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 couldn’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 45º and 70º 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 50º 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.
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