FIXING, WASHING, AND DRYING 391 



gelatin may become softened, and other undesirable effects may take place. It is a 

 good rule to keep the fixing bath between 65 and 70°F, The fixing bath should not 

 be colder than 60°F. 



A practical problem in the use of fixing baths is the determination of their exhaus- 

 tion point. As the hypo bath is used, two important changes take place. First of all, 

 the sodium thiosulphate is gradually used up in dissolving the unexposed grains of 

 silver halide from the developed emulsion. With the decreasing amount of thiosul- 

 phate available for dissolving the silver salt, the time of fixing grows longer until 

 finally the point is reached where the time of fixation exceeds the practical limit for 

 efficient or economic fixing. The second change which occurs is a gradual diminution 

 of the acidity of the fixing bath as a result of alkali being introduced into the bath 

 from the developer. The hardening properties of the bath usually decrease fairly 

 rapidly, and, when the acidity is below a certain point (usually near the neutral point), 

 the bath is rendered useless by the formation of a sludge of aluminum sulphite which 

 destroys the hardening properties of the bath. Consequently the bath should be 

 discarded when the time of fixation becomes excessive, the practical limit usually being 

 set at 10 to 15 min., or upon the formation of a precipitate of aluminum sulphite. 

 The bath should also be discarded if it becomes dark or discolored and stains the 

 negatives or prints, if it becomes frothy, or if the bath sulphurizes. 



A simple method of determining whether or not a fixing bath has been exhausted is 

 to immerse a test strip of film in the fixing solution until cleared of opalescence. The 

 strip is then washed thoroughly in nmning water, after which it is immersed in a 

 1 to 3 per cent solution of sodium sulphide. If the test strip remains clear, the fixing 

 bath may be regarded as satisfactory. However, if the test strip becomes colored 

 brown or yellow, this discoloration is an indication that the soluble halides have not 

 been entirely dissolved in the fixing bath, and consequently a new solution should be 

 prepared. Slight changes in discoloration may be observed if only one-half of the 

 test strip is treated in this manner; the untreated portion then acts as a control. 



For purposes of economy and rapidity in fixation, the practice followed by some 

 commercial firms, of using two fixing baths in cascade, has much to recommend it. 



The film or print is placed first in a partially exhausted fixing bath where it is 

 permitted to remain about 5 min., after which it is removed to a fresh fixing solution. 

 When the first bath is exhausted, it is replaced by the second, and the second bath is, 

 of course, replaced by a freshly compounded solution. The use of two baths insures 

 more rapid fixing of the film than can be accomplished through the use of only one 

 bath. Moreover, in being transferred to the second bath, the films do not carry over 

 any alkali from the developer, but only the partially exhausted fixing solution. Con- 

 sequently, the life of the second bath is longer than if the first or buffer bath were not 

 employed. Another advantage of the use of two fixing baths is that, if the hardening 

 properties of the first bath are impaired, the second bath may be used to provide the 

 necessary amount of hardening. 



Precautions should be taken to see that the first fixing bath does not produce 

 dichroic fog, stains, or discolorations, since these will not be removed in the second 

 bath. Both fixing baths should be maintained acid and should be discarded when 

 any of the troubles mentioned above occur. The films should be agitated when being 

 immersed in the fixing solution. This is especially true with respect to immersion in 

 the first fixing bath. 



Completion of Fixation. — A question of importance is the determination of com- 

 pleteness of fixation. Warwick ^ states that the clearing of the negative is a sign, not 

 that the action between the silver bromide and the sodium thiosulphate is complete, 

 but only that the opalescent layer of silver bromide has become so tenuous as to be 



1 Warwick, A. W., The Laws of Fixation, Am. Phot., November, 1918. 



