archaeological samples and textual accounts do not appear until the 13th century. The primary extant specimens of cannons from the 13th century are the Wuwei Bronze Cannon dated to 1227, the Heilongjiang hand cannon dated to 1288, and the Xanadu Gun dated to 1298. However, only the Xanadu gun contains an inscription bearing a date of production, so it is considered the earliest confirmed extant cannon. The Xanadu Gun is 34.7 cm in length and weighs 6.2 kg. The other cannons are dated using contextual evidence. The oldest representation of a bombard can be found in the Chinese town of Ta-tsu. In 1985, the Canadian historian Robin Yates visited the Buddhist cave temples when he saw a sculpture on the wall depicting a demon holding a hand-held bombard. The muzzle seems to have a blast and flames coming from it which some believe is proof of some type of super gun. Yates examined the cave and believed the drawings dated back to the late 12th century. Cast iron: Confirmed by archaeological evidence, cast iron, made from melting pig iron, was developed in China by the early 5th century BC during the Zhou Dynasty (1122–256 BC), the oldest specimens found in a tomb of Luhe County in Jiangsu province; despite this, most of the early blast furnaces and cupola furnaces discovered in China date after the state iron monopoly under Emperor Wu (r. 141–87 BC) was established in 117 BC, during the Han Dynasty (202 BC – 220 AD); Donald Wagner states that a possible reason why no ancient Chinese bloomery process has been discovered thus far is because the iron monopoly, which lasted until the 1st century AD when it was abolished for private entrepreneurship and local administrative use, wiped out any need for continuing the less-efficient bloomery process that continued in use in other parts of the world. Cast iron is comparatively brittle and is not suitable for purposes where a sharp edge or flexibility is required. An important Chinese innovation was the development of malleable iron in the 4th century BC, which enhanced the mechanical properties of cast iron through an annealing process. Furthermore, Wagner states that most iron tools in ancient China were made of cast iron in consideration of the low economic burden of producing cast iron, whereas most iron military weapons were made of more costly wrought iron and steel, signifying that "high performance was