DEFECTS IN NEGATIVES AND PRINTS 533 



the film is first immersed in the sohition or when the negative is withdrawn from the 

 sohition. The scum may be composed of oil, grease, insoluble particles of the process- 

 ing solution, oxidization products of the developer or other processing solutions, etc. 

 A frequent cause of scum on negatives is the precipitation of hardener in the fixing 

 bath by excess developer carried over by the negative. 



3. Prevention. — The formation of scum may be prevented by washing the process- 

 ing equipment with a hot solution of sodium hypochlorite. Scum may be removed 

 from the surface of the solution by a skimming device consisting of a wire framework 

 over which fine-mesh cloth has been stretched. "Patting" of the surface of the 

 solution with a blotter or cloth is also effective in removing surface scum. 



4. Remedy. — The remedy for scum, if any, will depend upon the nature of the 

 scum deposit. Oil and grease may be removed with carbon tetrachloride. Alcohol 

 is a solvent for some waxes and resins but should not be used on acetate films. The 

 use of a scum-resistant hardening fixing bath, such as the Eastman F5 or FIO will 

 help if the scum is due to precipitation of hardener in the fixing bath. 



Silvery Deposit. 



1. Distinguishing Characteristic. — A silvery deposit or scum on the surface of the 

 negative. 



2. Cause. — Such a deposit may be (a) silver sulphide formed by the interaction 

 of hydrogen sulphide of the air with silver salts of the developer, the silver sulphide 

 resulting from the sodium sulphite of the emulsion or (b) metallic silver precipitated 

 during development as a fine colloid and settling on the film. 



3. Prevention.— (a) The formation of silver sulphide may be prevented by scouring 

 the processing equipment with a hot solution of sodium hypochlorite. (6) The silver 

 is probably caused to precipitate owing to the presence of a silver solvent, such as hypo. 

 The preventive measure is therefore to scrub the developing tanks thoroughly to 

 free them from hypo. Agitation of negatives during development aids in preventing 

 formation of silver deposit. 



4. Remedy. — The deposit of metallic silver can be removed by rubbing it off the 

 film if this is done before the film has dried. 



Slime. 



1. Distinguishing Characteristic. — Deposit of slime on negative. 



2. Cause. — The slime formed on the walls of tanks containing running water may 

 become loose and deposit on the film. 



3. Prevention. — Slime may be prevented by cleaning the tanks periodically, espe- 

 cially if they are made of wood, and sterilizing with a solution of sodium hypochlorite. 



4. Remedy. — If discovered while the negative is still wet, the slime maj^ usually 

 be washed off with clear water. 



White Deposit Covering Negative. 



1. Distinguishing Characteristic. — White granular deposit on negative. 



2. Cause. — Sulphur deposited on the surface of the negative during fixing when an 

 improperly compounded or exhausted fixing bath is used. The deposit may also result 

 from incomplete washing. 



3. Prevention. — Properly compounded, fresh fixing bath, and complete washing. 



4. Remedy. — None. 



White Opalescent Deposit on Film. 



1. Distinguishing Characteristic. — White opalescent deposit on fUm or plate. 



2. Cause. — Often caused by use of undiluted alcohol and too high temperatures 

 in quick drying. This causes excessive desiccation of gelatin producing opalescence. 



