Iron and Steel
During Song times, heavy industry — especially the iron industry — grew astoundingly. Iron production reached around 125,000 tons per year in 1078 CE, a sixfold increase over the output in 800 CE.
Iron and steel were put to many uses, ranging from nails and tools to the chains for suspension bridges and Buddhist statues. The army was a large consumer: steel tips increased the effectiveness of Song arrows; mass-production methods were used to make iron armor in small, medium, and large sizes; high-quality steel for swords was made through high-temperature metallurgy. Huge bellows, often driven by waterwheels, were used to superheat the molten ore.
From Charcoal to Coal
At first charcoal was used in the production process, leading to deforestation of large parts of north China. By the end of the 11th century, however, coal had largely taken the place of charcoal.
According to Marco Polo
The sight of these “black stones ... which they dig out and burn like firewood” was something else that amazed Marco Polo:
It is a fact that all over the country of Cathay there is a kind of black stones existing in beds in the mountains, which they dig out and burn like firewood. If you supply the fire with them at night, and see that they are well kindled, you will find them still alight in the morning; and they make such capital fuel that no other is used throughout the country. It is true that they have plenty of wood also, but they do not burn it, because those stones burn better and cost less.
[Moreover with that vast number of people, and the number of hot baths that they maintain — for every one has such a bath at least three times a week, and in winter if possible every day, whilst every nobleman and man of wealth has a private bath for his own use — the wood would not suffice for the purpose.]