The photogravure process (cont.)
After drying, the copper plate with the attached resist is placed in a ferric chloride solution. The ferric chloride passes through the gelatin resist and, in between the baked asphaltum particles which are resistant to it, eats away the copper beneath producing a random pattern of tiny pits in the plate. The depth to which these pits are etched is inversely proportional to the thickness of the resist above them --- the ferric chloride reaches the copper in the thinner areas of the resist sooner than in the thicker areas and hence etches the copper longer and produces deeper pits in those areas. This results in the variations in tonality of the final print.
The rate at which the ferric chloride penetrates the gelatin resist is determined in large part by its specific gravity or the amount of water it contains. By controlling this factor, the practitioner can affect the contrast of the final print.
Note: The use of asphaltum powder is the traditional way to produce the necessary pits in the plate during the etching process. While many of my plates were indeed produced in this manner, I no longer do it this way. Instead, I give the sensitized resist a second exposure through a computer-generated stochastic mezzotint screen which is just a large transparenciy consisting of random tiny squiggles. This exposure creates tiny hardened lands in the developed resist which serve the same purpose as the baked mounds of asphaltum powder in resisting the etching action of the ferric chloride.
After etching, the resist is removed and the copper plate is inked and wiped. Ink is removed from the original surface of the plate, but remains in the pits. The deeper the pits, the more ink they hold. The plate is then run through a press in contact with a sheet of paper. Under pressure the ink is transferred from the pits in the plate to the paper where it produces the illusion of continuous tones.