Photogravure is an intaglio printmaking or photo-mechanical process whereby a copper plate is coated with a light-sensitive gelatine 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.
The photogravure process had its beginnings in the mid-19th century when Fox Talbot devised a method of producing intaglio (etching) plates by etching through a bichromated gelatine film. In 1879 Karl Klic introduced the first efficient and reliable method of producing photogravures.
The Process in Brief
1. A film positive from a photographic negative or artist’s marks on a transparent material is created. The film positive is made much like a gelatine silver print, the only real difference being that the positive is made on transparent film.
2. A sheet of gelatine tissue is sensitized in a bath of chilled potassium dichromate. The sensitized gelatine tissue is squeegeed gelatine side down to a sheet of Plexiglas and allowed to dry.
3. The dried gelatine tissue is exposed to the positive. During exposure the gelatine is hardened in proportion to the amount of light the gelatine receives through the positive. The action of light on the sensitized gelatine renders the more exposed areas of gelatine harder than the less exposed areas.
4. The exposed gelatine tissue is placed in a bath of chilled water or a mix of alcohol and water upon a sheet of mirror-finished copper. The tissue and plate are removed from the water and the tissue is squeegeed firmly on to the copper sheet.
5. The sandwich of exposed gelatine tissue and copper is allowed to dry from 5 to 45 minutes. During the drying period the gelatine contracts, creating a firm bond with the copper.
6. The dried gelatine on copper is submerged in a tray of hot water. The hot water dissolves the lesser-exposed gelatine and the image is revealed in relief in gelatine. Once development is complete the image is clearly visible in densities of gelatine on the plate.
7. The plate with the image in gelatine is allowed to dry in a temperature and humidity controlled environment for a minimum of 30 minutes.
8. The plate is placed in an aquatint box and a dusting of rosin is allowed to settle on the plate. The particles of rosin create acid resistant dots on approximately 50% of the surface of the plate. Once the dusting is complete the plate is placed in an oven. The heat of the oven melts the particles of rosin, adhering them to the gelatine tissue.
The aquatint box contains a wind vane and a few pounds of finely pulverized rosin powder. The wind vane once set in motion creates a cloud of aquatint particles inside the box. The plate is then placed in the box.
9. The aquatinted plate is placed in a room in which humidity and temperature are controlled and allowed to sit for a minimum of two hours. During this time the temperature and moisture content of the gelatine tissue equilibrates with that of the room.
10. The back of the plate and any other areas of the copper not to be etched are protected with an acid resistant paint.
11. The plate is etched in a series of ferric chloride acid baths. The acid penetrates the gelatin tissue and around the aquatint particles leaving tiny pits in the plate. The darkest areas of the image are the most deeply etched. The highlights receive the least etching.
12. The plate is rolled with etching inks, wiped by hand with a starched cloth and run through an etching press with a sheet of art paper. The ink is transferred from the plate to the paper. Gravures are printed in much the same manner in which Rembrandt’s or Goya’s etchings were printed.