THE PREPARATION OF PHOTOGRAPHIC EMULSIONS* WASHINGTON, D. C. BURT H. CARROLL,U. S. BUREAU OP STANDARDS, After a brief explanation of the status of the literature on emulsion making, modern theories of photographic sensitivity are outlined. The processes of emulsion making are described in order: rmulsificatia and ripening, w s h ing, after-ripening, and additions before coating. Emphasis i s placed on the underlying chemical and fihysical principles as far as they are known.
. . . . . . Introduction
Photographic "emulsions" are the sensitive coatings of films, plates, or papers. They are not truly emulsions a t all, since they are dispersions of crystalline solids, protected by lyophile colloids, but the term "emulsion" is applied a t all stages of manufacture, including the finished products which have been dried to moisture equilibrium with the air. This paper is concerned with emulsions protected with gelatin and developed after exposure. While emulsions of this type can be made from salts of other metals such as mercury and thallium ( I ) , for practical purposes we are concerned only with those of the silver halides. In the slow emulsions used on paper the halide may be chloride, a mixture of chloride and bromide, or bromide, the sensitivity generally increasing in the order given. Chlorobromide and pure bromide emulsions are also used for very slow plates such as lantern slides, but all the fast emulsions and many of the slower ones, such as motion picture positive film, contain bromide and small amounts (seldom over 5%) of iodide. The emulsion is usually 3 5 4 % silver halide and 6560% g e l a t i (air-dry); it also contains small amounts of soluble bromide, traces of silver in other forms (detectable only by the most refined methods of analysis (Z)),and, if sensitivity to the longer wave-lengths is desired, very small amounts of special dyes. All practical emulsions may be assumed to have been made by "emulsification" or precipitation of the silver halide in solutions containing gelatin; "ripening" by heat treatment, which begins during the emulsification and may be continued after the washing; and (except for some of the silver chloride emulsions) washing to remove soluble salts before d ~ n g .The object of this paper is to summarize the available knowledge of the process, considered in terms of the chemical and physical principles involved. This has been done but seldom, considering the interesting possibilities of the subject. Bancroft (3) published an extensive review of the literature up to 1910, using i t as material for a new theory of sensitivity, but most of the quantitative data have been obtained since that date. The situation is best explained in terms of its historical development. The invention of the silver-bromide-gelatin emulsion may be dated from 1864, 1871, or later, * Publication approved by the Director, Bureau of Standards, U. S. Department of Commerce. 2341
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depending on the point of view, but its practical use began in the early seventies and not before 1878 was the supremacy of the collodion wet plate overthrown. The development of the process at this stage was in the hands of numerous enthusiastic and ingenious, hut frequently unscientific investigators. They were largely amateur photographers or scientists and published their results freely in the numerous photographic journals of the time. We are indebted to Eder for the enormous labor of collecting this material in his "Ausfiihrliches Handbncb der Photographie," as well as for the very important experimental work at the Graphic Institute of Vienna under his direction. The first edition of Eder's "Photographie mit Bromsilber-Gelatine" appeared in 1880;in the preface of the second edition in 1882 he states that trade secrets are taking an ever-increasing place in emulsion making. By the end of the eighties new literature on emulsion making had almost vanished and while in the last ten years there has been an encouraging improvement in number and quality of such articles, the situation for the last generation is reflected by the sixth (1930) edition of Eder's book (rewritten by Wentzel), which contains a wealth of information on everything connected with manufacturing procedure, but perhaps less of the principles of emulsion making than the 1903 edition. This was inevitable in the nature of the subject. Emulsion making is ideally adapted to become a secret art; it is carried on under such conditions that essential details may readily be concealed even from those actually doing the work, and little or nothing can be learned about the process by analysis of the product. Above all, the number of variables involved and the difficulties in accurate measurement of the results are such that only an organization with ample resources and unusual willingness to wait for returns can undertake anything like systematic and thorough research. Occasional statements have been made that scientific investigation of the emulsion is impossible. Empirical formulas have been the shortest route to commercial success. Containing little or nothing subject to patent protection, they have been most jealously guarded. As a result, material which must be common knowledge among emulsion makers has been unavailable to scientists interested in the subject. Most of the data used in this article have been published; some few relate to experiments by Mr. Donald Hnbbard and the writer in the photographic emulsion laboratory of the Bureau of Standards, and will be described in full in the Bureau of Standards J o u m l of Research. Commercial formulas are unknown to the writer, but, as far as experimental conditions permit, conclusions have been based on emulsions of commercial quality. Opinions expressed are those of the writer, and are not official statements of the Bureau of Standards. Photographic characteristics will be described as far as possible in the terms originated by Hurter and Driffield, and described in any standard text on photography. Numerical values for speed given by different in-
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vestigators unfortunately are based on widely varying constants and cannot be directly compared, but density, gamma, fog, and scale are almost always according to the original definitions. Present Theories of Sensitivity While the study of emulsion making should be one of the most valuable methods of attacking the problem of photographic sensitivity, the present theories have necessarily been developed without it. We can, therefore, logically take the theories up at this point, and use them to explain the photographic results which follow given chemical and physical changes in the emulsion. I t should perhaps first be emphasized that sensitivity is measured in terms of the change produced by development after exposure. There is no direct measure of that change produced by light which is known as the latent image; it is readily conceivable that one emulsion might undergo more chemical change than a second on the same exposure, but have less sensitivity in the photographic sense. Discussion of the various theories of the latent image would take too large a space; we shall accept for use in this paper the view based on the pioneer work of Carey Lea and developed by Liippo-Cramer, that it is metallic silver, produced by photochemical decomposition of the silver halide, and made much more inert to many reagents by its adsorption on the unchanged silver halide. By means which are perhaps the greatest mystery of photography, it enormously accelerates the reduction of the silver halide by the empirically chosen materials which we know as developers. In the opinion of the writer, the latent image is not necessarily the primary product of the action of light. In all but a very few of the photochemical reactions which have been quantitatively studied, secondary reactions follow the primary photochemical process and modify the yield from the theoretical value of one molecule per quantum absorbed. Such reactions are almost inevitable in a system such as the emulsion; their influence is probably detected in the effect of intermittent exposure (4) and it is reasonable to expect that certain factors influence sensitivity through these secondary reactions. The emulsion is a dispersion of silver halide in gelatin, somewhat coarser than is usually described as colloidal, but still fine enough to pass readily of silver halide per through filter paper. There are about 1 X 10~;~rains of plate surface. These are, in general (5), independent units. Under ordinary conditions of development, a grain develops completely if it carries enough latent image to start development at all. As exposure is increased, the number of developable grains per unit area increases, with corresponding changes in developed silver a d density, until solarization begins, when the number of developable grains and density begin to fall off again. If the grains of an emulsion are closely alike in sensitivity, it
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will have high contrast, since the number of developable grains will increase rapidly when the exposure necessary to make any of them developable has been exceeded. On the other hand, if an emulsion is to have a long scale, there must be a wide range of grain sensitivity to cause a steady increase in density over a long range of exposures. The theory of sensitivity, therefore, logically begins with examination of the grains of silver halide. Emulsion grains are definite crystals of the regular system. They are of various shapes. If formed in the absence of ammonia, flat plates predominate, with occasional needles. In those examined by Sheppard and Trivelli ( 6 ) octahedral faces were developed to the exclusion of the cubic. The presence of ammonia during emulsification alters the external habit of the grains to a more rounded form. X-ray analysis (7) shows that the bromide and brom-iodide crystals are of the simple cubic (NaCl) lattice, so that the octahedral faces are of a single ionic species, Ag+ or Br-. Silver iodide a t room temperature crystallizes in the hexagonal system, but up to thirty mol per cent AgI it forms mixed crystals with the bromide, with the same type of lattice as pure AgBr. The spacing of the lattice increases with increasing amounts of iodide; Trivelli (8) predicted the existence of such a distortion of the bromide lattice with consequent increase in sensitivity. In hrom-iodide emulsions the iodide is not distributed uniformly, but is present in greater concentration in the large crystals (9). Any emulsion contains grains of a wide range of size. The size-frequency distribution has been most thoroughly investigated at the research laboratories of the Eastman Kodak Company (10). I t is found to be a random distribution which can he fitted approximately by the ordinary Gaussian error function, but more accurately by the modified formula =.
~~-k(losa-4'
in which y is the percentage of grains falling in the size-class x. The average grain size of Eastman emulsions (10) examined by Sheppard, Wight~ a highman, and Trivelli ranged from 0.4,i in a lantern-slide plate to 1 . 7 in speed type. (0.12 to 2.2p2 projective area.) I t has long been known that slow emulsions of high contrast have small uniform grains and that fast emulsions have coarser grains with a wider range in size, but only recently has there been established any definite connection between grain size and sensitivity. The first quantitative investigation by the use of the one-grain layer was published by Svedberg (11). He diluted commercial emulsions, coated them in a layer not over one grain thick, and determined the number and size-distribution of grains in a field by photomicrography with red light and panchromatic plates. The one-grain layer plates were then given graduated exposures and de-
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veloped. Developed grains were dissolved out by chromic acid and the number and size of remaining grains again determined. In any given emulsion the larger grains are invariably the more sensitive to any radiation. The uneven distribution of iodide among the grains (9) introduced an unknown factor into most investigations, but the behavior of pure bromide emulsions is quite similar (12). The results with or-particles (13) can be satisfactorily explained by the assumption that one or more particles striking any grain make it developable. The sensitivity of a grain is then directly related to its projective area, a, by the probability that that area will be struck, P = 1 - e - O N where N is the number of or-particles per unit area. Svedberg originally concluded that a similar relation held for visible light, sensitivity being a function of area alone. More extensive studies in the Eastman and the British Photographic Research Association laboratories showed that this is not the case. The sensitivity of the larger grains is greater than can be explained by area alone, whether the radiation is assumed to be a continuous flood or discrete quanta. There are several other reasons sufficient to void any theory which assumes that all grains are identical except for size. Toy (14) found that geometrically identical grains in the same emulsion had a range of sensitivities similar to that of the emulsion as a whole, although more restricted. As first discovered by Liippo-Cramer (15), the sensitivity of fast emulsions is greatly reduced by oxidizing agents, even those, such as chromic acid, which do not react with silver bromide; differencesin sensitivity between emulsions are much less after desensitization. Practical experience with emulsions had made i t evident that batches of gelatin varied enormously in their photographic usefulness although chemical and physical tests were practically the same; this was demonstrated in quantitative form by Sheppard, Elliott, and Sweet y
