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Calcining Alberta Slip


The Alberta Slip powder on the right has been calcined. You may need to do this for part of what you use.

Alberta Slip is a clay, if used raw in high percentages it can crack during drying so calcination may be needed.
Alberta slip is much more plastic than Albany was (and therefore has a higher drying shrinkage and dry strength). This gives you the option of mixing 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 a larger proportion of 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 you may find this requirement to be an extra hassle, but remember that this technique is giving you fine control of slurry properties.

To calcine a ceramic powder simply fire it high enough to destroy the plasticity but not so high as to sinter it (producing particle bonding that creates grit that could affect glaze surface quality). Fire the uncompacted powder in bisque vessels to 1000F (Cone 022 or red heat). For large or heavy-walled vessels fire slower (e.g. 200F per hour) but for smaller ones (especially thin-walled ones holding 500-1000g) you can fire much faster (e.g. 500F/hour). Hold for the time necessary for the heat to penetrate (start with 30 minutes). If any black powder remains in the center extend the soak time next firing.

Remember that Alberta Slip has an LOI, 9% of its weight gasses off during firing. That means you should use 9% less calcine powder. How do you calculate that? Suppose you are using a 50:50 ratio of raw:calcine and the recipe calls for 1000 grams of Alberta Slip. Supply 500 grams of raw Alberta Slip and 500*0.91=455 grams of calcine.

Alberta Slip is a product of Plainsman Clays and is mined in North America's premier "Clay Country" (near Ravenscrag, Saskatchewan, Canada and also in Montana, USA)