The collotype is a photomechanical process combining planographic and photographic technologies. Printing plates are produced based on the light sensitivity of chromium salts in gelatin – a mixture referred to generally as a dichromated colloid. From the plates, images are printed using greasy ink in a manner similar to that of lithography.

Invented by Alphonse Poitevin in 1855, the collotype process was capable of rendering fine photographic detail with less work than other high quality photomechanical processes such as the Woodburytype. The earliest demonstration of the process was a portrait by F. Joubert featured in The Photographic Journal described as a “phototype.” Many subsequent improvements and alterations were made to the process, most notably by Joseph Albert in 1868. Albert introduced the use of glass printing plates and a dichromated albumen-gelatin substratum (subbing) layer to improve the adhesion of the relief gelatin to the plate. The resulting process was patented as the albertype and initiated the widespread commercial application of the collotype.

Production of a collotype began with the thorough cleaning of a glass or metal plate followed by application of the subbing layer. The subbing layer typically consisted of sodium silicate in albumen or gelatin, a variation on Albert’s original dichromated formula capable of hardening without exposure to light. Above the hardened subbing layer an even coating of a light sensitive solution composed of water, gelatin and potassium or ammonium dichromate was applied. The sensitized plate was then leveled and dried in a drying box for 2-3 hours at roughly 122°F. Once dry, the plate was placed in a copy frame and exposed beneath a laterally reversed negative. The dichromated gelatin hardened in proportion to the light it received: shadows became hard and highlights remained soft. The plate was “fixed” in a rinse of cold water to remove excess dichromate and also to create randomized fissures in the gelatin. These fissures—or reticulation—served as a screenless matrix to control the amount of ink accepted or rejected by the gelatin during printing. 

Following washing and drying, the plate could be used for printing. Prior to inking, a damping solution (usually glycerin and water) was coated over the gelatin to moisten the soft regions. A stiff oil-based ink was then applied using a velvet or leather roller. Where the gelatin was wet (highlights) the ink was repelled; where gelatin was dry (shadows) the ink adhered to the plate. Unlike lithographic plates, collotype plates had a graduated capacity for absorption: water was absorbed in proportion to the amount of light the plate received during exposure. Ink was similarly adhered or repelled, resulting in a continuous tone image when transferred to paper.

The collotype’s reticulation pattern created a fine grain suitable for duplicating photographs and works of art. The process was also used for printing postcards, posters, catalog and book illustrations and advertisements. Inks, papers and varnishes were often selected to mimic photographic processes such as albumen, silver gelatin and platinum. Color collotypes were achieved via hand-painting, colored inks or inclusion of other processes such as chromolithography.