MgO

Notes

-Together with SrO, BaO and CaO it is considered one the Alkaline Earth group of oxides.

-Like CaO, MgO is refractory at lower temperatures, so much so that it can be used to increase opacity, as a matting agent (i.e. magnesium carbonate), and as a check to glaze fluidity in a manner similar to alumina (i.e. to prevent devitrification, that is, the tendency to produce crystalline surfaces). When mixed with CaO, it is not as refractory.

-It can act as a catalyst in low temperature bodies assisting in the conversion of quartz to higher expansion cristobalite which reduces crazing.

-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.

-It is valuable for its lower expansion and crazing resistance. When introduced into a glaze it should preferentially replace calcia, baria, and zinc before the alkalis to maintain surface character. Adding too much will generally move the surface texture toward matte or dry.

-MgO is a light oxide and generally is a poor choice for glazes to host bright colors. However, it does work well in earthtone and pastel glazes, especially in high temperature reduction firing. Likewise, it may be harmful to some under-glaze colors.

-Does not volatilize.

Properties
Surface Modifier	Magnesia is well known for the pleasant vellum 'fatty matte' and 'hares fur' tactile and visual effects that it produces around 1200C, especially in reduction firing (dolomite matte). However, it can produce matte effects at all temperatures.