Cristobalite
A crystalline form of silica (quartz is also) available as a raw material and formed by natural processes during firing of certain bodies. During cooling cristobalite changes from beta to alpha form around 220C. This change is accompanied by a 3% volume contraction.
Cristobalite forms spontaneously at temperatures above 1100C from very fine quartz found in some clays, from finely ground silica, and from molecular silica liberated during the formation of mullite from kaolin. This formation is a menace to most because its side effects make the body susceptible to dunting at 220C (cooling cracks, cracking during use due to sudden cooling). If feldspar is present in the body then any available molecular silica is taken up in the formation of silicates, and thus cristobalite does not form. If it does then it too is taken into solution. A good strategy in formulating a body is to use enough spar or naturally fluxed clays to be sure that any potential cristobalite is drawn into body glass (check with dilatometer test) and then re-establish fit with fine quartz. In this way quartz is compressing the glaze at 573C rather than cristobalite at 220C. A typical cone 10 porcelain with 25-30% feldspar will show no evidence of cristobalite on its expansion curve (as measured in a dilatometer). Conversely, high iron often non-vitreous stoneware bodies can generate high cristobalite levels.
A classic way to recognize a raw material (e.g. a ball clay) that forms significant cristobalite on firing is to note any significant shivering that occurs with a typical stoneware glaze. Some clays generate so much cristobalite that they will literally shed all of their glaze during final stages of cooling.
As a raw material added to earthenware bodies, cristobalite improves craze resistance after glazing because the sudden contraction puts the glaze into compression. Talc contains mineral species that, when added to earthenware bodies, act as a catalyst to the natural formation of cristobalite. This approach is necessary in low temperature ware because quartz inversion at 573C typically finds glazes still somewhat fluid, having not reached their set point (quartz inversion is used to advantage to put high temperature glaze in final compression). Out Bound Links
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