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I am using this on Polar Ice porcelain. This adds 20% black stain, 1.5% CMC gum and 5% bentonite to a 90:10 MNP and Nepheline Syenite mix. We mix this to have as high a specific gravity as possible (the CMC is an enabler for that). The lower water content is a key to this working well, it enables putting it on thick enough in a single brush stroke (the commercial one we have won't do that). The freedom to fiddle with the recipe enables really tuning the way it goes and and fires.
The commercial product has two serious issues. First, it is just not covering well enough, jet black requires three or four coats. Second, transparent dipping glazes do not cover well over it, even when the underglaze is bisque fired (upper left). By contrast, our own black (90% MNP, 10% Nepheline Syenite, 10% black stain, 1.5% CMC gum, 5% bentonite) overglazes perfectly (upper right). And one brush stroke covers enough. A further test with 20% black stain demonstrated that to be too high a percentage, it reacts with the overglaze.
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.
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). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with eachbrush stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow down its drying and harden it well at the dried state).
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