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Underglazes should normally be made as high specific gravity slurries. This is so that they will go on as thick as possible with one brush stroke (an obvious need for fluid brushwork designs). Something went wrong with this commercial red product, it is painting on far too thin and has very poor physical covering power and adhesion compared to the black underneath (of the same brand). Why? Because the specific gravity is only 1.22! A practical solution is to allow it to evaporate for a few days to raise that as high as possible while still being brushable.
Black brushwork needs to go on thick enough in one brushstroke. Commercial products we have don't do that - thus my motivation to work on this. Another issue is that they try to cover too wide a firing range (thus they melt too much at the high end and not enough at the low end). I am experimenting on cone 6 Polar Ice porcelain jiggered bowls using G2926B dipping glaze. The base underglaze recipe here is a 90:10 MNP:nepheline syenite mix (you could use your own porcelain instead of MNP and feldspar instead of nepheline). To that, I add 15% black stain, 1.5% CMC gum and 5% bentonite. With the CMC gum and bentonite, and blender mixing, a brushable consistency that stays put can be achieved at a fairly low water content compared to commercial products (enabling it to go on thicker in a single brush stroke). Assuming application at leather hard state, the drying and firing shrinkage can be matched to the body by varying the plasticity of the porcelain used (e.g. the percentage of bentonite it contains). And the percentage of stain can be tuned for enough color but no bleeding, bubble clouding or crystallization. And we can adjust the degree of maturity by varying the proportions of MNP and Nepheline (commercial underglazes often melt too much by cone 6 and fade and diffuse as a result), this one stays opaque black.
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