Alternate Names: Cal Kaolin, Meta Kaolin
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|GSPT - Frit Softening Point||2930F|
Calcined kaolin is a powdered white non-plastic material. It is raw kaolin that has been fired (in a rotary calcining kiln) high enough to remove the 12% (approx) crystal water. If you are a potter you can make your own 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). 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 maintain fired properties.
This material is often under-utilized in traditional ceramics, people do not realize its true value. Kaolin is pure clay mineral, having a fired chemistry of 1 part Al2O3 and 2 parts SiO2. But the raw clay crystals are hydrated, having 12% crystal-bound water. This is the secret to their plasticity. Al2O3 is essential to the chemistry of the vast majority of glazes and kaolin is ideal source material (because all glazes also need the SiO2 that it supplies and it readily decomposes in the melt). The other principle affordable and readily meltable supplier of Al2O3 are feldspars, however they also supply lots of KNaO (and in many cases oversupply it to get the needed Al2O3). Raw kaolin also supplies suspension to the glaze slurry and it hardens the dry glaze layer. However once raw kaolin percentages pass 20% in a recipe shrinkages can be to high (causing crawling). In these cases substituting part of the raw kaolin for calcined material solves the problem, maintaining the chemistry of the glaze but reducing the shrinkage and cracking. In other words, by substituting some of the raw kaolin for calcined the physical properties of the glaze slurry can be controlled without impacting the chemistry of the fired melt. Of course, mixing the raw and calcined materials must take into account the LOI of the raw material (12% less calcined is needed).
Calcined kaolin has many other uses in ceramics. They key property it has is that it is refractory. It softens at about cone 35 and it thus useful in refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, and investment casting (see molochite).
Calcined kaolins are also useful in tuning the shrinkage and plasticity of slips (engobes) which are applied to wet, leather hard or dry ware. Engobes contain higher clay percentages than glazes and it is more important to control their drying shrinkage. As with glazes, they can be substituted for part of the regular kaolin to tune drying while maintaining fired properties.
Body Thermal Expansion - Low
A useful material for refractories and low expansion applications.
If your drying glaze is doing what you see on the left, 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. This is Ravenscrag Slip being used pure as a glaze, it is shrinking too much so I simply add some calcined material to the bucket. That reduces the shrinkage and therefore the cracking (trade some of the kaolin in your glaze for calcined kaolin to do the same thing). Glazes need clay to suspend and harden them, but if your glaze has 20%+ kaolin and also bentonite, drop the bentonite (not needed). Other causes: Double-layering. Putting it on too thick. May be flocculating (high water content). Slow drying (try bisquing lower, heating before dipping; or glaze inside, dry it, then glaze outside).
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 1200F.
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.
These three cups are glazed with G1916S 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 3195 and 3110 plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (L3685T), center is Plainsman L212 and right a vitreous terra cotta (L3724F). 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.
Out Bound Links
The hazards of using this material in the ceramic industry and process
In Bound Links
Glomax Kaolin, Ajax Glomax Kaolin
The calcining process is used to remove some or all unwanted volatiles from a material (e.g. H2O, CO2, SO2) and/or to convert a material into a more stable, durable or harder state. Varying temperatures are employed to calcine various materials, depending on the decomposition temperature of the vol...
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
In contrast to man-made materials (like frits), ceramic minerals have a highly ordered atomic structure and a specific range of crystalline manifestations. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry when recipes are mixtures of ...