When was first clay used?

Clay was originally used, way back around 6,000 B.C. to line the inside of baskets to render them more leak proof. The people found out that when these baskets were discarded into the bon fire, when no longer needed, the reeds and grasses of the baskets burned off, but the clay was transformed by the fire, into a strong vessel or bowl that could be used for food or water storage. This happened purely by accident.