|Monthly Tech-Tip |
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 made from 90% MNP, 10% Nepheline Syenite and 6% black stain (plus some CMC gum and bentonite) performed better. It overglazes perfectly (upper right) and one brush stroke covers (although not as black as multiple coats of the commercial material. A further test with 20% black stain demonstrated that to be too high a percentage, it reacted with the overglaze. Black is a very important color for underglaze decoration, making your own affords the ability to tune the amount of gum and water for optimal usability. Next job: Get a better green!
A intensely pigmented and highly opaque brushing compound meant to be applied to leather hard pottery and covered with a transparent overglaze.
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.