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Michele Dupas

Interior Glaze Bled Through The Exterior Terra Sigillata

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I made a teapot and the exterior has bisqued with terra sigillata. I then glazed the interior with my clear glaze and fired to cone 6. The interior clear glaze seeped through in spots leaving dark patches. I believe these areas are where the interior glaze was the thickest. Any idea what caused this and if it is a terra sig/glaze reaction or the terra sig is so poreous that regardless of what glaze I choose it will bleed through?

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The glaze recipe, if you have it, might also be of some help.  Sometimes a glaze can have soluble/semi-soluble components and these can cause what you're talking about.  In my experiments with borax fluxed glazes, I've found this phenomenon especially common--the soluble flux is absorbed into the body through along with the moisture.  We could maybe help reformulate with a non-soluble source of flux.

 

Doesn't terra sig lose its sheen at a very low temperature, though?

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Doesn't terra sig lose its sheen at a very low temperature, though?

 

Yup, ^02 is about as high as it will go without loosing its shine. According to Vince Pitelka above that the clay shrinkage will cause the shine to diminish and leave a satin surface. Also, if the terra sig is made with earthenware clay then at ^6 it won't bear any resemblance to terra sig.

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