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This body is made from approximately 50:45:2:3 ball clay:talc:whiting:bentonite. These SHAB test bars are fired from cone 2 to 9 oxidation and 10 Reduction (bottom to top). From them the porosity and fired shrinkage can be measured (the SHAB test). Notice that the fired shrinkage is pretty stable from cone 2 to 8, but accelerates at cone 9 oxidation. But in reduction this stage has not been reached yet. The same thing happens with porosity, the cone 9 bar is dramatically more dense than the cone 8 one. But in reduction, it is still porous.
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Terra cotta clays mature rapidly over a narrow range of temperatures, showing dramatic changes in fired color, density and strength. These Plainsman BGP SHAB test bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. At cone 06 (1830F/1000C) it is porous and shrinks very little. But as it approaches and passes cone 03 (1950F/1070C) the color deepens and then moves toward brown at cone 02 (where it reaches maximum density and strength). However, past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). The second bar up, cone 04, is a good compromise: Adequate strength, good color and low shrinkage. This “single suitable temperature” is completely different than white burning low fire bodies, they are refractory.
Materials |
Talc
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite. |
Glossary |
Reduction Firing
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state. |
Glossary |
Vitrification
A process that happens in a kiln, the heat and atmosphere mature and develop the clay body until it reaches a density sufficient to impart the level of strength and durability required for the intended purpose. Most often this state is reached near zero p |
Glossary |
Artware
Artware, to potters and hobbyists, is pottery made from white-burning clays fired at low temperatures with brightly coloured glazes and decorated using decals, underglazes, lustres, etc. |
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