|Monthly Tech-Tip |
Occurs in cooling clay bodies at around 225C (and on heat-up for vitreous ware being refired). It is accompanied by a sudden volume change. Cristobalite, a less stable form of crystalline quartz, can be present in the matrix of stoneware clays where sufficient fine quartz, time and temperature (above 1100C) are available during firing. The range of temperatures is given here because charts that plot the expansion against temperature vary with different sources. In addition variations in temperature in the cross section of ware changes in volume occur as waves across a piece and can crack it. Thus if one side of a piece is 250C and the other 200C, the inversion and associated volume change are happening in the center of the piece. Less cristobalite is formed in faster firings and in more vitreous clay bodies where the feldspar takes the small quartz grains into solution.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
|Temperatures||Quartz inversion (alpha-beta) (540-600)|
|By Tony Hansen|
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