essential" and preferred for the latter. Celadon: Named after a pale-tinted spring green colour, Chinese archaeologist Wang Zhongshu (1982) asserts that shards having this type of ceramic glaze have been recovered from Eastern Han Dynasty (25–220 AD) tomb excavations in Zhejiang; he also asserts that this type of ceramic became well known during the Three Kingdoms (220–265). Richard Dewar (2002) disagrees with Wang's classification, stating that true celadon—which requires a minimum 1260 °C (2300 °F) furnace temperature, a preferred range of 1285° to 1305 °C (2345° to 2381 °F), and reduced firing—was not created until the beginning of the Northern Song Dynasty (960–1127). The unique grey or green celadon glaze is a result of iron oxide's transformation from ferric to ferrous iron (Fe2O3 → FeO) during the firing process. Longquan celadon wares, which the archeologist Nigel Wood at the University of Oxford writes were first made during the Northern Song, had bluish, blue-green, and olive green glazes and high silica and alkali contents which resembled later porcelain wares made at Jingdezhen and Dehua rather than stonewares. Chain drive, endless power-transmitting: The Greek Philon of Byzantium (3rd or 2nd century BC) described a chain drive and windlass used in the operation of a polybolos (a repeating ballista), "but the chain drive did not continuously transmit power from shaft to shaft and hence they were not in the direct line of ancestry of the chain-drive proper". A continuously driven chain drive first appeared in 11th century China. Perhaps inspired by chain pumps which had been known in China since at least the Han Dynasty (202 BC – 220 AD) when they were mentioned by the Han dynasty philosopher Wang Chong (27 – c. 100 AD), the endless power-transmitting chain drive was first used in the gearing of the clock tower built at Kaifeng in 1090 by the Song Chinese politician, mathematician and astronomer Su Song (1020–1101). Chopsticks: The Han dynasty historian and writer Sima Qian (145–86 BC) wrote in the Records of the Grand Historian that King Zhou of Shang was the first to make chopsticks out of ivory in the 11th century BC; the most ancient archaeological find of a pair of chopsticks, made of bronze, comes from Shang Tomb 1005 at Houjiazhuang, Anyang, dated roughly 1200 BC. By 600 BC, the use of chopsticks had spread to Yunnan (Dapona in Dali), and Töv Province by the 1st century. The earliest