A terra cotta clay fired from cone 06 (bottom) to 4

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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Why this transparently-glazed terra cotta is better at cone 03 than 04

Two terra cotta mugs, the glaze on one is more glossy and the body color is richer

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Terra cotta bodies are typically fired between cone 06 and 04. That being said, many, like this Plainsman L215, develop richer color at cone 03 and fire much stronger. Glazes, of course, melt better and micro-bubbles pass through easier at cone 03. But this happens only if the body has not begun to decompose (and therefore generate a lot more gases of decomposition). Notice that crazing is beginning on the one of the left. Apparently the improved body:glaze interface and the development of better vitrification reduces the problem. Cone 03 is somewhat of a sweet-spot for this specific body, however firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 for ongoing success (because cone 02 is too high for this body, glaze will blister). Do you know what terra cotta actually is, if not click the link to learn more (this is a big topic).

The strange vitrification profile of a talc body

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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.

Vitreous fired ceramic that stays floating!

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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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Links

Glossary	Cone 1
Glossary	Firing Shrinkage During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Glossary	Vitreous The fired state of a ceramic when sufficient heat in the kiln has produced the density and strength common, and expected, in stonewares and porcelains.
Glossary	Maturity A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.

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