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.
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