The "carbon process" is a method for producing photographic prints
invented and patented by Alphonse Poitevin
The process is an improvement on the gum printing
process (invented by Mungo Ponton
As described in more detail in the gum printing node, gum printing takes advantage of the fact that a mixture of gum arabic and potassium dichromate coated onto a sheet of paper will harden and become
relatively insoluble by water when exposed to light.
Poitevin incorporated carbon powder into the mixture of gum arabic and potassium dichromate.
As with gum printing, the coated paper is exposed to light and the parts of the mixture which havn't hardened are washed off.
As the hardening is an analog process (i.e. the amount of hardening depends on the amount of exposure to light) and since carbon has been mixed into the coating, the resulting positive image has a full tonal range of white through to black.
The use of the word "carbon" is a bit of a misnomer as actually a variety of powders can be used in place of carbon.
By using coloured powders, it is possible to produce monochromatic images in a wide variety of colours.
One major problem with both gum printing and Poitevin's process was that the light tended to render only the top layers of the coating insoluble.
Since the lower layers (i.e. those in contact with the paper) were generally still soluble, the step of washing away the soluble parts would often wash away the entire image.
Another problem with Poitevin's process was that even if the image remained, the mid-range tones tended to be rendered poorly although much better than with the gum printing process.
In 1858, Blair, Schouwaloff, and Burnett
announced almost simultaneously that both problems could be solved only by exposing the paper from the back.
Unfortunately, exposing the paper from the back greatly increased the required exposure times and meant that any texture or impurities in the paper would be visible in the image.
A final solution would have to await the invention of the carbon printing process by Joseph Wilson Swan in 1864.