Porcelain with underglaze iron decoration In the 17th century, Korean potters adopted underglaze brown painted decoration using iron to mimic the look of cobalt, as China’s 1657–1684 ban on foreign trade, in response to social turmoil, limited supplies of the blue pigment. Notches in the foot rim helped maintain the jar’s shape by allowing heat and gases to escape, instead of becoming trapped beneath the vessel and warping its form. This innovation, not used in China, was introduced to Japan by Korean potters and remains a distinctive feature of some Japanese tea bowls.