Ceramic Colors

colored mineral substances that are resistant to high temperatures; they are used to color ceramic products, glazes, and glass. Ceramic colors are finely ground mixtures of mineral pigments with fusible glass (overglaze colors) or with powdered glaze (underglaze colors). There are also colored fusible glazes (majolica colors). Ceramic colors include powdered and liquid preparations of precious metals, such as gold, platinum, and silver.

In the manufacture of ceramic colors, colored mineral substances are obtained from oxides of heavy nonferrous metals or, more rarely, from sulfurous and selenious metal compounds, such as CdS and CdSe. They are also obtained from synthetic colored compounds of natural minerals, such as corundum, spinel, garnet, and zircon. Because spinel and zircon are the most resistant to high temperatures, colored compounds of these two minerals are widely used. Fusible glasses or fluxes are lead borosilicates or alkaline lead aluminoborosilicates.

Depending on their composition and on the article to which they are applied, ceramic colors are fired at different temperatures. Overglaze colors for earthenware are fired at temperatures of 740°–780°C; those used for porcelain are fired at temperatures of 790°–830°C. Underglaze colors used for faience tiles are fired at 950°–1150°C; those for faience plumbing fixtures, at 1100°–1250°C. When overglaze ceramic colors are fired, the flux melts and fuses the pigment to the surface of the glaze. Underglaze ceramic colors are applied directly to the surface of the article and then covered with the glaze.