The gelatin tissue used for photogravure—also called pigment paper—consists of a thin layer of colored gelatin on a paper backing. It is very similar to the tissue used for carbon printing. Similar to other colloid-based photography such as carbon printing and gum bichromate, photogravure’s gelatin tissue is sensitized using a solution containing a dichromate salt. Exposure to ultraviolet light causes the sensitized gelatin to become less soluble (raising its melting point) relative to the degree of exposure. The tanning or hardening effect is called insolubilization. This photochemical process produces a layer of gelatin that contains a hardened contour representation of varying densities, which correspond ...
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