• Materials for Ceramics

Alberta Slip 1900F Calcined

Alternate Names: Calcined Alberta Slip

Notes

The chemistry of this material is slightly different than for raw Alberta Slip (it does not lose any weight on firing, so it supplies more of each of the oxides to the fired glaze, 9% more). This chemistry provided here assumes complete calcination (to at least cone 04). However there is an issue. The page at albertaslip.com recommends roasting to only 1000F, at that temperature it looses only 3% weight on firing.

Another issue is particulates. When calcined at 1900F, particles sinter together into larger ones, requiring glaze ball milling. But at 1000F this does not occur.

Related Information

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

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Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about 1 kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying (if used raw the GA6-B and similar recipes will crack as they dry and then crawl during firing). Roasting eliminates that. Calcining to 1850F sinters some particles together (creating a gritty material) while roasting to 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.

Here is what happens when a glaze has too much raw clay

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This is an example of how a glaze that contains too much plastic clay has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and roasted clay. Enough raw clay is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. Perhaps you have been using a glaze having a high percentage of clay and this does not happen - the reason is likely that the clay is not highly plastic.

Why does the glaze on the right crawl?

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This is G2415J Alberta Slip glaze on porcelain at cone 6. Why did the one on the right crawl? Left: thinnest application. Middle: thicker. Right thicker yet and crawling. All of these use a 50:50 calcine:raw mix of Alberta Slip in the recipe. While that appears fine for the two on the left, more calcine is needed to reduce shrinkage for the glaze on the right (perhaps 60:40 calcine:raw). This is a good demonstration of the need to adjust raw clay content for any glaze that tends to crack on drying. The Alberta Slip and Ravenscrag Slip page have information about how to calcine and calculate how much to use to tune the recipe to be perfect.

Clay in a glaze recipes. Why is it there?

What happens when there is too much?

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If I put this on social, I might get 100 comments on the cause. Most would say it's applied too thick. That is not correct. At the top, it is too thick, but not at the bottom (where it is still cracking). Crawling during firing is almost certain.

Before offering any opinion, one needs to sanity check the recipe. This one, GA6-B, contains a lot of clay. Why do dipping glazes contain clay, usually kaolin, ball clay? To suspend the slurry, slow down drying, gel the slurry to make it thixotropic and harden the layer on drying. But their chemistry is also important, clays supply the all-important Al₂O₃ and SiO₂ to the melt (which would otherwise have to come from feldspar). Certain clays excel at suspending. 15-20% EP kaolin, for example, is all that is needed. It’s not that plastic, but sticky and thixotropic (Grolleg and New Zealand kaolins are similar or even better). 10% of a gelling ball clay might be enough. However, 50% of a silty non-plastic clay might be needed. When the clay has too much influence, glazes shrink too much as they dry and then crack like this. Ones lacking clay (or employing one that is not suitable) have poor application properties (for dipping) and are powdery. This glaze has 80% pure raw Alberta Slip. That is a plastic clay, no wonder this is happening! 50% of it needs to be roasted.

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