Cone 04 terra cotta cross section close-up with glaze
The glaze is well melted, but the interfacial zone with the body is very narrow. It is basically just stuck on the surface. The body is not developing any clearly visible glassy phases as does porcelain and stoneware, so not surprisingly, its strength is much lower than vitrifiedclay bodies at higher temperatures. However it is possible to add a frit and glass-bond the particles at cone 02 (at much higher cost of course). Not surprisingly, glazes must be more closely tuned to match the thermal expansion of the body for lower temperatures (since they are not stuck on as well).
Terra cotta 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.
Clay Body Porosity In ceramics, porosity is considered an indication of density, and therefore strength and durability. Porosity is measured by the weight increase when boiled in water.
Vitrification The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.