The glaze has a calculated thermal expansion of 8.8 (because of high KNaO and low SiO2). Very high. It is basically stretched on. These plates are not glazed on the bottom. The glaze on the inside of the upper plate fits, the base is flat. But the glaze on the inside of the lower plate is pulling the base upward. The built-in stresses will eventually cause the piece to fail (likely fracturing into many pieces) if bumped. It is also almost certainly crazing. And the low SiO2 implicates it for leaching. The solution? Reduce the KNaO in favour of MgO and increase the SiO2 as much as possible without compromising the fired character.
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