TASI
Hydrated magnesium silicate, and softer than any other mineral · The flux behind most low-fire white bodies · Raises a low-fire body's expansion, which is what stops crazing · A magnesia source in a stoneware glaze: silky mattes and pastel colour
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Talc is a hydrated magnesium silicate, and the softest mineral there is. It is the flux most low-fire white bodies are built on, and in a glaze it is the usual way magnesia is added.
Talc is the flux most low-fire white bodies are built on — commonly half their weight or more, and about a fifth in a red body — where it sinters the clay together rather than melting it. It also raises the body's thermal expansion, which is what keeps a low-fire glaze in compression and stops it crazing. In a stoneware glaze it works the other way round, bringing magnesia into the glass: that lowers expansion, mattes the surface to something smooth and silky, and pulls colour toward pastels, with cobalt coming out lavender rather than blue.
