Pottery is the process of forming vessels and other objects with clay and other ceramic materials, which are fired to give them a hard, durable form. Major types include earthenware, stoneware and porcelain. The place where such wares are made by a potter is also called a pottery (plural “potteries”). The definition of pottery used by the American Society for Testing and Materials (ASTM) is “all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products.” A different definition, used within the field of ceramics, is “everything which is not porcelain”. In archaeology, especially of ancient and prehistoric periods, “pottery” often means vessels only, and figures etc of the same material are called “terracottas”. Clay as a part of the materials used is sometimes said to be essential, but this is dubious.