Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
The collotype process, not to be confused with either calotype or kallitype, is a photomechanical process for printing photographic illustrations. It has a very definite resemblance and relationship not only to lithography, but also to such purely photographic processes as bromoil.
The process depends on the principle that oil and water do not mix. A photographic image is made in gelatin, which is treated in such a manner that the white areas absorb water while the dark areas repel it. Then, if the plate is inked with a greasy ink (as in lithography), the ink will be attracted to the dark portions and repelled from the whites. The ink is then transferred to paper in a press of fairly simple construction.
Collotype is a completely screenless process. Middle tone reproduction is accomplished by a special treatment of the gelatin coating on the plate which causes an exceedingly fine reticulation, similar in purpose to the dot pattern of the halftone, but practically invisible to the naked eye. In the best examples of collotype prints, even grain is not visible. It is normally no larger than the grain of a silver image.
Unlike bromoil, no silver image is used in this process. A plate of glass, aluminum, or zinc is coated with a layer of plain gelatin which has been sensitized with ammonium dichromate. This is then exposed under a negative to arc light for a time sufficient to harden the gelatin completely in the black areas. After exposure, a solution of pure glycerin in water is poured over the plate. This accomplishes two things; it causes the reticulation pattern to appear, and it soaks into the plate in proportion to the lack of hardening. The plate is then put in a press which has both ‘water’ and ‘ink’ rollers. This keeps the plate at the exact moisture content needed during the press run.
Printing is done ‘offset’; the plate does not print directly to the paper, but to a rubber blanket, which in turn, transfers its ink to the paper. The collotype process is mainly valuable for short runs where exceedingly fine reproduction is desired. For longer runs, several plates can be made and used in succession as they wear out.
A simplified version of the collotype process, known as photo-gelatin printing, was used for many years to make the colored pictures used in the lobby displays of motion-picture theaters.
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