These terra cotta clay bars were fired at cone 1. They have curled. That is ususual. It is happening because cone 1 is within the volatile firing range of this clay. Over a narrow range from cone 01-2 it shrinks drastically. These bars were fired on edge and the shelf heat-sunk the lower side, enabling the upper sections to get ahead (and shrink more, curling the bars). This is why this temperature would be avoided. Ware would warp. But at cone 02 it is stable. And at cone 2 it is also better. And vitreous.
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.