|Monthly Tech-Tip |
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).
Terra cotta bodies are more volatile in the kiln than stonewares. They mature rapidly over a narrower range of temperatures, that process is accompanied by dramatic changes in fired color, density and fired strength. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP (a quarry material), cone 02 finds it at maximum density (and fired shrinkage). 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 stoneware strength). However past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). This is typical of most terra cotta clay materials.
The body is Plainsman L215. We used the 04DSDH firing schedule. The glaze is inexpensive to make so we have a 2 gallon bucket. It has good dipping much like a stoneware glaze 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 06 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 a smooth, it has a natural charm, and the color is very rich. And that piece has stoneware durability and strength.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.