Grayish-blue earthenware is called grayish-blue pottery in China. The final stage of firing does not allow much air to flow through the kiln, which results in a grayish-blue color due to reduction in the kiln. It is a more advanced type of redware than redware, in which the iron in the clay is reduced to ferrous oxide and the vessel becomes well-fired, shrunken, and hardened. The clay is of fine quality, and in this respect differs from the coarse clay of colored ceramics. Ash ceramics first appeared in the Longshan Culture period and accounted for about half of the earthenware. The surface of the potter’s wheel was often polished, and the patterns were often stripes, latticework, or jomon woven patterns. In the Longshan period, there were no dings or ditto, and even in the Shang and Zhou periods, wares were made of ash ceramics, but those made of ash ceramics seem to have been different from those used for fire.
