gunpowder had a level of nitrate capable of bursting through cast iron metal containers, in the form of the earliest hollow, gunpowder-filled grenade bombs. In 1280, the bomb store of the large gunpowder arsenal at Weiyang accidentally caught fire, which produced such a large explosion that a team of inspectors at the site a week later deduced that 100 guards had been killed instantly, with wooden beams and pillars blown sky high and landing at a distance of over 10 li (~2 mi or ~3 km) away from the explosion. By the time of Hanzo Yu and his Huolongjing (which 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 to 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 invented how to create explosive round shot by packing their hollow shells with this nitrate-enhanced gunpowder. An excavated trove of early Ming land mines showed that corned gunpowder was present in China by 1370. There is evidence suggesting that corned powder may have been used in East Asia as early as the thirteenth century. Printing During the Tang dynasty, printing was created in China (AD 618–906). The first mention of printing is in an AD 593 imperial decree by the Sui Emperor Wen-ti, who mandates the printing of Buddhist pictures and scriptures. Woodblock printing Blocks made from wood were used in the oldest type of Chinese printing. Printing textiles and reproducing Buddhist scriptures were also done using these blocks. Short religious writings were carried as charms in this manner. The Chinese invention of woodblock printing, at some point before the first dated book in 868 (the Diamond Sutra), produced the world's first print culture. According to A. Hyatt Mayor, curator at the Metropolitan Museum of Art, "it was the Chinese who really invented the means of communication that was to dominate until our age." Woodblock printing was better suited to Chinese characters than movable type, which the Chinese also invented, but which did not replace woodblock printing. Western printing presses, although introduced in the 16th century, were not widely used in China until the 19th century. China, along with Korea, was one of the last countries to adopt them. Woodblock