392 HANDBOOK OF PHOTOGRAPHY 



invisible According to this view, from 5 to 12.5 per cent of the original bromide 

 remains unattacked. This view seems to have gained some support, for a practical 

 rule frequent Ij^ observed is to permit the film to remain in the fixing bath for a length 

 of time double that required for the film to become clear. On the other hand, experi- 

 ments by Bullock, by Lumiere and Seyewetz, and later by Crabtree and Hartt^ indi- 

 cate that a film which is cleared of its opalescence is completely fixed. This state- 

 ment applies, however, to fresh fixing solutions and not to partially exhausted baths. 

 If the bath contains more than about 2 per cent, i.e., 20 g. per 1., of the silver halide 

 Avhich has been dissolved from fixed plates, all the removable silver salts are not 

 removed from the plates. Prolonged immersion in a partially exhausted fixing bath 

 will not dissolve the remaining salt, but the residual salt may be removed by trans- 

 ferring the film to a fresh solution. If any doubt exists as to the degree of fixation of a 

 plate, it is common practice to permit the material to remain in the fixing bath for a 

 length of time equal to twice the clearing time. The use of two fixing baths in cascade 

 is a much better solution to the problem, however. 



Reducing Action of Fixing Baths. — -Prolonged immersion of the film in a fixing bath 

 has been found to reduce the density of the silver image. For many applications of 

 photography, this is not important. In other applications, such as sensitometry, 

 photographic photometry, or processing of sound motion pictures, this may be a point 

 of practical importance. The results of the reducing action of fixing baths have been 

 studied by Russell and Crabtree,^ and the following is summarized from their paper, 

 which should be consulted for additional data, especially of a quantitative nature. 



The rate of reduction of the density of the silver image in a given fixing bath is 

 found to be greater for fine-grain than for coarse-grain emulsions. The fixing bath 

 having the lowest rate of reduction of density is given as formula 16 in the compilation 

 of fixing baths. The highest rates of reduction were obtained with fixing baths con- 

 taining relatively high concentrations of sulphite and acid. 



Acidity was the factor which affected the rate of reduction to the greatest extent in 

 ordinary fixing baths. Ammonium chloride, potassium bromide, and potassium iodide 

 increased the rate of reduction. Ammonium sulphite, sodium chloride, sodium sul- 

 phate, glycerin, and sugar decreased the reduction rate. Oxygen and oxidizing agents 

 such as peroxides have no apparent effect on the reduction rate in highlj" acid baths, 

 but the presence of oxygen increases the rate of reduction in fixing baths containing 

 low concentration of sulphite and acid. 



It was found that the rate of reduction was also increased by an increase in temper- 

 ature of the fixing bath. 



Troubles with Fixing Baths. — The troubles which may be reasonablj' expected to 

 occur from time to time when using fixing baths are tabulated for convenience and 

 readj' reference use. 



Blisters. 



1. Distinguishing Characteristic. — Blisters form on the surface of the photographic 

 material, resembling blisters on the body. On dry film, blisters appear as craterlike 

 depressions when examined by reflected light. 



2. Cause. — Blisters are caused by the evolution of carbon dioxide gas when 

 sodium carbonate of the developer is neutralized by the acid of the fixing bath. 

 Blisters are especially likelj^ to occur if the gelatin is soft and is incapable of with- 



' Crabtree, J. I., and H. A. Hartt, Some Properties of Fixing Baths, Trans. Soc. Motion Picture 

 Engrs., November, 1929, pp. 364-305. 



' RussELi,, H. D., and J. I. Cr.^btree, The Reducing Action of Fixing Baths on the Silver Image, 

 J. Soc. Motion Picture Engrs., 25, (No. 3), 371-397 (1932). 



