Iron Oxide Red
Iron Oxide Red is also known as Ferric Oxide, RIO, Hematite. In oxidation firing iron is very refractory, so much so that it is impossible, even in a highly melted frit, to produce a metallic glaze. It is an important source for tan, red-brown, and brown colours in glazes and bodies. Iron red colours, for example, are dependent on the crystallization of iron in a fluid glaze matrix and require large amounts of iron being present. The red colour of terra cotta bodies comes from iron, typically around 5% or more, and depends of the body being porous. As these bodies are fired to higher temperatures the colour shifts to a deeper red and finally brown.