The term iron oxide usually refers to ferric oxide (Fe2O3), but there are also ferric oxide as ferric suboxide, and iron hydroxide and iron suboxide. Most clay materials contain some iron, which imparts a staining color to the fired product, and glazes using this as a coloring agent will have a light yellow to reddish brown color in oxidizing firing and a light gray to black or blue-green color in reducing firing. They may also have crystals. As overglazes, they are prized for their orange-yellow and red colors. Yellow clay (mizuochi), brown iron ore (oniita), and iron sand (kurohama) have been conventionally used as iron pigments, but iron oxide made by firing iron sulfate (melanterite) is used for the red of Hizen overglaze painting.
