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

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).

Related Pictures

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

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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.

G1916Q transparent on terra cotta body at cone 06, 05, 03

Three clear-glaze terra cotta mugs with rich red color

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The body is Plainsman L215. We used the 04DSDH firing schedule. The G1916Q glaze is inexpensive to make so we have a 2-gallon bucket. It has dipping properties much like a stoneware glaze (because of the 15% ball clay it is thixotropic) so it is easy to apply quickly and evenly. For most terra cottas, body strength increases dramatically by cone 03. However, the most transparent and glassy glaze surface happens at cone 06. Terra cotta bodies need to be bisque-fired fairly low (e.g. cone 06) to have enough porosity to work well with dipping glazes. After cone 04 they generate increasing amounts of gases (as various particle species decompose within), for this reason the glazes can have more micro-bubble clouding or tiny dimples in the surface. This glaze has 2% iron oxide added as a fining agent to remove the bubbles. That iron also reddens the color and variegates the surface somewhat. Even though the surface character at cone 03 is not as smooth, it has a natural charm, and the color is very rich. And that piece has stoneware durability and strength.

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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

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