Chemistry of Black and White Photograpy

Chemistry of Black and White Photograpy

Light exposes tiny crystals of AgBr that are suspended in the thin layer of gelatin coated on the surface of the photographic paper.
Exposure of the AgBr with crystals produces particles of several free silver atoms such as Ag₂^o, Ag₃^o, Ag₄^o. As light passes through the lightest regions of the negative, dark regions with the highest density of these metallic silver form -- the longer the exposure, the greater the number of silver particles. The dark regions of the negative allow less light to come into contact with the AgBr crystals. This latent image is not yet visible to the eye.
Developing enhances the latent image. More AgBr is converted to silver metal by the developer. Those grains of AgBr with metallic silver clusters l on their surface from the initial exposure of AgBr with light, react faster with the developer to produce more black silver metal. The rate of reaction is slower when AgBr grains have fewer silver metal clusters on them.

The stop bath, acetic acid, stops the developer from reacting with AgBr. The image does not darken after the print is placed in the stop bath.
The fixer, sodium thiosulfate, reacts with the remaining AgBr to form a water soluble salt that can be washed out in the final step in the water bath.

Toning

Iron:
Silver metal reacts with K₃Fe(CN)₆, potassium ferricyanide, to form Fe₄[Fe(CN)₆]₃ in the presence of the iron salt in the solution. The black silver metal is replaced by this Prussian blue colored insoluble salt. Copper:
The same reaction as above is carried out with copper sulfate, CuSO₄, and potassium ferricyanide. The salt formed is Cu₂Fe(CN)₆, Hatchett's brown. The brown salt replaces the silver metal of the black image to give a copper color.
Sepia:
Sodium thiosulfate reacts in the presence of acid to produce white sulfur. The sulfur then reacts with silver metal of the image to form brown Ag₂S.