MgO (Magnesium Oxide, Magnesia)
| COLE - Co-efficient of Linear Expansion |
0.026 |
| MLPT - Melting Point (MP) |
2800C (From The Oxide Handbook) |
Notes-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.
Mechanisms
Glaze Matteness - Fatty 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). The mechanism is phase separation of the suddenly melting MgO, but MgO can also produce matte effects at lower temperatures as a refractory melt-stiffening additive.
Out Bound Links
- (Library)
Dealing With Crazing
This chapter in the lessons section of the book (a... - (Materials - Material source)
Dolomite - CaCO3.MgCO3 or CaMg(CO3)2 - Double carbonate of magnesia/calcia
Calcium Magnesium Carbonate, Raw Limestone
- (Materials - Material source)
Talc - Mg3Si4O6 or 3MgO.4SiO2.H2O
Magnesium Silicate, Steatite, French Chalk, Hydrated talc
- (Materials - Material source)
Light Magnesium Carbonate - Mg5(CO3)4(OH)2.4H2O
Hydrated Magnesium Carbonate Mineral, Hydromagnesite, Magnesium Carbonate Light
- (Materials - Material source)
Magnesite - MgCO3
Mag Carb, MgCO3, Anhydrous Magnesium Carbonate
- (Materials - Material source)
Frit - Frit master
Ceramic Frits
- (Materials - Closest material equivalent)
Magnesium Oxide - MgO
Magnesia, Periclase
In Bound Links
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