sentences:  The "De'an Shoucheng Lu", an account of the siege of De'an in 1132 during the Jin–Song Wars, records that Song forces used fire-lances against the Jurchen.The proportion of saltpeter in the propellant was increased to maximize its explosive power.To better withstand that explosive power, the paper and bamboo of which fire-lance barrels were originally made came to be replaced with metal.