518 HANDBOOK OF PHOTOGRAPHY 



may, therefore, be produced wherever free silver is encountered such as in the develop- 

 ing or fixing baths. In development, dichroic fog is formed in the presence of some 

 silver bromide solvent such as ammonia, an excess of sulphite or alkali, or hypo. 

 Thus, in single-tank processing, residue of hypo in the tank may produce dichroic fog 

 the next time development is done. If the fixing bath is not acid, or if it is exhausted 

 or contains an excess of dissolved silver, dichroic fog may be produced in the fixing 

 process. 



3. Prevention. — Dichroic fog may be prevented from being produced in the devel- 

 oper by adding lead acetate to the developer or cleaning the processing system with a 

 hot sodium hypochlorite solution. This procedure prevents the formation of sulphide 

 accumulations. The addition of 1.5 g. of potassium iodide per Uter of developer will 

 tend to decrease formation of fog by converting dissolved silver bromide into relatively 

 insoluble silver iodide. In the fixing bath, dichroic fog may be prevented by main- 

 taining the bath at its normal acidity and by using an intermediate bath of clear water 

 between developing and fixing so as to diminish the amount of developer carried over 

 into the hypo. Hardening the gelatin before development also tends to reduce tend- 

 ency for the formation of dichroic fog. 



4. Remedy. — -Dichroic fog may be removed by rinsing the negative in a solution 

 of 0.5 gr. of potassium permanganate (KMn04) to each ounce of water until the fog 

 has disappeared. The brown stain of manganese oxide which forms during this rinse 

 may be removed by washing the negative in a 5 per cent solution of potassium meta- 

 bisulphite or of sodium bisulphite. 



Chemical Fog. — -(See Development Fog.) 

 Development Fog. 



1. Distinguishing Characteristic. — General or local veiling of image due to deposit 

 of silver on the negative. 



2. Cause. — Practically all fogging troubles not directly caused by light are grouped 

 together under the general classification of chemical fog or development fog. Devel- 

 opment fog may result from any number of factors (some of which are treated sepa- 

 rately in this section) and includes certain types of fog which are likewise treated 

 elsewhere in this section. Thus development or chemical fog may be due to (a) the 

 nature of the negative emulsion, (6) the factory processing of the emulsion (emulsion 

 fog), (c) the age and past history of the negative, {d) the nature of the developer, (e) 

 impurities in the developer, (/) the time of development processing, {g) the tempera- 

 ture of development processing, {h) the presence of metallic salts in the developer, 

 {i) the presence of sodium sulphide in the developer, {j) the general processing during 

 development which may give rise to aerial fog, or (/c) the nature and intensity of the 

 darkroom illumination, for example. 



3. Prevention. — The manner of preventing the various types of development or 

 chemical fog depends upon the specific cause of the fog. According to the classifica- 

 tion given above, the following preventive measures have been found useful. 



a. The amount of fogging of a negative is proportional to the speed of the negative 

 or its sensitivity to light. Consequently, most difficulty from fogging maj^ usually 

 be expected from fast panchromatic emulsions, with minimum fogging resulting when 

 slow noncolor-sensitive emulsions are used. Some fog always occurs, although under 

 proper conditions this may usually be neglected for practical purposes. 



h. Emulsion fog results from the presence of developable grains of silver halide in 

 the unexposed portions of the emulsion. Emulsion fog is usually greater in fast emul- 

 sions than in slow emulsions and may be produced in the process of manufacture by 

 contact with fogging materials or by deterioration with age. 



