For centuries, Bizen has been highly regarded for its dark red-brown stoneware made from iron-rich clay with a natural ash glaze. Bizen is unglazed but fired at such a high a temperature that a natural glaze is formed on the surface. Also upon firing, wood ash flies up in the kiln and is deposited randomly on the pottery, giving each piece its own individuality and vitality. The beauty inherent in the unglazed, red-brown clay forms was often left untouched, giving the vessels the luster of burnished bronze. The height Bizen pottery was attained in the 18th century, and specimens from that period are much finer than subsequent works.