These four commercial hobby underglazes were fired side-by-side in melt-fluidity testers at cone 8. The blue and green remain comparatively refractory, the pink has begun to soften and flow, and the black has become highly fluid. These differences reflect the behaviour of the complete formulations, not just the stains. But ceramic stains themselves differ greatly in color strength, refractoriness, particle characteristics and interaction with a ceramic medium.
Manufacturers face a difficult compromise because many underglazes are advertised for use across a very wide temperature range. Some colors need much more stain than others to remain intense, and each stain can change the suspension, drying and fired properties of the product. A broadly useful base will therefore be underfired for one color and excessively melted for another. While manufacturers can compensate by adjusting the base, stain loading or opacity, the results at any one temperature can still differ dramatically.
A DIY approach can target your actual firing temperature. Begin with a compatible engobe or underglaze base (e.g. L3954B) and estimate the stain requirement—for example, relatively little for a strong blue or black and considerably more for some yellows and pinks. Then adjust the frit/feldspar until the underglaze bonds well without developing enough to lose opacity. Finally, fine-tune the stain percentage and adjust the medium. Done well, strong blues and blacks can be bright and opaque with less than 10% stain.

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The red underglaze on this low-fired bowl is not properly fluxed (melted), it does not adhere to the body (this is a commerial product). The bottom-most contour of this bowl is concave and the transparent overglaze, which is under some compression, has popped right off! This is a serious hazard on the inside of functional ware. Each stain has it own melting temperature, and the underglaze formulation using that stain must employ a mix that supplies sufficient fluxes. So test your underglazes (by firing without an overglaze), even if they are a commercial product.

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Black brushwork needs to go on thick enough in one brushstroke. Commercial products we have don't do that - thus my motivation to work. I am experimenting on cone 6 Polar Ice porcelain jiggered bowls using G2926B dipping glaze. The ceramic medium here is a 90:10 MNP:nepheline syenite mix (use a ballclay/feldspar mix as an alternative). To that, I add 15% 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 thicker in a single brush stroke). Assuming application at leather hard state, the drying and firing shrinkage can be matched to the body by varying the percentage of bentonite. And the percentage of stain can be tuned for enough color but no bleeding, bubble clouding or crystallization. And I can adjust the degree of maturity by varying the proportions of MNP and Nepheline (commercial underglazes often melt too much by cone 6 and fade and diffuse as a result), this one stays opaque black.

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Since white burning slips are made from refractory materials they are not nearly as vitreous as terracotta bodies, they need help to mature and a frit is the natural answer. With the right amount, fired shrinkage can be matched to the body while the slip remains opaque.
L3685A slip on the left: An adjustment to the popular "Fish Sauce" recipe (which is basically a super plastic white body). 8% Frit 3110 replaces 8% Pyrax to make it glass-bond to the body better while remaining completely opaque.
Right L3685C: It has 15% frit, moving it into glaze territory, with translucency replacing opacity. Although expensive zircon could be added to restore opacity, that doesn’t fix the excessive firing shrinkage it now has, mismatching it with the body (which becomes more eventful the thicker it is applied).
| Glossary |
Underglaze
Understand pottery underglazes: Why they brush differently, how they fire, why clears fail over them, and how to make your own recipes. In technical rather than art language. |
| Glossary |
Silk screen printing
Silk screen printing is one of the best options for hobbyists and potters to reproduce crisp and detailed decoration. But there are many details to know. |
| 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. |
| Projects |
DIY Underglaze Development
I studied a range of underglazes to understand what they are. Instead, I found out what they should be and how DIY is the key to tuning the degree of fired maturity for my temperature, minimizing costs and tuning the working properties. |
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