404 HANDBOOK OF PHOTOGRAPHY 



hypo concentration of the partly washed materials. The same effect occurs if hypo- 

 contaminated fingers are placed in the wash bath. In this ease, to obtain a given 

 degree of hypo elimination, washing will be prolonged beyond that time required if 

 the bath is not contaminated. 



A practical means of reducing to a minimum contamination from either of the 

 causes mentioned above is to wash the photographic materials in cascade. Two 

 methods are possible. In one of these the wash water remains constant for each 

 bath, the baths being cascaded; in the other the water from one bath overflows into 

 the next bath and all baths have their water changed periodically. The second 

 method is to be preferred unless the amount of wash water is limited. The material 

 to be washed is placed in the first or bottom tank and is removed, in due time, to 

 higher tanks in which the water is successively less contaminated with hypo. Should 

 it become necessary to add more negatives during the washing process, the partly 

 washed negatives can be removed to a higher tray, whereas those fresh from the hypo 

 bath are placed in the bottom traj'. Thus the partly washed negatives or prints 

 receive fresh water, whereas those fresh from the hypo bath receive water which is 

 only slightly contaminated by hypo from the materials in the preceeding bath. 



Completeness of Washing Negatives.— It is of practical importance to know when 

 negatives are sufficiently thoroughly washed as to prevent hypo staining. Washing 

 longer than is necessary is time consuming and accomplishes no essentially useful 

 service. There are three methods in common use for determining the washing time. 

 The first depends upon experience. The other methods depend upon chemical or 

 electroh'tic conductivity tests for traces of hypo or other solute. 



The washing of negatives in trays is time consuming but is practical where only a 

 small amount of work is being done. A common general rule, based on experience, is 

 that a negative or print should pass through 6 to 12 changes, remaining in each bath 

 for 5 min. In warm weather this method may be productive of frilling because of the 

 frequent handling of the negatives which this process requires. 



Where negatives or prints are washed in trays or tanks of running water the 

 required time of washing may be determined from the time which it takes for a 

 colored solution in the tank to become completely replaced by clear, colorless water. 

 To this time should be added the minimum washing time for the material under 

 consideration, as given in Table III. For example, if it takes 10 min. for the colored 



Table III. — Minimtjm Washing Time for Various Photographic Materials 



Washing Time 



Material Min. 



Lantern-slide plates 3 



Other plates 5 



Film negatives of all kindt^ 7 



Single-weight bromide papers 20-25 



Double-weight bromide papers 35—60 



solution to be replaced bj^ clear water running into the tray, then film negatives 

 should be washed at least 17 min., single-weight bromide papers should be washed 

 30 to 35 min., and lantern slides should be washed 13 min. Any colored solution or 

 dye may be used to determine the length of time for the tank to become cleared, but 

 it is desirable that this solution be one which has no deleterious effects on the nega- 

 tives or prints. 



A test solution for hypo maj' also be used. This solution is made bj' dissolving 

 the chemicals in the following formula. 



Potassium permanganate . 3 g. 4 gr. 



.Sodium hydroxide 0.6 g. 8 gr. 



Distilled water to make 250 cc. 8 oz. 



