Right: My attempts to apply a layer of watery white underglaze to leather-hard clay. Not surprising that I measured the water content of this at 68%. Yikes!
Left: After adding about a gram of Epsom Salts (and blender-mixing it in to dissolve), the slurry became thixotropic and gelled enough to enable applying a thicker layer with no problem. Note how it stays on the spoon. However, there is still an issue: Multple layers are needed, with lots of drying time between them. The low solids loading of this product also makes it tricky to use on bisque ware (more time is needed between the thinner coats it creates). Allowing it to settle and pouring off water, or evaporating some of the water to raise the specific gravity, would make it even better.

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Underglazes should normally be made as high-specific-gravity slurries (more solids, less water than glazes). 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. Or, add a gelling agent.

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Amaco V-303 Terra Cotta underglaze was applied in one coat at the leather-hard stage (on these Plainsman Snow mugs). It appeared to be thick enough on application and after bisque (left), but the clear overglaze on the right has diffused into it to melt it enough to lose its opacity. A higher specific gravity (lower water content) would make it possible to apply it thickly enough in one coat; this is especially important because each layer rewets the piece and adds a lengthy drying period. An appropriate specific gravity is thus the difference beween practical and impractical. Acheiving a better terra cotta texture and color is a prime candidate for DIY underglaze.
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