|Co-efficient of Linear Expansion||0.026|
|Frit Softening Point||2800C (From The Oxide Handbook)|
|Dry M.O.R. (50% Silica)||1537C|
-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.
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.
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.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
This glaze consists of micro fine silica, calcined EP kaolin, Ferro Frit 3249 MgO frit, and Ferro Frit 3134. It has been ball milled for 1, 3, and 6 hours with these same results. Notice the crystallization that is occurring. This is likely a product of the MgO in the Frit 3249. This high boron frit introduces it in a far more mobile and fluid state than would talc or dolomite and MgO is a matting agent (by virtue of the micro crystallization it can produce). The fluid melt and the fine silica further enhance the effect.
We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company they need to know the chemistry of your glaze to help.
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 IWCT 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.
Because this glaze employs 10% dolomite instead of 10% calcium carbonate it has a lower thermal expansion and is less likely to craze. While the dolomite is contributing MgO, which normally mattes glazes, there is not enough to do it here.
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.
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 a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.
Left: G2934 magnesia cone 6 matte (sold by Plainsman Clays). Right (G2934D): The same glaze, but with 0.4 molar of BaO (from Ferro Frit CC-257) substituted for the 0.4 MgO it had. The MgO is the mechanism of the matte effect. Barium also creates mattes, but only if the chemistry of the host glaze and the temperature are right. In addition, barium mattes are normally made using the raw carbonate form, not a frit. In fritted form, barium can be a powerful flux when well dissolved in the melt and boron is present. This glaze is actually remarkably transparent. However, if this was fired lower it could very well matte.
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.
The Right Chemistry for a Cone 6 MgO Matte
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.
Refers to a group of ceramic fluxing oxides that contribute similar properties to fired glazes. They contrast with the alkalis which are stronger fluxes.
Dolomite matte glazes have the potential to be very silky and pleasant to the touch, while at the same time being hard, durable and non-crazed (if they are formulated correctly).
|Oxides||CaO - Calcium Oxide, Calcia|
|Media||Desktop Insight 3 - Dealing With Crazing|
|Materials||Light Magnesium Carbonate|
|Glaze Matteness||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.|