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Alternate Names: Cal Kaolin, Meta Kaolin
Kaolin is pure clay mineral, having a fired chemistry of 1 molar part Al2O3 and 2 parts SiO2. However the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Calcining the material removes that water and destroys the plasticity. Calcined kaolin is not commonly used in traditional ceramics, except for the specifics mentioned below, but it is a commodity material in many industries. Various grades of calcined kaolin are produced by varying parameters in the calcination process (e.g. retention time in the rotary kiln, firing temperature). Calcination initially converts it to metakaolin (whose reactivity makes it useful in cements). Further heating whitens it more and reduces reactivity while still keeping abrasiveness low. Still, more calcination converts the crystals to mullite, making them much more abrasive.
Calcined kaolin has an important use in ceramic glazes. Al2O3 is essential to the chemistry of the vast majority of glazes and its most readily-meltable natural supplier is feldspars, however they most often oversupply KNaO in providing the needed Al2O3. Raw kaolin is an ideal second or even primary source of Al2O3. Not only does it readily decompose in the melt but it also supplies SiO2 that all glazes need. And it acts to suspend the slurry and harden the dried layer. But there is a problem with raw kaolin: Once recipe percentages pass 20% shrinkage becomes too high (causing crawling). In these cases substituting part of the raw kaolin for calcined kaolin solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed).
Calcined kaolins are also useful in tuning the shrinkage and plasticity of slips (engobes) which are applied to wet or leather-hard ware. Engobes contain higher clay percentages than glazes and it is more important to control their drying shrinkage. Thus, like for glazes, they can be substituted for part of the raw kaolin to tune drying while maintaining fired properties.
Calcined kaolin is refractory and softens at about cone 35. It is thus useful in refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, and investment casting (see molochite).
You can make calcined kaolin by simply bisque firing any raw powdered kaolin (in a small enough bisque vessel and slow enough ramp that the heat penetrates well). Roasting the powder at red heat, e.g. F, is sufficient to destroy the plasticity. It is a good idea to measure weight loss on calcined or roasting a compensate by that amount when substituting for raw kaolin. The material is a good example of how we can alter the mineralogy of a material to affect its working properties while maintaining the chemistry to retain fired properties.
If your drying pottery glaze is doing what you see on the left, a spider web crack pattern, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. The first thing to check is water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing. But that was not the issue here. Glazes need clay to suspend and harden them, but if there is too much it can mean trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove it.
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These are two 10 gram GBMF test balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).
It was spray applied on the dried bowl (no bisque fire) an was too thick (not to mention under fired). But the main problem was a glaze recipe having too high a clay content. If a glaze has more than about 25% clay, consider a mix of the raw clay and calcined. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about F.
These three cups are glazed with GS at cone 03. The glaze is the most crystal clear achieved so far because it contains almost no gas producing materials (not even raw kaolin). It contains Ferro frits and plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (LT), center is Plainsman L212 and right a vitreous terra cotta (LF). It is almost crystal clear, it has few bubbles compared to the kaolin-suspended version. These all survived a 300F/icewater IWCT test without crazing!
Calcined kaolin has zero plasticity. 25% bentonite had to be added to make it plastic enough to make this piece. Why bother? Because this will flash heavily in reduction firing.
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). GZ is high calcium and a high silica:alumina ratio. 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 GC 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 GC 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.
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