Collotype Printing

Collotype printing is a screenless photomechanical process that prints photographs in oil-based ink from a plate coated with light-hardened dichromated gelatin, rendering continuous tone without a halftone dot. The name derives from the Greek kolla, "glue." Alphonse Poitevin patented the principle in 1855, but a copper-plate version of 1865 held the gelatin too poorly for long runs. In 1868 Joseph Albert and Jakub Husník bonded it to glass through a hardened subbing layer, and their Albertype made commercial editions practical. Ernest Edwards's heliotype variant printed Oscar Gustave Rejlander's photographs for Charles Darwin's The Expression of the Emotions in Man and Animals (1872). From about 1870 to 1930, collotype was widely used to print photographs directly on the page in books, catalogs and postcards.

The plate is coated with dichromated gelatin, dried with gentle heat and exposed in contact with a reversed negative, so the gelatin hardens in proportion to the light it receives. Washing in cold water removes excess dichromate and opens a fine, irregular reticulation that acts as the printing grain. Once the plate is damped with glycerin and water, soft highlight areas repel the stiff ink while hardened shadows accept it in graduated amounts. Unvarnished collotypes have a velvety matte surface, and under a loupe the image resolves into a worm-like grain lying level with the paper. Nineteenth-century printers often used black-violet ink and shellac varnish to imitate gold-toned albumen prints. The 781 plates of Eadweard Muybridge's Animal Locomotion (1887) form one of the most complex collotype productions.

Collotype was long treated as a means of reproducing photographs rather than as an original photographic medium, and few photographers adopted it as their primary process. Todd Walker's 1970s collotypes are a notable exception, and Kyoto's Benrido atelier still prints editioned portfolios with contemporary photographers. Photogravure differs as an intaglio process, holding ink in etched recesses over an aquatint grain. Woodburytype casts a grainless image with slight surface relief from a lead mold filled with pigmented gelatin rather than ink. Halftone breaks tone into a regular dot screen, a pattern that also marks halftone-collotype hybrids such as Aquatone.