According to surviving reports, the incident was caused by inexperienced gunpowder makers hired to replace the previous ones, and had been careless while grinding sulfur. A spark caused by the grinding process came into contact with some fire lances which immediately started spewing flames and jetting around "like frightened snakes." The gunpowder makers did nothing as they found the sight highly amusing, that is until one fire lance burst into a cache of bombs, causing the entire complex to explode. The validity of this report is somewhat questionable, assuming everyone within the immediate vicinity was killed. By the time of Jiao Yu and his Huolongjing (a book that describes military applications of gunpowder in great detail) in the mid 14th century, the explosive potential of gunpowder was perfected, as the level of nitrate in gunpowder formulas had risen from a range of 12% to 91%, with at least 6 different formulas in use that are considered to have maximum explosive potential for gunpowder. By that time, the Chinese had discovered how to create explosive round shot by packing their hollow shells with this nitrate-enhanced gunpowder. Europe and Japan Gunpowder may have been used during the Mongol invasions of Europe. "Fire catapults", "pao", and "naphtha-shooters" are mentioned in some sources. However, according to Timothy May, "there is no concrete evidence that the Mongols used gunpowder weapons on a regular basis outside of China." Shortly after the Mongol invasions of Japan (1274–1281), the Japanese produced a scroll painting depicting a bomb. Called tetsuhau in Japanese, the bomb is speculated to have been the Chinese thunder crash bomb. Japanese descriptions of the invasions also talk of iron and bamboo pao causing "light and fire" and emitting 2–3,000 iron bullets. Hand cannon Traditionally the first appearance of the hand cannon is dated to the late 13th century, just after the Mongol conquest of the Song dynasty. However a sculpture depicting a figure carrying a gourd shaped hand cannon was discovered among the Dazu Rock Carvings in 1985 by Robin Yates. The sculptures were completed roughly 250 km northwest of Chongqing by 1128, after the fall of Kaifeng to the Jin dynasty. If the dating is correct this would push back the appearance of the cannon in China by a hundred years more than previously thought. The bulbous nature of the cannon is congruous with the earliest hand cannons discovered in China and Europe. Archaeological