 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.