396 HANDBOOK OF PHOTOGRAPHY 



3C. Prevention. — The following steps are recommended to prolong the life of the 

 fixing bath and prevent formation of aluminum sulphite, (o) Developers containing 

 an excess of alkali should be avoided. (6) Rinse the film between the developing and 

 fixing, to remove as much developer as possible. The rinse may be made using clear 

 water or an acid stop bath. Stop baths should be used with discretion, however, as an 

 excess of acid may be carried over to the fixing bath, thereby causing sulphurization 

 of the latter. 



4C Remedy. — The stains of aluminum sulphite may be removed by hardening the 

 film in an alkaline solution of formalin, after which the film is bathed for a few minutes 

 in a 5 per cent solution of sodium carbonate. The film should finally be thoroughly 

 washed. 



ID. Distinguishing Characteristic. — Fairly opaque stains on the film, having a 

 dirty yellow color. 



2D. Cause. — Such stains are produced when silver compounds are left in the 

 film as a result of incomplete fixation. They may be due to (a) Insufficient agitation 

 when immersing the film initially in the fixing bath. In such cases the developing 

 solution, coming into contact with the film, reduces the silver haUde (dissolved out 

 by the sodium thiosulphate) back again to yellow metallic silver. (6) Use of old or 

 exhausted fixing solution containing excess silver in solution. Some of this silver salt 

 remains on the film if it is not thoroughly washed. Originally this compound is 

 colorless, but upon exposure to the air a yellow stain results from the formation of 

 silver sulphide. 



3D. Prevention. — (a) Agitating the film when it is first immersed in the fixing bath 

 will aid in preventing stains. In this case, the tendency to form a stain is greatest 

 when strongly alkaline developing solutions are used and when the fixing baths are 

 neutral or alkaline instead of being acid. Consequently the fixing baths should be acid 

 and a developer having low alkaline content should be used, or the film should be rinsed 

 in water between development and fixing, (b) Use only a fresh acid fixing solution. 



4D. Remedy. — ^The following methods are frequently helpful in removing silver 

 stains : 



a. Wash the film thoroughly to remove any traces of sodium thiosulphate, and then 

 bathe in a 1 per cent solution of potassium cyanide. (Caution. Cyanide is a deadlj^ 

 poison. In solution with water, cyanide produces deadly fumes of hydrocyanic acid. 

 Consequently, this material must be carefully handled in well- ventilated rooms.) The 

 cyanide will dissolve away the silver thiosulphate and some of the silver sulphide. 

 Cyanide is also a solvent for metallic silver. Therefore, as soon as any signs of reduc- 

 tion of the silver image appear, the film should be removed from the cyanide bath and 

 washed thorovighly in clear water. 



b. Treatment with a weak solution of permanganate acid, followed by a thorough 

 wash in clear water and then immersing the film in cyanide will often be useful in 

 removing stains from old negatives. 



c. In those cases in which the silver stain cannot be completely removed, it may be 

 possible to make a new negative, free from stains, by copying the negative or print 

 through a yellow filter, such as the Wratten G filter. 



Water Supply. — Water is so frequently used in photographic processing that it is 

 advisable to pay considerable attention to the purity and adequacy of the supply. 

 For photographic purposes water may be used as a con.stituent of most processing 

 solutions, or it may be used merely as a solvent or wash. In either case impurities 

 in the water may result in imparting undesirable characteristics to the processing solu- 

 tions, to the formation of deposits on the photographic materials, or to other undesir- 

 able effects. As a general rule water which is suitable for human consumption will 

 also be satisfactory for photographic purposes. The filtered water obtained from 



