Dolomite

Dolomite as a ceramic material is a uniform calcium magnesium carbonate.  In ceramic glazes it is used as a source of magnesia and calcia.  Other than talc, dolomite is the principle source of MgO in high temperature raw glazes.  Dolomite by itself is refractory, but when combined with the typical oxides in a glaze (especially boron) it readily enters the melt.