Cast iron

UNESCO Courier, Oct, 1988

The Chinese practised the technique of using blast furnaces for making cast iron from at least the fourth century BC. There were a number of reasons for this. China had good refractory clays for the construction of the walls of blast furnaces. The Chinese also knew how to reduce the temperature at which the iron would melt. They threw in something which they called 'black earth., which contained much iron phosphate. If up to 6 per cent of phosphorus is added in this way to an iron mixture, it reduces the melting point from the normal 1130 degrees C to 950 degrees C. This technique was used in the early centuries, ceasing before the sixth century AD, when proper blast furnaces came Into use which needed no such assistance.

Coal, which gave a high temperature, was used as a fuel from the fourth century AD, and probably earlier. One method was to put the iron ore in batteries of elongated, tube-like crucibles, and pack these round with a mass of coal which was then burnt. This had the extra advantage of excluding sulphur from the process.

The widespread availability of cast iron in ancient China had many side effects. It led to the innovation of the cast iron ploughshare in agriculture, along with iron hoes and other tools. Iron knives, axes, chi Bels, saws and awls all became available. Food could be cooked in cast iron pots, and even toys were made of cast iron. Cast iron statuettes of various animals have been found in Han Dynasty tombs dating between the second centuries BC and AD. Cast iron moulds for implements dating from the fourth century BC have also been discovered. Hoes and axes would have been cast in these, in either bronze or iron.

The expertise in cast iron enabled pots and pans to be made with very thin walls, impossible by other iron technology. One extremely important result was that salt could be mass-produced from evaporated brine, which can only be done in such thin pans. This in turn led the Chinese to exploit natural gas by deep drilling. This was in order to tap the energy from the burning gas to evaporate the vast quantities of brine required for the giant salt industry (which the Han Dynasty nationalized along with the iron industry in 119 BC). The salt and gas industries could not have existed without the cast iron industry.

In the third century BC, the Chinese discovered how to make a malleable cast iron by annealing (that is, by holding it at a high temperature for a week or so). It was then not so brittle, and would therefore not shatter if subjected to a violent shock. This meant that objects like ploughshares could survive striking large stones with considerable force. Cast iron had something of the elasticity of wrought iron, but with the much greater strength and solidity that came from being cast. It was almost as good as steel.

. . Some of the ancient Chinese feats of casting iron are so impressive as to be almost unbelievable, even when the results are before our eyes. For instance, there is the cast iron pagoda shown on opposite page.

Perhaps the grandest cast iron structure of all was not actually a building. The Empress Wu Zetian had an octagonal cast iron column built, called the 'Celestial Axis Commemorating the Virtue of the Great Zhou Dynasty with Its Myriad Regions". It was built in 695 AD upon a base of cast iron 51 metres in circumference and 6 metres high. The column itself was 3.6 metres in diameter and rose 32 metres in the air; on top was a 'cloud canopy' 3 metres high and 9 metres in circumference. On top of this in turn stood four bronze dragons each 3.6 metres high supporting a glided pearl, We have a record of the amount of metal used in this constructionabout 1,325 tons. The largest single cast iron object ever made (the pagodas were obviously not a single piece) was erected on the orders of the Emperor Shizong of the Later Zhou Dynasty in commemoration of his campaign against the Tartars in 954 AD. This extraordinary object, 6 metres tall, still stands and is known as the Great Lion of Zangzhou (Hebei). It is not solid, but its walls vary from 4 to 20 centimetres. in thickness.