406 HANDBOOK OF PHOTOGRAPHY 



case with plates. The removal of hypo retained by the paper base appears to be a 

 matter of time and not of the amount of water or the velocitj' employed. 



In general the conditions appljdng to the washing of negatives apply equallj' well 

 to the washing of prints, at least qualitatively. On the assumption that hypo would 

 diffuse from both sides of the print instead of from only one side, it has sometimes 

 been presumed that prints may be washed more rapidly than an equal number and 

 size of film. It has also been stated that prints require a longer time of washing than 

 an equivalent quantity of negatives on the basis that the fibrous texture of the paper 

 holds hypo more tenaciously than do negatives, so that complete washing is not so 

 readily accomplished. 



Experiments by Hickman and Spencer' indicate that both points of view are cor- 

 rect within hmits. A large percentage of the total hypo may be removed from prints 

 within a comparatively short time, but considerably greater washing of prints than of 

 negatives is required to remove the last traces of hypo. Consequently, for complete 

 removal of hypo, prints should be washed in running water for a longer time than 

 negatives or should be washed in more baths than an equivalent amount of negative 

 material. 



When prints are washed in running ^\ater, Warwick ^ gives as a practical rule the 

 formula 



/ = A' logio A (14) 



where t = the washing time, in minutes, required for satisfactory washing; 



A = reciprocal of the fraction of hypo concentration which it is desired to obtain, 

 i.e.. for attaining a final hypo concentration which is 1/50,000 of the origi- 

 nal hypo concentration, A = 50,000; 

 K = a constant to be determined experimentally, but which is approximately 

 15.5 for common practical cases of operation. 

 If, for example, we take if = 16, and A = 100,000, the washing time is 



t = 16 logio 100.000 = 16 X 5 = 80 min. (15) 



whereas if we take A = 10,000, the washing time becomes 



t = 16 logio 10,000 = 16 X 4 = 64 min. (16) 



Effect of Container on Washing.^ — While the amount of water which is changed 

 is of importance in the washing of photographic materials, Hickman and Spencer have 

 determined that the elimination of hypo is sometimes affected bj' the characteristics 

 of the wash water container in such a manner as to prolong the washing time much 

 beyond that required for a complete change of water in the washing device. For 

 example, these authors report that plates placed in a trough, through which a constant 

 amount of water was flowing, were found to be washed free of hypo more rapidly if the 

 trough was slightly inclined than if it was horizontal. The effectiveness of tank wash-, 

 ing devices were found to vary considerably. The inclined trough was found to be the 

 most effective, as well as one of the most simple, of washing devices. 



It has been assumed (although not explicitly stated) that the material out of whicli 

 the container is made had no appreciable eftect on the time of washing of prints or 

 negatives. This is a desirable condition and is realized to a very close approximation 

 where the washing vessels are made of glass, enameled metal, glazed earthenware, 



1 Hickman, K. C. D., and D. A. Spencer, Washing of Photographic Product-s, Phot. J., 65, 443 

 (1925). 



2 Warwick, AV. A., Scientific Washing of Negatives and Prints, Am. Phot., 11 (No. 6), 317-327 

 (1917). 



' A comprehensive study of the effectiveness of washing devices has been reported by Hickman and 

 Spencer, Phot. J., 62, 225 (1922). 



