Friday, September 9, 2011

Long Term Considerations in Ceramic: COE

I've seen more and more people putting crushed soda lime and other glass on ceramic. I've been hesitant to do so. My reasons follow.

Ceramic glazes are also "glass on ceramic." They are carefully calculated, so that they expand and contract at the same rate as the ceramic on which they are used. This is to make sure that they adhere to the clay surface over the life of the object, and don't come off. Glass can expand and contract with changes in the weather & the immediate environment (such as a warm body, pool, shower, jacuzzi, winter).

The COE (coefficient of expansion) of soda lime glass (bottle glass) is 9. The average COE of common commercial high fire clays is in the neighborhood of 7. Porcelain drops down around 4. Both bottle glass and ceramic will expand and contract over their lifetimes--but at very different rates when the COE's are disparate.

A somewhat poorly matched glaze will craze--the "crackle" effect. However, increases in mismatching between glaze and ceramic can actually result in the glaze coming off of the ceramic. This may not happen immediately; it can occur up to several years after firing, when the two materials have gone through numerous cycles of mismatched expansion and contraction. This knowledge makes me reluctant to risk using commercial glass as a glaze on ceramic pieces.

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