All Glossary

Cristobalite

In ceramics, cristobalite is a crystalline form of silica formed in the matrix of clay bodies as they fire in the kiln. Silica can also exist as quartz (the most common) and tridymite.

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Cristobalite is a crystalline form of silica. Silica normally exists in nature as quartz. But cristobalite is available as a powder, it is manufactured by mixing quartz with the right fluxes/catalysts and calcined to the necessary temperature (1100C+), then cooling it quickly. But such a material is not useful in traditional ceramics since it would convert back to quartz on heating.

Quartz particles in a clay body, such as a non-vitreous whiteware, can find themselves in a mix of clays and fluxes that provide an environment for conversion to cristobalite. The firing temperature, fineness of the quartz particles (finer converts better) and soaking time determine the completeness of the conversion. Completely vitrified or low silica bodies in rapid firing cycles develop the least cristobalite. High-quartz (or those having quartz sand added) low feldspar bodies that are cooled slowly through the range (1500-800C) form the most cristobalite.

Related Information

Dilatometer curve of vitreous porcelain (red) vs. stoneware body

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The 500-600C zone is the alpha-beta inversion of quartz. Notice the vitreous body experiences a bigger expansion change there. But in the 100-270C cristobalite inversion region the stoneware undergoes a much more rapid change (especially in the 100-200C zone). This information affects how ware would be refired in production to avoid cracking (slowing down in these two zones). In addition, that stoneware would not be a good choice for an ovenware body. Photo courtesy of AF

Low fire ware cracking during firing. Why?

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Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. While stoneware dunting happens between 950-1150F on the way down, this could be happening anywhere. A simple fix is to slow down the entire cooling cycle. Learn to program your kiln. Use a conservative cooling rate of about 200F/hr (even slower at 1150-950F). No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).

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