This little pot on the left is more than it appears. Both of these samples were fired at cone 01 and clear glazed in the green state (without bisquing). The one on the right, a typical unprocessed native terra cotta clay, is full of pinholes, the one on the left has none. But the one on the left has been pre-processed by mother nature: it is from a thick layer of clay found below a bog in northern Alberta. It has 21% water content, you just cut a piece out, wedge it and throw a pot! Although it contains some particulate, it is highly plastic yet dries well and fires to a dense stoneware-like hardness 11 cones lower than we normally make stoneware at.
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.
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.