CHAPTER VI 



PHOTOGRAPHIC MATERIALS 



By Haywood Pakker 



Many substances are sensitive to light and thus can be used to form photographic 

 images. The modern art and science of photography, however, is based almost 

 exclusively on the use of the light-sensitive gelatino-silver halide emulsions. The use 

 of various other materials, such as bichromated colloids, the ferroprussiate or blue- 

 print papers, the diazotype materials, and even the collodion wet plate, is restricted 

 to such specialized fields that they are not generally included in discussions of photo- 

 graphic materials. 



In general, photographic materials are made up of two basic parts: the light-sensi- 

 tive emulsion, in which the image is formed; and the base or mechanical support 

 which maintains the emulsion in the correct geometrical form. In addition to these 

 two essentials, practical materials often contain additional layers for various special- 

 ized purposes. 



Photographic emulsions in general fall into two classes: negative emulsions, very 

 sensitive to light and intended for use in the camera; and positive emulsions, much 

 less sensitive and used for preparing prints from the camera negative. Although 

 there is actually no sharp division between these two classes, in general their prop- 

 erties differ so greatly that they are usually considered separately. 



The Emulsion. — The light-sensitive layer, or photographic emulsion, consists of a 

 suspension of minute silver halide crystals in gelatin. Thus it is not a true emulsion 

 in the colloid chemical sense but, by virtue of widespread and long continued use, 

 the term has taken on this specialized meaning in photography, and there is Uttle 

 danger of confusion. 



Preparation. — Negative emulsions are prepared by adding a solution of silver 

 nitrate slowly to a solution of potassium bromide and gelatin, with continual stirring. 

 By double decomposition this produces a precipitate of silver bromide which is kept 

 dispersed and prevented from settling by the gelatin. Since free silver ions tend to 

 react with gelatin, care is taken to keep the potassium bromide present in excess 

 throughout the precipitation. 



After precipitation is complete, the emulsion is digested, by boiling if it is acid, 

 or at a moderate temperature if it is made alkaline with ammonia. During this treat- 

 ment, the smallest crystals tend to go into solution and to precipitate on the larger 

 crystals (Ostwald ripening); there is also a tendency for recrystallization through 

 collisions and coalescence.^ 



When the ripening has been carried to the desired point, the emulsion is cooled to 

 allow the gelatin to set. It is then shredded into small pieces and washed in cold 

 water to remove the soluble potassium nitrate formed as a by-product of the precipita- 

 tion reaction as well as the excess potassium bromide. 



After washing, the emulsion is again melted, and more gelatin and various special 

 agents such as hardeners, wetting agents, etc., are added. Next the emulsion is given 

 a further ripening treatment and is then mixed thoroughly, filtered, and coated in a 



1 Sheppahd, S. E., and R. H. Lambert, Grain Growth in Silver Halide Precipitates, "Colloid 

 Symposium Monograph," vol. 6, p. 265, 1938. 



116 



