
 giving properties of hydrochinon and the character- 
 istics of pyrogallic acid, viz., its propensity to dyeing 
 the gelatine, and its oxygen-absorbing activity when 
 in alkaline solution. 
 
 At first sight it may seem that this system of making 
 up solutions is complicated and abstruse. We shall 
 show in a very few words not only that this is not at all 
 the case, but that, on the contrary, it is the simplest 
 and easiest method in practice, if metric units be 
 adopted. Let us keep in mind, first, that a normal 
 solution, or a gram-molecular solution, of any sub- 
 stance signifies a solution containing a definite number 
 of grams in a volume of one liter ; and, second, that this 
 definite number of grams is always either the whole 
 molecular weight or some fraction of the molecular 
 weight of the substance. So in order to make the 
 preparation of normal or fractional normal and gram- 
 molecular solutions a simple matter, all that is necessary 
 to have is a list containing these definite weights per 
 liter for all the substances we wish to use. Such a 
 list we have already started, and it will be well to tabu- 
 late and extend it here. Further molecular weights 
 will be found in the tables at the end of this chapter. 
 
174 CHEMISTRY FOR PHOTOGRAPHERS 
 
 i.oN. potassium carbonate 6q.i grains anhydrous salt 
 
 per liter 
 r.oN. sodium carbonate 53 o grams anhydrous salt 
 
 per liter 
 i.oN. sodium sulphite 63.0 grams anhydrous salt 
 
 per liter 
 I.oN. potassium bromide i iq.o grams per liter 
 
 I.oN. potassium disulphite 55.5 grams per liter 
 
 i.oG.-M.S. amidol 1Q7.0 grams per liter 
 
 i.oG.-M.S. duratol iqq.i grams per liter 
 
 1 .0 G.-M.S. hydrochinon i lo.o grams per liter 
 
 I.oG.-M.S. metol 344-3 grams per liter 
 
 I.oG.-M.S. pyrogallic acid iz6.o grams per liter 
 
 Assuming that 3.0 normal solutions have been pre- 
 pared of sodium carbonate and sodium sulphite, by 
 dissolving 159 grams and iSq grams respectively, 
 and diluting each to i ,000 cubic centimeters, and that 
 we wish to make up 100 Cubic centimeters of pyro- 
 soda developer to use immediately, we should take, 
 referring to the fractional normal formula for this 
 developer given on page 172, 12 cubic centimeters of 
 3.0 normal sodium carbonate, 17 cubic centimeters of 
 3.0 N. sodium sulphite, i centimeter of i.o N. citric 
 acid (70 grams per liter), and add 0.6 gram of pyro- 
 gallic acid (100 cubic centimeters is o.i of one liter, 
 and therefore a o.oy G.-M. solution will have o. i X 
 o.oj X 126.0 grams, or 0.6 gram of pyro in 100 cubic 
 centimeters of solution), after which this mixture is 
 to be diluted with water up to a volume of 100 cubic 
 centimeters. The three-times normal carbonate solu- 
 tion may be made up in quantity, if it is kept in a 
 bottle having a tight stopper to prevent alteration of 
 the concentration by evaporation. A rubber stopper 
 is best for this bottle. If it be desired to make up 
 considerable amounts of three-times normal sodium 
 sulphite, the solution should be put in a number of 
 
CHEMICALS OF PHOTOGRAPHY 175 
 
 smaller sized, full bottles with tight-fitting stoppers. 
 The concentration of sulphite solutions will change 
 not only by evaporation, if precautions are not taken, 
 but also from spontaneous oxidation by the air. A 
 good plan to follow is to make up only a half liter or a 
 liter at a time, according to the amount of work that 
 is being done, since sodium sulphite keeps much better 
 in solid form than it does in solution. More of these 
 standard developer formulas, in the fractional normal 
 form, will be found in the appendix. The advantages 
 of this system are numerous ; it has both simplicity and 
 uniformity; a variety of different developers can be 
 easily had on hand and used without cumbering the 
 darkroom and its cupboards with a great array of 
 bottled developer solutions; the concentrations can 
 be quickly varied by definite amounts, if one be given 
 to experimenting; and it is economical, because there 
 are never any prepared solutions standing around to 
 become oxidized and thus wasted, if the photographer 
 takes a few weeks' vacation from his hobby. When a 
 particular formula has been selected for permanent 
 use, the proper amounts of developing agent to make 
 the desired volume of solution can be weighed out and 
 put up in cork-stoppered vials. A supply of these kept 
 on hand will be found very convenient. 
 
 For containing solutions in bulk it is desirable to 
 have a supply of 0.5 liter, liter, and 2 liter glass-stop- 
 pered bottles, and to use them in the following way for 
 making up solutions. Take one of them, say a 2 liter 
 bottle, and measure into it carefully with a graduate 
 1,000 cubic .centimeters of water. Stick upon the side 
 of the bottle across the water-level a label and mark 
 on this the level of the surface, preferably in indelible 
 
176 CHEMISTRY FOR PHOTOGRAPHERS 
 
 ink. Do the same for the 2,000 cubic centimeter level; 
 and similarly for the other bottles. If 2 liters of a 
 solution are required, dissolve in enough water con- 
 tained in a beaker or other convenient dish the weighed 
 solid, filter this solution upon filter paper in a funnel 
 supported in the neck of the bottle, pour a little water 
 upon the filter after the solution has run through to 
 rinse off the paper, and, after this also has run into the 
 bottle, make up to the mark on the label with water, 
 and mix the liquid thoroughly by shaking the stop- 
 pered bottle. Write upon a larger label gummed to 
 the bottle the name of the solution, i.e., "3.0 N. Sodium 
 Carbonate," for example. This is a general method 
 for preparing solutions, to dissolve the solid, or solids, 
 in a portion of the required water, then to filter, if 
 necessary, and finally to dilute up to the specified 
 volume with water. Sometimes the solids have to be 
 dissolved in a particular order to avoid precipitation, 
 as is the case with duratol-hydrochinon developer. 
 
 It is frequently useful to have at hand prepared 
 solutions of acids and alkalies of known approximate 
 concentrations. Subjoined are data for making up 
 such solutions. 
 
 1. Acetic acid, concentrated, qg.5%, 17.5 N; acetic acid, dilute, 
 
 5. N, 71 cc. of the concentrated acid diluted with water to 
 2 JO cc. 
 
 2. Ammonium hydroxide, concentrated, 28%, sp. gr. o.qo, 7.3 N; 
 
 ammonium hydroxide, dilute, 7%, sp. gr. 0.97, 2. N, 18 cc. 
 of the concentrated ammonia diluted to 250 cc. with water. 
 
 3. Hydrochloric acid, concentrated, 39%, sp. gr. 1.20, 12.8 N; 
 
 hydrochloric acid, dilute, 20%, sp. gr. i . 10, 6. N, 46 cc. of the 
 concentrated acid diluted with water to 250 cc. ; hydro- 
 chloric acid, dilute, 10%, sp. gr. 1.05, 3. N, 22 cc. of the 
 concentrated acid made up with water to ijo cc. 
 
CHEMICALS OF PHOTOGRAPHY 177 
 
 Nitric acid, concentrated, 70%, sp. gr. 1.42, 16. N; nitric acid, 
 dilute, 40%, sp. gr. 1.25, 8. N, 88 cc. of the concentrated 
 acid diluted with water to 2jo cc. ; nitric acid, dilute, 2J%, 
 sp. gr. 1 . 1 J, 4.4 N, JO cc. of the concentrated acid made up 
 to 250 cc. with water. 
 
 Sulphuric acid, concentrated, q6%, sp. gr. i .84, 18. N; NEVER 
 POUR WATER INTO CONCENTRATED SULPHURIC 
 ACID; sulphuric acid, dilute, 60%, sp. gr. i.yo, q. N, 122 
 cc. of the concentrated acid poured, a little at a time, 
 slowly and carefully, with thorough cooling of the mixture 
 during the operation, into cold water, and, when cold, 
 made up to 250 cc; sulphuric acid, dilute, 33%, sp. gr. 
 I .z$, 4. N, 56 cc. of the concentrated acid made up, as above, 
 to 250 cc. with water. 
 
178 CHEMISTRY FOR PHOTOGRAPHERS 
 
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Appendix 
 
 Table I 
 
 THE CHEMICAL ELEMENTS AND THEIR ATOMIC 
 WEIGHTS (1915) 
 
 Elements 
 
 
 At. Wts. 
 
 Elements 
 
 
 At. Wts. 
 
 Elements 
 
 At. Wts. 
 
 Aluminum 
 
 Al 
 
 27.1 
 
 Holmium 
 
 Ho 
 
 163.5 
 
 Rhodium 
 
 Rh 
 
 102.9 
 
 Antimony 
 
 Sb 
 
 120.2 
 
 Hydrogen 
 
 H 
 
 1.008 
 
 Rubidium 
 
 Rb 
 
 85.45 
 
 Argon 
 
 A 
 
 39.88 
 
 Indium 
 
 In 
 
 114.8 
 
 Ruthenium 
 
 Ru 
 
 101.7 
 
 Arsenic 
 
 As 
 
 79.96 
 
 Iodine 
 
 I 
 
 126.92 
 
 Samarium 
 
 Sa 
 
 150.4 
 
 Barium 
 
 Ba 
 
 137.37 
 
 Iridium 
 
 Ir 
 
 193.1 
 
 Scandium 
 
 Sc 
 
 44.1 
 
 Bismuth 
 
 Bi 
 
 208.0 
 
 Iron 
 
 Fe 
 
 55.84 
 
 Selenium 
 
 Se 
 
 79.2 
 
 Boron 
 
 B 
 
 U.O 
 
 Krypton 
 
 Kr 
 
 82,92 
 
 Silicon 
 
 Si 
 
 28.3 
 
 Bromine 
 
 Br 
 
 79.92 
 
 Lanthanum 
 
 La 
 
 139.0 
 
 Silver 
 
 Ag 
 
 107.88 
 
 Cadmium 
 
 Cd 
 
 112.40 
 
 Lead 
 
 Pb 
 
 207.10 
 
 Sodium 
 
 Nl 
 
 23.00 
 
 Caesium 
 
 Cs 
 
 132.81 
 
 Lithium 
 
 Li 
 
 6.94 
 
 Strontium 
 
 Sr 
 
 87.63 
 
 Calcium 
 
 Ca 
 
 40.17 
 
 Lutesium 
 
 Lu 
 
 174.0 
 
 Sulphur 
 
 S 
 
 32.07 
 
 Carbon 
 
 C 
 
 12.00 
 
 Magnesium 
 
 Mg 
 
 24.32 
 
 Tantalum 
 
 Ta 
 
 181.5 
 
 Cerium 
 
 Ce 
 
 140.25 
 
 Manganese 
 
 Mn 
 
 54.93 
 
 Tellurium 
 
 Te 
 
 127.5 
 
 Chlorine 
 
 CI 
 
 35.46 
 
 Mercury 
 
 Hg 
 
 200.6 
 
 Terbium 
 
 Tb 
 
 159.2 
 
 Chromium 
 
 Cr 
 
 52.0 
 
 Molybdenum Mo 
 
 96,0 
 
 Thallium 
 
 Tl 
 
 204.0 
 
 Cobalt 
 
 Co 
 
 58.97 
 
 Neodymium 
 
 Nd 
 
 144.3 
 
 Thorium 
 
 Th 
 
 232.4 
 
 Columbium 
 
 Cb 
 
 93.5 
 
 Neon 
 
 Ne 
 
 20.2 
 
 Thulium 
 
 Tm 
 
 168.5 
 
 Copper 
 
 Cu 
 
 63.57 
 
 Nickel 
 
 Ni 
 
 58.68 
 
 Tin 
 
 Sn 
 
 119.0 
 
 Dysprosium 
 
 Dy 
 
 162.5 
 
 Niton 
 
 Nt 
 
 222.4 
 
 Titanium 
 
 Ti 
 
 48.1 
 
 Erbium 
 
 Er 
 
 167.7 
 
 Nitrogen 
 
 N 
 
 14.01 
 
 Tungsten 
 
 W 
 
 184.0 
 
 Europium 
 
 Eu 
 
 152.0 
 
 Osmium 
 
 Os 
 
 190.9 
 
 Uranium 
 
 U 
 
 238.5 
 
 Fluorine 
 
 F 
 
 19.0 
 
 Oxygen 
 
 
 
 16.00 
 
 Vanadium 
 
 V 
 
 51.0 
 
 GadoHnium 
 
 Gd 
 
 157.3 
 
 Palladium 
 
 Pd 
 
 106.7 
 
 Xenon 
 
 Xe 
 
 130.2 
 
 Gallium 
 
 Ga 
 
 69.9 
 
 Phosphorus 
 
 P 
 
 31.04 
 
 Ytterbium 
 
 Yb 
 
 172.0 
 
 Germanium 
 
 Ge 
 
 72.5 
 
 Platinum 
 
 Pt 
 
 195.2 
 
 Yttrium 
 
 Yt 
 
 89.0 
 
 Glucinum 
 
 Gl 
 
 9.1 
 
 Potassium 
 
 K 
 
 39.10 
 
 Zinc 
 
 Zn 
 
 65.37 
 
 Gold 
 
 Au 
 
 197.2 
 
 PraseodymiumPr 
 
 140,6 
 
 Zirconium 
 
 Zr 
 
 90.6 
 
 Helium 
 
 He 
 
 3.99 
 
 Radium 
 
 Ra 
 
 226.4 
 
 
 
 
 Table II 
 METRIC MEASURES AND THEIR EQUIVALENTS 
 
 Standards 
 
 The metric standard unit of length is the meter, 
 one ten-millionth part of the (assumed) length of the 
 quadrant of a terrestrial meridian. 
 
 The metric standard unit of weight is the weight of 
 one liter (i.ooo cubic centimeters) of pure water at its 
 greatest density (+ 4° C and 760 millimeters pressure) 
 and is called the kilogram, or kilo. 
 
 i8j 
 
1 84 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Measures of Length 
 
 Milli- 
 meters 
 
 Centi- 
 meters 
 
 Deci- 
 meters 
 
 Meters 
 
 Inches 
 
 Feet 
 
 Yards 
 
 1.0 
 
 10.0 
 
 100.0 
 
 1000.0 
 
 0.1 
 
 1.0 
 
 10.0 
 
 100.0 
 
 0.01 
 
 0.10 
 
 1.00 
 
 10.00 
 
 0.001 
 0.010 
 0.100 
 1.000 
 
 0.03937 
 
 0.39371 
 
 3.93708 
 
 39.37079 
 
 0.00328 
 0.03281 
 0.32809 
 3.28091 
 
 0.00109 
 0.01094 
 0.10936 
 1.09363 
 
 1 incti ' 
 1 foot = 
 
 2.S399 cm. 
 3.0479 dm. 
 
 1 yard 
 1 mile 
 
 0.9144 m. 
 
 1.6093 km. (Kilometer) 
 
 Measures of Volume 
 
 Cubic 
 centimeters 
 
 Liters, or 
 cubic deci- 
 meters 
 
 Cubic 
 inches 
 
 Cubic 
 feet 
 
 Drams 
 
 Ounces 
 
 Pints 
 
 . 1.0 
 
 10.0 
 
 100.0 
 1000.0 
 
 0.001 
 0.010 
 0.100 
 1.000 
 
 0.06103 
 
 0.61027 
 
 6.10271 
 
 61.02705 
 
 0.000035 
 0.000353 
 0.003532 
 0.035317 
 
 0.270 
 2.71 
 27.05 
 270.46 
 
 0.034 
 0.338 
 3.381 
 33.81 
 
 0.002 
 0.021 
 0.211 
 2.113 
 
 1 dram = 3.697 cc. 
 
 1 cu. in. = 16.3862 cc 
 
 1 ounce = 29.572 cc. 
 
 lou. ft. = 28.3153 1. 
 
 Ipint =473. 152 cc. 
 
 leu. yd. = 764.51341. 
 
 Measures of Weight 
 
 Milli- 
 
 Cg. 
 
 Dg. 
 
 
 Kilo- 
 
 
 Troy 
 
 Avoird. 
 
 Avoird. 
 
 grams 
 
 
 grams 
 
 
 Ounces 
 
 Ounces 
 
 Pounds 
 
 1.0 
 
 0.1 
 
 0.01 
 
 0.001 
 
 0.000001 
 
 0.0154 
 
 0.000032 
 
 0.000035 
 
 0.00000 
 
 10.0 
 
 1.0 
 
 0.10 
 
 0.010 
 
 0.000010 
 
 0.1543 
 
 0.000322 
 
 0.000353 
 
 0.00002 
 
 100.0 
 
 10.0 
 
 1.00 
 
 0.100 
 
 O.OOOIOC 
 
 1.5432 
 
 0.0032 IS 
 
 0.003527 
 
 0.00022 
 
 1000.0 
 
 100.0 
 
 lO.Ofl 
 
 1.000 
 
 O.OOIOOO 
 
 15.4323 
 
 0.032151 
 
 0.035274 
 
 0.00220 
 
 
 
 
 10.000 
 
 O.OIOOOO 
 
 154.3235 
 
 0.32151 
 
 0.35274 
 
 0.02205 
 
 
 
 
 100.000 
 
 O.IOOOOO 
 
 1543.2349 
 
 3.2151 
 
 3.5274 
 
 0.22046 
 
 
 
 
 1000.000 
 
 1.000000 
 
 15432.3488 
 
 32.151 
 
 35.274 
 
 2.2046 
 
 1 grain 
 1 troy oz. 
 1 av. oz. 
 
 0.06479895 g. 
 31.103496 g. 
 28.34954 g. 
 
 1 avoir, lb. = 
 1 avoir, lb. ■■ 
 1 cwt. 
 
 0.45359265 kg. 
 453.59265 g. 
 50.80237689 kg. 
 
 Millimeter = mm. 
 
 Centimeter = cm. 
 
 Decimeter » dm. 
 
 Meter «■ m. 
 
 Abbreviations 
 
 Cu. centim. = cc. 
 
 Liter = 1. 
 
 Milligram = mg. 
 
 Centigram ■» cg. 
 
 Decigram 
 
 Gram 
 
 Kilogram 
 
 dg. 
 
APPENDIX 
 
 185 
 
 •H^ 
 
 I I 
 
 CO 
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 ssl 
 
 < 
 
 CO 
 
 !:; 
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 ■8 
 
 
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 JQOQQQ 
 
 lis airs " 
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 15a 
 
 zo 
 
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 euta 
 
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 llllll 
 
 a 3 3 E g Sg. 
 aaisj§| 
 
 1 
 
 ll 
 
 
 M 
 
 
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1 86 CHEMISTRY FOR PHOTOGRAPHERS 
 
APPENDIX 
 
 187 
 
 
 la's"- 
 
 og,S 
 
 
 CO 
 
 C5ZZ 
 
 
 3^- 
 
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 2 
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 g g 
 
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 dbd 
 
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 o«o 
 
 ■■sg 
 
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1 88 CHEMISTRY FOR PHOTOGRAPHERS 
 
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 3 ^!2:^5 ^^jju 
 
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 9® b 
 
 oo-*^ t^t-iO 
 
 I 
 
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 s 
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 S 5, 
 
 is £ 
 
 ■pa "^ 
 
 «2 
 
 b2 
 
 ;.g.i 
 
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 o •- . 
 
 II" 
 
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 SftP.IS'g.-So 
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 cdutili 
 
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 tnta 
 
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APPENDIX 
 
 i8q 
 
 •« 
 
 I 
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 ei 
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 oi 
 
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 P4 
 
 
 •o 
 
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 r*5 1*3 ^-H»o & 
 
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 8m 
 
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 12 
 
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 ■■ssn 
 
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 5^^ 
 
 00 
 
 000 
 
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 000 
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 g 
 
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 dzzi 
 
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 CB fl tf c3 c! 
 
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 3^ o^P^ 
 
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 •°11S13 
 
iqo CHEMISTRY FOR PHOTOGRAPHERS 
 Table V 
 FIXING BATH FORMULAS 
 
 Sodium thiosulphate, crystallized, Na2S203.5H20; 
 molecular weight, 248.22; percentage of Na2S203 in the 
 crystallized salt, 63.709%. 
 
 For reduction purposes, where the thiosulphate is 
 oxidized, as by iodine, to tetrathionate, a i .0 N solution 
 contains 248.22 grams of the crystallized salt per liter 
 of solution. 
 
 For the reaction of metathesis whereby soluble silver- 
 sodium thiosulphate is formed, a i .0 N solution con- 
 tains 248.22 grams of the crystallized salt per liter of 
 solution. 
 
 1 . An average formula for plates ; also for lantemslides 
 Make an approximately 0.8 N thiosulphate solution: 
 
 Sodium thiosulphate 200 . grams 
 
 Water, to 1000 . cc. 
 
 2. For films; also for plates 
 
 Make an approximately i.o N thiosulphate solution: 
 
 Sodium thiosulphate 250. grams 
 
 Water, to 1000. cc 
 
 Leave plates and films in the solution until all creami- 
 
 ness has disappeared, and then for as long again. The 
 
 whole time will ordinarily be about 20 minutes. 
 
 3 . For bromide and gaslight papers 
 A 0.8 N thiosulphate solution: 
 
 Sodium thiosulphate 200 . grams 
 
 Water, to 1000 . cc. 
 
 Prints will be fixed in 10 to i j minutes. 
 
 4. For printing-out papers 
 
 A 0.4 N thiosulphate solution: 
 
 Sodium thiosulphate 100. grams 
 
 Water, to 1000. cc. 
 
 Soak prints for at least 10 minutes. 
 
 N.B. — These plain thiosulphate solutions are to be used at a 
 temperature of about + i8°C (65°F) ; and must be made up fresh 
 
APPENDIX iqi 
 
 FIXING BATH FORMULAS— (co«/t««ed) 
 
 5. Acid fixing-bath, for plates, films, and developing-out 
 
 papers. 
 An 0.8 N thiosulphate solution, containing potassium di- 
 
 sulphite (metabisulphite) : 
 
 Sodium thiosulphate 200 . grams 
 
 Potassium disulphite ay . grams 
 
 Water, to 1000 . cc. 
 
 6. For fixing the bleached print in the bromoil process : 
 Approximately 0.4 N thiosulphate: 
 
 Sodium thiosulphate 48. grams 
 
 Sodium sulphite 24. grams 
 
 Water, to joo . cc. 
 
 Leave the print in this bath z or 3 minutes. 
 
 7. Hardening bath, using formaldehyde. 
 
 Formaldehyde jo. cc. 
 
 Water, to yoo. cc. 
 
 Give 5 minutes; or use yi the above concentration 
 
 and soak plates or films for i y minutes. 
 
 8. Hardening bath, with chrome alum. 
 
 Potassium chromium sulphate 15. grams 
 
 Water, to 500 . cc. 
 
 Immerse for 10 to 15 minutes. 
 
 N.B. — Ordinary alum, potassium aluminum sulphate, should 
 not be used for hardening purposes, since the results are not so 
 permanent as with formaldehyde or chrome alum. 
 
 It is much better practice to keep the hardening solution sepa- 
 rate from the acid fixing bath. 
 
 g. Acid fixing and hardening bath, for negatives and develop- 
 ing-out papers. 
 
 Sodium thiosulphate (o.q N) 220 . grams 
 
 Potassium disulphite 28 . grams 
 
 Potassium chromium sulphate 28 . grams 
 
 Water, to 1000 . cc. 
 
 10 Cramer's acid fixing and hardening bath. 
 
 A. Potassium chromium sulphate i y . grams 
 
 Potassium disulphite 22 . j grams 
 
 Water, to 250. cc. 
 
 B. Sodium thiosulphate (i .0 N) 250. grams 
 
 Water, to 1000 . cc. 
 
 When solution and filtration have been effected, pour 
 A into B with constant stirring. 
 
iqi CHEMISTRY FOR PHOTOGRAPHERS 
 
 FIXING BATH FOIiMTTLAS— (concluded) 
 II. To test a fixing bath for exhaustion : 
 Place a drop of the solution on white paper; expose 
 it to light and air. If it turns brown, the solution is 
 
 worked out. 
 
 Table VI 
 
 FORMULAS FOR INTENSIFICATION 
 
 1 . Wellington Silver Intensifier. 
 
 No. I 
 
 Ammonium sulphocyanide 40 . grams 
 
 Sodium thiosulphate 40 . grams 
 
 Water, to 250. cc. 
 
 No. z 
 
 Silver nitrate 25 . grams 
 
 Water, to i jo . cc. 
 
 No. 3 
 
 Sodium sulphite 2.0 grams 
 
 Pyrogallic acid I . o grams 
 
 Water, to 10. cc. 
 
 No. 4 
 
 Ammonium hydroxide (cone.) 10. cc. 
 
 Water, to 100 . cc. 
 
 For use, add 1 5 cc. No. 2 to the same volume of No. i 
 with stirring. Enough of No. i is then to be added just 
 to dissolve the precipitated silver sulphocyanide, and no 
 more. To this mixture add 2 cc. each of No. 3 and No. 
 4 solutions. Put the dry negative into this bath. 
 Intensification will take place in from y to 20 minutes. 
 Greater rapidity and more density can be secured by 
 the addition of more of No. 4. 
 
 2. Uranium Intensifier. 
 
 No. I 
 
 Uranium nitrate i . gram 
 
 Water, to 100 . cc. 
 
 No. 2 
 
 Potassium ferricyanide i . gram 
 
 Acetic acid (Glacial) 18. cc. 
 
 Water, to 100. cc. 
 
APPENDIX iq3 
 
 FORMULAS FOR INTENSIFICATION— (coMfowwed) 
 
 Mix equal volumes of No. i and No. 2. After in- 
 tensification has reached the desired point, wash for 
 10 minutes in running water. 
 
 3. Chromium Intensifier. 
 
 No. I 
 
 Potassium dichromate • j . 5 grams 
 
 Water, to 100 . cc. 
 
 No. 1 
 
 Hydrochloric acid (cone.) J • 5 cc. 
 
 Water, to 100. cc. 
 
 Mix I volume of No. i, i volume of No. 2, and 2 
 volumes of water, just before using, as the mixture 
 does not keep well. Bleach the negative completely, 
 wash out the chromic acid, expose to daylight, and 
 re-develop in a developing solution suited to bromide 
 paper. 
 
 4. Mercury Intensifiers. 
 
 Negatives must be well washed after fixing and before 
 intensification. 
 
 A. Mercuric chloride. 
 
 Mercuric chloride 2 . grams 
 
 Water, to 100. cc. 
 
 B. Mercuric bromide 
 
 Mercuric chloride 2 . grams 
 
 Potassium bromide 2 . grams 
 
 Water, to 100. cc. 
 
 Bleach the negative thoroughly in either of the above 
 solutions; wash for 10 to 15 minutes, and develop the 
 white image in daylight either in sodium thiosulphate 
 solution ( I in 10), or in amidol developer. Wash and 
 dry. 
 
iq4 CHEMISTRY FOR PHOTOGRAPHERS 
 FORMULAS FOR INTENSIFICATION— (cowc/udetQ 
 
 C. Mercuric iodide. 
 
 Negatives do not need a careful washing before in- 
 tensification by this method. 
 
 Mercuric chloride 2 . grains 
 
 Potassium iodide 2 . grams 
 
 Sodium sulphite 20. grams 
 
 Water, to 100. cc. 
 
 The negative is intensified directly in this bath, but 
 must be subsequently treated with a developer to 
 prevent fading. 
 
 Table VII 
 FORMULAS FOR REDUCTION 
 
 1 . For uniform reduction, as of over-developed negatives. 
 Permanganate Reducer 
 
 Potassium permanganate 0.5 gram 
 
 Sulphuric acid (cone.) 2 . cc. 
 
 Water, to 200. cc. 
 
 Dissolve the permanganate in, say, half of the water, 
 add the sulphuric acid to this solution, and dilute to 
 the volume specified. 
 
 For use, dilute i volume of the stock solution up to 
 4 volumes with water. When the density has been 
 enough diminished, rinse the plate and put it for 5 
 minutes in an acid thiosulphate fixing bath. Wash well. 
 
 2. For reducing shadow details, i.e., increasing contrast, as in 
 over-exposed negatives. 
 
 Farmer's Ferricyanide Reducer 
 
 No. I 
 
 Potassium ferricyanide y . grams 
 
 Water, to yo. cc. 
 
 No. 2 
 
 Sodium thiosulphate 100. grams 
 
 Water, to 1000 . cc. 
 
 Since ferricyanide does not keep well in solution, it is 
 to be made up only as needed. 
 
APPENDIX iqj 
 
 FORMULAS FOR REDUCTION— (condwied) 
 
 To loo cc. of No. 2 add y to lo cc. of No i, immerse 
 the plate, and rock the tray until the negative has been 
 sufificiently reduced. Wash well. 
 
 3. For reduction of highlights, i.e., to diminish contrast. 
 
 A. Ceric Sulphate Reducer 
 
 Ceric sulphate (crystals) 10. grams 
 
 Sulphuric acid (cone.) 4. cc. 
 
 Water, to 100. cc. 
 
 Take i volume of the stock solution and dilute with 
 I volume of water. Rock the tray. 
 
 B. Bennett's Ammonium Persulphate Reducer 
 
 Ammonium persulphate 1 1 . grams 
 
 Sodium sulphite 2 . grams 
 
 Sulphuric acid (cone.) i . cc. 
 
 Water, to 100. cc. 
 
 Dilute I volume of the stock solution with 4 to 8 
 volumes of water. After reduction, rinse the plate and 
 put it for 6 minutes in an acid thiosulphate solution. 
 Wash well. 
 
 4. For making bright bromide prints from weak negatives. 
 
 Wellington Iodide Reducer 
 
 Potassium iodide 2.0 grams 
 
 Iodine 0.2 grams 
 
 Water, to 300 . cc. 
 
 After the bromide print, which must be somewhat 
 over-developed, has been fixed and washed, it is put in 
 the above bath and left until sufificiently lightened. 
 Then put it in fresh thiosulphate solution for a few 
 minutes and wash it thoroughly. In the iodide solution 
 the whites of the print turn a dark blue owing to the 
 formation of the iodide of starch with the sizing of the 
 paper. This blue compound is destroyed again by the 
 thiosulphate. 
 
igb CHEMISTRY FOR PHOTOGRAPHERS 
 
 Table VIII 
 
 FORMULAS FOR DYEING PLATES FOR COLOR COR- 
 RECTION 
 
 Prepare stock solutions of dyes, by dissolving i . gram 
 of each in qj% ethyl alcohol and diluting with the 
 same to i ,000 cc. 
 
 No. I 
 No. 2 
 No. 3 
 No. 4 
 No. 5 
 
 Acridine orange No. 
 Erythrosine 
 Orthochrome T 
 Pinaverdol 
 Pinacyanol 
 
 No. 6. Pinachrome 
 
 No. 7. 
 No. 8. 
 
 Homocol 
 Dicyanine 
 
 For blue-green and green 
 For greenish-yellow and yellow 
 For Blue-green, green, and yellow 
 For green, yellow, and orange 
 For red-sensitiveness; to the ex- 
 treme visible red. Gives very 
 fast plates 
 Sensitizes to line B. Plates are 
 
 faster than with No. 3 
 For red-sensitiveness 
 For infra-red 
 
 Use Distilled Water 
 
 Blue-green 
 Green 
 
 Greenish-yellow 
 Yellow 
 
 Green, yellow 
 Orange 
 
 Red* 
 
 Panchromatic t 
 Effects 
 
 No. 1 200 cc. 
 Dil. to 1 1. 
 
 No. 2 200 cc. 
 NHiOH 10 cc. 
 
 Dil. to 1 1. 
 
 No. 3 2 cc. 
 Dil. to 1 1. 
 
 No. 6 2 cc. 
 Dil. to 1 1. 
 
 No. 4 11 cc. 
 No. 5 14 cc. 
 
 
 No. 4 2 cc. 
 Dil. to 1 1. 
 
 Infra-redJ 
 
 No. 7 11 cc. 
 
 
 
 No. 8 2 cc. 
 Dil. 1 1. 
 
 NHjOH 33 cc. 
 
 
 
 Alrnliftl d.nn rr 
 
 
 Red* 
 
 Dil. to 1 1. 
 
 
 No. 7 2 cc. 
 Dil. to 1 1. 
 
 
 *Sensitize without light. 
 
 tUse plates that are not too fast; sensitize without light. 
 ^Make up just before using, since the sensitizing power of the solution 
 quickly diminishes. 
 
 When sensitizing, put the plates in a grooved trough 
 or tank containing the dye. A tray can be used, if it is 
 rocked constantly. Bathe the plates for 3 minutes; 
 wash in running water for 3 minutes; and put to dry 
 in a light-proof drying cupboard, or in a tight box with 
 fused calcium chloride as a dehydrating agent. 
 
APPENDIX 197 
 
 Table IX 
 
 LIST OF APPARATUS FOR EXPERIMENTS 
 
 I Alcohol lamp, or if available a Bunsen's burner with rubber 
 
 connecting tube 
 
 I Balance, capacity loo. grams to o.oy gram 
 
 3 Beakers, glass, lipped, loo. cc. capacity 
 
 3 Beakers, glass, lipped, ^$o. cc. capacity 
 
 3 Beakers, glass, lipped, joo. cc. capacity 
 
 I Cylinder, graduated, lo. cc. capacity 
 
 I Cylinder, graduated, yo. cc. capacity 
 
 1 Cylinder, graduated, 250. cc. capacity 
 
 2 Dishes, porcelain evaporating. No. o, 7.5 cm. diameter 
 
 1 Filter paper, package of 100 sheets, 18. j cm. diameter 
 
 2 Flasks, glass, conical, 125. cc. capacity 
 I Funnel, glass, 10. cm. diameter 
 
 I Litmus paper, tube of 100 strips, red 
 
 I Litmus paper, tube of 100 strips, blue 
 
 I Mortar and pestle, porcelain, ly. cm. diameter 
 
 6 Stirring-rods, glass, ij. cm. length 
 
 12 Test tubes, glass, ly. cm. x 2. cm. 
 
 I Weights, set, in block, yo. gram piece to o.oy gram 
 
 Table X 
 
 LIST OF CHEMICALS FOR EXPERIMENTS 
 
 N.B. — ^No amounts are suggested for those chemicals which 
 are commonly found on the photographer's shelves. 
 
 Quantity 
 
 Acetic acid, glacial, (Sp. Gr. i.oyS), C.P 
 
 Ammonium hydroxide, cone, (Sp. Gr. o.qo), C.P. . . 
 
 Citric acid, C.P yo. grams 
 
 Ethyl alcohol, gy% 
 
 Ferric ammonium citrate, C.P yo. 
 
 Ferric oxalate, C.P yo. 
 
 Ferrous sulphate, cryst., C.P yo. " 
 
 Gelatine, photographic yoo. 
 
 Hydrochloric acid, cone, (Sp. Gr. 1.20), C.P 
 
iq8 CHEMISTRY FOR PHOTOGRAPHERS 
 
 LIST OF CHEMICALS FOR EXPERIMENTS {concluded) 
 
 Magnesium, powder (from box of flashlight) 
 
 Mercuric chloride, C.P 50 . grams 
 
 Nitric acid, cone, (Sp. Gr. i .42), C.P 
 
 Potassium aluminum sulphate, cryst. C.P 50. 
 
 Potassium bromide, C.P 
 
 Potassium dichromate, C.P 50. 
 
 Potassium ferricyanide, C.P 50. 
 
 Potassium ferrocyanide, C.P 50. 
 
 Potassium iodide, C.P 25 . 
 
 Potassium oxalate, C.P 50 . 
 
 Potassium permanganate, C.P 50. 
 
 Pyrogallic acid 
 
 Silver nitrate, C.P 50. 
 
 Sodium carbonate, anhydrous, C.P 
 
 Sodium chloride, C.P jo. 
 
 Sodium sulphide, cryst., C.P 50 . 
 
 Sodium sulphite, dry, C.P 
 
 Sodium thiosulphate, cryst., photographic 
 
 Sulphuric acid, cone, (Sp. Gr. i .84), C.P 
 
 Uranium nitrate, cryst., C.P 25 . 
 
 Table XI 
 BIBLIOGRAPHY 
 
 ABC of Artistic Photography, Anderson, $2.50, '13, Dodd. 
 
 American Photography Exposure Tables, 25c, American Photog- 
 raphy 
 
 Book of Photography, Hasluck, $3.00, '07, McKay. 
 
 Bromide Printing, and Enlarging, Tennant, 2jc, '12, Tennant and 
 Ward. 
 
 Carbon Print, Buchanan, 50c, '10, J. B. Buchanan, Los Angeles. 
 
 Chemicals in Photography, Cowling, 25c, Tennant and Ward. 
 
 Chemistry for Photographers, Townsend, 50c, '06, Tennant and 
 Ward. 
 
 Developers and Development, Tennant, 25c, '12, Tennant and 
 Ward. 
 
 Elementary Chemistry of Photographic Chemicals, joc, '03, 
 Tennant and Ward. 
 
 First Book of Photography, Claudy, 75c, "12, Nast and Co. 
 
APPENDIX iqq 
 
 BIBLIOGRAPHY {concluded) 
 How to Make Good Prints, loc, '14, Am. Photographic Pub. 
 How to Make Oil and Bromoil Prints, Demachy and Hewitt, 
 
 2JC, Tennant and Ward. 
 Instruction in Photography, Abney, $1.50, '06, Lippincott. 
 Investigations on the Theory of the Photographic Process, 
 
 Sheppard and Mees, $1.75, '07, Longmans. 
 Lure of the Camera, Olcott, $3.50, '14, Houghton. 
 Manual of Photography, Fraprie and Miller, loc, 12, Am. 
 
 Photographic Pub. 
 Nature and the Camera, Dugmore, $1 .35, '02, Doubleday. 
 New Treatise on the Modem Methods of Carbon Printing, 
 
 Marton, $2.50, '12, Marton. 
 Oil and Bromoil Processes, Mortimer and Coulthurst, 500,' '08, 
 
 Tennant and Ward. 
 Photo-aquatint, or Gum Bichromate Process, Maskell and 
 
 Demachy, yoc, '01, Tennant and Ward. 
 Photographer's Handbook, Harrison and Douglass, $1.00, 08, 
 
 Lane. 
 Photographer's Note Book, Lambert, 50c, Tennant and Ward. 
 Photographic Chemicals and How to Make Them, Taylor, yoc, 
 
 '05, Tennant and Ward. 
 Photographing Flowers and Trees, McFarland, 50c, '11, Tennant 
 
 and Ward. 
 Photography, Hoppe and Others, $3. Jo, '11, Doubleday. 
 Photography for Bird Lovers, Beetham, $i.7j, '11, Scribner. 
 Photography for Students of Physics and Chemistry, Derr, $1 .40, 
 
 '06, Macmillan. 
 Photography, Its Principles and Applications, Watkins, $2.00, 
 
 '11, Van Nostrand. 
 Pictorial Landscape Photography, Anderson, $i.yo, '14, Photo 
 
 Era. 
 Platinotype Process, Warren, yoc, Tennant and Ward. 
 Platinum Printing, 2yc, 'i i, Tennant and Ward. 
 Saturday With My Camera, Johnson, $1.50, "14, Lippincott. 
 Sunlight and Shadow, Adams, $2.50, '12, Doubleday. 
 Unit Photography, Steadman, $2.00, '14, Van Nostrand. 
 Wellcome Photographic Exposure Record and Diary, 50c, 
 
 Burroughs, Wellcome and Co. 
 
INDEX 
 
 Page 
 
 Acetic acid 87 
 
 Acetylene 73 
 
 Acids ... .... .20 
 
 Adurol .... 75 
 
 Albumen no 
 
 Alcohol gi 
 
 Alkali 22 
 
 Amidol 75 
 
 Ammonium chloride 21, no 
 
 Ammonium dichromate 140 
 
 Ammonium hydroxide . . '9. 39 
 
 Ammonium persulphate reducer 106 
 
 Ammonium sulphocyanide qg 
 
 Apothecary weights 7 
 
 Arrowroot loq, 132 
 
 Auric chloride 114 
 
 Aurous chloride . . 114 
 
 Avoirdupois 7 
 
 Backed plates 60 
 
 Balance ... ... 7 
 
 Bases 22 
 
 Bennett's reducer 105 
 
 Benzene 72 
 
 Bleaches 86 
 
 Blueprint 130, 134 
 
 Bromine 49, J7 
 
 Bromoil 168 
 
 Calcium carbonate 114 
 
 Calcium chloride ... ... . . 142 
 
 Calcium hypochlorite 114 
 
 Carbon 10 
 
 Carbon dioxide 10, 28, 32 
 
 Carbon process 1 57 
 
 Carbon tissue 158 
 
 Celluloid 45, 5q 
 
 Cellulose log 
 
 Ceric sulphate reducer 105 
 
 Chemical change 11 
 
 Chemical reaction 11,22 
 
 Chlorides 16 
 
 Chlorine 17, 26, 30, 3q 
 
 200 
 
INDEX 20 1 
 
 Chrome alum 8q 
 
 Chromium intensifier loi 
 
 Chromium salts 153 
 
 Citric acid 87, 151 
 
 Collodion 1 10 
 
 Colloid 38, 153 
 
 Color sensitiveness, of emulsion yz 
 
 Color sensitiveness, of eye 52 
 
 Color sensitizing 53 
 
 Compound 14 
 
 Concentration 170 
 
 Constancy of composition 14 
 
 Constant density ratio .... 71 
 
 Copper sulphate q8, 125 
 
 Copper toning 124 
 
 Crystalloid 38 
 
 Cyanin 53 
 
 Definite proportions 14 
 
 Developer 62 
 
 Developing-out paper i iq 
 
 Development 61 
 
 Development of Carbon 159 
 
 Development of Gum 164 
 
 Dichromated gelatine . ... 1 57 
 
 Duratol 77 
 
 Elements 11 
 
 Emulsion 23, 46, 118 
 
 Energy change 22 
 
 Equation 14 
 
 Equivalents 19 
 
 Ether 159 
 
 Ethyl alcohol i y4 
 
 Exposure 54 
 
 Farmer's reducer loy 
 
 Ferric ammonium citrate ... ... 125, 130 
 
 Ferric ammonium oxalate 140 
 
 Ferric chloride 25 
 
 Ferric ferrocyanide 33 
 
 Ferric oxalate 3^, 34, '35 
 
 Ferric sulphate 33 
 
 Ferrous chloride 25 
 
 Ferrous ferricyanide 34 
 
 Ferrous oxalate 31, 3 J, 135, 136 
 
 Ferrous sulphate 33, 36, 43, 61 
 
 Fixing bath 80,81,117 
 
 Fixing process 80 
 
 Fog 6$ 
 
 Fractional normal formulas 172, 173, 174 
 
 Gaslight paper 120 
 
202 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Gelatine 37, 46, 50, 1J3 
 
 Gold chloride iij 
 
 Glass plate 45. JQ 
 
 Glycerine 145 
 
 Glycerine development 14 j 
 
 Gradation 51, 71 
 
 Grain, of emulsion 49 
 
 Gum arabic 38, i J3 
 
 Gum-bichromate process i6z 
 
 Halation 59, 7° 
 
 Hardening bath 8q 
 
 Heat energy 22, 27 
 
 Hydrazine 58 
 
 Hydriodic acid 31 
 
 Hydrochloric acid iQ. 39 
 
 Hydrogen 30 
 
 Hydrogen chloride 20, 3 1 
 
 Hydrogen dioxide 57 
 
 Hydrogen iodide 31 
 
 Hydrolysis 114 
 
 Hydrochinon 75 
 
 |[Hypo" 81 
 
 "Hypo' eliminator 90 
 
 Indicator 19 
 
 Intensification 93 
 
 Iodine 31, 50 
 
 Iron toning 127 
 
 Kallitype 147 
 
 Latent image 109 
 
 Latitude jy 
 
 Litmus paper ... 19 
 
 Magnesium 12, 23 
 
 Magnesium oxide 13, 2.3 
 
 Mercuric chloride 94 
 
 Mercurous chloride ... . . .... 95 
 
 Mercury 96 
 
 Mercury-ammonium chloride 96 
 
 Meta-compounds 75 
 
 Metathesis 23 
 
 Methyl alcohol 140 
 
 Metol 75 
 
 Metric system 169 
 
 Neutralization 22 
 
 Nitric acid 33 
 
 Nitrogen oxide 15 
 
 Normal solutions 170 
 
 Note book 6 
 
 Ortho-compounds 75 
 
 Ortol 75 
 
INDEX ao3 
 
 Overexposure 56, q3 
 
 Oxalic acid 140, 148 
 
 Oxidation 4, ^4, 32, 57 
 
 Oxidizer 24, 36 
 
 Oxygen 10, 15, 24, 31, 58 
 
 Ozobrome 167 
 
 Ozotype 166 
 
 Paper 45, log 
 
 Para-compounds 75 
 
 Percentage composition 14, 23 
 
 Percentage solutions 170 
 
 Permanganate reducer 104 
 
 Phenol 74 
 
 Phosphoric acid 1 50 
 
 Phosphorus . 28 
 
 Photographic image y 
 
 Physical change ... 10 
 
 Pigment 158, 162 
 
 Pinacyanol 53 
 
 Pinaverdol 53 
 
 Plain salted paper no 
 
 Platinic chloride 138 
 
 Platinotype 134 
 
 Platinum 134 
 
 Platinum recovery 138, 139 
 
 Platinum salts 117 
 
 Potassium alum 121 
 
 Potassium bromide 41, 46 
 
 Potassium chloroplatinite 136 
 
 Potassium chromium sulphate 8g 
 
 Potassium citrate 125 
 
 Potassium cyanide 80 
 
 Potassium dichromate 3?. 57, 1 54 
 
 Potassium disulphite 85. 87 
 
 Potassium ferricyanide 33, 125 
 
 Potassium ferrocyanide 33 
 
 Potassium iodide 41, 46 
 
 Potassium metabisulphite 8y 
 
 Potassitim oxalate 34, 136 
 
 Potassium permanganate 57, 83 
 
 Precipitate 16 
 
 Preparation of solutions 1 74 
 
 Preservative 86 
 
 Preserving platinum paper 142 
 
 Prussian blue 33, 131 
 
 Pyrogallic acid 62, 74, 77 
 
 Ray-filter 54 
 
 Reaction n, 2^ 
 
 Reaction, photo-chemical 27, 30, 32, 3q, 41 
 
204 CHEMISTRY FOR PHOTOGRAPHERS 
 
 Reaction, reversible 32., 4 1 
 
 Reducer 14. 57 
 
 Reduction > 24, 32, 61 
 
 "Reduction" (of negatives) 93, 103 
 
 Restrainer 68 
 
 Reversal y6 
 
 Ripening, of emulsion 48 
 
 Rochelle salt 1 50 
 
 Salt 22 
 
 Sawdust reduction (of platinum) 145 
 
 Scheele 3q 
 
 Selenium 2q 
 
 Selenium cell 29 
 
 Sensitizer 32, 36, 41 
 
 Sensitizing (platinotype) 140 
 
 Sepia 121 
 
 Silver 4, 14, 43, 97 
 
 Silver-ammonia chloride . . q6 
 
 Silver bromide 41,43,47,49,57,61 
 
 Silver chloride 17. 39. 43, n^ 
 
 Silver chromate 103 
 
 Silver iodide 41,43,47,49 
 
 Silver nitrate 14. 39. 46, 61 
 
 Silver-sodium thiosulphate 82, 97, 99 
 
 Silver sub-bromide 41.49. 57 
 
 Silver sub-chloride 40, 112 
 
 Silver sub-iodide 4^. 49 
 
 Silver sulphide 82. 121 
 
 Silver sulphite 64 
 
 Silver sulphocyanide 99 
 
 Sizing 133, 140 
 
 Sodium acetate 116, 150 
 
 Sodium carbonate 63 
 
 Sodium chloride 15 
 
 Sodium nitrate 18 
 
 Sodium potassium tartrate i yo 
 
 Sodium sulphide 1 22 
 
 Sodium sulphite 63 
 
 Sodium tetrathionate 97 
 
 Sodium thiosulphate 80, 87 
 
 Spectrum 51 
 
 Starch 109 
 
 Substitution products 73 
 
 Sulphide toning 121 
 
 Sulphur 28, 87, 88 
 
 Sulphur dioxide 86 
 
 Sulphuric acid 33 
 
 Sulphurous acid 86 
 
 Synthesis 23 
 
INDEX 205 
 
 Tartaric acid 86, 1 50 
 
 Toning iig, 122 
 
 Toning platinotype 145 
 
 Tumbufl's blue 34, 131 
 
 Under-exposure 5J. 93 
 
 Uranium intensifier 100 
 
 Uranium nitrate 3j, iiy 
 
 Uranium toning 126 
 
 Uranous ferrocyanide 35 
 
 Uranyl ferrocyanide . 35, 100 
 
 Vanadium toning 127 
 
 Washing, by decantation 16 
 
 Water, distilled 16 
 
 Weights 7