Carbon
prints were patented as a photographic process in 1864 by Joseph Wilson
Swan. However the basic principle goes back somewhat earlier to the invention
of gum printing by Mungo Ponton in 1839. As described in the section on
gum printing, a mixture of gum arabic, or gelatin and potassium dichromate
coated on a sheet of paper will harden on exposure to light so that the
mixture cannot be washed off in warm water. If exposed under a negative
only those parts of the emulsion which have received less light through
the denser parts of the negative will wash off.
Thus a reverse image of the original remains, giving a positive print. In 1855 Alphonse Poitevin patented a method of incorporating a carbon powder (black) in the mixture. This of course produced a print containing a range of tones from black through grey to white. Swans method improved on this by using a pre-coated paper which he called 'carbon tissue'. His method is very little different from what we use now, although 'carbon' is something of a mis-nomer, as a wide range of coloured pigments can be used in the tissue to form the image.