The terra cotta (red earthenware) body on the upper left is melting; it is way past zero water absorption, past vitrification. The red one below it and the third one down on the right have 1% absorption (like a stoneware), they are still fairly stable at cone 2. The two at the bottom have higher iron content and are also 1% absorption. By contrast, the buff and white bodies have 10%+ absorptions. Terra cotta bodies do not just have high iron content to fire them red; they also have high flux content (e.g. sodium and potassium-bearing minerals) that vitrifies them at low temperatures. White-burning bodies are white because they are purer (not only lacking the iron but also the fluxes). The upper right? Barnard slip. It has really high iron but has less flux than the terra cottas (having about 3% absorption).
| Glossary |
Clay Body Porosity
In ceramics, porosity is considered an indication of density and therefore strength and durability. Porosity is measured by water absorption, the weight increase when boiled in water. |
| Glossary |
Earthenware
What is the difference between earthenware and a regular stoneware body? Earthenwares lack the glass development to fill voids and glue particles. |
| 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. |
| Glossary |
Maturity
A term used in ceramics to signify the degree of vitrification in a fired clay. Mature clays are dense and strong; immature ones are porous and weak. |
| Articles |
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes taking place in the ware at each stage of a firing helps tune the curve and atmosphere to produce better ware |
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