A vessel being forced apart by the pressure of a low expansion glaze inside
Monday 17th October 2016
Many people would find the fired appearance of this insight-live.com/glossary/125">cone 10 reduction red fireclay (Plainsman FireRed) compelling. But it is not at all suitable for functional ware. This crack grew wider over a period of a week (after firing) because the inside glaze is exerting forces it cannot resist. Notice that where there is a glaze cover on the upper outside section there is no cracking. But the stresses within are still there, waiting for an opportunity for release. It is inherently risky to glaze ware only on the inside if you are not able to determine the fit. This is especially so if the body is not vitreous and does not have the strength to resist the outward pressure. But even if it is vitreous, the internal stresses will weaken its ability to withstand bumps.
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