When making your own black underglaze makes sense

Black brushwork needs to go on thick enough in one brushstroke. Commercial products we have don't do that - thus my motivation to work on this. I am experimenting on cone 6 Polar Ice porcelain jiggered bowls using G2926B dipping glaze. The base underglaze recipe here is a 90:10 MNP:nepheline syenite mix (you could use your own porcelain instead of MNP and feldspar instead of nepheline). To that, I added 20% black stain, 1.5% CMC gum and 5% bentonite. With the CMC gum and bentonite, and blender mixing, a brushable consistency that stays put can be achieved at a fairly low water content compared to commercial products (enabling it to go on thick enough in a single brush stroke). The freedom to fiddle with the recipe enables tuning both the application and firing properties. For example, assuming application at leather hard state, the drying and firing shrinkage can be matched to the body by varying the plasticity of the porcelain used (or the percentage of bentonite). And the percentage of stain can be tuned to get enough color but no bleeding, bubble clouding or crystallization (e.g. we later adjusted to 15%).

Related Pictures

Our own black underglaze better accepts the clear overglaze

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).

This underglaze red is not working at all. Why?

Underglazes should normally be made as high specific gravity slurries. This is so that they will go on as thick as possible with one brush stroke (an obvious need for fluid brushwork designs). Something went wrong with this commercial red product, it is painting on far too thin and has very poor physical covering power and adhesion compared to the black underneath (of the same brand). Why? Because the specific gravity is only 1.22! A practical solution is to allow it to evaporate for a few days to raise that as high as possible while still being brushable.

Very low specific gravities on two commercial underglazes

Commercial underglazes with low specific gravities

AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats and more time to dry and apply them). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with each brush stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow down its drying and harden it well at the dried state).

This is what happens when applying a non-gummed engobe or underglaze to bisque ware

The drying shrinkage of engobes and underglazes (and stickiness that goes with that) are important for adherence for application at the leather hard stage (they do not melt like glazes so do not glass-bond to the body). Wet adherence and shrinking together with the body during drying enhance the otherwise fragile interface. But not on bisque ware. If, however, the engobe is gummed it can appear to adhere to the bisque - but the fired bond with the body will not be as good as when applied at leather hard stage. Some underglazes employ calcined clays to minimize shrinkage for better compatibility with bisque, but it is still difficult to understand how they could develop any sort of quality fired bond!

Here is some motivation to make your own underglazes

Underglazes melting to different degrees

Commercial hobby underglazes are high in stain and very expensive. But does expensive mean suitable? To help answer we have over-fired these commercial products in a melt fluidity tester (to cone 8). They are recommended for use from cone 06-6 (some can go higher e.g. the green). Underglazes need to melt enough to bond with the underlying body, but not so much that opacity is lost (any melting loses opacity). Excessive melt can also cause design edges to bleed. To work well at greater thicknesses, underglazes need to have a firing shrinkage similar to the body (an ill-fitted underglaze and body forced into marriage are eventually going to divorce, in the form of flaking or cracking at their interface). Thus, while a regular glaze would melt enough to go well down the runway on this tester, an underglaze should not flow at all. At this temperature, none of these have achieved the right degree of maturity (the green is too refractory, the others over-melt to varying degrees). The only one that has a chance of suitability at cone 6, two cones lower than this, is the blue. Clearly, the base recipe and stain percentage in each underglaze recipe color needs attention, if that can be achieved all of these would mature to the same degree.

Links

Projects	A cereal bowl jigger mold made using 3D printing A new way to 3D-print your way to making jigger molds and templates. The molds are encased in a 3D printed shell that makes them fit perfectly into the cuphead. And the template is precise and very effectivwe.

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