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24 III. 16-MM FILM AND ITS CHARACTERISTICS
this point. The second ripening results in a very considerable increase in sensitivity without a serious increase in grain size ; this is especially true of negative-type emulsions. As in the case of the first ripening, the increase in sensitivity is due in appreciable measure to the sulfides produced as a result of the partial decomposition of the gelatin. The temperature and time of heating depend upon the gelatin used and the effect desired.
Over-ripening results in a serious increase in fog; under-ripening results in a slow continuation of the ripening process during subsequent storage, which continues even after the emulsion is coated on the base. Proper ripening is indicated by maximum sensitivity and minimum fog ; over-ripening results in an unstable emulsion that fogs rapidly in storage, under-ripening results in an unstable emulsion whose sensitivity increases during storage. As has been mentioned previously, after-ripening is extremely important for high-speed negative types of emulsions ; it is of far less importance with positive emulsions — especially those of the process type.
At the conclusion of the second ripening stage, the emulsion is chilled and stored until needed. Such an emulsion may serve as the basis for several products depending upon the nature of the sensitizing dyes and other ingredients that are added just prior to coating. Up to this point, variations in the individual steps of the process determine the essential character of the emulsion as to speed, contrast, graininess, and latitude.
Coating the Base with the Emulsion
Just prior to coating, the emulsion is treated for color sensitivity, keeping quality, fog, and — to a degree — speed, by the addition of very small quantities of other substances, including sensitizers. The materials used and the manner of application and treatment are closely guarded secrets of the film manufacturers ; very little has been published on the subject. The natural color sensitivity of silver halide emulsions centers in the region 350 to 450 millimicrons; positive-type emulsions such as fine-grain printing positive and raw film for sound negatives for variable-area recording are of this type that we may call blue sensitive.
The addition of small quantities of absorbing dyes (as small as 1 part in 100,000 parts of water or dilute alcohol solution equivalent) will cause an emulsion to show color sensitivity for other colors in addition to blue. Even though the dye used must actually stain the silver halide crystals in order to be effective, yet, of dyes that stain, only a comparatively few