Thinking at an elementary level about photographic processes, one can associate the physical makeup of photographs with the romance and glamour of precious metals such as silver, gold, and platinum. At their core, the processes which use these metals are inherently unstable and will deteriorate to greater or lesser degrees depending on their place in the periodic table. But what of the more humble and most common element in our universe, carbon? It doesn’t carry the same shine or mystique as precious metals, but as a means of reproducing photographs what it lacks in sparkle it makes up for in permanence.
Invented in 1855 by Alphonse Louis Poitevin, the carbon print process was a photomechanical process which did not come into practical use until 1864 when the revised process was patented by Joseph Wilson Swan. This is a drastic simplification of what is a complex history of the refinement of this process. A more complete chronology on Wikipedia will satisfy those readers who are keenly interested in the evolution of the process.
Suffice it to say that the process is based on the hardening action of light on gelatin which has been treated with potassium dichromate (or bichromate). More simply, a photographic negative could be placed on top of a sheet of this treated gelatin which was pigmented with carbon black, and following its exposure to light, a gelatin relief of the image would be formed which was then transferred to a final paper support. Where this process becomes quite complex is when experimental photographers such as the pictorialists at the turn of the century would place multiple layers of these gelatin reliefs on top of one another (or in combination with other processes) in perfect register in order to create more richly coloured and nuanced prints. Furthermore, once the science of three colour separation printing processes developed, the carbon process would evolve into even more complex tri-colour permutations.
In its more straightforward monochrome existence, the carbon print was in fact not restricted to being produced as a purely black pigment based image. It could, depending on the taste of the photographer/printer, take on any colour which they desired or felt best suited the image. However, it is the deep blacks and rich dark browns which are commonly associated with this most permanent of processes.
Although the steps in their creation are different, a finished carbon print can be quite similar to that of a woodburytype in both its appearance and technical identification. This difficulty continues to be a source of considerable challenge and amusement for cataloguers of these exquisite photomechanical reproductions.