MgO (Magnesium Oxide, Magnesia)

Notas

Magnesium oxide (MgO), commonly known as magnesia, is an alkaline earth oxide featuring a cubic crystal structure composed of a lattice of Mg2+ and O2- ions. While it is highly refractory on its own—melting at approximately 2800°C—it is a vital component in glaze chemistry due to its ability to form low-temperature eutectics with other oxides. In high-temperature ceramic applications, MgO begins fluxing at roughly 1170°C. MgO is highly valued for its role in reducing the thermal expansion of glazes, making it an exceptionally effective agent for preventing crazing when substituted for high-expansion alkali oxides. It does not volatilize during firing and contributes to melt stability, often acting to check fluidity and inhibit devitrification. When utilized in significant molar proportions (typically 0.3-0.4), MgO acts as a powerful matting agent by increasing the viscosity and surface tension of the melt. At these higher concentrations, it can also promote opacity due to incomplete glass development, while providing a stabilizing effect that prevents glaze running. Conversely, its high surface tension can promote crawling if the underlying glaze application is prone to shrinkage. MgO is particularly well-suited for earth-tone and pastel palettes, including specific cobalt-derived colors and nickel greens, though it can be chemically incompatible with chrome-based reds. It is typically sourced from minerals like dolomite or talc in high-fire glazes, and magnesium carbonate in lower-temperature formulations. In ceramic bodies, MgO can also serve as a catalyst, facilitating the transformation of quartz into cristobalite.