a type of heavy sticky earth that can be used for making pots, bricks etc.——朗文当代大辞典

A stiff viscous earth found, in many varieties, in beds or other deposits near the surface of the ground and at various depths below it: it forms with water a tenacious paste capable of being moulded into any shape, which hardens when dried, and forms the material of bricks, tiles, pottery, and ‘earthenware’ generally.——牛津当代大辞典  @en