Monthly Tech-Tip | No tracking! No ads! |
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 has two serious issues. First, it is just not covering well enough, to get jet black requires three or four coats. Second, it is intended for transparent brushing glazes over top - 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 almost covers enough (we later settled on 15% stain).
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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Troubles |
Bleeding colors
In ceramics, the edges of overglaze and underglaze color decoration often bleeds into the over or under glaze. How can this be avoided. |
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