While Chinese gunpowder formulas by the late 12th century and at least 1230 were powerful enough for explosive detonations and bursting cast iron shells, gunpowder was made more potent by applying the enrichment of sulphur from pyrite extracts. Chinese gunpowder solutions reached maximum explosive potential in the 14th century and at least six formulas are considered to have been optimal for creating explosive gunpowder, with levels of nitrate ranging from 12% to 91%. Evidence of large scale explosive gunpowder weapons manufacturing began to appear. While engaged in war with the Mongols in 1259, the official Li Zengbo wrote in his "Ko Zhai Za Gao, Xu Gao Hou" that the city of Qingzhou was manufacturing one to two thousand strong iron-cased bomb shells a month, and delivered them to Xiangyang and Yingzhou in loads of about ten to twenty thousand shells at a time.