524 HANDBOOK OF PHOTOGRAPHY 



during washing sufficiently well as to prevent formation of air bells. Shaded edges 

 which may be fuzzy rather than sharp indicate that the size of the air bell changed 

 during processing. 



3. Prevention. — Sufficient agitation of the negative in the processing solutions is a 

 general preventive for the formation of air or gas bells. Bubbles may also be elimi- 

 nated by brushing the surface of the negative with a soft camel's-hair brush during 

 processing. Formation of air bells during development may often be eliminated by 

 rinsing the negative in clear water prior to development. 



4. Remedy.- — ^Local reduction or intensification may be effective, although retouch- 

 ing will probably be required. 



Bare Spots. 



1. Distinguishi7ig Characteristic. — Spot of bare glass or film free from emulsion. 



2. Cause. — Defect in manufacture. Occurs very infrequently, especially with 

 negative materials manufactured by reliable concerns. 



3. Prevention. — None, so far as photographer is concerned. 



4. Remedy. — None, except possibly retouching. Manufacturers will usually 

 replace defective materials. 



Black Spots. 



1. Distinguishing Characteristic. — Dark or opaque spots of various shapes, usually 

 sharply defined at the edges. 



2. Cause. — Foreign matter imbedded in the emulsion during fixing, developing, 

 washing, or drying. 



3. Prevention. — Keep negative free from contamination of dust, lint, and other 

 particles. See that all solid particles are dissolved before using solutions. Filter all 

 solutions if necessary. 



4. Remedy. — Retouching. 



Brown Spots. 



1. Distinguishing Characteristic. — Brown spots, usualh^ sharplj'^ defined, irregular, 

 and often small. 



2. Cause. — Dry particles of developing agents settling on dry negative before 

 processing. 



3. Prevention. — Cleanliness in processing. Avoid mixing chemicals where they 

 may come into contact with photosensitive materials. 



4. Remedy. — Sometimes such spots may be removed by hardening the film in a 

 10 per cent solution of formalin for several minutes to prevent swelling and frilling, 

 after which the film is washed in water. The film is then bleached in a solution of 

 equal parts of (a) 5 g. of potassium permanganate per liter of water, and (6) 75 g. of 

 sodium chloride (common table salt) and 15 cc. of sulphuric acid per liter of solution. 

 The solutes (a) and (b) must be thoroughly dissolved before the solutions are 

 combined. After bleaching for 3 or 4 min., the brown stain of potassium perman- 

 ganate is removed bj^ immersing the negative in a 5 per cent solution of sodium bisul- 

 phite. The negative should then be rinsed in water and developed in strong light with 

 a nonstaining developer, such as metol-hydroquinone. 



Cinch Marks. 



1. Distinguishing Characteristic. — Fine transparent parallel scratches on emulsion. 



2. Cause. — From the friction produced when adjacent layers of film in a roll slide 

 past one another. Thej^ are frequently produced when attempts are made to tighten 

 a looselj'^ wound roll by pulling on the free end. 



