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FILM MANUFACTURE 23
heating. In the ripening process, the larger crystals become still larger and the smaller crystals smaller ; this is called initial ripening, digestion, or Ostwald ripening of an emulsion. During heating, the gelatin partially decomposes causing minute silver sulfide specks to form on the halide crystals. Although these silver sulfide specks (called sensitivity specks) play a very important part in the formation of the latent image, the exact nature of the very complex physicochemistry involved is just beginning to be understood.
In general, coarse-grained, higher speed emulsions result from the slow addition of the nitrate to the potassium bromide solution in addition to ripening for a relatively long time at a relatively high temperature. Finer grained, lower speed emulsions (such as process emulsions) result from more rapid addition of the nitrate together with ripening at lower temperatures. It is interesting to note that at this early stage of manufacture, the grain distribution of the finished emulsion is established and continues substantially unchanged throughout the remainder of the manufacturing process, and to a degree, during subsequent developing after exposure. After the initial ripening, gelatin of the chemically active type is added to bring the gelatin content of the heated mixture to approximately 10%. This step is very important since the subsequent sensitometric effects to be obtained depend upon the properties of the gelatins so added. After this gelatin is thoroughly dissolved, the emulsion is set in a cool place. In a few hours it sets to a stiff, jelly-like mass. At this point, the emulsion has low contrast and low sensitivity.
(b) Removal of Excess Soluble Salts. The emulsion at this point still contains the soluble potassium nitrate that was formed with the silver halide and an excess of potassium bromide. It is washed to remove these soluble salts; if this were not done, they would crystallize out during drying when the emulsion was being coated on the base. The presence of these salts would also interfere with proper second ripening. Water is ordinarily used ; washing is controlled by the rate of flow and by the temperature of the wash water used. In order to make washing easier, some emulsions are shredded into "noodles" by forcing the gelatinous mass through a wire screen or perforated die or plate. During the washing process, the halide crystals, being insoluble in water, are held mechanically in position by the gel structure and remain unaffected.
(c) After-Ripening and Sensitizing. After washing is completed, the emulsion is drained and then melted for the after-ripening; the remainder of the gelatin required for the finished emulsion is added at