The exact methods used in preparing commercial photographic emulsions are closely guarded trade secrets, but the basic procedures are well known. There are two general classes of emulsions, the characteristics of which are determined by the end use. They are “negative” emulsions that are used for exposure in cameras and produce a reversed or negative image, and print images that produce the final photograph that we show off to our friends and relatives.
Negative emulsions generally must exhibit a relatively wide flexibility in terms of sensitivity since they are used under conditions that are generally beyond the control of the casual photographer. It would be rather impractical if we couldnâ€™t take our vacation pictures in bright sunlight and shade with the same camera and film. The professional photographer has the option of having several thousands of dollars of cameras dangling around her neck to suit the conditions. Most of us do not.
Print or paper materials, on the other hand, are used under highly controlled conditions of exposure and can therefore have a much more limited “range” of sensitivity and provide the user with more direct control over the final results of the development process.
The preparation of a negative emulsion involves four distinct, but interrelated, steps: (1) the formation of silver halide crystals in gelatin through a precipitation process, (2) the recrystallization of the silver halide grains by physical or Ostwald ripening, (3)a washing process that removes excess soluble salts from the emulsion, and (4) a digestion or chemical sensitizing process involving the heating of the emulsion to increase its sensitivity to incident light.
The silver halide is formed through the reaction of a halide and ammoniacal silver nitrate, in a dilute (approximately 1.5 percent) solution of gelatin at a temperature between 45Âº and 70Âº C. The halide and the silver solutions may be added to the gelatin together, in what is termed a double-jet process, or separately (single-jet), in which case the halide is added first followed by the silver nitrate solution. The concentration of gelatin, the temperature, the concentrations of the two solutions and the rates of addition are important factors in determining both the average size and the size-distribution of the dispersion of silver halide and all must be carefully controlled.
After crystallization, the emulsion is stored for several hours at a moderate temperature during which the average crystal size increases via Ostwald ripening in which the smaller crystals tend to dissolve while the larger crystals grow as crystallization nuclei.
Following this ripening process the by-products are removed and the amount of free halide is reduced. Historically this was accomplished by chilling the emulsion to a gel and forcing it through a perforated screen to form “noodles” which were then washed in running water. Other methods, which are generally trade secrets, but include membrane filtration techniques, are now used in some cases.
After washing, additional gelatin is added to bring the emulsion to its final gelatin composition. Quite often, the added gelatin is rich in sulfur containing amino acids. The final emulsion is then heated to a temperature between 50Âº and 800C for about an hour to facilitate the interaction of the sulfur in the added gelatin and the silver crystals. The sulfur-silver halide interaction increases the number and size of the silver sulfide sensitivity centers and improves the characteristics of the grains. An alkaline gold thiocyanate may also be added at this stage to increase the sensitivity of the emulsion.
Other additives to the final emulsion may include:
1. stabilizers to retard changes in the size and size distribution of the grains;
2. antifogging substances to retard the development of unexposed grains of silver halide when the image is developed;
3. a gelatin hardener to prevent the gelatin from excessive swelling in processing;
4. surfactants and other components to control the wetting and other fluid characteristics of the emulsion during the coating operation;
5. surfactants, lubricants, and anti-static agents to control the surface properties of the dried emulsion;
6. color-sensitizing dyes that expand the range of light sensitivity of the emulsion.
Because the printing of images on paper is carried out in the darkroom under closely controlled conditions, the light sensitivity of the emulsion is not as critical as other visual aspects of the final print such as the tone and contrast of the image.
The emulsions used for developing papers differ from negative emulsions in a number of important respects. In preparing the paper photographic emulsion, the silver may be added to the gelatin solution containing the soluble halide, as in the preparation of a negative emulsion, the halide may be added to the gelatin silver solution, or the silver and soluble halide may be added simultaneously. The rate of addition and the concentrations involved are all designed to produce fine, uniformly sized crystals. In some processes, precipitation takes place in a slightly acid solution to inhibit recrystallization and growth of the crystal size. An excess of soluble halide is avoided for the same reason.
Paper emulsions are generally not heat ripened, as are negative systems, since that would result in larger average crystal sizes and tend to increase the sensitivity. Nor are they generally washed, because the concentration of salts is low and their presence tends to reduce further ripening in storage and changes in sensitivity. In addition, when the emulsion is coated on the paper, a significant amount of the soluble salts is absorbed by the paper stock and thus removed from the system. Those ions that remain in the emulsion may have a desirable influence on the color or contrast of the final image.
The speed or sensitivity of an emulsion can be adjusted in many cases by the use of color-sensitizing dyes (see below). Other additives may be antiseptics, such as phenol or thymol to inhibit the growth of microorganisms; hardeners, such as alum, formaldehyde and glyoxal to improve the physical characteristics of the emulsion during storage and development; and wetting agents, such as saponin or other surfactants to reduce surface tension and facilitate the coating of the emulsion.
Special papers with wider ranges of applicability in terms of contrast, spectral sensitivity, tonal qualities, etc., may be produced in one of two ways: (1) by the admixture of two emulsions of different contrast and color sensitivity, and (2) by sensitizing in such a way that the result varies with the wavelength of the exposing light.
The silver halide grains in a paper emulsion seldom exceed 0.01to 0.02 microns as compared with from 1.0 to 2.0 microns in a negative emulsion and the amount of silver halide in the coated paper per unit area is about one-fifth that of a negative material.
The rate of development of the individual grains in an emulsion is affected by so many factors, such as the rate of diffusion of the solution through the gelatin matrix, the adsorption of the developing agent, the solution of the silver halide, oxidation products of the developing agent and the accumulation of restraining by-products, that exact analysis of it is difficult. The time of appearance of a visible image is, within limits, a reliable indication of the rate of development. It varies with different emulsions and is quite different with different developing agents, but the variation with temperature, dilution and pH is almost directly related to the variation in the rate of development.
The rate of development, as determined from the change in the optical density of the developed image, is complicated by the fact that density increases in two different ways: (1) by the increase in the amount of silver as the grains develop and (2) by an increase in the number of grains in the process of development. Density grows rapidly at first and then slows down until development is complete and no further growth in density takes place. Prolonged development would, of course, increase overall density through the development of unexposed grains (fog).
Halting Development – The Stop Bath
Once the exposed image has been developed to the desired degree, it is necessary to halt the chemical process quickly to prevent over development and the production of fog. The solution used to that end is referred to as the “stop” bath. Since developers function a relatively high pHâ€™s, the typical stop bath is simply a solution of acetic or some other weak acid. The action of the acid is so rapid it usually requires only seconds for the process to be effectively halted.