(Sources: Talc, Dolomite, Magnesium Carbonate)

-Together with SrO, BaO and CaO it is one of the Alkaline Earth group of oxides. It has a cubic crystal structure.

-Like CaO, MgO is refractory at lower temperatures, so much so that it can be used to increase opacity, to perform as a matting agent (i.e. magnesium carbonate), and act as a check to glaze fluidity in a manner similar to alumina (e.g. to prevent devitrification or 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 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

• Fusion - 2800C

  From The Oxide Handbook

• Surface Modifier - Matte

  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.

Linked Articles

• Bringing Out the Big Guns in Craze Control: MgO (G1215U)

  MgO is the secret weapon of craze control. If your application can tolerate it you can create a cone 6 base glaze of very low thermal expansion that is very resistant to crazing.

Authors

• Tony Hansen (Owner)

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