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Underglaze brushstrokes were applied to this test cup at the leather hard stage (lower left) and then bisque fired on. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that it is certainly fusing enough to seal the surface of the bisque. Notice what happened on the upper left: Although the bisque piece was immersed in a dipping glaze the underglaze is not covering. Yet on the upper right a just-applied transparent brushing glaze has covered well (although three coats were needed with plenty of drying time between). Why is this? Brushing underglazes contain a lot of gum, it makes them spread well like paint - and dry hard, impervious and non-absorbent. And the frit they contain makes them fire even denser. Manufacturers assume that gummed brushing glazes will also be used over them and thus no coverage issues will occur.
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.
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. |
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