The carbon printing process is one that has a great many more steps than you will encounter in other alternative techniques. Each of these major steps has a plethora of simple little steps so be prepared to spend some time making a print. There are four phases to making a carbon print:
PHASE #1 - Sensitizing the carbon tissue
PHASE #2 - Exposing the carbon tissue
PHASE #3 - Mating the carbon tissue to the final support
PHASE #4 - Developing and drying the final image
Here’s a quick overview. A piece of very thin and flexible paper is meticulously coated with gelatin and a pigment. I strongly advise buying prepared tissue from Bostick & Sullivan rather than making your own. This prepared tissue is then sensitized in a dichromate solution. The sensitized sheet, which is called carbon tissue, is then dried, mated with a negative, and exposed with UV light in a manner similar to most other alternative processes.
The UV light causes the long chains of atoms making up the gelatin to become “cross-linked”. This entanglement hardens the gelatin, and the degree of hardening is relative to the amount of light received through the negative, i.e., like in the gum bichromate process. The exposed tissue is then immersed in a cool water bath and mated to a sheet of microporous gelatinized or resin coated paper and put under pressure for a period of time.
The mated pair is then immersed in a warm water bath until there is visual evidence that the tissue is ready to be separated from the support. This is often recognized as a delicate black cloud on the edges of the mated pair. The tissue is then peeled away from the support and washed until the unexposed gelatin is washed away - leaving a permanent carbon image. If additional clearing of the highlights is needed the print is immersed in a bath of dilute potassium metabisulfite.