The carbon process is based on the light sensitivity of chromium in the form of dichromate salts suspended in pigmented gelatin, known as a dichromated colloid. This class of photographic processes are also sometimes referred to as “pigment prints” because the visible image forming substance is pigment.

Carbon tissue, consisting of pigmented gelatin coated on a paper support, was commercially available in cut sheets or rolls. Before use, the photographer sensitized the tissue with a potassium dichromate solution. When exposed to light under a negative, the gelatin hardened in proportion to the amount of light received in the shadows and mid-tones and remained soft and water soluble in the highlights. The hardening action of light took place from the top down so that in the mid-tones the soluble gelatin was under the hardened portion of the gelatin. The exposed tissue then had to be transferred to another support before development so that all the soluble gelatin could be removed, preserving the mid-tones.

The single transfer method required the exposed carbon tissue to be transferred to a sheet of transfer paper, which consisted of a paper support coated with hardened gelatin. Each element was soaked in cool water and then brought into contact, squeegeed together, and set between blotters under weight for about twenty minutes. The tissue and transfer paper were then placed in a hot water bath, the carbon tissue’s paper backing was removed and discarded and the print was “developed” in hot water which dissolved the soluble gelatin in the highlights and mid-tones. Next, the print was “fixed” in a bath of alum which hardened the gelatin and dissolved residual chromium salts, and then the print was washed.  Single transfer prints were often laterally reversed, depending on the subject this may or may not be important. However, it could be rectified by several methods including printing through the base of the negative, which would cause some loss of sharpness and detail, or by using the double transfer method.

The double transfer method required the exposed carbon tissue to be transferred to a temporary support. The print was then developed, fixed and washed the same way as the single transfer method. The final transfer paper, consisting of paper coated with hardened gelatin, was soaked in water and brought into contact with the wet carbon print, they were squeegeed together and dried. After the print was dry, the temporary support was removed.

Initially the pigment used was carbon black, hence the name of the process—carbon. Any color pigment could be used and a wide variety of colors were commercially available. The most common colors were black, dark brown, and a purple/red-brown that resembled the color of printed out prints (albumen). Carbon prints could also be transferred to a variety of rigid supports, such as porcelain, glass, enameled wood, etc.

It is not possible to distinguish between a carbon and carbro print. Carbro was introduced later and is closely related to carbon. The main difference is that in the carbro process the gelatin was hardened by squeegeeing the sensitized carbon tissue against a silver bromide print. The name “carbro” is derived from the combination of carbon and bromide. Since carbon printing began to decline with the introduction of carbro, prints made after 1930 are likely carbro. One major advantage of carbro was the ability to make enlargements because the tissues were exposed in contact with a silver bromide enlargement rather than contact printed.