Magnesia replacing Calcia

Here we show two pairs of glazes. In each pair some of the calcia in the first glaze is replaced by magnesia to generate the second glaze.

Alternatively stated, both glazes have the same total MgO + CaO in their seger formula, but in different proportions. The first glaze has higher CaO, the second higher MgO.

In both cases, the second glaze with higher MgO is either more glossy and transparent or otherwise more fully fused.

Glazes are fired at cone 6 in an electric kiln.

The kiln is cooled slowly, at a rate of 25 deg F an hour in the temperature interval 1750 deg F to 1650 deg F, with a 1 hour hold at 1650 deg F.

First Pair

The glossy fully transparent phase visible in the first glaze, has increased in volume and become the dominant phase in the second glaze.

The low MgO glaze is alk_H90_L3_LLPFe, the high MgO glaze is alk_H90_LLPFeMg.

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Second Pair

The textured look of the first glaze, its slightly sand-papery feel resulting from fine grains protruding out of the surface are absent in the second glaze, which has a smooth buttery surface.

The low MgO glaze is perlGrey_180_2, the high MgO glaze is perlGrey_180_4.

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