But this does not happen when a piece is glazed only on the inside or outside. Why? As terra cottas approach vitrification they generate gases as particles within decompose. At cone 04 it is not an issue, transparent glazes will fire ultra-clear and defect-free. But fire that same body to cone 02 or 01 and the glaze can be filled with bubbles and surface blisters. If gases have no other way to escape the body except by bubbling up through the glaze, this is what happens. But, where adjacent unglazed surfaces exist, all the gases will route out through it, leaving the glazed one defect free. Of course that is not practical here. So the only solution is to fire lower. If vitrification is really needed then a frit must be added to the body to densify it at that lower temperature.
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.