DEFECTS IN NEGATIVES AND PRINTS 519 



c. During storage, emulsions may show a tendency to fog. This tendency is 

 aggravated in the presence of moisture and heat and in the presence of such gases and 

 vapors as hydrogen sulphide, coal gas, and fumes of turpentine and benzol. Conse- 

 quently, preventive measures should be those which will protect the stored negative 

 material from exposure to chemicals, gases, fumes, light, undue heat, and other radia- 

 tions. Keep on hand only such material as will meet normal demands for negative 

 material. 



d. When properly used, none of the common developers produce sufficient fog to 

 be serious in practical photography, except possibly in certain kinds of scientific 

 work. Fresh and properly compounded developers are the best preventive against 

 fog produced by the developer itself. The developer should be free from impurities, 

 especially those of metallic salts, and should be used at the normal recommended 

 temperature. This will ordinarily be between 60 and 70°F. 



e. Impurities in the developer may produce appreciable fogging. Excess of sul- 

 phite or the presence of metallic salts in the developer or the metals themselves are a 

 relatively common source of fog. The only preventive is to use properly com- 

 pounded developers of chemical purity and to ascertain that impurities do not con- 

 taminate the developing solution. Traces of metals or metallic salts may easily come 

 from metallic developing tanks. 



/. The amount of fog is roughly proportional to the time of development, so that, 

 so far as fog is concerned, it is inadvisable to prolong development beyond the normal 

 time recommended by the manufacturer of the photosensitive material. 



g. Development, as practically all chemical reactions, proceeds at a more rapid 

 rate at high than at low temperatures. Consequently the tendency toward fog 

 increases as the temperature of the developer is increased. For normal processing, the 

 usual recommendation of manufacturers is that the developer temperature should not 

 exceed 70°F. or 21 °C. 



h. The presence of metallic salts or of metals in the developers is to be avoided. 



i. The presence of excess sulphite in the developer should be avoided. 



j. Since aerial fog results from exposure of the wet negative to the air during devel- 

 opment, this type of fog may be reduced or eliminated by keeping the negative material 

 completely submerged in solution during development. 



k. Too intense a darkroom light or the wrong type of safe-light filter for the type 

 of emulsion being processed may result in light fog or in partial or complete reversal 

 of the image during development. The darkroom safe light should (1) pass only those 

 wavelengths for which the negative is insensitive or (2), in the case of panchromatic 

 emulsions, should transmit freely only in the region for which the human eye is most 

 sensitive at low light intensities. The recommendations of the manufacturer may be 

 taken for the type of safe-light filter to use with a given emulsion. Manufacturer's 

 recommendations concerning the amount of light to use may be taken as a general 

 guide, but practical tests will indicate whether or not any particular situation produces 

 fogging. 



Generally speaking, development or chemical fog may be eliminated or niinimized 

 by (1) using fresh, pure, and properly compounded developers, (2) developing for the 

 correct time and at the correct temperature, (3) keeping the negative completely 

 submerged in developer, (4) developing in absolute darkness or with a safe light which 

 has been proved satisfactory. 



4. Remedy. — General reduction of the negative may sometimes be helpful. 



Emulsion Fog. 



1. Distinguishing Characteristic. — General veil of fog on negative not accounted 

 for bj^ light or chemical fog. 



