"In parallel with superlative Ru wares from Henan province, and white porcelain in Xing and Ding from Hebei province, was another important ceramics tradition in north China, this was the manufacture of what used to be termed Northeren Celadon ware and more recently called the Yaozhou ware in shaanxi province. Yaozhou was made at the kilns in present day Tongquan county, the most important site is Huangbao and Chenlu. Like Ding ware the finest Yaozhou products were requisitioned for court, in Southern Song dynasty a writer called Gu Wenjian wrote. The present administration judge the white porcelain of Ding (zhou) unfit use, and thus ordered Ru (zhou) to produce celadon ware, there fore the kiln in Tangzhou in hebei, Dengzhou and Yaozhou all followed this order.." (Rose Kerr "Song dynasty ceramics" ISBN 1-85177-415-7)