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G3838A - Low Expansion Transparent for P300 Porcelain

Modified: 2018-06-16 22:53:31

A low expansion transparent cone 6 glaze for Plainsman P300 (and other kaolin only porcelains difficult to fit glazes to)

Notes

We have been recommending G2926J, an adjustment to G2926B to lower its thermal expansion. However J does not have as smooth of a surface or is as crystal clear. This recipe is perhaps a better alternative.

The 50:30:20 Frit3134:Kaolin:Silica recipe (77CO₄A) was a popular clear base that Plainsman Clays recommended in the early days. It has not been used in recent years because of the high frit content (expense and perceived issues with durability). However we have come to appreciate in recent years how difficult it is to make a good transparent glaze on porcelains (one that has a high gloss and does not craze and works with many colors). A significant soda/potash content seems vital in getting good gloss and color, but these contribute to high expansion (and thus crazing). However, we have also realized that higher boron (to get better melting) enables higher alumina and silica (all three of these are low thermal expansion and therefore counteract the KNaO and the latter two contribute to durability). The 77CO₄A recipe has high boron, so we used it as a starting point.

A preliminary change had encouraging results, but the glaze was clouding on slow cool (in large, heavily packed kilns). To help prevent that crystal growth (that causes the clouiding) we increased the Al₂O₃ a little and added some MgO (to stiffen the melt). The frit supplying the MgO has brought along a little extra boron but we want better melting to improve the gloss (that, plus a little less SiO₂, should help).

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