DOLO

Dolomite is a natural double carbonate of calcium and magnesium, mined as rock and ground. It is one of the main magnesia sources in a glaze, and the material behind the soft, silky surface of a dolomite matte.

Dolomite is mined as rock and ground, and it carries calcium and magnesium together in the one mineral, which is why it brings both oxides into a recipe at once. That is what separates it from talc and magnesium carbonate, the other two routes magnesia takes into a glaze. Magnesia is usually the reason to reach for it. At sufficient levels it takes a glaze matte, and the surface it gives is a particular one, smooth and thick and often described as silky or buttery — strongest on a slow cool, since the matting comes from crystals growing as the glaze cools rather than from underfiring. The name has stuck to the effect: a magnesium matte with its magnesia from dolomite is a dolomite matte. Magnesia also changes what colourants do, holding a glaze in the pastel range, with cobalt in particular moving off blue toward lavender and then grape purple as the magnesia climbs. And magnesia has the lowest thermal expansion of the common glaze fluxes, so raising it pulls a crazing glaze back toward 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 alongside boron or a frit it enters the melt readily, and in a clay body it fluxes at stoneware temperatures.