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It took a lot to be able to throw this moderately bellied vessel because the clay is pure calcined kaolin. It has zero plasticity. Actually it is worse than zero. That is why 25% bentonite was needed to make it barely plastic enough. That 25% would have done much better with other non-clay materials like feldspar or silica! How can this be? In its natural state, kaolin’s plasticity comes from its layered crystal structure, the water both bonds the plate-like particles together and lubricates their lateral movement against each other. The chemically bound water in the natural kaolinite crystals, which are tiny water magnets, is the secret to their ability to create plasticity - calcining drives it off. This dehydroxylation also changes the crystal structure, converting kaolinite into non-crystalline metakaolin, a particle that is actually hostile to plasticity. Calcined kaolin is also subject to shear thickening, a thin slurry thickens when propeller mixed - the particles form structural resistance, the opposite of what raw kaolin does.
Glossary |
Flashing
A visual effect that occurs in wood and salt firing of ceramic ware. Many potters value the effect and use special materials and firing methods to enhance it. |
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Materials |
Calcined Kaolin
This is kaolin powder that has been fired in a furnace to remove the 12% crystal water and render it non-plastic. |
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