Analysis of information sources in references of the Wikipedia article "한국의 토기와 도자기" in Korean language version.
Koryo potters also experimented with the use of copper for red designs under the glaze, since ground copper pigment fires red in the reducing kiln atmosphere. This technique was started in the twelfth century. Many scholars agree that Chinese Yuan wares with underglaze red design were inspired by the Koryo potters' use of copper red at the time when the Yuan and Koryo courts had very close political ties.
