Sensitizing the Gelatin Tissue
The gelatin tissue used for photogravure-also called pigment paperconsists 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 colloidbased 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 to the tonalities of the positive. The more light the gelatin receives, the greater the depth of hardening. The less light that passes through the positive, the shallower the depth of hardening. The exposed side of the gelatin is then bonded to the copper plate. Finally, the paper backing and remaining soluble gelatin is removed with warm water. The resulting layer is a three-dimensional contour image that will act as a permeable membrane between the copper plate and the etching mordant (Figure 3-1).