Carbon Print

A carbon print is a photographic print whose image consists of pigment held in gelatin, formed when light hardens a dichromate-sensitized layer of pigmented gelatin exposed in contact with a negative. The name comes from the carbon black first used, though almost any pigment can serve. Alphonse Poitevin introduced the process in 1855, building on William Henry Fox Talbot's 1852 finding that dichromated colloids become insoluble on exposure, at a time when the fading of albumen and salted paper prints was a pressing concern. Because light hardens the layer from the surface down, early prints lost their halftones in washing. Adolphe Fargier's transfer method of 1860 solved the problem, and Joseph Wilson Swan's 1864 patent for ready-made carbon tissue made the process practical. The Autotype Company, which bought Swan's rights in 1868, commercialized it.

The tissue, a paper coated with pigmented gelatin, is sensitized in potassium dichromate, contact-printed under a negative, squeegeed onto a transfer support and developed in warm water that dissolves the unhardened gelatin. Single transfer yields a laterally reversed image, and double transfer through a temporary support restores its orientation. The result is a long tonal scale with no silver, deep shadows that stand in faint glossy relief above the highlights, and, with stable pigments, near immunity to fading. Tissues came in many colors, and images could be transferred to glass, porcelain or metal. Permanence made carbon the standard for art reproduction at firms such as Adolphe Braun's, and for the late-1870s carbon edition of Thomas Annan's The Old Closes and Streets of Glasgow, which replaced fading albumen prints.

Carbon belongs to the family of dichromated colloid processes. Gum bichromate and direct carbon methods such as the Fresson process develop on their final support without transfer, and carbro hardens the tissue through contact with a silver bromide print, which allows enlargement. Carbon and carbro prints cannot be told apart visually, and Woodburytypes, cast in pigmented gelatin from a relief mold, are almost identical in appearance and chemistry. The process declined by the 1940s and was revived in the 1970s by the alternative-process movement.