Why does transparently glazed terra cotta look better at cone 03 than 04?
Terra cotta bodies typically develop richer color at cone 03 and fire much stronger. Glazes melt better and thus micro-bubbles pass through easier, this produces better transparency and a more brilliant surface. Notice that crazing is beginning on the one of the left. But because of better body:glaze interface and development of better vitrification the one on the right is not crazing. Cone 03 is somewhat of a sweet-spot for this specific body, firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 with ongoing success.
Terra cotta 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.
Vitrification 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.