The commonly used term for a type of natural plastic clay that hardens when dried. When exposed to firing temperatures in excess of 600° C, terracotta increases in mechanical strength and, though porous, will not revert to its plastic state when saturated by water. The plastic property of terracotta clays has resulted in their use globally since Neolithic times in the production of vessels, votive objects, sculpture and architectural decoration. In ceramic terms, the composition of terracotta can be compared to that of earthenware. Earthenware clays are formed from sedimentary clays, which contain many organic and mineral impurities. It is these that determine the characteristic colour of the clay. The colour most commonly associated with terracotta is a rich red-brown, which is due to the presence of iron oxide in the clay that, when fired in an oxygen-rich atmosphere, produces the distinctive red colour. The presence of other minerals, the firing temperature and the atmosphere in the kiln all contribute to the final colour of terracotta, which can range from dark brown to pink, buff, tan, orange or even green.