394 HANDBOOK OF PHOTOGRAPHY 



4. Remedy. — The film may be hardened by hnmersing it in a hardening sohition 

 containing potassium alum, chrome alum, or formalin. 



Milkiness or Sludging. 



\A. Distinguishing Characteristic. — -Formation of milky precipitate which is pale 

 yellow and settles very slowly when the bath is standing, or yellow deposit of sulphur 

 precipitated on the gelatin which may later cause fading of the image. 



2A. Cause. — Such milkiness may be caused by (a) excess of acid in the hardener; 

 (6) too little sulphite, or sulphite of impure quality when compounding the bath solu- 

 tion. Some loss of sulphite occurs through oxidation, especially'' if the fixing bath is 

 stored in open tanks, but this loss can be prevented or minimized by covering the tank 

 or by placing a floating cover on the solution, (c) High temperature of the fixing bath. 



3A. Prevention. — Depending upon the cause enumerated above, milkiness may be 

 prevented by (a) using less acid in the fixing bath; (6) use of the proper amount of 

 sulphite, or sulphite of pure quality; or (c) keeping the fixing solutions, whenever 

 possible, between 65 and 70°F. 



4:A. Remedy. — -None, except to prepare a fresh fixing bath. 



\B. Distinguishing Characteristic. — White precipitate which disappears on stand- 

 ing for several hours. A white gelatinous sludge of aluminum sulphate may also 

 settle out. 



2B. Cause. — This milky sludge may be caused by (o) too little acid in the hardener, 

 (6) too little hardener in the fixing bath, or (c) exhausted fixing bath containing alum 

 and sulphite but no acid. 



3B. Prevention. — According to the cause enumerated above, milkiness may be 

 prevented by the following methods: (a) The use of the correct amount of acid in 

 compounding the fixing solution. Care should be taken, not only to observe that the 

 proper amounts of acid are used, but to use either glacial or 28 per cent acetic acid, as 

 called for in the formula. If 28 per cent acetic acid is used instead of the same amount 

 of glacial acetic acid, the solution will contain less than one-third of the required 

 amount of acid. (6) More hardener solution should be added to the fixing bath. 

 (c) Acid may be added to the bath from time to time to prevent sludging. 



4B. Remedy. — It is usually best to make up a fresh fixing bath. 



\C. Distinguishing Characteristic. — Greenish gelatinous precipitate sometimes 

 forming upon the film. 



2C. Cause. — -The sludge consists of chromium hydroxide and is caused by an 

 excess of developer in the fixing solution. 



3C Prevention. — Milkiness due to this cause may be prevented by rinsing the 

 film in water before fixing, by the use of an acid stop bath before fixing, or by the 

 revival of the bath with the addition of acid. Frequently the formation of the green 

 sludge may be prevented from forming on the film by agitating the film thoroughly 

 when it is first immersed in the fixing bath. 



4C. Remedy. — If the sludge is discovered while the film is still wet, it may fre- 

 quently be removed by swabbing with soft cotton. 



Scum. 



\A. Distinguishing Characteristic. — Metallic appearing scum deposit on the surface 

 of the fixing bath. 



2A. Cause. — The scum consists of silver sulphide formed by the reaction of silver 

 thiosulphate in the partially exhausted or infrequently used fixing solution, with the 

 hydrogen sulphide gas present in the air. 



3A. Prevention. — Protect the unused fixing baths from contact with the air by 

 means of a cover on the container. 



