The blue color of ceramic ware is caused by cobalt, iron, copper, and other elements. The blue of underglaze blue (the blue of underglaze painting) is cobalt blue, which is painted with gozu (a mineral containing cobalt). Gosu is a mineral containing cobalt. Sometsuke is probably used more than the sum of all other colors. Iron produces blue by reduction firing. This is the case with the blue glaze of Seto chajiri-ware, the Omakai glaze, and the blue anadare glaze of Iga. In alkaline glazes fired in oxidized iron, copper compounds produce a beautiful dark blue/turquoise color.
In the Orient, the term “blue” is used to cover a wide range of colors from indigo to green, and the color blue in overglaze enamels is rather correctly called green.
