The body is Plainsman L215 (bisque fired at cone 06). These commercial underglazes are not temperature-specific, they work the same across a wide range of temperatures. But the glaze is G2931K, it was designed for cone 03 and does not melt as well at cone 04 (for example, it goes milky if applied too thick), it was for use on Zero3 stoneware and Zero3 porcelain. This body is, however, a standard porous earthenware. While it is normally fired at cone 06-04, it tolerates cone 03. Actually, it benefits from cone 03, firing stronger and a deeper red color. So, since this glaze and body are both better at cone 03, it makes total sense to fire pieces like that at that temperature.
G2931K - Low Fire Fritted Zero3 Transparent Glaze
A cone 03-02 clear medium-expansio glaze developed from Worthington Clear.
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.
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