A reason for DIY underglaze: Black!
Better coverage, control of bleeding
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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A reason for DIY underglazes:
Higher specific gravity to go on thicker
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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.
Our own black underglaze better accepts the clear overglaze
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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).
Very low specific gravities on two commercial underglazes
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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
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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!
A reason for DIY underglazes:
Ability to use a lower percentage of stain
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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.
Silk screening using a professionally-made screen
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Silk screening is a popular decorating method. There is no better way to get a quality screen than having an aluminum-framed one made at a shop that specializes in this process (e.g. t-shirt printers). You can buy those hinges from a supplier online. This screen is 16x20 inches and I have multiple designs on it - I made them in Adobe Illustrator, which enables a vector graphic original that gives superior quality. I am about to screen lettering and a logo onto a tile. Although companies like Speedball, Amaco and Mayco make inks for this purpose, I am using an ink I made by mixing a black stain into a ceramic medium and adding enough water to achieve a viscous gel.
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