|Monthly Tech-Tip |
Underglaze brushstrokes were applied to this cup at the leather hard stage (lower left). It was then bisque fired. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that although not melting as much as a glaze, it is certainly fusing enough to seal the surface of the bisque where applied. Notice what happens on the upper right: The bisque piece was immersed in a dipping glaze for a few seconds - the underglaze is not covering. On the upper right a transparent brushing glaze has been applied over the underglaze brushstrokes. Notice that it has covered. But three coats were needed with plenty of drying time between them, especially over the brushstrokes.
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.