FIXING, WASHING, AND DRYING 403 



where ni is a constant depending upon the type of material being washed. For plates 

 and films washed in tanks, m = 1, whereas when washing is done in shallow trays, 

 m = 1.25. For washing prints in trays, m = 2. Equation (13) gives the number 

 of baths required (for specified values of A and V/v), under the assumption that the 

 film is washed thoroughly in each bath. 



Washing with Limited Water Supply. — It sometimes happens that the amount of 

 water available for washing is limited so that the question arises as to the most effec- 

 tive use of the available supplj\ Under such restrictions it can be shown that the best 

 utilization of a given water supply is that in which the total amount of water is divided 

 into as many separate baths as possible so long as the amount of water in each bath is 

 sufficient to cover the entire negative or print. Between wash baths the surface 

 water from the negatives should be removed. The removal of the svirface water is very 

 effective in obtaining completeness of washing but is a point which is frequently 

 neglected. 



The conditions outlined above for the optimum use of a limited water supply are 

 such that the reduction of hypo concentration per individual bath is not likelj'- to be 

 very high. Consequently a large number of baths is desirable. We may summarize 

 the conditions for optimum use of a limited wash-water supply as follows: (1) wash 

 the negatives or prints in the minimum amount of water in each bath which will com- 

 pletely cover all the surface area, (2) remove the surface water between baths by 

 sponging the photographic material with a viscose sponge or lintless cloth, and (3) 

 wash in as many baths as possible since each bath does not produce very much reduc- 

 tion in hypo concentration. 



Importance of Adequate Water Supply. — In any wash bath, the most fundamental 

 rule is to use plenty of water. For multiple-bath washing this general statement may 

 be modified slightly, as in this case it is more important that the ratio of the volume 

 of water in the bath be large compared to the volume of water carried over from one 

 bath to another by the film. Plenty of water is one answer to this situation; a better 

 answer, especially when the water supply is limited, is to make the ratio V/v as large 

 as possible. A very effective way of increasing V/v is to remoA^e as much of the surface 

 water as possible between wash baths. 



Washing Several Negatives in the Same Bath. — To save time in washing, it is common 

 practice to place several negatives or prints in the same wash baths at the same time. 

 So long as each sheet of material is always kept separated from all others, the general 

 principles outlined for multiple wash baths hold. 



On the other hand, if the sheets of photographic material are not agitated and 

 especially if thej^ are permitted to pile upon one another so that one sheet protects 

 another from being washed by fresh water, hypo will not be effectively removed from 

 the protected areas. Therefore such washing maj'- not remove any appreciable per- 

 centage of hypo from the protected areas. 



Even if the separate sheets are kept separated, it is still possible, if thej^ are not 

 thoroughly agitated, that some of the negatives or prints will sink to the bottom of 

 the tank or tray, whereas others may remain at a higher level of wash water in the 

 bath. Since the hypo solution is more dense than the fresh, uncontaminated water, 

 it will tend to settle at the bottom of the tank. Thus the sheets at the bottom of the 

 tank will receive less effective washing than those at the top. 



Contamination of Partly Washed Materials. — The addition of negatives or prints 

 fresh from the hypo bath into a bath of partiallj^ washed prints or negatives may 

 easily undo all the effects of previous washing, so that it may be necessary to start 

 washing all over again if complete washing is to be attained. The reason for this is 

 that the materials having the higher concentration of hypo add considerable hypo 

 to the wash water, so that the introduction of the hypo-laden materials may raise the 



