industry's demands for charcoal. However, by this time the Chinese had figured out how to use bituminous coke to replace the use of charcoal, and with this switch in resources many acres of prime timberland in China were spared. This switch in resources from charcoal to coal was later used in Europe by the 17th century. The economy of the Song dynasty was one of the most prosperous and advanced economies in the medieval world. Song Chinese invested their funds in joint stock companies and in multiple sailing vessels at a time when monetary gain was assured from the vigorous overseas trade and indigenous trade along the Grand Canal and Yangzi River. Prominent merchant families and private businesses were allowed to occupy industries that were not already government-operated monopolies. Both private and government-controlled industries met the needs of a growing Chinese population in the Song. Both artisans and merchants formed guilds which the state had to deal with when assessing taxes, requisitioning goods, and setting standard worker's wages and prices on goods. The iron industry was pursued by both private entrepreneurs who owned their own smelters as well as government-supervised smelting facilities. The Song economy was stable enough to produce over a hundred million kg (over two hundred million lb) of iron product a year. Large scale deforestation in China would have continued if not for the 11th-century innovation of the use of coal instead of charcoal in blast furnaces for smelting cast iron. Much of this iron was reserved for military use in crafting weapons and armoring troops, but some was used to fashion the many iron products needed to fill the demands of the growing indigenous market. The iron trade within China was furthered by the building of new canals which aided the flow of iron products from production centers to the large market found in the capital city. The annual output of minted copper currency in 1085 alone reached roughly six billion coins. The most notable advancement in the Song economy was the establishment of the world's first government issued paper-printed money, known as Jiaozi (see also Huizi). For the printing of paper money alone, the Song court established several government-run factories in the cities of Huizhou, Chengdu, Hangzhou, and Anqi. The size of the workforce employed in paper money factories was large; it was recorded in 1175 that the factory at Hangzhou employed more than a