URL: AMACO recommended specific gravities for Potters Choice glazes
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The specific gravities on two commercial underglazes might shock you
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats, and more time to dry and apply them). Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (but thinning down as it dries). With underglazes it is important to get adequate thickness with one brush-stroke, so a high specific gravity is important. This may be reason enough to consider making your own (by adding stain powders to a base). Actually, this technique of adding-stains-to-a-base-transparent is even more practical for making your own glazes, it just takes the right amount of gum to make them paint well.
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