Carbon print

In the original version of the printing process, carbon tissue (a temporary support sheet coated with a layer of gelatin mixed with a pigment—originally carbon black, from which the name derives) is bathed in a potassium dichromate sensitizing solution, dried, then exposed to strong ultraviolet light through a photographic negative, hardening the gelatin in proportion to the amount of light reaching it.

It was developed in the mid-19th century in response to concerns about the fading of early types of silver-based black-and-white prints, which was already becoming apparent within a relatively few years of their introduction.

Carbon tissue, a layer of unsensitized pigmented gelatin on a thin paper support sheet,[1] was introduced by British physicist and chemist Joseph Swan[2][3] in 1864.

The efficiencies gained through these more modern automated processes relegated carbon printing to the commercial backwaters in the latter half of the 20th century.

Because of the comparative insensitivity of the material, sunlight or another strong source of UV light is normally used to minimize the required exposure time.

They are developed in warm water, which dissolves the unhardened gelatin, leaving a colored relief image that is thickest where it received the strongest exposure.

A fourth black pigment "key" layer is sometimes added, as in mechanical printing processes, to improve edge definition and mask any spurious color cast in the dark areas of the image, but it is not a traditional component.

A more contemporary example of the color stability of pigments is found in the paints used on automobiles today, which must survive intense daily exposure to very harsh lighting, under extreme conditions.