. geek Md Pee fa ‘een. 4 ~y Elementary _ er Photographic 1 Chemistry «| Eastman Kodak Company | Rochester, N. Y. | RESEARCH LIBRARY THE GETTY RESEARCH INSTITUTE JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION Elementary Photographic Chemistry Eastman Kodak Company Rochester, N. Y. 1924 THE INST CONTENTS Chapter. I. An Outline of Elementary Chemistry- - - II. The Chemistry of Photographic Materials- - III. The Chemistry of Development - - - - IV. The Chemistry of Fixation - - - - - V. The Chemistry of Toning - - ee VI. The Chemistry of Intensification and enue VII. The Chemistry of Washing - - - - - Mite eormilas «is so ee eee IX. Preparing Solutions TN RAY RU EE TO tial > X. Simple Chemical Tests - - - - - - TS al es a INTRODUCTION Photography is so essentially a chemical process that every photographer should have an interest in the chemicals which he uses and in the reactions which they undergo. This book is written in response to a demand for a simple account of photographic chemistry, for the practical photog- rapher. No attempt has been made to give the chemical theory in full, for which textbooks on chemistry should be consulted. In Chapter I, a statement is given only of the chemistry which is necessary to an understanding of the remainder of the book. In the same way reference should be made to photo- graphic textbooks for general photographic practice, as this book treats only of photographic chemistry and not practical photography. In order to give the information about photographic chem- icals which is necessary for their intelligent use, the properties of each of the more important chemicals are given in a separate paragraph which is inserted in the section dealing with its use but is printed in a smaller distinct type face to facilitate reference; an index to the chemicals dealt with is given at the end of the book. No apology is needed for the insistence placed on the need for pure chemicals and on the advantage to be gained by using the Eastman Tested Chemicals, which are specially purified and tested for photographic use. EASTMAN KODAK COMPANY, RocuHEsTER, N. Y. April, 1924 CHAPTER I. An Outline of Elementary Chemistry All substances are made by the combination in various pro- portions of a limited number of elements, of which about eighty exist. These elements combine in definite proportions to form bodies of fixed composition, which are termed com- pounds. ‘Thus, one volume of the gaseous element hydrogen combines with one volume of the gaseous element chlorine to form two volumes of the compound hydrochloric acid gas. This combination can be represented by what is called a chemical equation. Thus, if we write H for hydrogen, Cl for chlorine and H Cl for hydrochloric acid, we can represent the above combination by the equation H + Cl HCl Hydrogen Chlorine Hydrochloric Acid Gas It will be seen that an equation such as that given above is really a shorthand method of stating what happens, the ele- ments which take part in the combination being designated by letters. These letters which stand for the elements are called the “symbols” of the elements. The elements which are of the greatest importance in photography and their symbols are as follows: Gases Name. Symbol. Remarks. Hydrogen H_ The lightest gas known. Nitrogen N_ Forms 80% of the air. (Approx.) Oxygen O Forms 20% of the air. (Approx.) Chlorine Cl Greenish - yellow poisonous gas. Bromine Br Poisonous brownish-red gas at high temperatures, liquid at ordinary temperatures. 6 EASTMAN KODAK COMPANY Non-metallic Solids Name. Symbol. Remarks. Carbon C Occurs in three forms: dia- mond, graphite, and char- coal or amorphous carbon. Sulphur S Yellowish-white, brittle solid. Iodine I Violet plate-like crystals, sim- ilar in chemical properties to chlorine and bromine. Metallic Solids Sodium Na_ Very light, attacked by mois- ture, kept under light oil. Potassium K_ Very light, attacked by mois- ture, kept under light oil. Calcium Ca _ Silvery white metal, attacked by moisture. Aluminum Al Very light, white metal. Iron Fe In the pure state it is called ) wrought-iron; when con- taining a small amount of carbon it forms cast-iron and steel. Copper Cu Reddish, tough metal. Silver Ag. White metal. Platinum Pt Valuable white metal, very heavy. Gold Au Reddish yellow metal, very heavy. Mercury Hg White metallic liquid, very heavy * These elements fall into two groups; those which are metals and those which are not metals. Apart from the ap- pearance of the elements, the classification of an element in one of these two groups depends upon its relation to oxygen. Many of the elements when heated in the presence of oxygen will combine with it and will form what are called oxides. Thus, carbon will burn in oxygen and will form a gaseous compound of carbon with oxygen called carbon dioxide. Iron will burn in oxygen and forms a solid iron oxide. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 7 Oxides of Elements Name. Symbol. Remarks. Hydrogen oxide (water) HzO Can be made by burning hy- drogen in air or oxygen. Acid Oxides Carbon dioxide CO. A heavy gas, is produced by burning carbon; e. g., char- coal. Nitric oxide NO Colorless gas, turns reddish- brown in contact with oxygen. Sulphur dioxide SO. Colorless gas. Produced by burning sulphur. Basic Oxides Aluminum oxide AlzO; White powder which forms when aluminum is burned in the air. Calcium oxide CaO Quicklime, obtained by heat- ing chalk. Iron oxide Fe20; Red powder formed when iron rusts. Mercuric oxide HgO Red powder formed by slow heating of mercury in the air. Many oxides are soluble in water, forming two classes of compounds, which are known respectively as acids and bases, the acid oxides being produced from the non-metallic elements and the basic oxides from the metallic elements. Thus, car- bon, nitrogen and sulphur all form acid oxides which dissolve in water to form acids, while sodium, potassium and calcium form typical basic oxides which dissolve in water to form bases. Bases are either alkaline or earthy, the alkaline bases being soluble, the earthy bases insoluble. The ordinary way of distinguishing between an acid and a base is to test the solution with a trace of certain dyes which change color according to whether the solution is acid or alkaline. Thus, if a piece of paper soaked in a solution of litmus, generally known as litmus paper, is put into a solution, it will turn red if the solu- tion is acid, and blue if the solution is alkaline. Thus, sodium forms an oxide which dissolves in water and makes a solution 8 EASTMAN KODAK COMPANY. of basic caustic soda, the caustic soda having the formula NaOH, and being composed of sodium, oxygen and hydrogen. On the other hand, sulphur combines with oxygen and the oxide dissolves in water to form sulphurous acid, this having the formula H:SO3 and being formed by the combination of water, H.O, with sulphur dioxide, SOz. Thus: SO2 + HO = H2SOz3 Sulphur Dioxide Water Sulphurous Acid All acids contain hydrogen and this hydrogen can be re- placed by a metal, forming a compound which is termed a “‘salt.’? Thus, if we have sulphuric acid and we dissolve a piece of iron in it, the iron will replace the hydrogen of the acid, which will be given off as bubbles of gas and a solution of the salt, iron sulphate, will be formed: H2S04 + Fe = Fe SO.4 + He Sulphuric Acid Iron _ Iron Sulphate Hydrogen Gas Salts are also formed by the direct union of an acid and a base. Thus, if we have caustic soda, NaOH, and sulphurous acid, H:SO3, they combine to form sodium sulphite, eliminat- ing water. Thus: 2NaOH oo H2SO3 = NasSO3 oa 2H20 Two parts of Sulphurous Acid Sodium Sulphite Water Caustic Soda It will be seen that the sodium sulphite is formed by the com- bination of the base derived from sodium with the acid derived from sulphur. Sometimes a non-metallic element forms two different oxides, and these in turn will form two different acids. When we burn sulphur in oxygen, for instance, each atom of sulphur combines with two atoms of oxygen and forms sulphur di- oxide: S +20 = SO2 and this dissolves in water to form sulphurous acid. If the sulphur dioxide is passed, with more oxygen over heated platinum, it is possible to make it combine with another atom of oxygen and form the compound sulphur trioxide, SOs, and this dissolves in water and forms sulphuric acid: SO3 + H20 = H2S0.4 so that from sulphur we not only get sulphurous acid but a second acid—sulphuric acid. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 9 Just as the hydrogen of sulphurous acid is replaced by sodium to form sodium sulphite, so the hydrogen of sulphuric acid is replaced by sodium to form sodium sulphate. Sulphur Dioxide SO2 Sulphur Trioxide SO3 Sulphurous Acid H2S8O3 Sulphuric Acid H2S04 Sodium Sulphite Naz2SOz2 Sodium Sulphate Na2SO4 Salts are usually neutral to litmus paper, though sometimes they are somewhat acid or alkaline. But in addition to the neutral salts, an acid in which there are two hydrogen atoms can have one of them replaced by a metal instead of both, and in this case we get acid salts, which are equivalent in their behavior to a mixture of equal parts of the acid and the neutral salt. For instance, from sulphurous acid if we replace both the hydrogens, we get sodium sulphite—Na2SO;—but if we replace only one of the hydrogens, we get the compound NaHSOs, which is called sodium acid sulphite, sodium hydro- gen sulphite or, more usually, sodium bisulphite. Sulphur forms a number of different acids. It forms not only acids from its two oxides SO2 and SOs, but it forms com- pound acids containing more than one atom of sulphur, and of these one is a very great importance to the photographer, namely, thiosulphuric acid, which forms a sodium salt, sodium thiosulphate, NazS2.03. It will be seen that this compound differs from sodium sulphite in having two atoms of sulphur instead of one, and it is the compound, generally known as “hypo,” which is used for fixing photographic materials. Some acids are formed not from oxides but by the direct combination of a non-metallic element with hydrogen, and of these the most important are the strong acids formed from chlorine, bromine and iodine, which three elements, because they occur in sea salt, are called halogens, from the Greek name for the salt sea. Thus, chlorine combines directly with hydrogen to form hydrochloric acid, H Cl, and if the hydrogen of this is replaced by metals, we get chlorides, of which the best known is sodium chloride, Na Cl, which is common salt. Similarly, bromine combines with hydrogen to form hydro- bromic acid, with which metals form bromides, and in the same way the iodides are formed from iodine. 10 EASTMAN KODAK COMPANY. The Halogens, Their Acids and Salts Halogen Element Acid Sodium Salt Cl Chlorine H Cl Hydrochloric Acid Na Cl Sodium Chloride Br Bromide H Br Hydrobromic Acid Na Br Sodium Bromide I Iodine HI Hydriodie Acid NaI Sodium Iodide Salts are soluble in water to different extents, the solubil- ity depending upon the nature of the salt. Some, such as hypo, are extremely soluble, hypo being soluble in less than its own volume of water, while others are only slightly soluble or even almost completely insoluble, silver chloride, bromide and iodide being well known examples of very insoluble materials. A solu- tion of a salt may be regarded as containing both the basic and the acid components of the salt in a more or less free condition. For instance, all copper salts in solution behave in much the same way, showing properties in common, due to the presence of the copper. In the same way all chlorides or sulphates show common properties in solution. Now, when we mix two solutions of soluble salts, and the base of one can form an insoluble salt with the acid of the other, then this rearrangement will take place and the insoluble substance will be thrown out of solution as a precipitate. Thus, silver nitrate and sodium chloride are both very soluble in water, but when the solutions are mixed the silver and the sodium change places so that silver chloride and sodium nitrate are formed, and the almost insoluble silver chloride is thrown out of the solution, leaving only the sodium nitrate behind. Ag NOs + Na Cl = Ag Cl + Na NO3 Silver Nitrate Sodium Chloride Silver Chloride Sodium Nitrate Soluble — Soluble Insoluble Soluble Precipitated This ‘‘double decomposition” is the simplest kind of chemical reaction and is the one with which we are most familiar. Other types of chemical reaction which are of great im- portance in photography are those of oxidation and reduction. The simplest example of oxidation is, of course, that in which an element combines with oxygen; but when an element forms two or more compounds with oxygen, then we are said to per- form oxidation when we raise the element from the level of oxidation of one of its compounds to another level in which it is combined with more oxygen. For example, by the oxida- tion of sodium sulphite, NazSO3, which is a compound formed from sulphur dioxide, SO2, we get sodium sulphate Na2SOu,, ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 11 which is derived from sulphur trioxide, SO3. This can be done by means of oxygen. If we pass air, which contains 20% of oxygen (the rest being chiefly nitrogen), through a sulphite solution, or even leave sodium sulphite exposed to the air for long periods, it will be oxidized into sulphate— NaeSO3 + O = NaeSOu Sodium Sulphite Oxygen Sodium Sulphate When metallic elements form two oxides with different amounts of oxygen, these two oxides will act as bases for two series of salts. Thus, iron forms Ferrous salts derived from Fe O, and Ferric salts derived from Fe20s. Thus, we have Ferrous Chloride, Fe Cle, green crystals, Ferric Chloride, Fe Cl;, red-brown crystals. Very often oxidation is accomplished not by the use of oxygen itself but by the use of some substance which itself is a higher compound of oxygen and which can be reduced to a lower com- pound of oxygen or to an element which contains no oxygen at all. Thus, for instance, when hydroquinone’ is oxidized, we get quinone, which we call the oxidation product of hydro- quinone, but if we add sulphite to quinone, the quinone ox- idizes the sulphite to sulphate and is itself reduced again to hydroquinone. In this case the sulphite acts as a reducing agent, reduction being the opposite to oxidation. Thus, a body which is easily oxidized will take the oxygen it needs from other substances and so acts as a reducing agent. Hydro- quinone is oxidized to quinone, which is reduced by sulphite to hydroquinone (hydrochinon). Conversely, the sulphite is oxidized by the quinone to sulphate. Similarly, if we add ferric salts to hydroquinone, they will oxidize it to quinone and will themselves be reduced to ferrous salts. The term reduction is applied especially to the liberation of metallic elements from their compounds. Thus, if we heat mercuric oxide, the oxygen is driven off by the heat and the mercuric oxide is reduced to mercury. Generally, reduction cannot be accomplished by heat alone, and it is necessary to have some substance present which can be oxidized in order to ee ’ *Chemists in America and Great Britain spell hydrochinon as hydroquinone. The spelling used generally by photographers is hydrochinon. 12 EASTMAN KODAK COMPANY. reduce a compound. Thus, to reduce iron from its oxide, of which iron ore is chiefly composed, we heat it with charcoal or carbon, which is oxidized to form carbon dioxide and which reduces the iron oxide to metallic iron. Chemical compounds consist of five great classes: 1. ACIDS, which are formed from non-metallic elements and : which contain hydrogen replaceable by a metal; 2. BASES, which are formed from the metallic elements, and which, when soluble in water, are called alkalis; 3. SALTS, which are formed from the combination of an acid and a base; 4. OXIDIZERS, which are substances containing an excess of oxygen and which can give up this oxygen to another com- pound; 5. REDUCERS, which are greedy for oxygen and which take the oxygen away from any compound containing an avail- able supply of it. ELEMENTS METALS NON-METALS form Oxides with Oxygen with Oxygen “ form Oxides ith H Oxides with \ Oxides with water form form water form Acids ee Salts * ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 13 CHAPTER II. ‘The Chemistry of Photographic Materials The art of photography is founded upon the fact that the compounds of silver, and especially its compounds with chlo- rine, bromine and iodine, are sensitive to light. The earliest photographs were made by coating paper with silver chloride and using this to form images by its darkening under the action of light, but the sensitiveness of the silver chloride was too slight to use it in this way to form images in the camera. To get results which require less exposure to light, advan- tage is taken of the fact that it is not necessary for the light to do the whole work of forming the image; it is possible to expose the silver salt for only a short time to the light and then to continue the production of the image by chemical action, the process being termed ‘“‘development.”’ Sensitive photographic materials therefore consist of paper, film, or glass coated with a layer in which is suspended the sensitive silver bromide or silver chloride. This layer is called the emulszon. ‘This emulsion consists of a suspension of the silver salt in a solution of gelatin. It is made by soaking gelatin in water until it is swollen and then dissolving it by gently warming and stirring. The necessary bromide or chlo- ride, e. g., potassium bromide or sodium chloride, is then added to the solution and dissolves in it. Meanwhile; the right amount of silver nitrate to react with the amount of salts used has been weighed out and is dissolved in water. The silver nitrate solution is then added slowly to the solution of gelatin and salt and produces in it a precipitate of the silver compound, the mixing being done in the dark-room, since the silver com- pound produced is sensitive to light. IfsiHere were no gelatin in the solution the silver compound would settle down to the bottom and an emulsion would not be formed, but the gelatin prevents the settling so that as the silver nitrate is added a little at a time evenly precipitated silver salt is uniformly distributed through the solution. If this emulsion is coated on a support, such as paper or film and then cooled, the gelatin will set.,to nd when the jelly is dried we get a smooth coating of lsion of the sensitive silver compound. @ 14 EASTMAN KODAK COMPANY. Photographic materials which are to be developed must contain no excess of soluble silver and the emulsion must be made so that there is always an excess of bromide or chloride, since any excess of soluble silver will produce a heavy fog over the whole of the surface as soon as the material is placed in the developer. In the case of Solio paper, however, which is not used for development but which is printed out, a chloride emulsion is made with an excess of silver nitrate. This causes rapid darkening in the light, so that prints are made upon Solio paper and not developed, the visible image being toned and fixed. Solio paper can be developed with certain precautions, such as the use of acid developers or after treatment with bromide to remove the excess of silver nitrate. In the early days of photography prints were usually made on printing-out papers, but at the present time most prints are made by the use of developing-out chloride and bromide papers, which are chemically of the same nature as the negative making materials and are coated with emulsions containing no free silver nitrate. Negative making materials such as plates and films, always contain silver bromide with a small addition of silver iodide. The different degrees of sensitiveness are obtained by the tem- perature and theduration,of heat which the emulsions undergo during manufacture, the most sensitive emulsiac ns s being heated to higher temperatures and for a longer t the emulsions. If a slow nai is coatec “rial is wn as bromide paper and is used for pri noting especial fo making enlarge me Bis. The less sensi which commonly used for eo: @ light contain silver chloride in t In‘order to obtain silver nitrate the firs vie a is to dissoly metallic silver in nitric acid. The silver replaces the hye of the acid and fo ilver nitrate, th composing a furdesportion of the nitrate is crystallizéd out of the solution a less, transparent plates SILVER NITRATE for photographie has to be extremely pure, and since metallic silver usually cont a small quantity of other metals, such as copper and lead, it is necess these impurities. This is accomplished by recr ion, so that the silver nitrate is finally obtained in a perfectl m. ind obtain in color- O free it from * ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 15 In order to ensure the purity of the silver nitrate which it uses, the Eastman Kodak Company prepares its own and is the largest maker of silver nitrate in the world, using about one-twenty-fifth of all the silver mined in the United States, the mint being the only larger consumer. Silver nitrate is very soluble in water. It attacks organic material, and blackens skin, wood, cloth, and other similar substances on expo- sure to light. When a solution of silver nitrate is added to a solution of a bromide or chloride of another element, a reaction occurs and the insoluble silver bromide or chloride is precipitated. Thus, if we add silver nitrate to potassium bromide, the re- action occurs according to the following equation: Ag NOs +e K Br ss Ag Br 4. KNOs Silver Nitrate Potassium Bromide Silver Bromide Potassium Nitrate The potassium nitrate formed remains in solution, but if the solution is at all concentrated, the silver bromide is thrown down to the bottom of the vessel as a thick, curdy precipitate. The bromides and chlorides used in photography are chiefly the salts of potassium and sodium. Both the bromides and the chlorides are obtained from naturally occurring salt deposits. but, whereas these deposits consist chiefly of chlorides, they contain only a very small quantity of bromide, and bromide is - therefore a very much more expensive material than chloride. The elements chlorine, bromine and iodine are ah atural salt or from the sea, iodine being der dform. Chlorine is‘a yellow- and poisonous, bromine gives more noxious than chlorine and ne forms shining, black crystalline x give a violet vapor. The chief and iodides used in é otography are the AMMONIUM CHLORIDE: Made from ammonia and hydro- chloric acid, should have no smell, and when evaporated by heat should leave no residue behind. White crystals soluble in water. AMMONIUM BROMIDE: Very similar to the chloride, which is the only impurity ly to be present. [UM IODIDE: Should consist of colorless crystals. ht and is stained yellow by the iodine liberated. rater and deliquescent (see p. 24). Soluble in alcohol. r 7 | 16 EASTMAN KODAK COMPANY. SODIUM CHLORIDE: Ordinary table salt is fairly pure sodium chloride and a very pure salt is easily obtained. The pure salt is stable and not deliquescent. Soluble in cold water to the extent of 35%. Solubility increases very little on heating. SODIUM BROMIDE: Is a white salt, similar to the chloride but more soluble. Is generally pure but may contain chloride. POTASSIUM CHLORIDE: White salt, very similar to sodium chloride. POTASSIUM BROMIDE: Occurs as colorless cubical crystals and is generally pure. Very soluble in water. POTASSIUM IODIDE: Similar to bromide. Very soluble. May contain as impurities carbonate, sulphate and iodate, but is usually pure. A solution of potassium iodide dissolves iodine, which is in- soluble in water, and is therefore used to prepare a solution of iodine. The gelatin which is used to emulsify the sensitive silver salts is a very complex substance which is obtained from the bones and skins of animals, and it has some curious and valu- able properties. In cold water it does not dissolve but it swells as if, instead of the gelatin dissolving in the water, the water dissolves in the gelatin. If the water is heated, the gelatin will dissolve, and it will dissolve to any extent. It cannot be said that there is a definite solubility of gelatin in water in the same sense as salts may be considered to have a definite solubility. As more gelatin is added, the solution becomes thicker. If the gelatin solution is heated, it will become thinner and less viscous when hot, and will thicken again as it cools, but it will not recover completely. It will remain thinner than if it had not been heated, so that the heating of the gela- tin solution produces a permanent change in its pro rties. If a gelatin solution is cooled, t the solution in a dry state process is continued long enough, the set and will remain as a thick liquid. Gelatin belongs to the class of subs colloids, the name being derived from a Greek word meaning “oummy.’ When a gelatin jelly is dried it shrinks down and forms a horny or glassy layer of the gelatin itself, smooth and rather brittle. This dry gelatin, when placed in water, will at once absorb the water and swell up again to form a jelly. This swelling of gelatin when wet, and shrinking when dry, is of great importance in photography. When a photo- ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 17 graphic material with an emulsion made with gelatin is placed in water, the film will swell up and will continue to absorb more water and swell for a long time, finally becoming soft and even dissolving, the extent to which this occurs depending on the temperature and the nature of the solutions in which it is placed. A small amount of either an acid or alkali will produce _a considerable increase in the swelling, and since the developer is alkaline and the fixing bath is acid, both these solutions have a great tendency to swell the gelatin, especially when they are warm. In order to avoid difficulty from this course, gelatin emulsions have a hardener added before they are coated, gelatin being hardened and made more resistant to swelling by the addition of alum. Under ordinary circumstances no difficulty is experienced by the photographer due to the soften- ing of the gelatin, but when photographic materials are ex- posed to extreme temperatures, care must be taken in handling them. Hardening agents such as alum must be added to the fixing bath, and all solutions must be kept at the same tempera- ture in order to avoid sudden contractions or expansions of the gelatin which may result in detaching the film from its sup- port or in the production of reticulation, 1. e., a coarse wrinkling all over the film. 18 EASTMAN KODAK COMPANY. CHAPTER IIL. The Chemistry of Development When a light sensitive material is exposed for a short time to light, although the change which takes place may be so minute that it cannot be detected by any ordinary means, if the exposed material is placed in a chemical solution, which is termed the ‘‘developer,”’ the chlorine or bromine is taken away from the silver, and the black metallic silver which remains behind forms the image. This image is, of course, made up of grains, because the original emulsion contains the silver bromide in the form of microscopic crystals, and when the bromide is taken away from each of these, the crystal breaks up and a tiny coke-like mass of metallic silver remains behind in exactly the same position as the bromide crystal from which it was formed, so that, whereas the original emulsion consisted of microscopic crystalline grains of the sensitive silver salt, the final image consists of equally microscopic grains of black metallic silver. This removal of the bromide from the metallic silver is known chemically as reduction. (It must be remem- bered that chemical reduction has nothing to do with the photo- graphic operation known as the reducing of a negative, that is, the weakening of an over-dense negative, where the word simply refers to the removal of the silver and is not used in the chemical sense.) Chemical reducers are substances which have an affinity for oxygen and which can liberate the metals from their salts, such as the charcoal which, as explained in Chapter I, is used to reduce iron from its ore. A developing solution is therefore one which contains a chemical reducer. All substances which are easily oxidized are, however, not developers, since in order that a reducer may be used as the photographic developer it is necessary that it should be able to reduce exposed silver bromide but should not affect unexposed silver bromide, so that its affinity for oxygen must be within certain narrow bounds; it must be a sufficiently strong reducer to reduce the exposed silver salt, and at the same time must not affect that which has not been exposed. For practical purposes the developing agents are limited to a very few substances, almost all of which are chemically derived from benzene, the light oil which is dis- tilled from coal tar. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 19 The commonest developing agents are pyrogallol (pyro), hydrochinon, Elon, para-aminophenol or Kodelon, and dia- minophenol. PYROGALLOL (or pyrogallic acid) is made from gallic acid, which is obtained from gall nuts imported from China. The gall nuts are fermented to obtain the gallic acid, and the gallic acid is then heated in a still from which the pyrogallol is distilled over. Before the war most of the pyrogallol used in this country was made in Europe, but the shortage was met by the erection of a plant by the Eastman Kodak Company which to-day makes all the pyrogallol needed for its customers. Pyrogallol is made in two forms: a flaky powder form and a crystal form. When the powdered pyrogallol is opened in the dark-room or studio, the fine particles fly about and are likely to settle on paper or plates, producing spots on the photographs. For this reason the Eastman Kodak Company supplies pyrogallol in the crystal form, which can be handled without any danger of par- ticles flying about and giving trouble. HYDROCHINON is made from benzene which is first converted into aniline and then oxidized. It is now made in several places in the United States, as well as by the Eastman Kodak Company. Although it is somewhat less powerful as a reducing agent than pyro, it gives no stain and when used in conjunction with Elon or Kodelon it is a very useful developer, in fact, it is a constituent of a majority of the better known commercial developers in use today. It keeps very well when used in tank developers because it does not oxidize as readily as pyro and is generally used in motion picture work. Its purity is very 1m- portant and Eastman Tested Hydrochinon may be relied upon for use in all formulas. Some time after pyrogallic acid and hydrochinon were in general use by photographers, there were introduced a number of new developing agents made from coal tar, which are very useful as supplements to the older developers. Several of these are based on a substance called para-aminophenol, which is made in the manufacture of dyes. When para-aminophenol is treated with methyl alcohol the methyl part of the alcohol attaches itself to it and forms a compound called methyl- para-aminophenol, which is a more active developing agent than the para-aminophenol itself. Another developing agent of the same type is diaminophenol, and is prepared in a way similar to para-aminophenol. Para-aminophenol, methyl-para-aminophenol and diamino- phenol are all bases and the developing agents are their salts, the oxalate of para-aminophenol, the hydrochloride of dia- minophenol being used, and the sulphate of methyl-para- aminophenol. PARA-AMINOPHENOL (OXALATE) is manufactured by the Eastman Kodak Company under the name of Kodelon. Many of 20 EASTMAN KODAK COMPANY. the so-called “‘new’’ developing agents on the market consist entirely or mainly of para-aminophenol. A good sample should be light in color and should burn entirely when heated to redness, leaving no ash behind. MONOMETHYL PARA-AMINOPHENOL SULPHATE is manu- factured and sold by the Eastman Kodak Company under the name of Elon. Monomethy] para-aminophenol sulphate is distinguished sharply from para-aminophenol (oxalate) by the fact that it is soluble in the cold in its own weight of strong hydrochloric acid, whereas the para- aminophenol (oxalate) is’ insoluble. DIAMINOPHENOL HYDROCHLORIDE is sold by the Eastman Kodak Company under the trade name of Acrol. It is a steel gray powder, darkening easily in the air and is oxidized so rapidly in solu- tion that it is usual to dissolve it only when required for use. Different reducing agents behave differently as develop- ers. We cannot use Elon in the place of hydrochinon and get the same effect. An image developed with Elon comes up very quickly all over the plate and gains density slowly, while the hydrochinon image comes up very slowly but gains density steadily and rapidly. A very little change in the temperature ~ affects hydrochinon a good deal and affects Elon very little, and in the same way a small amount of sodium or potassium bro- mide affects hydrochinon and does not affect Elon nearly so much. These differences in the developing agents depend upon the chemical nature of the substances themselves, and the particular property to which these differences are due is called the “reduction potential” of the developer. The reduction potential alone does not determine the speed with which the developer develops the image, because this depends chiefly upon the rate at which the developer dif- fuses into the film and on the amount of developing agent and other substances in the developer. A high reduction potential enables a developer to continue to develop more nearly at a normal rate under adverse circumstances, such as low tempera- ture or the presence of bromide. The reduction potential of a developer, in fact, may be compared to the horse-power of an automobile which for other reasons than the power of its engine is limited in speed. If we have two automobiles and they are confined to a maximum speed of twenty miles an hour, then on level roads the one with the more powerful en- gine may be no faster than that with a weaker engine, but in a high wind or on a more hilly road the more powerful engine will allow the automobile to keep its speed, while the machine with the weaker engine will be forced to go more slowly. We could, indeed, measure the horse-power of an automobile by the ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 21 maximum grade which it could climb at a uniform speed of 20 miles an hour. In development, the analogy to the hill is the addition of bromide to the developer, since the addition of bromide greatly retards development, and it is found that the higher the reduc- tion potential of a developer, the more bromide is required to produce a given effect. If we measure the developing agents in this way, we shall find that hydrochinon has the lowest re- duction potential, then pyro, then Kodelon, and finally Elon, which has the highest. Hydrochinon has so low a potential that it is rarely used alone but is generally used with Elon. Kodelon can be substituted for Elon but more Kodelon has to be used in order to produce a developer of the same strength. Developers with a high reduction potential such as Elon, and to a less extent Kodelon, make the image flash up all over at once, because they start development very quickly even in the lesser exposed portions of the emulsion, while developers of low reduction potential, like pyro and especially hydrochinon, bring up the highlights of the image first and the shadows do not fully appear until the highlights are somewhat developed. Most developing agents cannot develop at all when used by themselves. With the exception of Acrol, developing agents, in order to do their work, must be in an alkaline solution, and the energy depends upon the amount of alkali present. The developers of higher reduction potential, which bring up the image very quickly, require less alkali than those of lower re- duction potential. For instance, hydrochinon is often used with caustic alkalis, while the other developing agents require © only the weaker carbonated alkali. The amount of alkali governs the energy of a developer, and if too much alkali is present, the developer will tend to pro- duce chemical fog, while if too little alkali is present, it will be slow in its action. Alkalis also soften the gelatin of the emulsion, and consequently too alkaline a developer will pro- duce over-swelling and will give trouble with frilling or blisters ‘in warm weather. The alkalis used in development are of two kinds: the caustic alkalis and the carbonated alkalis. Caustic alkalis are produced when the metal itself reacts with water, the metals from which the alkalis generally used are derived being potassium and sodium. These metals are so easily oxidized that they have to be preserved from all contact with air or water by immersion in light oil or gasoline. 22 EASTMAN KODAK COMPANY. If we take a small piece of sodium and place it on the sur- face of water in a dish, it will react with the water with great violence, melting with the heat produced and sputtering about the surface; while if we restrict its movement, the development of heat will be so great that the hydrogen produced will burst into flame. In the case of potassium, the reaction is even more violent than with sodium and is always accompanied by flame. The reaction may be represented by the equation— Na + H20 = NaOH oe H Sodium Water Caustic Soda Hydrogen the sodium combining with the water to form caustic soda and liberating hydrogen, which comes off as gas, and, as has already been stated, catches fire and burns in the air. This is, of course, not the method by which the alkalis are actually pro- duced. As a matter of fact, the metals are produced by elec- troplating the metal out from the melted alkali. CAUSTIC SODA is made either by the passage of an electric current through a solution of common salt, when the soda separates at one electrode and chlorine gas is liberated at the other, or from sodium carbonate, which is causticized by means of lime. Lime is calcium oxide and is prepared by heating limestone, which is calclum carbonate, the carbon dioxide being driven off from the limestone by the heat. When the lime is added to sodium carbonate, the lime removes the carbon dioxide from the carbonate, and leaves the sodium hydrate in the solution, which is then evaporated to get the solid sub- stance. At present, caustic soda is easily obtained in a very pure state, and there is usually no difficulty in getting good caustic soda for photo- graphic work. It must be protected from the air, since it easily absorbs moisture and carbon dioxide. As its name indicates, it is very caustic and attacks the skin, clothing, etc. CAUSTIC POTASH is very similar to caustic soda and is pre- pared in the same way. Fifty-six parts of caustic potash are chemi- cally equivalent to forty parts of caustic soda. An alkali which was often used with pyrogallol in the early days of photography, but which is rarely used nowadays, is ammonia. Nitrogen combines with three times its volume of hydrogen to form a gas, NH3. This gas is known as ammonia and is very soluble in water, its solution being strongly alkaline. Ammonia combines directly with acids to form salts which are analogous to the salts of sodium and potassium. Thus with hydrochloric acid it forms ammonium chloride, which is similar to sodium chloride and potassium chloride: NHs = HCl = NH.Cl Ammonia Hydrochloric Acid Ammonium Chloride ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 23 Ammonia is a somewhat weaker alkali than soda or potash but stronger than the carbonates. For use in development it has the disadvantage that being used in the form of a solution of a gas its strength is somewhat uncertain and variable, the am- monia escaping from the solution. Also, it is a solvent of silver bromide and tends to produce colored fogs which are not so easily produced with other alkalis. AMMONIA SOLUTION is commercially prepared from the am- moniacal liquor obtained in the distillation of coal for coal gas. The liquor is neutralized with sulphuric acid, the ammonium sulphate crystallized out, and the ammonia gas liberated from the sulphate with lime and led into water, in which it dissolves. The solution is usually free from impurities. Ammonia solutions are prepared Ee aa in two strengths “ammonia water,’”’ containing 10% of ammonia gas by weight bad having a specific ‘gravity of .96, and ‘‘stronger ammonia water’ con- taining 28% of ammonia by weight and having a specific gravity of .90. The alkalis generally used for photographic work are not the caustic alkalis but the carbonates, which are salts of car- bonic acid, H2CO3. Carbonic acid is a very weak acid, so that in solution the carbonates are not neutral but alkaline because of the predominance of the strong base over the weak acid, the carbonate being, to some extent, split up into the bicarbonate or acid carbonate and the caustic alkali. The use of a carbon- ate in development therefore represents a sort of reservoir of alkali, only a small amount of alkali being present at any time, but more being generated by dissociation of the carbonate as itis used up. If instead of using carbonate we were to use for development a solution containing a proportional amount of caustic alkali, we should have only a small amount of alkali present, and it would soon be exhausted. The use of carbonate, therefore, enables us to employ a small concentration of alkali and yet to keep that concentration nearly constant during use. When a salt is dissolved in water at a high temperature un- til no more will dissolve and then the solution is allowed to cool, the salt will generally be deposited in crystals; sometimes, as in the case of silver nitrate, the crystals consist of the pure substance, but more often each part of the salt combines with one or more parts of water to form the crystals. This com- bined water is called “‘water of crystallization.”’ Thus, crystals of sodium carbonate formed from a cool solution contain ten parts of water to one of aroon ales and their composition should be written: NazCO2 e 10H20 24 EASTMAN KODAK COMPANY. What is called in the last paragraph a “‘part”’ of sodium car- bonate, NazCOs, will weigh 106 units, while a ‘‘part’”’ of water, H.0, weighs 18 units, so that the crystals of sodium carbonate contain 106 parts by weight of sodium carbonate and 180 parts by weight of water, and consequently crystallized sodium car- bonate contains only 37% of dry sodium carbonate. If sodium carbonate is crystallized from a hot solution only one part of water is combined in the crystals with each part of sodium carbonate so that they have the composition NazCO; . H2O and contain 85% of dry carbonate. Sodium carbonate con- taining ten parts of water of crystallization loses nine of them by drying in the air and breaks up, forming the compound with one part of water. This last part of water is only removed with difficulty by heating in the air, when the dry carbonate is formed, containing only a small residual amount of water and about 98% carbonate. When exposed to the air chemicals often either absorb or give up water. Those which absorb water are said to be “hy- groscopic,”’ and if they absorb so much that they dissolve and form a solution they are said to be ‘‘deliquescent.’”’ Chemicals which give up water to the air, so that the crystals break down and become covered with powder, are called “efflorescent.” SODIUM CARBONATE comes on the market in three forms: Crystals with ten parts of water, NazCO3. 10H2O containing 37% of the carbonate; crystals with one part of water, NazCO3.H2O0, con- taining 85% of the carbonate, and the dry powder containing 98% of the carbonate. The carbonate is made by treatment of salt solution with ammonia and carbon dioxide which reacts with the salt to produce sodium bicarbonate, NaHCO3. The bicarbonate is heated and half of the carbonic acid is driven off, producing crude sodium carbonate, which at this stage is known as “‘soda ash.” This is then dissolved in water, and crystals of ‘‘sal soda,”’ containing ten parts of water, are pro- duced. From this a crystalline salt with either one or ten parts of water is prepared for photographic use, but owing to the uncertainty of the composition of these crystals it is better to prepare the pure dry carbonate. This is obtained by heating the pure bicarbonate which can be precipitated from a solution of sal soda by means of ear- bon dioxide gas. When the bicarbonate is heated in the air, half of the carbonic acid is driven off, and sodium carbonate, NazCOs, is pro- duced according to the equation: SNaHCOs ot see 4. as Paired Sodium Bicarbonate Sodium Carbonate Carbon Dioxide Water The exact amount of heating is very important. If it is not done for sufficient time there will be a large amount of bicarbonate left in the product, and bicarbonate is practically useless as an alkali in photog- raphy. On the other hand, if heating is continued too long, caustic ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 25 soda will be produced. In the preparation of photographic carbonate the heating should be continued so that the material is almost pure sodium carbonate containing practically no bicarbonate but is very slightly on the alkaline side. Much caustic soda would be fatal, but it is better to have a trace of caustic soda than bicarbonate. The prepar- ation of carbonate of soda is a matter to which the greatest attention is given by the Eastman Kodak Company, and the E. K. Tested Car- bonate is specially prepared to meet the needs of the photographer. POTASSIUM CARBONATE is sometimes substituted for sodium carbonate in developer formulas. Although it is more soluble and is a somewhat stronger alkali than sodium carbonate, it has the dis- advantages of being more expensive and absorbs water very readily. It must, therefore, be kept in well-sealed bottles. Owing to the fact that developers are necessarily substances which have a great affinity for oxygen and that the air contains oxygen, developing solutions containing only the developing agent and alkali would be rapidly spoiled from oxidation by the air. In order to make the developer keep there is added to the developing solution, in addition to the reducing agent and alkali, some sulphite of soda. Sulphite of soda has a very strong affinity for oxygen, being easily oxidized to sulphate of soda (see page 11), so that it protects the developer from the oxygen of the air, thus acting as a “‘preservative.”’ This action of the sulphite is very easily seen with the pyrogallol developer. The oxidation product of pyrogallol is yellow, and this oxida- tion product which is formed in development is deposited in the film along with the silver, so that if we use a pyrogallol devel- oper without sulphite we shall get a very yellow negative, the image consisting partly of silver and partly of the oxidized pyrogallol. If we use sulphite in the developer, the image will be much less yellow because the pyrogallol will be prevented from oxidizing, the sulphite being oxidized instead, and finally if we add a great deal of sulphite, we shall get almost as blue an image as with Elon, the oxidation product of which is not deposited in a colored form with the silver. SODIUM SULPHITE is prepared by blowing sulphur dioxide as into a solution of carbonate of soda. When sulphite is crystalized rom the cooled solution it forms crystals containing seven parts of water to one of sulphite, of the composition NazSO3 . 7H20 which contain, when pure, 50% of dry sulphite. These crystals give up water when kept in the air and form a white powder on the surface. Since sulphite, when exposed to the air, has a tendency to oxidize to the sulphate, and as the sulphate is not a preservative, it is well to view with suspicion sulphite which has effloresced to a great extent. A quick rinse in cold water will remove the white powder from the crystals. Sulphite free from water is produced by two methods: by drying the crystals, which produces what is called the “‘desiccated”’ salt, 26 EASTMAN KODAK COMPANY. containing about 92% of pure sulphite, and by precipitation from hot solutions which gives a compound generally called ‘‘anhydrous” sulphite, and which contains as much as 96.5% of sulphite. _ Eastman Tested Sulphite is the desiccated salt, and is prepared in a very pure state almost free from sulphate. If If prepared in this aay as a dry powder the sulphite will keep well for a long time. Seis forms a number of compounds with sulphurous acid in addition to sodium sulphite itself. Thus we have sodium acid sulphite or bisulphite, NaHSO:, which may be regarded as a compound of sodium sulphite with sulphurous acid: Na2SO3 +. H2SO3 = 2N aHSOs3 Sodium Sulphite Sulphurous Acid Bisulphite of Soda Again we have sodium metabisulphite, which is a compound of sodium sulphite with sulphur dioxide: Naz2S205 Na2SOz + SO2 Sodium Sulphite Sulphur Dioxide Saat Metabisulphite These acid sulphites are very similar in their properties and probably form the same solution when dissolved in water. POTASSIUM METABISULPHITE is often used as a preserv- ative. It forms good crystals and is convenient in use but is very costly in comparison with sodium bisulphite. SODIUM BISULPHITH, when pure, is a white salt which has an acid reaction, often containing a slight excess of sulphur dioxide. Since sodium sulphite is an alkaline salt, owing to the predomi- nance of the strong base, soda, over the weak sulphurous acid, a neu- tral solution can be produced by adding a small amount of bisulphite to sulphite, and this neutral solution has found extensive application as a preservative for a pyro developer. Bisulphite is used very largely as a preservative for fixing baths, supplying both the sulphite and the acid necessary. It is difficult to prepare bisulphite free from iron, and any iron in the bisulphite produces a dark color when used for making up a pyro solution. The Eastman Kodak Company has an entirely satisfactory bisulphite and lists it among its Tested Chemicals. It is often customary to substitute sodium bisulphite for potassium metabisulphite weight for weight. It really sim- mers down to a matter of dollars and cents because either chemical is quite satisfactory for the purpose but, as a rule, sodium bisulphite ranges in cost from 4 to \% that of potas- sium metabisulphite. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 27 Since sodium bisulphite may be considered as a compound of sodium sulphite and sulphurous acid, while sodium sulphite is alkaline, bisulphite is preferable as a preservative in the case of a two-solution developer, since oxidation progresses less readily in acid than in alkaline solution. In the case of a one-solution developer containing, say, sodium sulphite, sodium bisulphite and sodium carbonate, the bisulphite is converted to sulphite by the sodium carbonate according to the following equation: Sodium Bisulphite + Sodium Carbonate = Sodium Sulphite + Sodium Bicarbonate so that a corresponding amount of sodium sulphite might just as well have been added in the first place. Sodium bisulphite also neutralizes or destroys an equivalent amount of sodium carbonate, thus reducing the proportion of alkali and therefore exerts an apparent restraining action, while the developer ap- parently keeps longer because some of the carbonate has been destroyed. 28 EASTMAN KODAK COMPANY. CHAPTER IV. The Chemistry of Fixation After development, the undeveloped silver bromide is re- moved by immersion of the negative or print in what is called the “‘fixing’”’ bath. There are only a few substances which will dissolve silver bromide, and the one which is universally used in modern photography is sodium thiosulphate, NazS2Os, which is known to photographers as hyposulphite of soda, or more usually as hypo, though the name hyposulphite of soda is used by chemists for another substance. THIOSULPHATE OF SODA or HYPO can be made by boiling together sodium sulphite and sulphur, the sulphur combining with the sodium sulphite according to the equation Naz2SO3 7 S = Na2S203 Sodium Sulphite Sulphur Hypo In practice it is generally made from calcium sulphite residues, the calcium thiosulphate being then converted into the sodium salt by treatment with sodium sulphate. The hypo comes on the market in clear crystals and is usually fairly pure, any foreign substance present being more often due to accidental contamination than of a chemical nature and consisting of dirt, straw or wood dust due to careless handling. Sometimes, however, the hypo contains calcium thiosulphate, which decomposes much more readily than the sodium salt. On the whole, it is not difficult to obtain good hypo; the East- man Tested Hypo is prepared in the form of granular crystals, easy to dissolve, and free from accidental contamination. In the process of fixation the silver bromide is dissolved in the hypo by combining with it to form a compound sodium silver thiosulphate. Two of these compound thiosulphates exists, one of them being almost insoluble in water, while the other is very soluble. As long as the fixing bath has any ap- preciable fixing power the soluble compound only is formed. Fixing is accomplished by means of hypo only, but mate- rials are usually transferred from the developer to the fixing bath with very little rinsing so that a good deal of developer is carried over into the fixing bath, and this soon oxidizes in the bath, turning it brown, and staining negatives or prints. In order to avoid this the bath has sulphite of soda added to it as a preservative against oxidation, and the preservative action is, of course, greater if the bath is kept in a slightly acid state. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 29 In order to prevent the gelatin from swelling and softening it is also usual to add some hardening agent to the fixing bath so that a fixing bath instead of containing only hypo will con- tain in addition sulphite, acid, and hardener. Now, if a few drops of acid, such as sulphuric or hydro- chloric acid are added to a weak solution of hypo, the hypo will be decomposed and the solution will become milky, owing to the precipitation of sulphur. This is because the acid con- verts the sodium thiosulphate into the free thiosulphuric acid, and this substance is quite unstable, decomposing into sul- phurous acid and sulphur according to the equation: H28203 = H2S8O3 + S Thiosulphuric Acid Sulphurous Acid Sulphur The change of thiosulphate into sulphite and sulphur is rever- sible, since, if we boil together sulphite and sulphur we shall get thiosulphate formed, so that while acids free sulphur from the hypo, sulphite combines with the sulphur to form hypo again. Consequently, we can prevent acid decomposing the hypo if we have enough sulphite present, since the sulphite works in the opposite direction to the acid. An acid fixing bath, therefore, is preserved from decomposition by the sulphite, which also serves to prevent the oxidation of developer carried over into it. The developer which is carried over into the fixing bath is, however, alkaline and consequently a consider- able amount of acid is required in a fixing bath which is used for any length of time, since if only a small amount is present, it will soon be neutralized by the developer carried over. We are, therefore, in the difficult position that we require a large amount of acid present, and yet the fixing bath must not be strongly acid. The solution of the difficulty is found by taking advantage of the fact that there are some acids which are very weak in their acidity and yet can neutralize alkali in the same way as a strong acid, so that a large amount of these acids can be added without making the bath so acid that sulphur is pre- cipitated. The strength of an acid depends upon the fact that when it is dissolved in water some of the hydrogen contained in it dissociates from the acid and remains in the solution in an active form, and the acidity of the solution depends upon the proportion of the hydrogen which is dissociated into this active form. The amount of alkali which the acid can neutralize, however, depends upon the total amount of the hydrogen pres- ent, and not on the dissociated portions only. The strongest 30 EASTMAN KODAK COMPANY. acids are the mineral acids, such as sulphuric and hydrochloric while the weakest acids are the organic acids, such as citric and acetic acids. Since a large amount of a weak acid is required, the best acid for the purpose is acetic acid. ACETIC ACID is prepared by the fermentation of apple juice, yielding a product commonly called vinegar. In addition to acetic acid, vinegar also contains many impurities and the acid strength is from 4% to 8%. The stronger acid is made from acetate of lime which is prepared either by neutralizing vinegar with chalk or, more commonly, by neutralizing with lime the crude acetic acid prepared by the destructive distillation of wood. Acid thus prepared may contain as high as 99.5% acetic acid and is usually called glacial acetic acid because, at moderately low temperatures, it freezes to a solid. Dilutions of the glacial acid are commonly supplied contain- ing 80% and 28% acetic acid. This 28% acetic acid, prepared by diluting the pure glacial acetic acid, must not be confused with “commercial 28% acetic acid”’ which is prepared by redistilling the acid obtained by the destructive distillation of wood and contains many impurities which have a decidedly deleterious effect on photo- graphic materials. When acetic cannot be obtained for the fixing bath, the only substitute which appears to be generally available is sodium bisulphite. Bisulphite of soda, NaHSOs, is intermediate between sulphite of soda and sulphurous acid, and is, therefore, equal in acidity to a mixture of equal proportions of these two substances. It makes a satisfactory acid fixing bath but does not give quite as good a reserve of available acid in the bath as acetic acid does. This is of importance particularly in connec- tion with the hardening agent used in the fixing bath. The commonest hardening agent is potash alum, the alums having the property of tanning gelatin. ALUM is a compound sulphate of sodium, potassium or am- monium with aluminum. If the hydrogen in sulphuric acid be re- placed by potassium, we get potassium sulphate, K2SO., while if it be replaced by aluminum, we get aluminum sulphate, Ale (SOx)s. The aluminum sulphate combines with other sulphates to form the alums, of which the commonest are potassium alum and ammonium alum. Sodium alum does not crystallize well, but the potassium and ammonium salts crystallize in large, clear crystals, and are convenient in use. POTASSIUM CHROME ALUM, which is often used in the place of ordinary alum, does not contain any aluminum in spite of its name. It is a compound sulphate of potassium sulphate with chromium sulphate, of which the formula is Cre(SO.)3, the chromium taking the place of the aluminum present in aluminum sulphate. Chrome alum is prepared commercially in large quantities and of ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 31 a high degree of purity. It occurs as violet crystals soluble in water, its solution in cold water being violet but going green on heating owing to the change in the composition of the salt. AMMONIUM CHROME ALUM is chemically very similar to potassium chrome alum except that it is the compound of ammo- nium sulphate with chromium sulphate. Despite its very close chem- ical similarity to the potassium compound, it is found that it has a decided tendency to produce fog, even though the fixing bath is {maintained acid. Just why this is has not been satisfactorily ex- plained. However, this warning is given because at times, due to price fluctuations, the used ammonium alum is sometimes slightly advantageous from the point of view of dollars and cents, but it is always safer to use a potassium salt even though it may cost one or , two cents a pound more. In the presence of sodium sulphite a solution of chrome alum loses its hardening properties somewhat rapidly, depend- ing upon the concentration of the chrome alum and the sodium sulphite. A fresh chrome alum fixing bath containing hypo, chrome alum, and sodium bisulphite loses its hardening prop- erties in the course of one or two days even if the bath is not used. A chrome alum fixing bath containing from 1 to 2% chrome alum is only useful when used immediately after preparation, although a bath containing from 5 to 10 per cent chrome alum will maintain its hardening properties for two or three days. Chrome alum is most useful as a hardening bath between developing and fixing. A plain solution of chrome alum retains its hardening properties indefinitely, though with use when developer is carried over by the plates and films, the hardening properties of the bath fall off owing to the presence of sodium sulphite in the developer. FORMALIN is a solution of formaldehyde, a gas having a very strong odor. The commercial solution contains 40% of formalde- hyde and has the property of hardening gelatin very powerfully, a 5% solution rendering the gelatin of a film completely insoluble in boiling water in less than a minute. It should be used only in alkaline or neutral solutions because in acid solutions it does not harden. Formalin, however, irritates the mucous membrane of the nose and throat and is very objectionable to certain individuals. It is important not to overwork a fixing bath, because as the fixing bath becomes saturated with silver the film or paper will carry this silver into the wash water with it and if not properly washed the silver salt will remain in the finished pho- tograph and will decompose into silver sulphide in time, pro- ducing stains. A gallon of the standard strength fixing bath will fix one hundred 8 x 10 prints, and when these have been fixed the bath should be changed. 32 EASTMAN KODAK COMPANY. CHAPTER V. The Chemistry of Toning The operation of toning consists in the deposition on the silver image of another substance having a different color, in order to get a more pleasing result, or of the transformation of the silver image into another substance for the same purpose. There are four principal methods of toning: A. Toning by the replacement of the silver by other metals; B. Toning by the deposition of salts of metals; C. Toning by the transformation of the silver image into some substance to which dyes will attach themselves in an insoluble form; D. Transformation of the silver image into a stable, strongly colored salt of silver. A. In the case of prints which are made by the printing- out processes, the silver compound produced by the action of light is colored, and after fixation the image left is usually of an unpleasant color,—a yellow or yellow-brown—and in order to change this to a more satisfactory color it is toned by means of gold or, more rarely, platinum. When a silver image is placed in a solution of gold or platinum the silver will replace the metal in solution, going into solution itself, and the gold or platinum will be deposited in the place of the silver. The rate at which these metals are deposited is very important, especially in the case of gold toning. If the gold is deposited too slowly, it will be deposited in a very fine condition, and in the case of finely divided metals, their color depends upon the fineness of the division. Finely divided gold is red, which is not as pleasing as the blue gold obtained by more rapid deposition. In order to ensure rapid deposition it is necessary that the bath should be kept alkaline, and consequently borax or sodium acetate is added to the gold chloride to make a toning bath, while sometimes substances having a weak reducing action are added, such as sulphocyanides or formates. Platinum toning baths are used in an acid condition. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 33 The chemicals used for making up these toning baths must be of high purity, and it is best to get tested chemicals in all cases. GOLD CHLORIDE is made by dissolving gold in a mixture of hydrochloric and nitric acids and evaporating the solution. It forms brownish crystals, rapidly absorbing water, which contain 65% metallic gold. The salt is sold in small glass tubes containing 15 grains, and in order to use it, the label is removed from the tube and the tube is broken in a bottle containing a known amount of water so that a solution of definite strength is obtained without danger of losing the precious material. GOLD SODIUM CHLORIDE is a double chloride of gold and sodium which occurs in yellow crystals and contains 49% of metallic gold. It has the advantage over the pure chloride of gold that itis neither acid nor deliquescent. POTASSIUM CHLOROPLATINITE is the double chloride of platinum and potassium, and is the form in which platinum is used for a toning bath. It occurs in reddish crystals, and is supplied in sealed glass tubes like gold chloride. LEAD NITRATE and LEAD ACETATE. These colorless salts of lead are sometimes used for toning baths. They are both soluble in water and the solutions are very poisonous. SODIUM ACETATE, SODIUM PHOSPHATE and BORAX are all weak alkalis and are used in gold toning baths for this reason. They occur as white salts, soluble in water. Borax occurs as a mineral and is largely used in industry. Only the pure salt should be used for photographic purposes. AMMONIUM SULPHOCYANATE, SULPHOCYANIDE or THIOCYANATE, is a salt occurring in very deliquescent crystals. In order to be at all certain of its strength it must be preserved with great care, out of contact with the air. It is one of the most popular salts for use with gold chloride in toning baths. B. A good many metallic compounds are colored, and if the silver image is replaced by these colored compounds, wholly or in part, a colored image is obtained. In most of the toning processes based upon the use of colored compounds, ferro- cyanides of metals are employed, the silver image being first transformed into silver ferrocyanide, the silver in the silver ferrocyanide being then substituted by another metal of which the ferrocyanide is colored. The ferro- and ferricyanides are very complex compounds. The cyanides themselves are compounds containing carbon and nitrogen, and have a curious resemblance to chlorides and bromides. Hydrogen unites with carbon and nitrogen to form an acid, HCN, which is called hydrocyanic acid, and which is known popularly as prussic acid. The hydrogen in this can be substituted by metals to form cyanides such as potassium 34 EASTMAN KODAK COMPANY. cyanide, KCN, which is analogous to potassium chloride, KCl, or potassium bromide, KBr, and on adding a solution of silver nitrate to a soluble cyanide, silver cyanide, AgCN, is precip- itated as an insoluble salt, just as silver chloride or silver bromide is precipitated. There is one respect, however, in which hydrocyanic acid and the cyanides differ from the corresponding chlorine or bromine compounds, and this is that they are extremely poi- sonous. A trace of cyanide swallowed will cause death. Cyanide solutions are solvents for the silver halides, form- ing soluble double compounds with the insoluble silver salts. Potassium cyanide is employed for fixing wet collodion plates, which, being made from silver iodide, are not easily fixed in hypo. Whenever cyanides are used by photographers, their extremely poisonous nature should be remembered and every possible care taken in keeping and using them. The cyanides easily form complicated double compounds. With sulphur, for instance, they form sulphocyanides, and ammonium sulphocyanide has already been referred to as being used in gold toning baths. The cyanides unite with iron cyanides to form two important groups of compounds called ferrocyanides and ferricyanides. These differ from each other in their degree of oxidation, the ferricyanides being more highly oxidized than the ferrocyanides, so that when a ferri- cyanide is reduced a ferrocyanide is formed. POTASSIUM FERROCYANIDE is yellow. It is known as ve ey prussiate of potash” and has very_little application in photog- raphy. POTASSIUM FERRICYANIDE or RED PRUSSIATE OF POTASH is prepared by passing chlorine gas into a solution of the ferrocyanide and is deposited from concentrated solution as red crystals. The crystals are soluble in water to a yellow solution which does not keep well. The value of ferricyanide in photography lies in the fact that ferricyanide oxidizes the silver image and forms silver ferrocyanide from it, so that if a negative is placed in a solu- tion of ferricyanide, it is slowly bleached to silver ferrocyanide. This property can be made use of in various ways. The silver ferrocyanide is soluble in hypo so that if we use a solu- tion of potassium ferricyanide and hypo instead of plain potassium ferricyanide, we shall not get a white image produced but the silver image will be slowly dissolved, since it will be converted into the silver ferrocyanide by the ferricyanide and ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 35 then the silver compound formed will be dissolved in the hypo. This mixture of ferricyanide and hypo is known as Farmer’s Reducer, and will be referred to in the next chapter. Again, if we add bromide to our ferricyanide solution, silver bromide is more insoluble than silver ferrocyanide and consequently the silver ferrocyanide as it is produced will be transformed into silver bromide. This operation of transforming a silver image into a bromide image is generally known as bleaching. If we combine with the potassium ferricyanide a salt of a metal which gives an insoluble colored ferrocyanide, then we shall get the silver ferrocyanide formed, and this will be converted into the ferrocyanide of the metal whose salt has been added to the bath. If we add an iron salt, such for instance as iron citrate, to the potassium ferricyanide, we shall get a blue iron ferrocyanide formed and the image will be toned blue. If we use uranium nitrate, we shall get the reddish brown uranium ferrocyanide, while if we use copper citrate, we shall get the red copper ferrocyanide. Sometimes instead of using the metal salt in the same bath as the ferricyanide the operation is done in two steps, the silver being first bleached to silver ferro- cyanide, and this being then combined with a salt of the metal to form the colored metallic ferrocyanide. C. The range of colors which can be obtained by the use of colored metals or metallic compounds is rather limited, and in order to get a wider range, especially for motion picture and lantern slide work, experimenters have tried to find methods of using dyes and attaching them to the image. It has been found that this can be done by transforming the silver image into silver iodide, which can be accomplished, for instance, by treatment of the image with a mixture of potassium ferricyanide and potassium iodide. The silver iodide image formed in this way will mordant basic dyes and attach them to the image so that the image assumes the color of the dye. The Eastman Kodak Company has recently worked out a new process in which instead of transforming the silver image into silver iodide it is treated with a copper toning bath and transformed into copper ferrocyanide, and then the basic dyes are mordanted on to the copper ferro- cyanide image. The copper ferrocyanide is red; but very little is required to mordant a good deal of dye and the image is very transparent, so that this new process makes it possible to obtain very good results by dye toning. Full particulars of this process are given in our booklets on lantern slide making and on the toning of motion picture film. 36 KASTMAN KODAK COMPANY. D. Silver sulphide is a very insoluble compound of silver, and consequently if a silver image or a silver halide salt is treated with sulphur or a sulphide respectively they will at once be transformed into silver sulphide. Silver sulphide has a color varying from light brown to black, according to its state of subdivision, and the transformation of the image into silver sulphide is by far the most popular method of toning developing-out paper prints, the prints so toned being generally known as “‘sepia’”’ prints. There are two general methods of transforming the image into silver sulphide: A. Direct toning, with the hypo alum bath; and B. Bleaching and redevelopment. A. As was explained in the chapter dealing with fixing, when an acid is added to a solution of hypo, it tends to precip- itate sulphur. Now, asolution of alum in water is weakly acid, so that if alum is added to plain hypo without any sulphite present, the solution will, after a time, become turbid and pre- cipitate sulphur. This solution of alum and hypo at the point where it is ready to precipitate the sulphur may be considered as having free sulphur in solution, and if prints are immersed in a hot solution of alum and hypo, the silver image will be converted directly into silver sulphide and the prints will be toned brown. Only one precaution is necessary in order to obtain successful results with the hypo-alum toning bath. The bath tends to dissolve the image and consequently if a fresh bath is used, it will weaken the print, eating out the highlights. In order to prevent this a little silver must be added to the bath either in the form of silver nitrate or by toning a number of waste prints or by throwing in old Solio prints, which contain free silver. A bath lasts for a long time, and as a general rule a hypo-alum bath which has been somewhat used works better than a fresh bath. B. The greatest objection to the hypo-alum bath is that the bath has a somewhat disagreeable odor, sulphur compounds being liberated from it, and it is rather troublesome to use a bath which has to be heated, so that while hypo-alum toning is used on the large scale, smaller quantities of prints are com- monly toned by bleaching the silver bromide print in a bath of ferricyanide and bromide, and then treating the bleached print after washing, with sodium sulphide, which converts the silver bromide directly into silver sulphide. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 37 SODIUM SULPHIDE occurs in white, transparent crystals, which have a strong affinity for water and quickly deliquesce unless kept carefully protected from the air. It is best kept in a strong stock solution. So much trouble has been caused by impure sodium sulphide that of recent years the Eastman Kodak Company has been supplying a sulphide which they have fused so that it will contain no moisture and be of definite purity. One part, by weight, of the fused sulphide is equivalent to three parts, by weight, (approx.) of the crystals. Sodium sulphide often contains impurities, chiefly iron, though by dissolving in hot water the iron sulphide quickly separates out as a black sludge, leaving a clear solution which should be decanted. Old sodium sulphide often contains hypo, since hypo is produced in the oxidation of sulphide, and if hypo is present in any considerable amount, some of the silver bromide will be dissolved by it and the sais will lose strength in the highlights and give a very inferior result. All sulphides give off a certain amount of hydrogen sul- phide, which smells offensively, and which is extremely dan- gerous to photographic materials, since a very small amount of hydrogen sulphide will convert enough of the silver bro- mide or chloride of the material into sulphide to produce a severe fog. No photographic materials should therefore be stored in a room where sulphides are kept or where sulphide toning is done. It has already been explained that the color of silver sul- phide depends upon its state of division, and since the state of division of the toned image depends upon that of the untoned image and this again upon the treatment of the material, it is evident that the exposure and development of the print will have an effect upon the result obtained. As a general rule, it may be stated that to get good colors in sulphide toning it is necessary that a print should have been fully developed and not over-exposed; a print which is very fully exposed and then developed for a short time will not give a good tone. 38 EASTMAN KODAK COMPANY. The Chemistry of Reduction and Intensification REDUCTION. By reduction in photography is meant the removal of some silver from the image so as to produce a less intense image. Thus, in the case of an over-developed plate there will be too much density and contrast, and the negative may be reduced to lessen this. In the case of an over-exposed negative there may not be an excess of contrast but the negative will be too dense all over, and in this case what is required is the removal of the excess density. It is unfortunate that the word “reduction” is used in Eng- lish for this process. In other languages the word ‘‘weakening”’ is used, and this is undoubtedly a better word, because the chemical action involved in the removal of silver from a nega- tive is oxidation, and the use of the word reduction leads to confusion with true chemical reduction, such as occurs in de- velopment. All the photographic reducers are oxidizing agents, and almost any strong oxidizing agent will act as a photographic reducer and will remove silver, but various oxidizing agents behave differently in respect to the highlights and shadows of is image. Reducing solutions can be classified in three classes: a. Cutting reducers b. True scale reducers c. Flattening reducers. A. The cutting reducers remove an equal amount of sil- ver from all parts of the image and consequently remove a larger proportion of the image from the shadows than from the highlights of the negative. The typical cutting reducer is that known as Farmer’s Reducer, consisting of a mixture of potassium ferricyanide and hypo, the potassium ferricyanide oxidizing the silver to silver ferrocyanide and the hypo dis- solving the latter compound. Farmer’s Reducer will not keep when mixed, decomposing rapidly, so that it is usually made by making a strong solution of the ferricyanide and then add- ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 39 ing a few drops of this to a hypo solution when the reducer is required. It is especially useful for clearing negatives or lan- tern slides which show slight fog, and is also used for local reduction, the solution being applied with a brush or a wad of absorbent cotton. Another cutting reducer is permanganate. The perman- ganates are very strong oxidizing agents, and if a solution of permanganate containing sulphuric acid is applied to a nega- tive, it will oxidize the silver to silver sulphate, which is sufficiently soluble in water to be dissolved. Permanganate has only a very weak action on a negative if no acid is present and this may be made use of for the removal of ‘‘dichroic”’ fog, the yellow or pink stain sometimes produced in development. Dichroic fog consists of very finely divided silver and this is attacked by a solution of plain per- manganate which will have no appreciable action on the silver . of the image. An important difference between the behavior of ferricyan- ide and permanganate when used for reducing pyro-developed negatives should be noted. In a negative developed with pyro the image consists partly of the oxidation product of the pyro associated with the silver. (See p. 25). When such a negative is reduced with ferricyanide the silver is removed but the stain is unattacked so that the negative appears to become yellower during reduction, though the ferricyanide does not really pro- duce the color, only making it evident by removal of the silver. Permanganate, on the other hand, attacks the stain image in preference to the silver and consequently makes the negative less yellow. Permanganate can also be used as an alternative to ferricyanide for bleaching negatives, since if bromide be added to the solution silver bromide will be formed and the same bleaching action obtained as with ferricyanide. POTASSIUM PERMANGANATE occurs in dark purple crystals which dissolve to form a purple solution. It is easily ob- tained pure but there is a good deal of impure permanganate on the market; Eastman Tested Permanganate is a pure product. In addition to its use for reduction and bleaching, perman- ganate is employed as a test for hypo, since it is at once reduced by hypo, and the colored solution of the permanganate, there- fore, loses its color in the presence of any hypo. It may con- sequently be used to test the elimination of hypo from negatives or prints in washing. When permanganate is reduced in the absence of an excess of free acid, a brownish precipitate of 40 EASTMAN KODAK COMPANY. manganese dioxide is obtained and sometimes negatives or prints which have been treated with permanganate are stained brown by this material. Fortunately, manganese dioxide is removed by bisulphite, which reduces it still further, forming a soluble manganese salt. The brown stain can, therefore, be removed by immersion of the stained material in a solution of bisulphite. A very powerful cutting reducer is made from a solution of iodine in potassium iodide, to which potassium cyanide has been added to dissolve the silver iodide formed during reduc- tion. Iodine is not soluble in water but is soluble in a solution of potassium iodide, and to make up the reducer a few iodine crystals are dissolved in a 10% solution of potassium iodide, and five parts of this are added to one part of a 10% solution of potassium cyanide, making up to 100 parts with water for use. B. Proportional reducers are those which act on all parts of the negative in proportion to the amount of silver present there; hence they exactly undo the action of development, since during development the density of all parts of the nega- tive increases proportionally. A correctly exposed but over- developed negative should be reduced with a proportional re- ducer. Unfortunately, there are no single substances which form exactly proportional reducers, but by mixing perman- ganate, which is a slightly cutting reducer, with persulphate, which is a flattening reducer, a proportional reducer may be obtained. See formula (R-5) p. 50. C. In order to have a flattening reducer, we require one which acts very much more on the heavy deposits than on the light deposits of the negative, and which will consequently reduce the highlights without affecting the detail in the shad- ows. Only one such reducer is known, and this is ammonium persulphate. Ammonium persulphate is a powerful oxidizing agent and attacks the silver of the negative, transforming it into silver sulphate, which dissolves in the solution. It must be used in an acid solution and is somewhat uncertain in its behavior, occasionally refusing to act, and always acting more rapidly as the reduction progresses. AMMONIUM PERSULPHATE is a white crystalline ne stable when dry. It has been found in the Research Laboratory of the Eastman Kodak Company that the action of persulphate depends largely upon its containing a very small amount of iron salt as an impurity, and that its capricious behavior is due to variations in the amount of iron present. The persulphate supplied as an Kastman Tested Chemical may be relied upon to give a uni- form action in reduction. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 41 INTENSIFICATION. Intensification is photographically the opposite of reduc- tion, the object being to increase contrast. This is done by the deposition of some other material on the silver image. A silver image, for instance, can be very much intensified by toning it with uranium (see page 35), the reddish-brown uranium ferrocyanide having very great printing strength and convert- ing a weak negative into one having a great effective contrast for printing purposes. Usually, however, intensification is performed by depositing silver or mercury upon the image, and most photographic intensifiers depend upon the use of mercury. : Mercury is a metal which forms two series of salts, the mercuric salts, which are in a higher degree of oxidation, and the mercurous salts. Many of the mercuric salts are insoluble in water, but mer- curic chloride is sufficiently soluble for practical use, and when a silver image is placed in a solution of mercuric chloride, this reacts with the silver and forms a mixture of mercurous chlo- ride and silver chloride. The bleached image, which appears white, can then be treated in various ways. If it is developed, for instance, both the silver chloride and the mercurous chloride will be reduced to the metal, and in addition to the silver, with which we started, we shall have added to every part of silver an equal part of mercury. Instead of using a developer we may blacken the image with ammonia, which forms a black mercury ammo- nium chloride and produces a high degree of intensification. MERCURY BICHLORIDE is a virulently poisonous salt, known popularly as ‘‘corrosive sublimate.”’ Its only use in photog- raphy is for intensification, and it is obtained in white, heavy crystals which are soluble with some difficulty in water. This may be obtained in satisfactory purity by ordering Eastman Tested Mercury Bichloride. For many purposes separate bleaching and redevelopment is inconvenient, and for this reason the EHastman Intensifier has been placed on the market, this consisting of a mercury solution in which the intensification proceeds continuously so that it can be stopped at any time. This does not give quite so great an intensification as the use of the two solutions, but it is far more convenient in operation. A very powerful method of intensification, used chiefly for negatives made by photo-engravers, is obtained by bleaching with mercuric chloride and blackening with silver dissolved in 42 EASTMAN KODAK COMPANY. potassium cyanide. The use of the cyanide cuts the shadows very slightly at the same time that the highlights are intensi- fied, so that a great increase in the contrast of the negative is obtained. This is usually known as the ‘‘Monckhoven” Inten- sifier. The only other intensifier which calls for notice here is the chromium intensifier. The silver image is bleached with a solu- tion of bichromate containing a very little hydrochloric acid, bichromate being an oxidizer of the same type as permanganate or ferricyanide. The image is then redeveloped and will be found to be intensified to an appreciable extent. This inten- sifier has found increasing favor owing to the ease and cer- ‘tainty of its operation. POTASSIUM BICHROMATE is made by the oxidation of chromium salts. It forms orange-red crystals, stable in air, and is easily soluble to a yellow solution. It is obtained in a pure form by crystallization. Potassium bichromate is used in photography both for bleaching negatives and for sensitizing gelatin, fish glue, etc. When gelatin containing bichromate is exposed to light it becomes insoluble in water and in this way images may be obtained in in- soluble gelatin. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 43 CHAPTER VII. The Chemistry of Washing It may seem strange that a chapter dealing with washing should be inserted in a book on photographic chemistry, because washing is not usually regarded as a chemical operation. Nevertheless, the laws governing washing are distinctly chem- ical in their nature, and the importance of washing in photog- raphy justifies greater attention than is usually paid to the subject. As a general rule the object in washing negatives or prints is to remove from them the chemicals of the fixing bath which they contain. In the first place, it must be pointed out that it should not be necessary to wash out silver compounds but only the chemicals of the fixing bath. If an exhausted fixing bath is used silver compounds will be present during washing and must be very completely removed, so that if work has to be hurried and the time of washing must be cut down, it is most important that fixing should be complete. The best way of ensuring complete fixing is to use two fixing baths, and to transfer the negatives or prints to the sec- ond bath after they have been fixed in the first. Then, when the first bath begins to show signs of exhaustion and refuses to fix quickly, it should be replaced by the second, and the new, clean fixing bath should be used in the place of the second bath again. This procedure ensures that no material can be removed from the fixing bath until the first insoluble compound of silver and hyposulphite has been converted into the second soluble compound. It must be remembered that this first insoluble compound is invisible and that if a negative is transferred to the washing tank as soon as it is visibly clear, some of the in- soluble silver hypo compound will remain in the negative when itis dry. If the negative is transferred to a second fixing bath instead of the washing tank, this compound will be entirely removed and the task of washing will be much simplified. The rate of washing depends entirely upon the diffusion of the hypo out of the film into the water. This diffusion rate has nothing to do with solubility. The solubility of a substance fixes the proportion of the substance which can go into solution. 44 EASTMAN KODAK COMPANY. There are a number of errors which are current concerning washing. It is commonly believed, for instance, that plates and paper can be washed more rapidly in warm water than in cold. This is a mistake. It is true that any salt will diffuse more rapidly in warm water than in cold, but when washing a photo- graphic material the diffusion has to take place in gelatin, and the warmer the water in which the gelatin is placed, the more it swells, and its swelling hinders diffusion in about the same proportion as the rise in temperature accelerates it, so that, as a matter of fact, washing goes on at about the same rate at all ordinary temperatures. It is sometimes stated that material which has been hard- ened in the fixing bath washes more slowly than material which has not been hardened. This, too, is incorrect. Gelatin is like a sponge; the effect of hardening it is to contract all the network of the sponge, but in so doing the gelatin as a whole is not contracted and there is no difference in the diffusion be- tween gelatin, which has not been hardened and which has been hardened, unless the gelatin has been dried after harden- ing. If a negative is thoroughly hardened in the fixing bath and then is dried down, it will not expand much when soaked again and consequently diffusion through it will be difficult, but be- fore drying the hardening does not affect diffusion and the materials which wash most quickly are those in which the gela- tin has not been swollen in its treatment, either in development or fixation, but has been kept in a firm, solid condition. The actual rate of washing may be understood by remem- bering that the amount of hypo remaining in the gelatin is continually halved in the same period of time as the washing proceeds. An average negative, for instance, will give up half its hypo in two minutes, so that at the end of two minutes half the hypo will be remaining in it, after four minutes one-quarter, after six minutes one-eighth, after eight minutes one-sixteenth, ten minutes one-thirty-second, and so on. It will be seen that in a short time the amount of hypo remaining will be infinitesi- mal. This, however, assumes that the negative is continually exposed to fresh water, which is the most important matter in arranging the washing of either negatives or prints. If a lot of prints are put in a tray and water allowed to splash on the top of the tray, it is very easy for the water on the top to run off again, and for the prints at the bottom to lie soaking in a pool of fairly strong hypo solution, which is much heavier than water and which will fall to the bottom of the ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 45 tray. If the object is to get the quickest washing, washing tanks should be arranged so that the water is continuously and completely changed and the prints or negatives are subjected to a continuous current of fresh water. If water is of value, and it is desired to economize in its use, then by far the most effective way of washing is to use successive changes of small quantities of water, putting the prints first in one tray, leaving them there for from two minutes to five minutes, and then transferring them to an entirely fresh lot of water, repeating this until they are washed. The progress of the washing can be followed by adding a lit- tle permanganate solution to the wash water after the prints are taken out of it in order to see how much hypo is left in it, the presence of hypo being seen by decoloration of the perman- ganate. An even simpler test is to taste the prints. Six changes of five minutes each should be sufficient to eliminate the hypo effectively from any ordinary material. 46 EASTMAN KODAK COMPANY. CHAPTER VIII. Formulae It is always best to use the formule for solutions recom- mended in the instructions issued by the maker for the use of photographic materials; these formule are often adjusted to the properties of the particular materials concerned and will give better and more certain results than can be obtained with any other formule. It is often convenient, however, to have available standard formule, and the following formule are therefore given: DEVELOPING FORMULAE FOR FILMS AND PLATES. STANDARD A. B. C. Pyro. (Formula D-1) Stock Solution A. Avoirdupois Sodium Bisulphite or Potassium Metonieuiente - 140 grains Pyro- - - ~ - 2 OZS8. Potassium Bromide: mel atl a 16 grains Water to make SL Ee ee 32 OZS. Stock Solution B. Water I RE ee PR 32 OZS. Sodium Sulphite (E. K. Co.) - = i mS tome Stock Solution C. Water - Sh ic) ss 32 ozs. Sodium Cartan ste (B, K. Co. ) - = = = -« 24 ons, Dissolve chemicals in order given. For Tray Development— Take 1 part of A, 1 part of B, 1 part of C, and 7 parts of water. For Tank Development— Take 1 part of A, 1 part of B, 1 part of C, and 25 parts of water. Two SoLuTION Pyro TRAY DEVELOPER. (Formula D-21) Stock Solution A. Sodium Fasulpbive or Potable Metabayinnas - 140 grains Pyro- - - - 2 ozs. Potassium Bromide. SM i er 16 grains Water to make — = oe i 2 32 ozs. Stock Solution B. Water ~ wm ait ie sts a 32 ozs. Sodium Sulphite (E. K. Co.) - - - - - 34 ozs. Sodium Carbonate (E. K. Co.) - - 2% ozs. For use, take 1 part of A, 1 part of B, aad 8 parts of water. ELEMENTARY PHOTOGRAPHIC CHEMISTRY. 47 (Elon-Hydrochinon) TANK OR TRAY DEVELOPER. (Formula D-61A) Stock Solution. Hot Water Spout ani Bey Vine i Uy Gute ileey ace i |e 16 ozs. Elon - Se yy eS 45 grains Sodium Sulphite (B. K. Go. ‘i ere one OE Be 3 02S. Sodium Bisulphite- - - - - - - - 30 grains Hydrochinon -— - Se ee rm ete md sk 85 grains Sodium Carbonate (E. K. Co. pe ee OR) ai ead? ALG eraing Potassium Bromide - ssh Moe! Bae WY om 24 grains Cold watertomake - - - - - - - 32 ozs. For tray, use 1 part of stock solution to 1 part of water. For tank, use 1 part of stock solution to 3 parts of water. (Elon-Hydrochinon-Pyro) Amateur Finishers (Formula D-18) Hot Water (about ) meh cae bie ay a a aie 1 gallon lon- - - - - = = 100 grains Sodium Sulphite (E. K. Co.) - = =: = = 12) ozs. Sodium Bisulphite - - - - - - - 1650 grains Hydrochinon -— - a ee ye) nae 1 oz Sodium Carbonate oe K. Co. ) - - - = = 6% ozs Pyro- - = 8 eo eed = Droge Pan Bromide. wim! te Hew ee ot 60 grains Cold water tomake - - - - - = = 10 gallons Process TRAY DEVELOPER. (Formula D-9) (Hydrochinon-Caustic) Water - a ees ee ee 16 ozs Sodium Bieniohite Set ta eee i 34 OZ. A. 4 Hydrochinon Rem alia myn, aime th a 34 OZ. Potassium Bromide - - - - - = -« 34 OZ Watertomake - - - - - - = = 32 ozs B. fCold Water -— - a 32 OZS. Sodium Hydroxide (Caustic Soda) - - - = 1% ozs. Use equal parts of A and B and develop for three minutes at a temperature of 65° Fahrenheit. Process TANK OR TRAY DEVELOPER. (Formula D-11) ee aes Hot water (about 125° F.) - - - - - 64 ozs. Elon - = - eee ea as 60 grains Sodium Sulphite (E. K. Co. ) eh Eee ee fy ae ad iae 10 ozs. Hydrochinon -~ - - - - Ii ozs. Potassium Carbonate (or Sodium Carbonate) - - 8 ozs. Potassium Bromide - - - 34 OZ. Cold water to make - - ~ - - 1 gallon Used at 65° Fahrenheit, in Stier tank or tray this developer will give very good contrast in five minutes. The developer 48 EASTMAN KODAK COMPANY. is recommended for use with Process plates or films and with Process Panchromatic Plates. When less contrast is desired, the developer should be diluted with an equal volume of water. X-Ray DEVELOPER. (Formula D-19) (Elon-Hydrochinon) Hot water (about 125° EF.) =2F Sia Ve