Sandy King has an extraordinarily detailed article about the carbon transfer process on the «Alternative Photography» site [here], including a historical survey, detailed instructions for mixing your own chemicals, process notes and tons of references. The process itself is painstaking and suitable only for dedicated obsessives. But the results are awesome.
The negative (sorry, the diapositive) from which the color carbon print of Frida Kahlo was made was taken in 1939. Artist: Nickolas Muray. A rough guess: the negative was an 8 by 10 inch Kodachrome. Kodachrome Professional film was supplied in 8×10 inch sheets with a very, very slow speed of ASA 8 (yes, eight, for daylight illumination) and ASA 10 for Type B (tungsten bulb, used indoors). For more work by this and other artists, see the Art and Soul site [here].
This is what Sandy King has to say, in part:
The carbon transfer process is considered by most persons who know it to be one of the most beautiful of all photographic processes. Carbon prints are capable of a wide range of image characteristics, they can be virtually any color or tone, and the final image can be placed on a wide variety of surfaces, including glass, metal, paper, as well as various kinds of synthetic surfaces. When the final support has a smooth surface carbon prints have a highly unique quality, a discernible relief that gives them a real dimensional quality, especially prominent when the photograph is held sideways to the light. Carbon is without question the most distinctive and stable of all photographic processes, with the capability of presenting images with a wide range of image characteristics, of virtually any color or tone, on a wide variety of surfaces. Finally, carbon transfer prints, which are made up of inert pigment(s) suspended in a hardened gelatin colloid, are the most stable of all photographic prints.