A reason for DIY underglazes:
To better accept a clear dipping overglaze
The commercial product has two 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, my 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 for the black (I later settled on 15% stain for consistent dense color with one stroke).
Related Pictures
A reason for DIY underglaze: Black!
Better coverage, control of bleeding
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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.
Underglaze decoration difficult to cover with clear overglaze
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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 may not cover underglazes well
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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 covered. 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 CMC gum, it makes them spread well like paint - and dry hard (but also denser and less absorbent). Manufacturers of brushing underglazes assume that gummed brushing glazes will also be used over their products. How can coverage be achieved using a DIY transparent? Mixing your glaze as a thixotropic slurry, a first coat dipping glaze or even as a brushing glaze.
Dipping glaze not working over solid underglazes:
What can be done to fix this?
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This is what typically happens when applying a dipping glaze over an underglazed bisque-fired piece - it does not stick. Commercial underglazes impede the absorptive powder of the bisque, but it still might be possible to make it work. Here are some options:
- Bisque the under-glazed ware to a low temperature (e.g. cone 010 or lower) and apply dipping glaze to that.
- Make your own underglaze for dipping, leaving out the CMC gum.
- Make your glaze thixotropic - because it is gelled, it may hang on in an even layer over the underglaze.
- Make a brushing glaze and paint that over the underglaze. 1%-1.5% CMC gum will make it paintable. 1% additional Veegum will gel it enough that it can tolerate more water and go on thinner; this enables applying multiple coats and gives good control of the final thickness.
A reason for DIY underglazes:
Commercial products don't adapt the medium
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All of these commercial underglaze colors. Underglazes are not pure stains; they are a blend of stain powders with a host recipe (a ceramic medium, which is a porcelain-like mix of clay, frit, silica). But the recipe of that medium should be different for each stain type because they have different potencies, effects on rheology and influences on fired maturity. The stain/medium mix is supposed to mature enough to fuse to the body but not so much that it loses opacity, or worse, melts. And the recipe should be different for each temperature!
They all have a powder carrier; that is, they are suspended in a water/gum solution so they brush like paint and dry slowly. Suppliers have got the carrier working great. But the medium is a work in progress! Adapting or optimizing the medium recipe to each color is beyond the ability of most. It is so much easier to use one medium and the same percentage for each stain.
To demonstrate, I fired these...
to cone 8 in a melt flow tester. The blue, green and red are from one manufacturer. The pink/maroon one needs a lot less flux, the blue a little less and the green needs more. The black underglaze (D) 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 because a much lower stain percentage is needed).Links
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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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