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These stain-water mixes were painted onto bisque and an overglaze was applied. Pure stains are refractory, thus do not glass-bond with the body. And they repel being wetted by the molten glaze on top. Problems like this are the result. Stains need to be mixed with a stain medium that both supplies a bonding glass (for firing) and a clay to suspend the slurry and dry-harden and dry-bond it. One medium option used to be Gerstley Borate, it was both plastic (so it dried and hardened like a clay and suspended the slurry), and it was a melter. It was just a matter of tuning the percentage to optimize performance. One problem was that the best mix of those ended up using 70% stain (whereas 15-20% is theoretically enough).
A better option is to use a full-fledged medium, a recipe. Each of the ingredients is there for a purpose. This provides the ultimate flexibility to tune the mix properties for brushing and firing to enable minimizing the percentage of stain needed.

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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 but on others, it is full of microbubbles and pinholes. The potter has not had success finding a transparent overglaze that works consistently. Stain manufacturers do not mix stains with porcelain to making underglazes.
So, although closer control of the transparent glaze thickness or a more fluid melt glaze recipe might help, the real solution may lie with the underglaze recipes used here. An ideal bisque-stage underglaze is sinter-bonded but not sealed (therefore not accepting glaze water). An ideal fired underglaze also has controlled maturity: enough glass development to bond well to the body and promote glaze acceptance, but not so much that edge-bleeding and opacity loss occur. This state of 'controlled maturity' is also more likely to match body thermal expansion. The cost savings and the potential to fine-tune each color to your exact needs can be powerful motivations to use DIY underglazes.
| 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. |
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