Very low specific gravities on two commercial underglazes

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 each brush 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).

Related Pictures

The glaze firing reveals that the specific gravity of this V-303 terra cotta under glaze is too low

Amaco V-303 Terra Cotta underglaze was applied in one coat, at 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 difused into it to melt it enough to lose the 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.

When making your own black underglaze makes sense

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.

Underglazes suitable for making silk screen transfers are another special case. The ideal one needs to cover well like this one. But it also needs to gel and harden enough to hang onto the paper but not so hard that it does not separate and transfer to the ware. Commercial products for brushing are unlikely to be optimal so it makes sense to mix your own and experiment with different amounts of gum.

Links

URLs	https://www.amaco.com/clay_how_tos/207 AMACO recommended specific gravities for Potters Choice glazes
Glossary	Underglaze An intensely pigmented highly opaque non-melting ceramic material mix meant to adhere best to leather hard pottery and fire-fit the body. Often transparently overglazed. Starter recipes.
Glossary	Brushing Glaze Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are also issues compared to dipping glazes. You can also make your own.
Glossary	Specific gravity In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency.

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