Alternate Names: Calcined Alberta Slip
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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.
Roasted Alberta Slip (right) and raw powder (left). These are thin=walled 5 inch cast bowls, fired to 1000F and held it for 30 minutes. Why calcine? Why 1000F? Because Alberta Slip is a clay, it shrinks on drying. Roasting eliminates that, a 50:50 raw:roast mix works well for most recipes having high percentages. And 1000F? Calcining to 1900F sinters particles together (creating a gritty material) while 1000F just does not. But note the difference: The raw powder has an LOI of 9% (looses 9% weight on firing). 1900F-calcine has zero%. 1000F-roast has only 3%. So we need 6% less (9%-3%) of it than of the raw to supply the same chemistry. For example, if a recipe calls for 1000 grams of Alberta Slip: Use 500g raw and 456g 1000F roast (500*.94).
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. Albertaslip.com and Ravenscrag.com both have pages about how to calcine and calculate how much to use to tune the recipe to be perfect.
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 calcine material. Enough raw 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. The Alberta Slip website has a page about how to do the calcining.
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 calcined Alberta Slip (explained at albertaslip.com). The calcined 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.
Out Bound Links
Archie Bray Slip
In Bound Links
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Roasted Alberta Slip