Photogravure, sometimes known as heliogravure (particularly hand photogravure), is an intaglio printmaking or photo-mechanical process whereby a copper plate is coated with a light-sensitive gelatin tissue which had been exposed to a film positive, and then etched, resulting in a high quality intaglio print that can reproduce the detail and continuous tones of a photograph.
First, a continuous tone film positive is made from the original photographic negative. A smaller negative can be enlarged onto a sheet of film, which is then processed to a range of continuous tones with specific densities. The copper plate must be thoroughly cleaned, its surface highly polished, and its edges beveled (to avoid damaging the paper during printing). Next it is evenly dusted or sprayed with an acid resist of rosin or asphaltum, and heated to make the resist adhere. This procedure is identical to that of aquatint print-making, so early photogravures were sometimes called photo-aquatints.
The second stage is to sensitize a sheet of pigmented gelatin tissue by immersion into a 3.5% solution of potassium dichromate for 3 minutes. Once dried against a Plexiglas surface, it is ready for the next stage. In the same time, the image is also prepared. A positive transparency is made from either an original negative or a copy negative. This film positive, which must be made the size desired for the final print, is then contact-printed under ultraviolet (UV) light to a gelatin-coated paper (known as carbon tissue) which was previously made light sensitive by soaking it in a solution of potassium bichromate, then dried. In this process, the action of the light through the film positive changes the melting temperature of the gelatin. Areas exposed to light have a higher melting temperature and are said to be "hardened" and less exposed areas stay the same.
Next the image must be transferred to the prepared copper plate. The image-carrying tissue is adhered to the plate. This tissue/plate is then soaked in hot water softening the unexposed gelatin and allowing the paper base of the tissue to separate. Portions of the gelatin that received little or no light during exposure to the transparency remain soluble and slowly wash away, leaving a gelatin image that will act as an acid resist when the plate is etched. The gelatin image on the copper plate, now called a resist, is then dried. The gelatin tissue is adhered or "laid down" onto the highly polished copper plate under a layer of cool water. It is squeezed into place and the excess water is wiped clear.
The next stage is to etching the plate in a series of ferric chloride baths, from the densest to slightly more dilute, in steps. The density of these baths is measured in degrees Baum.. The viscosity of the Ferric Chloride controls the speed with which the solution penetrates the gelatin. The ferric chloride migrates through the gelatin, etching the shadows and blacks under the thinnest areas first. The etching progresses through the tonal scale from dark to light as the plate is moved to successively more dilute baths of ferric chloride. The result is a plate with many minute reservoirs or cells of varying depths. During printing, the deeper cells hold more ink and thus transfer more ink to the paper, creating the darker areas of the image. The "wells" which hold the ink vary in depth, a unique aspect of photogravure.
Printing a photogravure is similar to printing any other intaglio plate, especially a finely etched aquatint.
This procedure produces "grain" gravures, so called because of the random dots created by the dusted rosin. Photogravure is a time-consuming, labor-intensive, costly process used today only by fine-art photographer-printmakers. Alternatives are: Digital direct-to-plate photogravure and Photo-polymergravure.