• Materials for Ceramics

Calcined Kaolin

Alternate Names: Cal Kaolin, Meta Kaolin

Notes

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 alters the particle structure and shape, thus destroying 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 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 can become too high (causing cracking and subsequent crawling in the firing). 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 should 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, as 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. 1000F, is sufficient to destroy the plasticity. As already noted, it is a good idea to measure weight loss on calcined or roasting and 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.

Related Information

Badly crawled glaze fired at cone 5 reduction

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.

Original container bag of Glomax calcined kaolin

This is the back side of the bag, the front side is blank. Notice it contains kaolin and titanium dioxide. The latter is added to improve performance in paints.

Glaze cracking during drying? Wash it off and then do this.

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.

What material makes the tiny bubbles? The big bubbles?

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).

This piece is thrown from calcined kaolin

It took a lot to be able to throw this moderately bellied vessel because the clay is pure calcined kaolin. It has zero plasticity. Actually it is worse than zero. That is why 25% bentonite was needed to make it barely plastic enough. That 25% would have done much better with other non-clay materials like feldspar or silica! How can this be? In its natural state, kaolin’s plasticity comes from its layered crystal structure, the water both bonds the plate-like particles together and lubricates their lateral movement against each other. The chemically bound water in the natural kaolinite crystals, which are tiny water magnets, is the secret to their ability to create plasticity - calcining drives it off. This dehydroxylation also changes the crystal structure, converting kaolinite into non-crystalline metakaolin, a particle that is actually hostile to plasticity. Calcined kaolin is also subject to shear thickening, a thin slurry thickens when propeller mixed - the particles form structural resistance, the opposite of what raw kaolin does.

Will this crawl when fired? For sure!

This high-Alberta-Slip glaze is shrinking too much on drying. Thus it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe (common with slip glazes). Clay is needed to suspend the other particles, but too much causes the excessive shrinkage. The easiest way to fix this is to use a mix of raw and roasted Alberta Slip. The roasted Alberta Slip has no plasticity and thus much less shrinkage (but it still has the same chemistry). Many matte glazes have high kaolin contents and recipes will often contain both raw and calcined kaolin for the same reason.

From Scribbles to Success: Fixing This Glaze Recipe

If you do DIY pottery glazing you may have recipes scribbled onto cards like this. But the card is not the big issue here, it is that recipe! It really needs some work. Here is what could be done.
-Add this as a recipe in an account at insight-live.com (and assign it a code number) to start a testing project. Along the way document it with pictures, firing schedules, general notes, etc.
-With all that feldspar it is sure to craze, reducing the high thermal expansion K₂O it contributes in favor of low expansion MgO (from talc) is the most likely fix.
-With all that clay (29 total) it is likely to crack while drying (and then crawl during firing), split it into part calcined kaolin and part raw kaolin (the ball clay is not needed).
-And those colorants: It is better to use cobalt oxide than carbonate. Perhaps the burnt umber could be increased to eliminate the need for both the manganese and iron (since it supplies both and has zero LOI). Better yet, remove all four and use a black stain (7% would likely be enough).

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