Our own black underglaze better accepts the clear overglaze

The commercial product has two serious issues. First, it is just not covering well enough, jet black requires three or four coats. Second, transparent 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 covers enough. A further test with 20% black stain demonstrated that to be too high a percentage, it reacts with the overglaze.

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Black is a case where making our own underglaze makes sense

I am using this on Polar Ice porcelain. This adds 20% black stain, 1.5% CMC gum and 5% bentonite to a 90:10 MNP and Nepheline Syenite mix. We mix this to have as high a specific gravity as possible (the CMC is an enabler for that). The lower water content is a key to this working well, it enables putting it on thick enough in a single brush stroke (the commercial one we have won't do that). The freedom to fiddle with the recipe enables really tuning the way it goes and and fires.

Underglaze decoration difficult to cover with clear overglaze

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.

Why dipping transparent glazes do not cover underglazes well

Underglaze fails converage by transparent

Underglaze brushstrokes were applied to this test cup at the leather hard stage (lower left) and then bisque fired on. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that it is certainly fusing enough to seal the surface of the bisque. Notice what happened on the upper left: Although the bisque piece was immersed in a dipping glaze the underglaze is not covering. Yet on the upper right a just-applied transparent brushing glaze has covered well (although three coats were needed with plenty of drying time between). Why is this? Brushing underglazes contain a lot of gum, it makes them spread well like paint - and dry hard, impervious and non-absorbent. And the frit they contain makes them fire even denser. Manufacturers assume that gummed brushing glazes will also be used over them and thus no coverage issues will occur.

Underglaze color mayhem at cone 5!

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.

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