|Monthly Tech-Tip |
An excellent example that demonstrates the brittleness typical of vitrified terra cotta bodies. This bowl was fired to cone 02 and rung like a tuning fork when struck with a spoon. The body is dense like a porcelain and at appeared to be incredibly strong (this body is much more vitreous than an average terra cotta would be). However after a few more taps with the spoon it broke in two! It is brittle! Very hard, but brittle. At first I thought it might be that the glaze is under compression, but when I dropped the halves they did not shatter in the manner characteristic of compressed glaze, and they broke with razor sharp edges (like a vitrified porcelain does). So firing for this body must stop short of the most dense matrix possible to avoid this brittleness.
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.