The red coloration of ceramic ware is due to iron, chromium, copper, selenium, tellurium, uranium, and gold. Iron compounds exhibit a red color on oxidizing sintering, and the color tone is governed by conditions such as the state of iron compounding, the proportion of iron compounds, the state of subdivision of ferric oxide micromolecules, the state of firing, and the effects of other coexisting minerals. Red iron oxides can be used as overglaze or underglaze pigments at temperatures below 1,000 degrees Celsius, but above 1,000 degrees Celsius they cannot be used as paints because they transform into magnetite. Chromium oxide produces a good red color in alkaline lead-containing glazes. Copper oxide in alkaline glazes produces a fine red in reduction. Chinese porcelain oxblood red and fire-red are both copper red. Various gold compounds produce red under certain conditions. Red can be obtained by heating a mixture of selenium and cadmium sulfide.
