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MgO (Magnesium Oxide, Magnesia)
Notes-Together with SrO, BaO and CaO it is one of the Alkaline Earth group of oxides. It has a cubic crystal structure. Zircon and Magnesia melt at 2800C, making them the highest melting oxides. Remarkably, MgO readily forms eutectics with other oxides to produce melts at surprisingly low temperatures.
-When employed as a secondary flux in high temperature glazes it melts well (beginning action about 1170C) and can be present in glossy glazes. In frits at medium temperatures it acts in a similar manner.
-MgO is best known for its ability to matte glazes in larger proportions. The mechanism of this is different at higher temperatures (vs. low). At medium and high temperatures it is sourced mainly from dolomite and talc, as the proportion of MgO is increased (against other fluxes) the viscosity (and surface tension) of the melt increases and the glaze produced is more matte. As proportions rise even further (e.g. above 0.4 molar) the glaze becomes more opaque (as a product of incomplete glass development). For this reason, MgO is one of the most effective matting agents. For example, glazes having almost no alumina, very slow silica and very high boron that would otherwise run right off the ware can be completely stabilized with an MgO content of 0.3-0.4 molar. Unlike a refractory stabilizer, in many formulations MgO does not significantly impede melting and glass development when it mattes the glaze. At lower temperatures the matting mechanism of MgO is that it simply stiffens and opacifies the glaze due to its refractory nature. It is common to source it from magnesium carbonate in these ranges. Like CaO, MgO it is very refractory by itself, around 2800C melting point!
-Since the surface tension of MgO-containing melts does increase with its proportion, this can cause crawling if the glaze laydown has any shrinkage issues. This is less of a problem in reduction and most problematic in low fire.
-MgO is very valuable for its lowering effect on glaze thermal expansion (this is one reason why MgO mattes can be made very resistant to crazing). Low expansion frits are invariably based on MgO. Theoretically, surface character is best maintained when introducing MgO into a glaze to replace calcia, baria, and zinc. Still, replacing the alkalis with MgO is the single most effective strategy to reduce crazing, this works so well because oxide molecules of the highest possible expansion are being replaced with ones of the lowest. But this must be done with caution since having more than about 0.1 molar will begin to affect gloss. Also, you will have to determine if color is detrimentally affected. Yet many liner glazes have plenty of gloss and color is not an often an issue, they are excellent candidates for MgO strategies to deal with crazing issues.
-MgO is a light oxide and generally is a poor choice for glazes to host bright colors. However, it does work well in earth tone and pastel glazes, especially in high temperature reduction firing. Likewise, it may be harmful to some under-glaze colors.
-Since MgO stiffens the melt it can be used simply to check glaze fluidity (in a manner similar to alumina) and to prevent devitrification (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).
-Does not volatilize.
Compare two glazes having different mechanisms for their matteness
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived and 300F to ice water plunge without cracking). This looks like it could be a great liner glaze.
Classic dolomite glaze at cone 10 reduction on a speckle producing clay body (10R). The magnesia flux in dolomite creates a silky matte surface.
A magnesia matte that breaks on contours
GR10-G Silky magnesia matte cone 10R (Ravenscrag 100, Talc 10, Tin Oxide 4). This is a good example silky matte mechanism of high MgO. The Ravenscrag:Talc mix produces a good silky matte, the added tin appears to break the effect at the edges.
GR10-B transparent glaze using 10% talc instead of 10% calcium carbonate. This version has a lower thermal expansion and is less likely to craze.
An example where adding silica really helps a glaze
The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.
Frits melt so much better than raw materials
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
The difference between dolomite and calcium carbonate in a glaze
These glaze cones are fired at cone 6 and have the same recipe: 20 Frit 3134, 21 EP Kaolin, 27 calcium carbonate, 32 silica. The difference: The one on the left uses dolomite instead of calcium carbonate. Notice how the MgO from the dolomite completely mattes the surface whereas the CaO from the calcium carbonate produces a brilliant gloss.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
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