MANGANESE DIOXIDE BRICKOX
MnO2—flexible colorant—with alkaline fluxes gives purple and red colours—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to colour basalt bodies. Concentrations of more than 5% may promote blistering. Toxic in inhalation and ingestion. Fumes from firing are especially toxic. (Source: Clay: A Studio Handbook) Manganese Dioxide is available as a pure material or as a ground ore (pyrolusite). In glazes Manganese Dioxide will behave in a refractory manner (stiffening the melt). Because to the expulsion of oxygen at 1080, glazes using manganese should avoid this temperature range to reduce the chance of blistering and ruining of the glaze surface. Manganese dioxide is the key to Rockingham brown wares. Manganese browns have a different, often more pleasant character than iron browns.
A fine black powder which when used in conjunction with cobalt and iron produces black. In lead and low alkaline glazes it produces a dull brown, while in high alkaline glazes along with tin, the colour is purple. In magnesium and barium glazes, colours ranging from lilac to violet will develop. With small amounts of cobalt oxide, a deep violet can be obtained. Other interesting colour effects are obtained with copper in crystalline glazes (2-5%).