Directions for the Home Manufacture of Carbon Tissue for Photogravure Printing
First, a note about gelatins. Gelatin is extracted from many sources, including the hide, skin, white connective tissue, and bones of animals. It is useful in carbon photography and in gravure work because it absorbs water and swells with increased temperatures until it reaches a melting point, at which point it forms a colloid; when again cooled, this colloid will set, even at low concentrations, and the cycle can be repeated. Certain chemicals, including chrome alum, potassium alum, formalin, and the various dichromates (ammonium, potassium, and sodium) serve to either harden, reduce, or altogether eliminate the ability of gelatin to absorb water.