The body is Plainsman L215. With the 04DSDH firing schedule. The glaze is G1916Q. The body is processed at 42 mesh, so it has lots of coarse gas-generating LOI particles. But two factors can minimize them the create a more transparent glass.
The right mug: Cone 03 is working, but this in on blistering and pinholing tipping point. Cone 04 would be better.

This picture has its own page with more detail, click here to see it.
Terra cotta bodies are typically fired between cone 06 and 04. There are many reasons not to venture outside this range. But some can be pushed into stoneware territory at cone 03 (e.g. Plainsman L215). However, it’s a tipping point because going beyond cone 03 really turns on LOI gas generation (and the danger of glaze blistering). But here, the onset of some vitrification has improved the body:glaze interface (to reduce the crazing which is happens with the cone 04 mug on the left). Needless to say, accurate firing is needed to avoid over firing.

This picture has its own page with more detail, click here to see it.
Tin oxide is a powerful opacifier, but the 5% in this Frit 3124 based Arbuckle Majolica cone 04 recipe on the left is not sufficient. Adding 10% zircon (e.g. Zircopax, Superpax) produces the one on the right. Would just using 15% zircon be better? No. More zircon means a stiffer melt and more issues with crawling (notice the small bare spots on the right edge). This could be managed by the addition of CMC gum or a higher clay content to get better adhesion and dry hardness). But another improvement would be worth trying first: Use 4% tin and 8% zircon. That should opacify enough and present fewer problems with crawling.
| Recipes |
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry. |
| Glossary |
Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve. |
![]() 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