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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.
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 much 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.
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