FIXING, WASHING, AND DRYING 395 



4A. Remedy. — The scum may be removed from the surface of the bath by drawing 

 the edge of blotting paper or a piece of cheese cloth over the surface of the bath. 



IB. Distinguishing Characteristic. — Greenish white scum, often precipitated on the 

 surface of the film when chrome alum fixing baths are employed. 



2B. Cause. — The scum consists of basic chromium sulphite or chromium hydroxide. 



3B. Prevention. — To remove chromium scum, films which have been treated with 

 a chrome alum fixing or hardening bath should be wiped or squeegeed carefully between 

 the washing and drying operations. If this scum is not completely removed, the scum 

 which remains will leave a stain on the negative and, after the film has dried, the scum 

 will be difficult to remove. The scum may be removed from the wet film by swabbing 

 the negative with moist absorbent cotton. Formation of the scum may often be 

 prevented by (a) agitating the film thoroughly when first immersing it in the stop bath 

 and (6) using a developer containing as little alkali as possible and by reviving the 

 fixing bath at intervals with acid. 



45. Remedy. — None. 



Sludging. — (See Milkiness.) 



Stains. 



I A. Distinguishing Characteristic. — Green stain (when using chrome alum baths) 

 imparting to the gelatin an appearance as if colored by green dye. 



2A. Cause. — According to Crabtree and Russell, the intensity of the stain is 

 influenced by the relative proportions of carbonate and sulphite in the developer. An 

 excess of carbonate increases the intensity of the stain, whereas an excess of sulphite 

 decreases the stain intensity. The intensity of the stain is also materially increased 

 at high temperatures (80 to 90°F.) 



3A. Prevention. — Increasing the carbonate content of the hardening bath will 

 reduce or eliminate the stain. 



4A. Remedy. — By treating the film with a 5 per cent solution of sodium citrate 

 or a 50 per cent solution of potassium hydroxide, the stain can usually be removed. 

 This treatment destroys the hardening of the gelatin, however. Provided the stain 

 is uniform over the entire film, a stain is not ordinarily objectionable from the photo- 

 graphic standpoint. If the stain is not uniform, prints will show the presence of the 

 stain. ^ 



IB. Distinguishing Characteristic. — Yellowish-white opalescent stain on film. 



2B. Cause. — This stain is caused by the precipitation of sulphur on the gelatin. 

 It maj^ be due to (a) the use of an improperly compounded fixing bath which contains 

 too little sulphite or a relative excess of acid or (6) a fixing bath Avhich is too warm. 

 When either of these conditions are encountered, colloidal or finelj^ divided sulphur 

 may be formed, which deposits on the gelatin. 



35. Prevention. — According to the cause listed above, the stain may be prevented 

 by (a) properly compounding the fixing bath or (fe) by maintaining the solution at 

 temperatures between 65 and 70°F. 



45. Remedy. — These stains may be removed bj^ first hardening the film in an 

 alkaline solution of formalin, after which the film is bathed in a 10 per cent solution 

 of sodium sulphite at a temperature of about 100 to 120°F. These stains cannot be 

 removed by water or acid baths. 



IC. Distinguishing Characteristic. — White powdery stains of aluminum sulphite. 



2C. Cause. — Owing to insufficient acidity of the fixing bath. This may occur as 

 a result of incorrect compounding or partial exhaustion of the bath. 



1 For other stains, see S. E. Sheppard and A. Ballard, Chemistry of the Acid Fixing and Hardening 

 Bath, J. Franklin Inst., 200, 537 (1925). 



