Cobalt oxide is the main ingredient of the pigment gozu, used in Oriental ceramics for dyeing or painting with gozu. It is stable as an underglaze decorative underglaze pigment at firing temperatures above 1250 degrees Celsius. When highly pure cobalt oxide is used as pigment and fired in reducing fire, the color becomes purplish indigo, and when fired in oxidizing fire, the color becomes darker. In addition to cobalt oxide, pigmented gosu contains impurities of metal oxides such as nickel, copper, iron, and manganese. Each of these impurities affects the coloration of cobalt oxide. Iron oxide gives a pale blue color in reduced burns and a yellow or brownish color in oxidized burns, while manganese oxide gives a grayish purple color in reduced burns and a grayish brown color in oxidized burns. Copper oxide volatilizes at high temperatures and has little effect on the coloration of cobalt. The composition of the glaze also affects the coloration of gouzu. (1) If there is too much silicic acid or magnesia, the color of cobalt becomes darker. (2) If there is a lot of alumina, the violet-blue color becomes brighter. (3) When there is a lot of lime, the color becomes darker and darker. (4) When zinc oxide is used as a base, the indigo color becomes sky blue. Cobalt oxide is used as a pigment in two types: kaibi (CoO,Al2O3) and yakinuki gozu, which is made from cobalt oxide and kaolin.
（The (CoO-Al2O3-SiO2) dark blue is used as glass enamel, (CoO-ZnO-SiO2) kaebi is used as overglaze enamel, and the (ZnO-CoO-Al2O3-Cr2O3) blue-green or peacock (CoO-MnO-Fe2O3-Cr2O3) black are also used. Cobalt oxide also makes a pink crystalline glaze.
