May 2026: We are continuing a major code rewrite. Please contact us if you find issues. Thank you.
A stain-water mix was painted into recesses and an overglaze was applied. This stain is refractory, thus it does not glass-bond with the body. And it repels being wetted by the molten glaze. This problem is the result. The stain needs 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 it. One medium option used to be Gerstley Borate, it was both plastic, for suspension, and a melter. It was just a matter of tuning the percentage to optimize performance. However, Gillespie Borate does not work as well.
A better option is to use a full fledged medium, a recipe. Each of the ingredients is there for a purpose so there is even more flexibility to tune the properties. Mix it to work like a brushing glaze will further improve it.

This picture has its own page with more detail, click here to see it.
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. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk