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Potters often encounter the problem shown here. These pieces are fired at cone 6. They are decorated with underglazes made from a mix of porcelain powders and stains. The transparent glaze works over certain colors (e.g. the light blue), but over others, it is full of microbubbles and pinholes. The potter has not had success finding a transparent overglaze that works consistently. As can be seen here, stain types behave differently; they are not just inert powders of different colors. As an extreme example, MnAl pink body stain is very refractory; it won't work well in glazes (however, other stains classified for use in bodies do work in glazes e.g. blues).
While close control of glaze thickness and a more fluid melt recipe might help, a more likely solution is a better base in which to mix the stain powders (the Underglazes Glossary entry, below, has more information). Selection of stains and tuning their percentage is another important factor.
| Glossary |
Stain Medium
It is a mistake to use pure stains for decorating ware. Stains need to be mixed with a ceramic carrier and a working medium to work and fire well. |
| 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. |
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