Note:  Do not rely on this information. It is very old.

Carbon Process

Carbon Process, a photographic printing process, which depends on the fact that gelatine becomes insoluble if mixed with potassium bichromate and exposed to light. The paper ("pigmented paper," or "carbon tissue") is therefore prepared by coating it with gelatine, well-mixed with some finely-powdered pigment, as Venetian red, bone black, alizarin lake, etc. It is then sensitised by floating it on a solution of potassium bichromate, and dried. To obtain the print it'is exposed to light under the photographic "negative." As no visible change occurs, the length of exposure must be gauged by experience, or by means of an instrument called the "actinometer." When fully printed it requires to be "developed." To do this the gelatine is transferred face downwards to another sheet of paper by pressing it on when wet, and peeling off the original paper support. It is then washed with hot water, which dissolves off the gelatine and pigments in the parts unexposed to light. It is next immersed in alum solution to harden the film, washed well in cold water, and dried.