Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
Apart from the fog caused on negatives and prints by halation, light leaks, and overexposure, there is chemical fog. Fog is a deposit of silver not forming part of the true image.
Chemical fog occurs as a result of the following:
Overdevelopment or forcing of an underexposed negative.
Developer which is too strong, without sufficient dilution.
Impurities in the chemicals of the developer, especially metallic foreign matter, and hypo.
Too much alkali and too little restrainer in the developer.
Developer used at too high a temperature.
Sulfide in the developer due to bacterial action, especially in large wooden tanks.
Prevention is always better than the cure… clean utensils, careful control of time and temperature, and accuracy in compounding developers will avoid chemical fog in nearly all cases. Where fog has resulted, a quick dip in dilute Farmer’s Reducer may sometimes remove it without too much damage to the image.
In the case of sulfide fog in large developing tanks, the developer can often be restored to usefulness by the addition of lead acetate, which renders the sulfide as harmless lead sulfide. But the best cure is to dump the tank and sterilize it with sodium hypochlorite before it is filled with fresh developer.
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