Insights into the photogravure grew out of early attempts to create a photographic image. The refined process begins by first dusting a metal plate with the same fine resin powder as used in aquatint to form an acid resist. Once the rosin has been melted to the plate’s surface to create a random dot pattern, a photosensitive gelatin emulsion is applied. When dry it is then exposed to a positive transparency where the gelatin light hardened areas form an additional acid resist. The remaining gelatin is wash away in proportion to the density of the transparency that covered it, either nearly or completely exposing the metal surface of the plate. When placed in successive acid baths of decreasing strength, the metal will first dissolve in the exposed areas between the rosin dots. The thinner areas of gelatin will eventually wear away in proportion to their exposure to light and reveal more bare metal of the plate underneath. Because these areas have less contact time with the acid they will not etch as deeply, and these shallow wells will hold less ink producing lighter tones when printed. Photogravure produces thousands of irregular ink cells in varying depths that can merge into a subtle continuous toned image with very rich blacks.
Use the link below for more insights into this process.