|Monthly Tech-Tip |
These are commercial underglaze colors fired in a flow tester. Underglazes are not pure stains, they are a blend of stain powders with a host recipe (a porcelain-like mix of clay, frit, silica) that matures enough to fuse to the body but not so much that it melts. The blue, green and red are from one manufacturer. Stain powders have different melting temperatures and require differing percentages to get color intensity, the base recipe of the medium should compensate for that. That is not being done here, the pink one needs less flux and the green needs more. The black underglaze (D) (from a second manufacturer) has liquefied, gassed out and is about to head down the runway! The E black (a third manufacturer) has not even started to melt or even sinter. The black engobes were plastic, the colored ones were not, likely an indication that black requires a much lower stain percentage in the engobe recipe.
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.
In ceramics, the edges of overglaze and underglaze color decoration often bleeds into the over or under glaze. How can this be avoided.