Ceramics Monthly recently received an email containing an excellent list of questions about calcined kaolin. “Why,” asked the writer, “do some recipes call for calcined clay and others not?” Other equally fundamental and significant questions followed. This article provides answers to those important queries.
Take Control of Shrinkage
Why would one want to use calcined clay? The answer is because any wet clay shrinks as it drys. However, the same clay, when calcined, shrinks less during the process of drying. In the case of glazes, drying shrinkage can become problematic as the clay content of the glaze rises above 10% of the recipe. If a glaze contains so much clay that it shrinks too much as it air dries, it will crack and may even fall off the ware. Calcined clay does not behave in the same way, yet delivers the same amount of silica and alumina as the raw clay originally called for, so the recipe has the benefit of high alumina and silica but the application and drying properties of a recipe containing a lesser amount of clay.
Single firers commonly use what are called slip glazes for application to leather-hard ware. Slip glazes have intentionally high shrinkage so they shrink with the greenware to which they are applied. When converting a greenware slip glaze to a glaze to be applied to bisqueware, calcined kaolin is frequently substituted for some of the clay fraction to prevent shrinkage cracks.
Slip glazes are essentially a slurry of clay and water containing a small amount of quartz plus feldspar or other fluxes. Such a slip-glaze recipe will typically contain from 25% to as much as 70% clay. Slip-glaze recipes that have a clay content on the low end of the range usually also contain bentonite because a small amount of it gives the glaze a relatively high shrinkage.
To achieve the high silica and alumina content of a slip glaze with a high clay content, yet avoid the shrinkage involved, some of the raw clay in the recipe can be replaced with calcined clay. Slightly less calcined clay than raw clay is used because the calcining process drives off water and makes the calcined clay lighter for the same amount of alumina and silica it contains. In other words, calcined clay contains more alumina and silica per gram than uncalcined clay does.
The benefits of reduced shrinkage when using calcined clay comes with a trade off. Wet clay is a relatively plastic material. Calcined clay is not.
As a real world example of how calcining clay works, Alberta Slip is a clay that is mined as an alternative to the now-unavailable Albany slip clay. However, Alberta Slip is much more plastic than Albany slip was. Thus, if replacing a high percentage of Albany slip in a recipe with Alberta Slip, the glaze may crack as it dries. Tony Hansen of Plainsman Clays Limited offers an excellent explanation of how to use part of the Alberta Slip in the recipe in calcined form to prevent this problem.
Calcining Alberta Slip*
Alberta Slip is much more plastic than Albany slip 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 (50% or more if no other clays are present), then the Alberta Slip is likely too plastic to use raw, and part of it must be calcined (see below). You will learn the proportion of calcined to raw Alberta Slip to use in each recipe after testing (start by using a 50:50 mix). If the glaze cracks on drying, then a larger proportion of calcined material or clay is needed (1); if it is too powdery, then more raw material is needed. Of course, if you are applying the glaze to leather-hard ware, raw Alberta Slip might be better. Although calcining can be an extra hassle, remember that this technique is giving you more control of the slurry properties.
To calcine Alberta Slip, fire the uncompacted powder in previously bisque-fired vessels to 1000°F (cone 022 or red heat) at 100°F per hour. Hold for the time necessary for the heat to penetrate, so that little or no black powder remains in the center. This might require an hour or two of hold time. The calcined material turns red and is devoid of moisture (2). If you fire in larger or considerably thick containers, you will have to soak longer, long enough so that little or no black powder remains in the center.
Remember that Alberta Slip has an LOI of 9%, meaning that percentage of its weight gasses off during firing. That means you should use 9% less calcined powder in a recipe that calls for raw clay. 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.
*Text and recipe by Plainsman Clay Limited. Alberta Slip is a product of Plainsman Clays Limited and is mined near Ravenscrag, Saskatchewan, Canada. To learn more about Plainsman Clays Limited and to see more recipes using Alberta Slip, check out http://plainsmanclays.com/albertaslip.
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