PHOTOGRAPHIC MATERIALS 117 



thin uniform layer on the properly prepared support material. Here it is set by 

 chilling and dried. The drying conditions must be carefully controlled to prevent any 

 sudden changes in the rate of drying, which would cause variations in the sensitivity 

 over the surface of the emulsion. 



For most negative emulsions a small percentage of potassium iodide is mixed with 

 the potassium bromide. This produces mixed crystals of silver bromoiodide which 

 are more sensitive to light than the pure silver bromide. The ripening treatments also 

 cause a great increase in sensitivity, partly because of the increase in grain size but 

 largely due to a reaction with certain components of the gelatin. 



Positive emulsions are prepared in a similar manner, except that chlorides or 

 mixtures of chloride and bromide are normally used instead of bromide and iodide. 

 Chloride emulsions for contact printing papers are often coated without washing to 

 remove the soluble salts. 



In actual practice, each of the above steps may involve several operations, and all 

 the steps are mutually interdependent. They must all be carefully adjusted to one 

 another in order to obtain the properties desired in the finished emulsion. While a 

 great deal of research has been carried out by photographic manufacturers on the 

 effects of variations in the different steps, very little of the information gained has 

 found its way into the scientific literature. Actually this is of little importance since 

 it is a knowledge of the properties of the finished materials, rather than the details of 

 manufacture, which is of interest to the user of photographic materials. 



Photographic Gelatin. — The unique importance of gelatin in photography is due to 

 its desirable physical characteristics, combined with certain peculiar and very valuable 

 chemical properties. 



Physically, gelatin has the property of absorbing water and swelling. Below a 

 certain temperature range (ordinarily 35 to 40°C.) this swelling is limited, and the 

 gelatin remains in the semisolid gel form. Above this temperature the swollen gelatin 

 melts to the sol form, or a colloidal solution, which will take up water without limit. 

 When the sol is cooled, however, it will again set to a firm gel even in concentrations 

 as low as 1 per cent. Thus the gelatin is particularly suited for use as the binder for 

 the sensitive silver halide grains, since it can be made alternately liquid or solid as 

 required in the various steps of manufacture, and after exposure of the finished mate- 

 rial it will again swell to allow penetration of the developer chemicals to the exposed 

 grains. 



In addition to its action as a mechanical binder, it also acts as a protective colloid. 

 If aqueous solutions of silver nitrate and potassium brromide are mixed, the resulting 

 minute particles of insoluble silver bromide rapidly agglomerate into a curdy precipi- 

 tate which settles out. Also, the particles are spontaneously developable, i.e., a 

 normal photographic developer solution will reduce all the silver bromide particles to 

 metallic silver, even without exposure to light. However, if even a small quantity of 

 gelatin is present, it prevents the agglomeration of the particles, and it also prevents 

 any action by the developer until the grains have been rendered developable by 

 exposure to light or by some chemical action. 



An equally important property, however, is the effect of gelatin on the sensitivity 

 of the grains. When the silver bromide is first precipitated, it is comparatively 

 insensitive. During the digestion, however, the sensitivity may be increased from 

 100 to 1000 times, provided a suitable gelatin was used and the initial precipitation 

 was made in the proper manner. This sensitizing action is partly due to the presence 

 in the gelatin of minute quantities of certain types of organic compounds containing 

 labile sulfur which, under the conditions of the digestion is capable of reacting to form 

 specks of silver sulfide on the surface of the silver bromide grains. ' 



1 Sheppard, S. E., Photographic Gelatin, Phot. J., 66, 380 (1925). 

 , Some Factors in Photographic Sensitivity, J. Soc. Motion Picture Engrs., 24, 500 (1935). 



