Alternate Names: Calcined Alberta Slip
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The chemistry of this material is slightly different than for raw Alberta Slip. This calcine material does not lose any weight on firing, so it supplies more of each of the oxides to the fired glaze (almost 10% more). In some situations where the chemistry of the glaze is critical to specific effects and you are doing glaze chemistry calculations, it would be good to employ this chemistry in recipes that use this material (rather than using the chemistry of the raw Alberta Slip).
Alberta slip is much more plastic than Albany was (and therefore has a higher dry shrinkage and dry strength). This is actually an advantage because you can mix raw and calcined material in the needed proportion to adjust the drying shrinkage of your glaze. A key point to remember: If a glaze has a significant proportion of Alberta slip (e.g. 50% or more if no other clays are present or even less if there are other clays in the recipe), then the Alberta slip is likely too plastic to use raw, part of it must be calcined (see below). You will learn the proportion of calcine:raw to use in each recipe after use (start by using a 50:50 mix). If the glaze cracks on drying, then more cacline is needed; if it is too powdery, then more raw is needed. Of course, if you are applying the glaze to leather hard ware, raw Alberta Slip might be better. At first this calcining requirement might seem to be an extra hassle, but remember that extra control it provides (over the slurry properties).
To calcine a ceramic powder simply fire it high enough to destroy the plasticity but not so high as to sinter it (the sintered powder will have aglomerated particles that have bonded, this grit will not pass your screen and will thus affect glaze surface quality). In the Plainsman studio we fire uncompacted powder in thin slip-cast bisqued bowls that hold about 650 grams each. We fire them to 1000F (Cone 022 or red heat) at 100F per hour and hold for 10 minutes (the calcined material turns red and is devoid of moisture). If you fire in larger or heavier containers soak longer, long enough so that little or no black raw powder remains in the center.
Calcined Alberta Slip (right) and raw powder (left). These are just 5 inch cast bowls, I fire them to cone 020 and hold it for 30 minutes. Why calcine? Because for glazes having 50% or more Alberta Slip, cracking on drying can occur, especially if it is applied thick (Alberta Slip is a clay, it shrinks). I mix 50:50 raw:calcine for use in recipes. However, Alberta Slip has an LOI of 9%, so I need to use 9% less of the calcine powder (just multiply the amount by 0.91). Suppose, I needed 1000 grams: I would use 500 raw and 500*.91=455.
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 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 dry glaze is shrinking too much, 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 (they would quickly settle to the bottom of the bucket without it), but too much causes excessive shrinkage. Fixing this problem is not nearly as difficult as most people think. You can reduce shrinkage by calcining part of the clay or swapping a clay component for another of similar chemistry but lower shrinkage. The best way: Use glaze chemistry to source some Al2O3 (contributed by the clay) from feldspar instead. Of course this involves juggling amounts of other materials in the recipe to maintain the overall chemistry.
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Archie Bray Slip
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Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.