The carbon printing process was invented and patented in 1864 by Joseph Wilson Swan. The process is an enhancement to the carbon process invented in 1855 by Alphonse Poitevin which is, in turn, an enhancement of the gum printing process invented in 1839 by Mungo Ponton.

Before we describe the process, we need to be clear on an important point which is probably best made using a demonstration: take a photographic transparency (e.g. a colour slide or a negative) and hold it out in front of you. Orient it so that the top of the image on the transparency is upwards. Make sure that you are looking at the transparency from the non-emulsion side (i.e. from the shiny side). If you inspect the image, you'll see that its orientation is correct (i.e. left on the image will be to your left and right on the image will be to your right).

The carbon printing process works as follows:

  1. A "carbon tissue" is prepared in advance by applying a coloured gelatin coating onto one side of a fairly stiff sheet of paper (strictly speaking, the carbon tissue is the thin layer of coloured gelatin).
  2. A second and possibly a third sheet of paper are prepared in advance with a clear gelatin coating on one side.
  3. The tissue and backing paper is soaked briefly in a dilute solution of potassium dichromate. This renders it sensitive to light (i.e. the parts of the tissue which are subsequently exposed to light will become relatively insoluble in water).
  4. A negative is placed onto the tissue with the emulsion side of the negative in close contact with the tissue and the "sandwich" is exposed to a bright UV light source for a while (e.g. bright sunlight for a few minutes).
  5. The negative is removed and the tissue is washed to remove the dichromate. The tissue will be dissolved in inverse proportion to the amount of light that the tissue was exposed to (i.e. tissue which was covered by relatively dark areas on the negative will be washed away more than tissue which was covered by relatively transparent areas on the negative). Note that if the tissue and backing paper were to be held up and viewed at this point with the top of the image facing upwards then the image would be properly oriented.
  6. The second sheet of paper and the tissue are brought together (gelatin-to-gelatin) underwater and squeezed together quite firmly.
  7. The resulting "sandwich" is allowed to soak in warm water for a short while and then the backing paper is removed from the tissue and discarded.
  8. additional gentle washing (i.e. soaking, not rubbing) in warm water dissolves the remaining unexposed gelatine away and leaves the image on the second sheet of paper.
  9. The second sheet is allowed to dry and then washed in potassium metabisulfite to remove the remaining potassium dichromate.
  10. The sheet is washed again in clear water and dried.
The resulting image isn't the final image as it is reversed (i.e. if viewed with the top of the image pointing upwards then left in the image will be to the viewer's right). To "correct" this, before drying the image for the last time, the tissue is transferred to the third sheet of paper by essentially repeating the last five steps.

An alternative approach which is sometimes used avoids the double transfer by placing the negative in contact with the carbon tissue with the emulsion side away from the tissue. This approach results in a degradation of resulting image quality because of the gap between the emulsion on the negative and the carbon tissue. The degradation may be acceptable if the negative is quite thin but it becomes hopeless with negatives on glass plates.

Swan's innovations over Poitevin's carbon process were:

  • the use of a carbon tissue that is transferred between the sheets of paper. This ensured that the side of the tissue which was least exposed to light (i.e. the side originally touching the carbon tissue's backing paper) and hence most soluble were most exposed to the washing process which only starts in ernest when the "carbon tissue" has been transferred to the second sheet of paper.
  • the double transfer of the carbon tissue (from the first "backing paper" sheet to the second sheet and then from the second sheet to the third sheet) to yield a correctly oriented positive image.
Swan's process is still used today with very minor changes. It results in long lasting photographic prints of exceptional tonal quality.

Sources: the two main sources were a web page titled "carbon print" located at http://photography.about.com/library/glossary/bldef_carbon.htm (last accessed 2002/09/28) and a document titled "Gelatin in Photography / Chapter 1 / History of the Applications of Gelatin in Photography" which can be found in PDF format at http://rmp.opusis.com/documents/gelatininphotography.pdf (last accessed 2002/09/28).

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