A unique aspect of the photogravure process is connected to the properties of ferric chloride, the mordant that penetrates the gelatin to etch the copper beneath. The correct chemical name is iron(III) chloride; however, in this text we will use the traditional term ferric chloride to avoid confusion and maintain consistency with other literature. In photogravure, a progression of ferric chloride solutions of varying densities are used to etch the copper. The densities of the solutions are measured on the Baumé scale using a Baumé hydrometer. See Appendix D for the chemical formulae of a ferric chloride solution and its various reactions.
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