Synthetic red iron oxide

The most common colorant in ceramics is synthetic red iron oxide, which contains the most iron. It is commercially available as a soft and very fine powder produced by grinding ore or heat processing ferrous/ferric sulphate or ferric hydroxide. All irons decompose and produce similar colors in glazes and clay bodies during firing.
Red iron oxide comes in a variety of shades ranging from a bright light red to a deep red maroon. These are normally designated by a scale ranging from about 120-180 (this number designation should be on the bags from the manufacturer, darker colors are higher numbers), but in ceramics, these different grades should all fire to a similar temperature because they contain the same amount of iron. The various raw colors are caused by the degree of grinding.