 Gunpowder was difficult to use in mines because it is hygroscopic, easily absorbing water from the atmosphere, and when wet is no longer explosive.A 14th-century military treatise, the "Huolongjing" (Fire Dragon Manual), describes hollow cast iron cannonball shells filled with gunpowder.The wad of the mine was made of hard wood, carrying three different fuses in case of defective connection to the touch hole.