Thomas T. Hill1 995 Highland Avenue Rochester, New York
Laboratory-Scale Photographic Emulsion Technique
Photographic emulsions are used for so many technical and scientific purposes that even the great variety commercially available will not serve all needs. Since simple instructions on the preparation of such emulsions are not easily found in the literature, the following discussion and references are offered to the teacher, student, and experimenter, who, with some practice, will he able to obtain fairly repeatable results. A photographic emulsion is a suspension of lightsensitive silver halide crystals in a matrix of gelatin, coated upon an inert, stable support. Some of the many variables which affect its properties will he discussed; only experience will bring home to each experimenter the necessity for extreme purity of raw materials and extremely close adherence to instructions. Since the silver halides are insoluble in water, they are prepared in situ, by reacting soluble silver nitrate and alkali halides in the presence of gelatin as a protective colloid. Then a complex series of treatments determine the exact photographic properties, before the mixture is coated in a thin layer upon a support and dried. The effects of these treatments were studied in detail by a group a t the Bureau of Standards some forty years ago, and subsequently discussed in a series of papers by Carroll and his associates (5a-d); this data will not be repeated here. Little has been published since. Typical Emulsion Formula
The following formula is typical for a simple emulsion for use on a photographic paper support. Stelgmann General Chloride Test Emulsion Formula ( 1 % )
Prepare the three "parts" under normal room lighting, and then mntinue in a photographic darkroom, using a safelight such as Wratten OA, or equivalent.
Part I-Cold dist. or deionized water (@5 to 10°C) Gelatin; photographic quality Allow to soak 30 min, warm to solution, and add: Sodium chloride, AR Cadmium chloride, AR Citric acid, anhyd. AR Potassium bromide, AR Part I1 Dist. or deionized water Silver nitrate, AR P a t 111Gelatin, photographic, in a 1200 ml vessel (glass, ceramic, or stainless steel)
840 MI 30 g 15 g 0.5 g 6g 0.5 g 200 ml 30 g 100 g
Proeedwe: Under safelight illumination, adjust temperature of the two solutions to 50°C, stir Part I rapidly and pour Part I1
Presented before the Division of Chemical Education at the 150th meeting of the A.C:S. a t Atlantrc C ~ t ySeptember , 1965. ' I n addition to being a oonsultant, the author is a lecturer a t the Rochester Institute of Technology.
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into it, taking approximately 3-5 see. "Ripen" for ten min, by keeping mixture at 5 0 T in a waterbath, with very slow stirring. Then allow to cool to room temperature (20°C), for about 1 hr, before proceeding. Warm up to full solution (3C-40°C) and add 1000 ml of this mix to 100 g of photographic gelatin (Part 111). Stir thoroughly, and allow to soak at room tem~erature(20°C). for at least 15 min. Return vessel to the wstkrbath, wirm p; to 55'C, with frequent stirring. "Digestion" proceeds a t 5 5 T , without stirring, until maximum speed and gradation are attained, without fog formation (i.e., with a fog level on a coated, processed strip not over 0.02 ASA Density units). This digestion (or secondripening) will take approximately 1 hr, but exact time must be determined by tests, and depends much upon the gelatin used. Such a test consists of hand-coating test-strips a t 10 or 15-min intervals, chilling them to set the layer, exposing and procwing them, to produce a stepwedge image far visual and/or densitametric evaluation. When an optimum digestion time h a been determined, cool the digested lot of emulsion to about 3 5 T , and coat it upon a prepared support. If more convenient, the completed emulsion may he chilled to set the gelatin, and stored in a dark refrigerator (at about 5 T ) , for up to a week, before coating it. If this is done, the set emulsion must be warmed up in a waterbath to a solution, at about 35-40"C, and coated immediately, since longer standing a t that temperature would result in changes by further digestion processes.
The main ingredients of this emulsion are the sodium chloride and the silver nitrate, which form a suspension of small crystals of silver chloride in the protective colloid, gelatin. Since AgBr is "faster" in its reaction to light, a little bromide is added to increase exposure speed. A small amount of cadmium chloride is also added to give increased contrast to the final image. Citric (01. other) acid is added to adjust the pH to below 7.0. A study of published emulsion formulas wiU show a great variation in the amounts of ingredients. Muchof this is the result of empirical development which occurred over the years, and which was thought at the time to be carefully controlled. There were, however, many then unknown causes of variations, particularly the gelatin and its minor impurities. However, in most formulas a little excess halide (narticularlv bromide) is called for, over the equimolecular amount for reaction with the AgN08, since a little soluble bromide (or other restrainer) is necessary for good storage properties, and for clean whites in the final image. A great deal of excess halide would simply act as development restrainer and slow down the resulting emulsion. While pure silver iodide emulsions have not been found useful, a small amount of AgI in AgBr emulsions increases their speed. Up to about 5% is often used, particularly in fast negative-film emulsions. Most commercial enlarging papers are mixtures of AgCl and AgBr, in varying ratios; the slower Contact-exposure papers are coated with emulsions like the formulas just described, mainly silver chloride.
~.
This formula, as given, is suitable for many test uses, such as those upon photographic gelatins to determine their suitability for this and similar formulas, upon basepapers to be coated with production-lots of emulsion, upon materials of construction, or upon other proposed ingredients for production-lots of emulsions. Such test mixes will show whether the "new" material under test causes any photographic effects, such as fog, changes in speed or gradation, contrast, or any other photographic property which may be sometimes desirable, sometimes not wanted. The formula, as given, is not suitable for production of papers for sale, but could be the basis for such a use with careful, tested, additions of the necessary "finals" including antifoggants, hardeners, restrainers, bactericides, coating aids (surfactants), and stabilizers. Without such additions, coated sheets of this emulsion may be stored (before use) for only a few weeks, and care must be taken in processing not to have the solutions too warm, or the emulsion layer will melt and flow from the support. However, for testing purposes as mentioned above, it is better not to include these additives, as their normal effects could mask the differences being looked for, between a "new" component and the previously used one. Glossary of Terms
Many of the terms used in photographic emulsion work are either taken directly from chemistry, but with changes in meaning, or are new to chemists in general. The term Emulsion itself never fits the chemist's definition, although it is sometimes similar in appearance to a true emulsion. A photographic emulsion starts out as a colloidal suspension of silver halide crystals in water, in the presence of the protective colloid, gelatin. When the thin coated layer of this mixture is dried, the separation of these crystals, or grains, of silver halide is maintained, but we still do not have a true emulsion. To complicate matters, the photographer who used the material speaks of "an emulsion" referring to a particular type or lot of photo film or photo paper (identified on the package by an "emulsion number" affixed to the package by the maker). Anti-foggants are compounds added to an emulsion to prevent or to lessen the formation of fog in the processed image. They may be inorganic, such as a bromide, or organic such as benztriazol. They will also have side-effectssuch as slowing up the emulsion, changing the image-color, etc. They are sometimes called restrainers. Active gelatins are gelatins which contain a high proportion (although still measured as parts per million) of the natural "impurities" having photographic effects, especially ones having a more sensitizing (speeding up) effect than a restraining (slowing down) effect. An emulsion made with active gelatins will have more speed and usually more contrast than "average" gelatins give. Coating aids are additives, usually surfactant compounds, which improve the evenness of coating of an emulsion onto a support, and help reduce any bubbles or other surface defects. Cmtrmt in a photographic image is a qualitative description of the difference between the darkest and the lightest portions. It is expressed quantitatively as "gamma," measured from the "Characteristic curve" plotted from density measurements, especially from an
exposure through a step-tablet which is a series of evenly varied shades of gray. Contrast is low if the gamma value is below 1.0; that is "normal" for the subject. It is high if the gamma value is above 1.0. Except for graphic-arts films it is seldom over 3.0, but the latter may measure as high as 9.0, indicating a great difference between the blackest black and whitest white in the image. However "gamma" can be misleading, as it is really only a measurement of the contrast due to development; the subject itself has high or low contrast, as does the lighting by which i t is photographed. A lowcontrast image is often spoken of as "soft"; a highcontrast one as "hard," but these terms are only relative. The choice of gelatin used in an emulsion has a definite effect on the maximum or minimum contrasts available. "Finals" are the final additions made to an emulsion mix just before it is coated upon a support. They are added at that time because they may be unstablein solution, or volatile, or would interfere by side-reactions if added earlier. For example, a hardener will increase the viscosity of the gelatin (and the emulsion made with it) too much if allowed to react very long with the melted, liquid emulsion. Or the "final" may be a coating aid, or gelatin added only to adjust the coatingviscosity. Most sensitizing dyes which are now so universally used in commercial production emulsions are added as finals, but are usually then allowed to react for a definite period, such as 15 min, before coating is started. Fog is silver in the developed emulsion where i t was not exposed to light; that is, nonimage silver which will blur or interfere with the wanted image. There are many types of fog, with varied causes. Hardeners are tanning agents which harden the gelatin layer so that it can survive the processing in highly alkaline and acid solutions, often a t elevated tempers, tures, as well as reduce any scratching of the processed emulsion during handling. Many types of hardeners are used, including organic ones such as aldehydes, and inorganic ones such as alums. These are used frequently in combination. "Inert" gelatin is a term, controversial now, that was originally intended to describe highly purified gelatin having no photographic effects whatever; it cannot be used without either "artificial" sensitizers and other additives, or with an "active" gelatin. This is a relative term, not quantitative. Processing Solutions are required in the several steps taken to process a photo emulsion. Development (using a developer) is followed by a stop-bath (a weak acid solution to neutralize the alkaline developer), a fixing solution containing usually a thiosulfate (to solubilize the unused silver halide), and a washing in running water. Newer techniques for rapid access photography may shorten or skip some of these steps, but usually a t the sacrifice of permanence of image. Sensitizers in photo emulsion work include compounds added singly or in combination to increase the sensitivity to light of the finished emulsion, and thus its "speed." They may be natural impurities in the gelatin, some of which are now identified, or "synthetic" sensitizers, including not only organics but metals and metal salts such as gold, tin, or others. Stabilizers are chemicals added to the emulsion to Volume 43, Number 9, September 1966
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prevent or retard changes during storage before exposure which may result in fog and other defects. I n comparison to anti-foggants, stabilizers usually do not slow up the emulsions. Speed is the qualitative measure of the sensitivity of an emulsion to light; it is increased by longer digestion. Effect of Variables
Although the photographic emulsion involves mainly the preparation of a precipitate of silver halides in a sol of gelatin in water, many other "minor," but very important, chemical reactions also take place during its preparation. At the start the gelatin is performing several of its many functions in this application. It is the emuldfication agent, having excellent protective colloidal properties; it contributes minute amounts of photographically active impurities to the newly-formed silver halide crystals; it also takes part in some of the reactions itaself. The two "cooking" periods, spoken of as ripening and after-ripening (or digestion), contribute very much to the eventual phot,ographic properties. The first ripening, often called "Ostwald ripening," is mainly concerned with the increase in the size of the silver halide crystals and with the distribution of the range of resulting crystal sizes. This helps to determine whether it will bc a fine-grained slow emulsion, or a coarse-grained fast one, as well as to determine other properties. Up to this point, in the preparation, the minimum amount of gelatin sufficient for properly protected emulsification is present; more gelat>inis added before digestion. In the above formula, the same gelatin type is used. In other emulsion formulas, a more or less active gelatin is specified, since its impurities contribute to the reactions taking place during the digestion. In such a case a third addition of gelatin may be made before coating out, to adjust the viscosity of the mix. If too little gelatin is present at the time of emulsification, the silver halide crystals will agglomerate and clump unevenly, resulting in a general and heavy fog. If too much is present a t that time, the undesirably small silver halide crystals will form, and the "Ostwald ripening" process will be interfered with. The digestion, or second-ripening, is a primary factor in developing the speed and gradation of the resulting product, particularly in the presence of an active gelatin or of artificial sensit,izers. The increase of sensitivity to light is a function of the combined conditions of digestion: temperature and time of treatment. If digestion is carried too far, the result will he too high a fog, the onset of which can be sudden and disproportionately large with ovcrdigestion. Normally with good formula and gelatins, the increases in speed, gradation, and contrast during digestion are more rapid than the tendency to develop a high fog. Hence the instructions say to digest until highest speed and gradation are obtained, just prior to fog. This means until, on processing a test-coating, an ASA density of more than 0.01 or 0.02 ASA units iri the unexposed areas is found. This instruction thus presupposes some information as to an approximate time of digestion. In its absence, it is necessary to run a preliminary "digestion-time series" by coating test-strips a t intervals during a long digestion time for later processing, in order to determine an approximate opt,imum by visual or instrumental examina494 / Journol of Chemical Education
tion. The optimum is only approximate because different lots of the same type of photo gelatin will affect it. Therefore, an experienced worker will start making small test-coatings every time an emulsion is prepared, at about 80% of the anticipated optimum time of digestion, for quick exposure and processing, in order to check the results. I n a first trial, or where the exact digestion time is critical, as in cases where very short differences in time make large differences in results, the emulsion may be cooled to about 20°C to stop the action of digestion, until the coated test-strip has been processed and evaluated. The emulsion is then rewarmed to the proper temperature for continuation of digestion, if necessary. "Faster" emulsions are more sensitive to the many variables, so that often even successive emulsion preparations using identical ingredients will differsome in the optimum digestion time. Most formulas specify the exact temperature to he used for digestion, especially those which must he treated at lower or higher than usual temperatures, such as ammonia-containing emulsions (like the Formula #3 given later in this paper) which do require lower temperatures. Thus the time of digestion is the variable left to the worker to control. While the physical sizes and types of crystal have been determined largely during the first ripening, the light-sensitive properties of these crystals are changed during digestion by many reactions taking place a t the same time, including the adsorption of extraneous ions and molecules onto the silver-halide grain surfaces, forming complexes with the surface ions of the crystal lattice. These extraneous ions or molecules may be present as a result of specific additions made a t this time, or because of those important "impurities" in the photographic gelatin. I n the latter case they may not be immediately available to take part in reactions, but are formed in situ because of slow decomposition of precursors as a result of the slow "cooking" going on a t this point. Although much research has been done on this aspect of the "impurities" in gelatins, and their replace ment by better-controlled and purer individual chemicals, much of the practice is still empirical, since most of what has been learned on this subject by the research in the main industrial laboratories has not been published. Among the many reactions proceeding a t the same time during digestion are some of which cause an identical chemical to he a desirable sensitizer in one case, or an undesirable desensitizer in another. For example, some aldehydes present in photo gelatin will act as sensitizers in many formulas, and are preferred. I n an emulsion formula using a reduction-sensitizer (such as a tin salt), these aldehydes result in high fog levels, because the reducing agent decomposes them to fog-promoters. Hence the literature is full of contradictory statements concerning various "natural" or synthetic added ingredients, and materials which have been avoided for years because of trouble in the past, turn out to be useful in new and differentemulsion formulas. An important aspect in emulsion preparation which must be mentioned is the control of pH and pAg a t various stages of the process. Some of the formulas in the literature specify this; older formulas in general ignore the subject. The subject is too long to cover here, but one of Dr. Carroll's papers (6e) gives the fundamental data, still of great value.
Difficultly Controlled Variables
While this paper a t least mentions the more important variables, or factors affecting results, there are so many, usually interrelated, that it is impossible to cover them all in a short discussion, and it must be admitted that some are not yet well understood and explained. The variables mentioned so far are the ones a chemist would think of first, since any chemical reaction is affected by time, temperature, and environment. And, as in many other areas of chemistry, there are many complex reactions taking place a t the same time, so that control procedures affecting one reaction may not control another. I n photographic emulsion-making the experimenter soon finds that there are countless other variables-some controllable, some not, and some not even identified as yet! First among the additional variables are the differences due to the raw materials, aside from differences due to identity (such as those found in emulsions based on silver chloride versus silver bromide). These impurities may be present in the raw materials to the extent of a few parts per million, or even a part per billion, levels far below normal analytical control ranges. Even with industrially available fine chemicals, such as potassium bromide or silver nitrate, emulsion makers must often specify extra purification. Analytical reagent grade chemicals often are usable, but not always. Impurities of no importance to their normal analytical uses can vary widely in the parts-per-million range, especially when not normally included in the standard analysis scheme. There are an astounding number of "photographically active impurities," organic and inorganic, some desirable when controlled, and some undesirable at any level. A few metallic salts, such as those of tin and mercury, are dangerous at a part per million, although they can be added intentionally at a part or two per billion with desirable results. Lead, copper, iron, and other "heavy metals" do not give serious trouble up to a few parts per million, although the exact limits vary with the emulsion type. Some organic impurities are also desirable within narrow ranges of amounts but are definitely deleterious at higher levels. For example, sodium thiosulfate has been found to be extremely active as a sensitizer, and has a synergistic action with other compounds such as gold salts. I t has therefore been studied extensively, particularly as it appears to be one of the important "natural" sensitizers present in photographic gelatins. For some uses, little is needed; one specification calls for it in the range of 10 to 25 parts per million, no more, no less. Such compounds may be present not only in the gelatin, but in inorganic raw materials from particular places, especially sulfur compounds from ores. The important point here is that "normal" industrial purification is seldom enough for ingredients for photographic emulsions. On the other hand, a trace of a photographicallyactive compound may get into an emulsion preparation as contamination in the water or air of the laboratory. For example, one should not attempt to prepare photo emulsions in a laboratory which has been used for organic preparations, or organic analyses. Minute amounts of sulfur compounds, or of mercury vapor from droplets spilled from manometers or broken thermom-
eters will seriously affect the emulsions. Mauy common organic solvents, or their vapors, have photographic effects, usually causing general fog. Some discussion of this can be found in the literature. The hazards of radioactivity are well known today, not only health hazards, but also damaging effects on light-sensitive materials. Twenty years ago there was a classic case of a large amount of high-quality X-ray film ruined by many tiny fog spots. They were traced eventually to radioactive contamination of the strawboard used in packaging. The strawboard was made from straw grown in the midwest during 1945, at a time when there was fallout from the first bomb tests (15). Since then, careful monitoring of air, water, and raw materials used in emulsions, or near them, has been a regular procedure, not only in emulsion coating plants, but a t factories producing their raw materials, such as gelatin or basepaper. While it has been known for some time that many cosmetics contain photographically active impurities, resulting in special preca~t~ions a t the larger film producers, new problems keep turning up. Silicones now used in clothing as waterproofing, or as crease-retainers, mill cause fog in photo materials, even though only minute an~ountsare rubbed off into the air. Even as remote an item as the plasticizer in the plastic lining of a steel drum, used to ship a liquid hardener for gelatin in the subcoating on photo base-paper has resulted in fog in emulsion layers coated over it later, a t a photo enlulsion production plant. To summarize: all raw materials must be controlled; all surrounding conditions in the mixing area and all equipment must be carefully checked arid control maintained. This may mean also, for example, not changing lots of a raw material during the progress of a series of tests, and checking each new lot of raw material in an actual emulsion mix before using it in an important procedure. This is particularly important in the case of gelatin, which, being a natural product, varies from lot to lot in spite of the most careful production techniques. In plants producing commercial emulsions only about one-third of the gelatins used in a single type of emulsion is allowed to "run out? at a time, so that no single emulsion batch is made up with entirely uew gelat,irls. And in spite of a great deal of work to develop analytical procedures using chemical testing, no series of such tests has yet. been able to replace the photo emulsion itself as the mly way t,o completely characterize a hatch of gelatin. The emulsion as a test method not only answers the questions specifically asked, but also tells if an unknown detrimental impurity has unexpectedly contaminated a particular batch of gelatin. We speak quantitatively about the properties of a photo gelat,in and its trace impurities, but we really do not. yet know all their identities, nor the quantities of a specific ingredient; we really deal with the overall combined effect of the mixture present in the gelatin. These problems have occupied many researchers for many years, and a few hooks have been published on photographic gelatins, specifically a new one from the Focal Press in London and New York (7). Yet there is much more work still to be done. In the meantime, the specification to a supplier of just what one is intending to prepare, and close cooperation with his technical representative is the only way to obtain satisfactory photographic gelatins. One should not try to use food Volume 43, Number 9, September 1966
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gelatins. Some lots may accidentally give promising results, hut such results will he impossible to duplicate with another batch. For other emulsion ingredients, Analytical Reagent grades or better are needed. Silver nitrate is available in selected AR lots; some users recrystallize it later. One brand of AR halide may d i e r photographically from another, even though they are identical for the analytical purposes for which they are made. Therefore one should not use a different grade or brand without testing the compound in emulsions against a known lot, and suppliers should be asked to report when they have made a change in their procedures, even though the resulting chemical may he better for other customers' purposes. Glass, ceramic, and stainless steel equipment, as well as many items made of plastic, may he used for laboratory emulsion preparation. However, stainless iron, plated metals, and soldered joints must he avoided. Plastics loaded with excess plasticizer should not be used, although a thorough soaking in some "scrap" emulsion will often leach out any deleterious materials. This is worth the effort, since plastic items shown in the accompanying illustrations are much safer to use in a photographic darkroom than items made of glass. Only the specifically prepared photographic safelights available from photo dealers should he used. Red or amber bulbs are usually unsafe photographically. Washing of Photographic Emulsions
One operation not needed in the first emulsion example given earlier hut needed in the examples which follow, is that of washing out the soluble potassium nitrate formed in the reaction which produces the silver halides. Otherwise it will crystallize out when the coated layer is dried,. giving - an uneven surface and other difficulti&. The traditional method of washing a photographic emulsion is to chill the mix to set the eelatin into a s t 8 gel after the first ripening, and then to cut it up into small shreds or "noodles" to increase the surface for faster leaching out of the soluble salts. Or small cubes can be formed with a stainless-steel food cutter, such as the one shown in Figure 1. The cubes are put into a
-
Figure 1. Nonmrrorive food-diserr used to cut up photogrophie emulrion before worhing procedure. The lower one is rhown on blwk of emulion ready for cutting. The cutting is done on a gloss plate for cleanliness. Caution: Avoid sunerr of iron or tin-plated met& or they will corrode and sontaminote the emulsions. Cutter rhown is Mognerium alloy, with itoinlerr-steel wirer.
container having about twice the volume of the chunks of emulsion, and subjected to a current of cold water until tests show that all soluble ingredients have been 496
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removed. Such tests include the Nessler reagent test if the emulsion contained ammonia or ammonium salts and the use of a conductivity meter. While a tall glass funnel, or percolator, has traditionally been used for this, I have found that it is much easier and safer to use plastic vessels, such as those shown in Figure 2. These are made from quart-size
Figure 2. Plastic ware "red in photo-emul=ionlobomtory work, for rofety and convenience. At left is o standard "meowrematic dkpenrer'' which con be set for omovnh of from 5 ml to 25 ml for outornotic filling and dispensing in the dim illurninalion necessary in o photographic workroom. The four 1 -liter bottler ore rtondord wash bonier odapted touse for washing emulsions. Several con b e "red in tandem os shown with the sold worhwoter fed in through the dispeming tube (with tip cut off), and Rowing out ot top. The newer type at left is more convenient; the other three have hod holes drilled in the bottle rhovlderr for o u l b w of woter. Such em& rion-warhen are l e t in o rink when in use.
washbottles by cutting holes in the bottkshoulder, and trimming off the fine tip on the tube, to which the watersupply hose is attached. This allows the water to enter at the bottom through the tube and exit through the holes. Water for this use must be free of photographically-active ingredients. If it must he distilled or deionized to remove any that are present, it will then be necessary to 'Lartificiallyharden" it with an inert salt, such as an acetate of a sulfate. Otherwise it would swell the gelatin too much, slowing down the washing, and altering the total material-balance of the mix. I t should he cold, 15°C or less, so that it will not dissolvc and remove too much of the gelatin. It will take from two to six hours to remove the soluble salts from a liter of noodled emulsion. Other methods for the removal of these soluble by-products have been developed, to simplify commercial emulsion manufacture. One way is by salting-out with sodium sulfate, or with zirconium sulfate (14d). Another is to use a gelatin derivative which will coagulate at a change in pH, allowing decantation-washing by a short rinse, and redispersal by changing pH again (1). Another simple emulsionwashing apparatus is discussed and illustrated by Dr. Carroll in Reference (5f). Washing, if required, is performed before the digestion step. Emulsion coating, after digestion and addition of any needed "finals," is done by various methods-by hard or machine, and on glass, paper, or plastic film bases. These supports must he first prepared by coating with a "subbing layer" of gelatin, using hardeners and other
ingredients, (see Fig. 3). I n the case of papers, this subbing usually contains purified barium sulfate in suspension, to insulate and to insure a smooth surface. The purpose of the subbing is to assure an even, smooth, and well-adhered layer of the emulsion. It is usually applied by the manufacturer of the support. Glass may be obsolete for most photographic uses, but it is handy for laboratory experiments, since the emulsion layer may he more quickly dried for test-exposure and pro* essing, and is self supporting in subsequent operations.
Full-Ammenia Bromide Emulsion Formulo (PA01 3.4)
Part I:
Prepare and hold at 40°C Distilled or deionized water, cold 450 ml Potassium Bmmide, AR 92 g Potassium Iodide 1.7 g Gelatin (photographic) 30 g (allow gelatin to soak in cold water 30 min, then stir whilewarning to 40°C to dissolve) Part 11: Prepare and hold at 30°"C 300 ml Distilled or deionized water Silver Nitrate, AR 100 g Ammonium Hydroxide, AR; 29% Approx. 100 ml Add the ammonium hydroxide slowly with stirring, until the brown silver hydroxide precipitate which
Gelatin, photographic ( ~ e s t )
Figure 3. Lobclratory emulsion cooling block (with separate adurtoble hopper). A simple block with stainless steel top on which paper or other "bore" moteriol is held down b y suction (note groove inride the edge, fed b y tube to vowurn-pump) and with channels indde to have hot woter Rowing during eooting, with quick change to sold woter to chili m d set emulsion or soon oscoded.
Other Formulor
The emulsion formulas given below are typical of three additional types; many more will be found in the refererices cited. N e d r a l Bromide Emulsion Formda (PAW 3.2)
90 g
Pmcedure: In a photogrephihic darkmom under safelights (Wratten 1, 2, or 6B or equivalent), stir Part I rapidly and add half of Part 11. Raise temperature to 40PC, and ripen 45 min without stirring. Then, stirring rapidly, add rest of Part 11. Pour 800 ml of this mix into Part 111, warn to 40°C, and ripen for 15 min. Cool until gelatin sets. Store overnight in dark refrigerator (5'). Shred or noodle the set emulsion, and wash in running water until wash water has conductivity of about 1800 X 10-%ho/om, which will take about 3 h n , with water a t 15% Drain and bring the weight of the hatch to 1400 g with distilled or deionized water. The washed noodles may he stored &gain a t @5'C, or melted immediately for the secnnd ripening (digestion). W a r n mix to 40°C in waterbath, add 20 g more gelatin, allow to oak 15 min, and then ripen a t 53-C. This will take from 1 to 2 hrs. To determine optimum time, make small test coatings a t 45, 60, 90, and 120 min; digest longer if optimum has not been reached. Optimum is the highest exposure speed before fog level rises above 0.1 A.S.A. units. For coating, bring mix to 20W g with distilled or deionized water, adjust temperature to ahout 32-35"C, and coat by hand or in machine onto a prepared support. Chill coating and dry. Note: Additional speed may be ohtained by gold semitizing this formula, by which treatment it will approach the speed of some commercial negative emulsions. Sasitizino: Just before the dieestion. add 1.6 e of standard wmrw smnmnwn, I I I I O ~ . W U > P., h t m n ( J r ~ i g ~ m n n )This . in preb;lml Ilg adding I ; l J m l of 11 I ' ; gcdd uldtri
