FIXING, WASHING, AND DRYING 



379 



3. A hardening stop bath should have properties similar to those of a plain acid 

 stop bath and in addition should produce satisfactory hardening throughout its life. 

 Since the hardening produced by alum mixtures varies with the quantity of developer 

 or alkali added, it is apparent that an acid hardening stop bath will not produce 

 uniform hardening except for a limited time unless it is suitably revived. 



Composition of Stop Baths. — The composition of a number of suitable stop baths is 

 given in Table I. The chemicals should be mixed in the order in which they are given. 



The 28 per cent acetic acid required for one of these stop baths may be made from 

 glacial acetic acid bj' diluting 3 parts of glacial acetic acid with 8 parts of water. 



Table I. — Composition of Stop Baths 



Constituents 



Recommended for. 



Sodium sulphate, g 



Sulphuric acid, cc 



Chrome alum, g 



Sodium bisulphite, g 



Acetic acid (28 per cent), cc. . 

 Potassium metabisulphide, g. 

 Potassium chrome alum, g. . . 

 Water to make, 1 



Nonhardening 



Lantern 

 slides 



192 



83.. 5 



1 .0 



50 

 1.0 



Bromide prints 



48 



1.0 



38.5 



Hardening 



Films and plates 



20 

 20 



30 



1.0 



1 Does not keep well. 



With the acetic acid stop bath, negatives or prints should be immersed in the stop 

 bath for about 5 sec. before fixing. 



The potassium chrome alum stop bath is especially suitable for use in hot weather. 

 When immersed in this hardening stop bath the negatives should be agitated for the 

 first 30 or 40 sec. They should remain in this bath for 3 min. Agitation of the film. 

 assists in the prevention of blisters due to the decomposition of carbonate in the 

 developer by the chrome alum and prevents stains due to the precipitation of chro- 

 mium hydroxide by the alkali of the developer. This bath may require frequent 

 renewal. 



Processing Conditions. — ^To reduce to a minimum all tendency toward reticulation 

 or uneven swelling of the gelatin, it is desirable that all processing solutions be main- 

 tained at the same temperatures. Since the temperature at which the processing 

 solutions are used is usually determined by the temperature of satisfactory operation 

 of the developer, which is usually between 65 to 70°F., it is desirable that the stop 

 baths be used in this temperature range. The potassium chrome alum stop bath, 

 however, may be used at temperatures considerably above this if it becomes necessary 

 to operate with warmer solutions. 



It is advisable to agitate the photographic materials immediately upon immersion 

 in the stop bath. 



The stop bath should be discarded when a sludge forms either in the bath or on the 

 films or, in the case of hardening baths, when the bath fails to harden the gelatin 

 emulsion. Some revival of baths may be accomplished by adding sufficient acid to 

 maintain the original acidity. In many cases however^ it will be more satisfactory 



