This body is 70:30 kaolin:feldspar (fired at cone 6 with glaze G2926B). The fired body has a vitreous porcelain-like surface. But, this dense craze pattern indicates a very serious fit problem. The COE (thermal expansion) of this body is much too low. There isn't a glaze-fix solution to this. Reformulating it enough to fit, while maintaining the firing characteristics, would be well beyond the expertise, material availability and budget of any typical pottery or manufacturer. All that work would just boil down to "compensating for an unusually low-expansion body". Adding quartz to the body to raise its COE is the appropriate conventional solution. An initial test would be to substitute 25 of the kaolin for 200 mesh silica. It is refractory, and while the finer particles will be dissolved by the feldspar, enough of the larger pure quartz ones will remain to impose their very high COE on the fired matrix.
The other issue is with the flat particle shape of kaolin. The throwing process has lined up the predominant kaolin particles concentric to the centre. During drying, and especially firing, more shrinkage occurs across them than along them. All ten of the cups, which I made and dried in my normal, careful fashion, cracked like this! The solution is adding a filler, one with non-platy particles. Silica is perfect; the grains act like aggregate in concrete, strengthening the matrix and separating the clay particles, forcing them to orient more randomly.
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Porcelain
How do you make porcelain? There is a surprisingly simple logic to formulating them and to adjusting their working, drying, glazing and firing properties for different purposes. |
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Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen. |
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