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The underglaze was painted on to bisque ware (has not be fired on). This is a problem. It has a high gum content and has sealed the surface so the porous body underneath is unable to pull water out to dry it quickly. During the slow dry the little absorption that is taking place is generating air bubbles from below and these are producing bare spots. The solution is to either make your own underglaze having a lower gum content or decorate ware in the dry or leather hard stage so the bisque fire will neutralize the gum.
The commercial product was just not covering well enough. To get jet black required three or four coats. And we found that transparent dipping glazes did not cover well over it, even when it was bisqued on (upper left). By contrast, our own black (started from 90% MNP, 10% Nepheline Syenite and 10% black stain plus some CMC gum and bentonite) overglazed perfectly (upper right) compared with the commercial ones (upper left). And one brush stroke covered (although not as black as multiple coats of the commercial ones on the left). A further test with 20% black stain demonstrated that to be too high a percentage, it reacted 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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Materials |
CMC Gum
CMC gum is indispensable for many types of ceramic glazes. It is a glue and is mainly used to slow drying and improve adhesion and dry hardness. |
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