400 HANDBOOK OF PHOTOGRAPHY 



prior to fixing. The general principles of washing are the same, however, no matter 

 in which stage of processing washing takes place. 



General Remarks on Washing. — The washing process is simply one of diluting or 

 dissolving the material and carrying away the diluted products with the disposal of 

 the wash water. No chemical action takes place during washing, or, at least, those 

 chemical actions which may take place are accidental and undesired. 



So far as concerns the removal of the product to be eliminated, the washing of 

 photographic materials is largely independent of the temperature of the wash water. 

 Most substances are more soluble in hot than in cold water, so that it might be 

 expected that they would diffuse from the gelatin more rapidly in warm than in cold 

 water. Any attempt to wash photographic materials more thoroughly or more 

 quickly bj^ elevating the temperature of the water is counteracted bj' the swelling of 

 the gelatin at the increased temperature. This swelling tends to inhibit diffusion. 

 Both of these counteracting effects occur at about the same rate with a rise in tempera- 

 ture, so that the rate of washing is largely independent of the temperature, at least 

 within the range of temperatures usually employed for photographic work. At 

 elevated temperatures the gelatin may become soft, and reticulation may take place. 

 For this reason it is advisable that the temperature of the wash water be kept suffi- 

 ciently low as not to soften the gelatin. Temperatures of 60 to 70°F. are usually 

 satisfactory. 



Hardening of the gelatin has little effect on the rate at which photosensitive mate- 

 rials may be washed, unless the material has been dried between the time of hardening 

 and that of washing. If the gelatin has been hardened and dried before washing, it 

 will not expand much when subsequently soaked in water, so that diffusion through 

 it will be difficult. Before the gelatin has dried, hardening does not affect the diffusion 

 through the gelatin. 



The completeness of washing does depend upon the length of washing time and 

 upon the agitation of the photographic materials in the wash water. It also depends 

 upon the method of washing. As a first approximation, it may be stated that in 

 general the longer the photographic material is washed, the more completely are the 

 adhering chemicals removed. The removal of chemicals progresses geometrically 

 rather than arithmetically; i.e., in a given time interval, a certain percentage (rather 

 than a certain amount) of the remaining chemicals are removed. By washing for a 

 sufficiently long period of time, any desired degree of removal may be attained. The 

 more actively the photographic materials are agitated in the wash solution the more 

 quickly the chemicals are removed, as a general rule. 



The most fundamental requirement in washing, however, is to provide an adequate 

 supply of wash water. This may be done by immersing the photographic materials in 

 running water, or by successively removing the photographic materials from one bath 

 to a fresh bath. 



Mechanism, of Washing. — Two methods of washing are in extensive use: (1) the 

 continuous-flow method and (2) the multiple-bath method. Both methods are 

 capable of yielding satisfactory and thorough washing of the photographic material, 

 but the physics of the essential operations is slighth' different in these two cases. 

 An understanding of the mechanism of washing is useful in obtaining the most com- 

 plete washing for a given amount of water and in a given length of time. 



In the continuous-flow system, fresh clean water is made to flow steadily over the 

 negative or print. So long as fresh water is supplied, washing continues indefinitely; 

 the prints are more or less continuously agitated with respect to the water flow, and 

 the operation is carried out automatically. All continuous-flow methods may not be 

 equally effective, however, and care should be taken to see that the method employed 

 in continuous-flow washing is such as to actually fulfill the requirements given above. 



