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HANDBOOK OF PHOTOGRAPHY 



such as the various classes of sulphur compounds which apparently act by favoring 

 the growth of sensitivity specks and which cause a general increase in sensitivity, 

 or "optical," such as the dyes which extend the absorption bands of the dyed grains, 

 thus extending the sensitivity to new regions of the spectrum. Antifogging agents of 

 various chemical types are added to restrain the formation of spontaneously develop- 

 able fog grains and to allow the digestion process to be carried as far as possible. 



The physical properties of the emulsion are modified by hardening agents that 

 decrease the swelling and raise the melting point of the gelatin so that it can withstand 

 higher temperatures in processing. Chrome alum was originally widely used, though 

 more recentl}^ organic tanning agents have been introduced. Wetting agents which 

 reduce the surface tension of the melted emulsion are added to facilitate the coating. 



Printing-paper emulsions contain other addition agents, or doctors, of various 

 types to control such properties as image color, contrast, surface texture, etc. 



250 

 200 

 150 

 100 

 50 



o.\ 



0.3 05 0.7 0.9 I.I 1.3 1.5 1.7 1.9 

 Grain Size (Area of Grains in Square Microns) 



Fig. 3. — Size-frequency curves for the grains of two typical emulsions. Curve B is for slow 

 lantern-slide emulsions, while curve A is for portrait-film emulsion. 



Physical Properties of the Emulsion. Thickness. — Negative emulsions are usually 

 coated comparatively thick, 0.0012 to 0.0016 in. (0.03 to 0.04 mm.) when dry, in order 

 to obtain a long exposure scale (good exposure latitude). ^ Positive emulsions are nor- 

 mally somewhat thinner, while emulsions designed for reversal processing are much 

 thinner, so that in the first exposure the high-light areas are exposed practically com- 

 pletely, giving clean high lights in the positive. With images viewed by reflected light, 

 such as prints on paper, the maximum density is limited by reflections from the sur- 

 face, rather than by total concentration of silver. Therefore paper emulsions may be 

 coated very thin to give conveniently rapid processing. 



Because of the nature of the coating process, emulsions on continuous strips of 

 support, such as paper or film base, can be coated with extremely uniform thickness, 

 while with emulsions on plates there may be some variations in thickness, unless 

 polished plate glass is used. 



Melting Point. — Emulsions with plain unhardened gelatin usually melt in pure 

 water at about 95°F.; however, most emulsions have at least some hardening, so that 

 the average melting point is more nearly 105 to 110°F., and some special emulsions 

 have much higher melting points. The succession of processing solutions, at various 

 pH values and with various salt concentrations causes a cycle of swelliiig and deswell- 

 ing changes which may test the tenacity of the gelatin to the utmost. Therefore, 



1 Sheppard, S. E., "Behavior of Gelatin in the Processing of Motion Picture Film," Trans. Soc. 

 Motion Picture Engrs., 11, 707 (1927). 



