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.

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This flaking is happening at the interface between the body and orange commercial underglaze (the firing is cone 05). The reason is simple.
Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a powder medium, designed to impart the missing properties. However, suppliers like to quote products as suitable over a wide range (e.g. cone 06-6). Of course, at cone 06, they have almost no glass development for bonding to the body and at cone 6 they may well be melted. In this case, the stain used is refractory and a high percentage is needed. So the bond with the body is very poor. A thick layer coupled with an incompatible thermal expansion can make the underglaze/glaze layer pop right off.
Is a DIY version possible? Imagine if you created a powder medium (e.g. L3685Z3) containing enough frit to counter the refractory character of the stain and develop the needed glass to bond with the body. That would solve this problem.

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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).
| 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. |
| 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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