|Monthly Tech-Tip |
Alternate Names: Cal Kaolin, Meta Kaolin
Calcined kaolin (also called metakaolin) 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.
Calcination makes the powder whiter and more chemically inert. That makes it useful in a wide variety of products and industries. But strangely, it 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 an ideal source (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 imparts suspending properties to the glaze slurry. And it hardens the dry glaze layer. But there is a problem with raw kaolin: Once percentages pass 20% in a recipe 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. This enables controlling the physical properties of the glaze slurry 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 other uses in ceramics. It 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).
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. Thus they can be substituted for part of the raw kaolin to tune drying while maintaining fired properties.
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. 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!
|Materials||Glomax LL Calcined Kaolin|
|Materials||Burgess Iceberg Calcined Kaolin|
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Pure clay mineral, there are many brand names of varying purity and iron content.
Materials that melt at high temperatures. These are normally used for kiln bricks, furniture, etc. or for ceramics that must withstand high temperatures during service.
Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
The calcination of kaolin
|Frit Softening Point||2930F|
|Body Thermal Expansion||A useful material for refractories and low expansion applications.|