Dolomite

Dolomite is mined as rock and ground, and because calcium and magnesium are combined in the one mineral it supplies both oxides to a recipe at once. Talc and magnesium carbonate are the other two common sources of magnesia, and the magnesia is usually the reason to use dolomite. At high enough levels it makes a glaze matte, and the surface is a particular one, smooth and thick and often described as silky or buttery. The effect is strongest on a slow cool, because the matting comes from crystals growing as the glaze cools, not from underfiring. A magnesium matte with its magnesia from dolomite is called a dolomite matte. Magnesia also changes what colourants do. It keeps a glaze in the pastel range, and cobalt in particular moves from blue towards lavender and then grape purple as the magnesia increases. Magnesia has the lowest thermal expansion of the common glaze fluxes, so raising it pulls a crazing glaze back towards fit. Nearly half the weight of dolomite leaves as carbon dioxide during the firing. On its own it stays refractory through the low fire range, though with boron or a frit alongside it enters the melt readily, and in a clay body it fluxes at stoneware temperatures.