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22 III. 16-MM FILM AND ITS CHARACTERISTICS
(a) It acts to adhere to and cooperates mechanically with the base.
(b) It shows minimum mechanical distortion when swelled (as during developing) and shrunk (as during drying and subsequent use). Not only must it suffer minimum distortion itself during such swelling and shrinking, but it must also cause the halide crystals and their corresponding silver nuclei to suffer minimum distortion and shift in orientation.
Gelatin is usually made from selected clippings of calf hide and ears, and cheek pieces and pates. For photographic purposes, all pieces must be free of bacterial infection. Two common processes are used in refining gelatin ; they are known as the lime-acid process and the soda process. Both processes require soaking of the pieces for an extended period (several months) ; as the character of the gelatin has a marked effect upon the sensitometric characteristics of a finished emulsion, the particular process used to refine the gelatin depends upon the effects desired in the emulsion to be manufactured.
Emulsion Preparation Stages. There are three important stages in the preparation of an emulsion :
(a) Emulsification and the initial ripening;
(b) Removal of excess soluble salts;
(c) After-ripening and the sensitizing.
(a) Emulsification and Initial Ripening. Emulsions are usually prepared by mixing solutions of 10% or more of soluble halides with silver nitrate solution. (Silver bromide is one of the common halides in most emulsions whether of the negative or of the positive type ; most negative types also contain some silver iodide.) To prepare the emulsion, a small amount of relatively inactive gelatin is swelled in cold water and then heated to form a melted gelatin solution ; this gelatin is only a small percentage of the total required for the finished emulsion.
The alkali halides are then added ; an excess of potassium bromide is always used. Silver nitrate solution is then added while the gelatin solution is continuously stirred ; it is necessary that there be an excess of bromide even locally where the nitrate enters the gelatin solution. The halide precipitates formed are always microcrystalline ; the crystals when first formed are so fine that when viewed by transmitted light, the solution appears deep red. When more nitrate is added, some of the precipitate forms around the fine crystals already present — increasing their size — while the remainder of the precipitate forms small new crystals.
Upon completion of the addition of the nitrate solution, there is still an excess of bromide; under this condition the emulsion is ripened by