Blue-glazed jar, Tang dynasty (618–907). Earthenware, 11.7 x 12cm. Ashmolean museum, EA1956.1076 © Ashmolean museum 

This Tang dynasty earthenware jar has a distinctive blue glaze from the inside of the lip spreading unevenly down three quarters of the body. To achieve this effect, cobalt is used as a colouring agent. This method has been used to create blue pigments since ancient times.

The cobalt ores used during this period were from mines in Persia were extremely rare, and therefore expensive. However, the development of the Silk Road assisted with the import of these ores to China. This allowed China to create the earliest applications of cobalt to porcelain.