Etching the Plate
The varying densities of ferric chloride solutions are used to etch the copper plate. There are three reasons to use ferric chloride as the mordant. First, it can be mixed with water to various dilutions for controlled penetration rates through the gelatin resist. Second, it etches the metal beneath without producing gasses or other reactions that would disrupt the gelatin layer. Third, it produces a straight etch, with little lateral etching. The etching process must be controllable and precise so that the depth of the etched wells is inversely proportional to the various thicknesses of the gelatin resist. This results in a tonal equivalent to the film positive. The most common way of etching flat plate photogravures is the multiple-bath method. The plate is advanced through a controlled progression of ferric chloride baths, from less dilute (high Baumé) to more dilute (low Baumé) solutions.