|Monthly Tech-Tip |
This term refers to an abrupt change in volume that occurs in quartz crystals when they are heated from the room temperature stable alpha phase to the beta crystal phase that exists above a theoretical temperature of 573C. It is referred to as an inversion because the process is reversed when the temperature falls back below 573C (although the reversal expansion curve is not identical to the heatup one). Since the change occurs across a narrow range of temperatures, ware will crack if there are significant temperature gradients within it. For example, if one side of a piece is at 573 and the other at 600, then as the piece cools further the volume change will move horizontally across it and could start a crack at the first weakness it encounters. The range we indicate for quartz inversion recognizes the fact that kiln gradients almost always exist and that quartz inversion does not happen at one specific temperature as is often suggested (the curve of temperature vs expansion is simply much steeper across a narrow range than it is outside that range, about 50 degrees).
Quartz inversion can be beneficial because it can put the glaze under compression and thus prevent crazing.
Wikipedia quartz inversion
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.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.
|Temperatures||Cristobalite inversion (alpha/beta) (210C-280C)|