A certain amount of unselective reduction of the silver halide occurs during the development of most photographic emulsions.
This "fog" production acts as a limiting factor in the time of development, and may interfere with the achievement of shadow detail.
The importance of fog control in the practice of photography scarcely needs to be emphasized. However, the amount of material pertinent to a study of the mechanism of fog formation is small.
The existence of different kinds of fog is well known. Dundon and Crabtree 1 suggest the following classification.
(I) Emulsion fog, which results from the presence of "developable" grains in the unexposed emulsion.
(2) Aerial fog, which results from aerial oxidation of the developer in contact fi$ ,tbe,;mulsion.
(3) Chemical development fog, which merely a non-selective reduction, such as occurs with silver halides without the presence of gelatin, or reduction preceded by a nucleation effect which is the result of a chemical property of the developer."
(4) Solvent fog, such as dichroic fog, "which is the result of physical development by silver dissolved in the developer due to the presence of a silver solvent such as sulfite or ammonia."
(5) Light fog, caused by accidental exposure to actinic light.
The preceding listing contains several causes of fog which are secondary and accidental.
Light fog is merely the accidental production of a latent image by light action, and dcvelopment of this latent image proceeds in the normal way. Xerial fog is a secondary and controllable type which occurs, if at all, only in the presence of oxygen.
Certain impurities sometimes present in the developing solution may promote * Communication No. 865 from the Kodak Research Laboratories.