The white engobe was applied by pouring at leather hard stage. The underglazes were also painted on at leather hard. The mugs were then dried, cleaned, bisque fired, dipping in clear glaze and final fired to cone 03. The clay and engobe have frit additions to make them vitrify at low temperatures.
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.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
Plainsman Zero3 L,K,H and Firing Schedule
G2931K - Low Fire Fritted Zero3 Transparent Glaze
A cone 04-02 clear glaze developed from Zero3 which in turn was developed from Worthington Clear. This employs frit instead of Ulexite.