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The reason this floats is two-fold: It has a low density and it is highly vitreous. This is made from a mix of 60% Plainsman BGP terra cotta clay, 35% Fillite and 5% bentonite (by weight). By volume there is more Fillite than clay. The Fillite reduces the plasticity of the material considerably, but it was still possible to form this ball and the SHAB test bars without problem. This was fired at cone 6, this clay would normally be melting by that temperature. But instead, it acts as a vitreous bonding agent to hold together the millions of tiny glass spheres of the Fillite. The result is a material that does not absorb water easily, this ball floated for more than 8 hours before sinking.
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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.
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Fillite Standard
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