MgCO3. In high temperature glazes it acts as a flux (beginning action about 1170C) producing viscous melts of high surface tension and opaque and matte glazes. Like CaO, its melting action drastically accelerates at high temperatures.
The surface tension of MgO-containing melts is less of a problem in reduction. Zircon and Magnesia melt at 2800C making them the highest melting oxides. Remarkably, MgO readily forms eutectics with other oxides to melt at surprisingly low temperatures.