Effect of Furnace Atmosphere on Color of Copper Glaze

Condition of Atmosphere to Produce Red Copper Glaze

Abstract

Porcelain bodies applied with copper glaze were heated to 1300°C under various atmospheres in the same firing schedule. Colors of the glaze samples obtained were classified by reflectance spectroscopy. The mode of the copper distribution and its oxidation state in the glaze were examined by EPMA, EDX and ESR. The copper glaze was grey and green for reducing and oxidizing atmospheres respectively. Red copper glaze was obtained by heating in reducing atmosphere followed by cooling in oxidizing one. From ESR analyses, it was found that the red glaze consisted of Cu²⁺ as well as Cu⁺, as was assumed from the equilibrium condition. The result denied the conventional theory that the red coloration is caused mainly by metallic copper colloid. The red chromophore in red copper glaze was assumed to be Cu₂O. The copper in glazes volatilized remarkably during the reducing firing.