The carbro print process of 1919 is an outgrowth of the earlier Carbon-Print process of the l860s and led via the ozotype and ozobrome processes to the color carbro process. A monochrome carbro print begins with a finished GELATIN SILVER BROMIDE print, which is a print made on paper bearing a highly light-sensitive silver-bromide emulsion. This print, when wet, is pressed together with a sensitized carbro tissue. The tissue is a thin paper coated with pigmented gelatin that has first been sensitized by immersion in a solution of potassium bichromate and then immersed in a bath containing bleaching agents. Through chemical action, the gelatin of the carbro tissue hardens in proportion to the selective bleaching out of the silver of the bromide print. The print and the tissue are separated and the tissue placed face down on a sheet of transfer paper for about twenty minutes. The tissue and transfer paper are then separated in warm water, leaving the gelatin layer attached to the transfer paper. The paper is then bathed until the excess softened gelatin is washed away, leaving the partially hardened gelatin image. The print is further hardened in an alum bath; then it is dried. As it is silver-free, the monochrome image thus produced is permanent, that is, it does not fade. The original bromide print, now bleached, can be redeveloped and used to make as many as five additional carbro prints. Tricolor carbro prints are made possible by a further evolution of the process, in which three bromide prints of one subject are used. The negatives for these prints are made by photographing the subject or a color image of the subject through a red, a green, and a blue filter. From these "separation negatives" the bromide prints are made. Each is then placed in contact with a thin bichromated gelatin tissue, pigmented in accordance with the color of the filter used on the negative. The process continues as described above, and the resultant three transfer papers, each carrying a different color, are stacked together in careful alignment, that is, in registration, to produce a single colored image. Like a black-and-white carbro print, this image is permanent.

Baldwin, Gordon   LOOKING AT PHOTOGRAPHS   Getty/BMP 1991