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Cast Iron in Ancient China

Cast Iron in Ancient China

This article is a revised version of a talk Mr. Donald B. Wagner gave at the one-day Symposium on Cast Iron in Ancient China. We listed some main part of content to share with the friends working in iron casting industry.

We can see that casting steel, with up to about 1.5% carbon, requires temperatures well over 1500 centigrade. Such temperatures were often reached in early times in various parts of the world, but the refractory materials necessary to manipulate molten steel and cast it into useful artifacts were not developed until the late 19th century, in Britain.

With a higher carbon content, 3 - 4%, practical casting is much easier, requiring temperatures around 1300 centigrade. Iron with this carbon content is called "cast iron" because it is easy to form by casting. It lends itself well to large-scale production, which has always been important in ancient China. It was undoubtedly the use of cast iron in ancient China that made it possible for every peasant to have iron implements.

White Cast Iron

The use of cast iron makes it possible to mass-produce implements cheaply, but for most implement types it has inferior mechanical properties. All pre-modern Chinese cast iron has a very low silicon content, usually under 0.5% Si, and therefore solidifies as "white cast iron", in which the carbon is present as cementite, Fe3C, which is extremely hard, harder than quartz, and this makes the iron brittle.

An example is this mattock-head. The micrograph shows that it is white cast iron. Its hardness may be an advantage with such an implement, for it would be very abrasion-resistant. But the worker would have had to be careful, for if it hit a rock it might shatter.

Coins were usually made of copper alloys, but sometimes shortages of copper made it necessary to cast them of iron. Here are three iron coins, from the Western Han, Song, and Qing periods.

Mike Wayman and Helen Wang studied 37 iron coins of the Song period in the British Museum, London. They are all of white cast iron, and their article has a great deal of detail on the types of white cast iron found here. 

An interesting aspect is revealed by X-rays of some of the coins: these show that there are sometimes quite large casting bubbles in the coins, making them feel distinctly light in the hand. It seems possible that the coin-founders intended these bubbles, in order to save material.

This is a cast-iron mirror – probably Han-dynasty (206 BC – AD 220), probably white cast iron.

An interesting characteristic of traditional Chinese metallurgy is that cast iron is often combined with other materials. The legs of this vessel are cast iron, while the body is bronze. I have not seen a metallurgical investigation of a vessel of this type, but the legs are most probably of white cast iron.

The vessel is a Ding from a grave excavated at Yutaishan in Jiangling, Hubei, dated to the 4th century BC.

Malleable Cast Iron

These crossbow-bolts have bronze tips and iron shafts – presumably because bronze has better casting properties than iron, while iron is cheaper than bronze.

Lian Haiping in Shanghai has studied some examples of this type of artefact and found that the shafts are of cast iron which has been decarburized in the solid state – this type of iron is called “whiteheart malleable cast iron”. She reported this at the BUMA conference in Beijing in 2006.

This implement-cap is “blackheart malleable cast iron” – it was first cast, then annealed at a high temperature, probably around 950°C, for a period of days. This treatment caused the carbon in the iron to precipitate as graphite, making a material which has much better mechanical properties than ordinary cast iron (white or grey).

These gads from a copper-mine site show how tough the ancient Chinese cast iron could be. They are obviously subject to very hard punishment, being hammered into cracks in the rock. 

But they are of cast iron – again annealed at a high temperature for a period of days, this time decarburizing the iron at the surface and precipitating graphite farther in.

These scissors are also of cast iron. They were cast, then annealed in an oxidizing atmosphere to decarburize to a uniform carbon content around 1%, with a few very small grapite nodules, then bent into shape by a smith.

Grey Cast Iron

That in itself is surprising enough, but note that the iron is very low in both carbon and silicon, and nevertheless the structure is grey cast iron.

Normally such an alloy would solidify as white cast iron – only very slow cooling would allow it to solidify as grey cast iron.

This must have been cast in a massive heated ceramic mould and allowed to cool very slowly, over a period of days.

The only ancient artefacts I know of which are grey-cast are moulds, and I believe the reason is that the moulds must be very tough, to tolerate the thermal shock when molten iron is poured in, but they could not be made of malleable cast iron, because the annealing process tends to warp castings slightly, so that it would be difficult to fit the parts of the mould together.

More Uses for Cast Iron in China

Cast Iron Woks

Here is a wok – that is the usual English word, which comes from the Cantonese pronunciation, which in Mandarin is pronounced huo. It is broken, so it was possible to measure the thickness at its thinnest part.

How they were cast in the 19th century can be seen here. They are still being cast today, probably by different methods. It is interesting to note that cast-iron woks are now being imported to Europe in large numbers and used as portable fireplaces.

Monumental Castings

Another major use for cast iron was for statues and other large monuments. This is a quite famous cast iron statue, one of four at the Zhongyue Temple in Dengfeng, cast in AD 1024.

By studying the mould-seams on statues like this one and the next we can learn a lot about how these statues were cast.

In this case we have a very interesting phenomenon. The photograph shows what looks like a repair to the statue, but in fact it represents a repair to the mould.

Something happened to the mould – it broke and had to be repaired. The bumps which can be seen are probably the heads of spikes that were used to hold the repair in place.

Cast Iron Bells

I think it is only in China that bells have been made of cast iron.

 

This broken bell is interesting because we can see in the fracture that something has gone wrong. Those air bubbles at the surface would (I believe) have had a bad effect on the tone of the bell. 

Probably the founders added sulphur to the iron to assure that it would solidify as white cast iron, but added too much, so that the bubbles formed. 

Cast Iron Cannons

The ancient founders' experience in casting statues and such came in very handy when they began casting iron cannons. The first iron cannons in China come at the very beginning of the Ming Dynasty, in the 1370's.

The cannon above is probably a combination of materials – wrought iron and cast iron – as we can see at the right: two cannons with wrought iron inside and cast iron outside. George Banks sketched these at the Dagu Forts, in modern Tianjin, in 1860. He believed that these were 'evidently very old', from the +17th century or before; while this dating is quite plausible it is not clear what evidence he could have had for it. He does not mention any inscriptions.


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