The Collaborative International Dictionary of English v.0.48:

Carbon process \Car"bon process\ (Photog.)
   A printing process depending on the effect of light on
   bichromatized gelatin. Paper coated with a mixture of the
   gelatin and a pigment is called

   carbon paper or

   carbon tissue. This is exposed under a negative and the
      film is transferred from the paper to some other support
      and developed by washing (the unexposed portions being
      dissolved away). If the process stops here it is called
      single transfer; if the image is afterward transferred
      in order to give an unreversed print, the method is called
      double transfer.
      [Webster 1913 Suppl.]