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One small pinhole in a terra cotta mug and we have a problem

This is L3724E terra cotta stoneware. The inside slip is L3685S, a frit-fluxed engobe that is hard like the body and attaches well to it (engobes are often insufficiently fluxed). The glaze (G1916Q) is Frit 3195, Frit 3110 and 15% ball clay. The body has about 3% porosity, enough to make very strong pots. However that porosity is still enough to absorb water (and coffee). Although not too visible here, the pinhole in the inner surface has enabled absorption and there is a quarter-sized area of discoloration below the glaze. The piece could possibly be fired a cone higher, but testing would be required to see if the slip is still firing-shrinkage and thermal-expansion compatible with the body and that the body would not be over-fired. A better solution is adjust the firing curve to heal the glaze better. High temperature stoneware can easily have a 3% porosity also, so this is not just a low fire issue.

One small pinhole in a terra cotta mug and we have a problem

Links

Recipes L3724F - Cone 03 Terra Cotta Stoneware
An experimental Zero3 using Plainsman 3D clay
Recipes L3685U - Cone 03 White Stoneware/Engobe
A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on a matching red stoneware.
Glossary Engobe
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
Glossary Clay Body Porosity
In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
Glossary Pinholing
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.

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