The use of gelatin in photographic emulsions dates back to about 1870 when Dr. Maddox of England replaced the collodion wet process with a gelatin emulsion which could be dried and was not required to be used immediately. Gelatin emulsions have, through the years, been continually improved in quality and speed. Gelatin is still the best medium known for making photographic emulsions (68-71).
Gelatin for photographic use is primarily Type B alkaline processed gelatin, especially for emulsion preparation. Type A gelatin has limited application for top coating and subbing. Although cattle hides have been used, Type B photographic gelatin is generally made from ossein derived from bone. Preparation and extraction of the raw materials are done under carefully controlled
conditions to produce gelatin with desired photographic properties, such as varying degrees of sensitivity or inertness with minimal fogging properties.
Gelatin serves several functions in the preparation of the silver emulsions. It acts as a protective colloid during the precipitation of the silver halides; it is an important factor in controlling the size of the silver halide grains; and it protects the halide grains in the reducing action of the developer so that the reduction of these grains to metallic silver is in proportion to their exposure to light.
Formulas for photographic emulsions and procedures for their preparation can be found in the literature and patents. First, the emulsifying gelatin is dissolved in water and a solution of the required halide salts is added. Next a solution of silver nitrate is carefully added at a specified rate and with constant agitation. The mixture is then heated at a predetermined temperature up to 50°C for a set time. The salts are removed by decantation and washing after the gelatin containing silver halide is precipitated by coagulation. More gelatin and water are added to reconstitute to a proper consistency before chemical sensitization. Variations of the basic process to control silver halide crystal size distribution and size include processes to control nucleation and halide concentration during precipitation.
A final ripening and sensitization then takes place by heating to 50°C or above to reach the maximum or desired sensitivity. This procedure is used for the so-called boiled emulsion.
An ammonia emulsion is prepared similarly, but with the addition of ammonia in the early blending, and the use of lower ripening temperatures.
The emulsion is now ready to be coated on the desired batching film, paper or metal.
Gelatin itself contains natural ingredients which, though present in minute amounts, act as sensitizers in an emulsion. Other substances present naturally act as restrainers: they play an important role in emulsion preparation to offset reactions which cause fog.