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A Re-study of the 'Tinned Coins' of the Song Dynasty

Dai Zhiqiang, Zhou Weirong (China Numismatic Museum, Beijing)

The 'tinned coins' were a kind of specific coins issued in the period of Huizong Emperor of the Song Dynasty, which had close connections with the history of Chinese metallurgy. However, for a long time it has been a debatable problem due to the confusion of the narration of the related historical literature. In order to get a clear understanding of the 'tinned coins', recently we have done a series of work. It comes to the conclusions that so-called 'tinned coins' should be a kind of iron coins, made of a kind of specific iron alloy (Fe-Sn-C) and having nothing to do with zinc and lead.

The first Chinese scholar who applied the 'tinned coins' as historical materials to inquire into some problems of the history of Chinese metallurgy was Prof. Hongzhao Zhang. In 1920s, according to "Fore one thousand coins, eight catties of copper, four catties of 'black tin' and two catties of 'white tin' are used" recorded in Songshi-Shihuozhi (the Economy Section of the History of Song Dynasty), Mr. Zhang put forth that so-called 'white tin' was zinc. At that time, someone happened to report a coin regarded as one of the Shaoshen coins of the Northern Song Dynasty which has such alloy composition as 55.49% Cu, 25.80% Pb and 13.15% Zn. Therefore, Mr. Zhang concluded that zinc had been used to cast coins in the Northern Song Dynasty. Here, we don't intend to discuss whether this coin and its composition is true or false. But, according to the results achieved by us from over ten years' researches, it was impossible to use metallic zinc to cast coins in the Northern Song Dynasty. It is the fact that we want to deal with is that the evidence quoted by Mr. Zhang to discuss the 'tinned coins' is wrong. So-called 'tinned coins' is a kind of iron coins of the Song Dynasty but not copper-based coins. This problem was ever demonstrated by Toshi Nakashima, Kuanghua Zhao and Shichang Ye in recent years. Their grounds are about the same, as follow.
(1) The copper-based coins of the Song Dynasty are lead-tinned bronze coins, certainly containing tin. Again emphasizing 'containing tin' is meaningless.
(2) Many historical documents of the Song Dynasty clarify that the 'tinned coins' are iron coins or tinned iron coins; only the 'tinned coins' are regarded as iron coins the related literature in the Economy Section of the Northern Song Dynasty can be successfully explained.
(3) If the 'tinned coins' are thought as bronze coins their weights are inconsistent with the regulations of the Government; but if they are iron coins their weights tally with the regulations.
(4) So-called 'tinned coins' can be covered some copper by a kind of ancient medicine (blue vitriol).

Objectively speaking, their deduction is right. But their identical view that they believe the 'tinned coins' as lead-tinned iron coins, an alloy of Fe-Pb-Sn, is wrong. The key reason which bring about such result is that they all consider that the copper above-mentioned in the documents about the coinage materials should be iron. This conclusion is incorrect. Checking the original documents, we find that the documents factually state two events: the 'tinned coins' and the big coins used as ten cashes. These two things have no relations each other. The question is that the statement in the Economy Section of the Song Dynasty lost several sentences. The 'tinned coins' are iron coins; the big coins used as ten cashes are bronze coins. The components described as above were used to cast the big bronze coins not for 'tinned coins'. There are no documents for the alloy proportion of the 'tinned coins' in the historical literature. In fact, such an alloy of Fe-Pb-Sn (8:4:2) doesn't exist.

What is the composition of the 'tinned coins'? In recent years, we analyzed more than 300 iron coins which are thought as 'tinned coins' by some Chinese numismatists, collected from Shaanxi, Shanxi, Guangxi, Ningxia and Gansu provinces. Of them, we found a coin named Zhenghe Tongbao which contains about 3% tin. With SEM determination, we found a high tin phase containing 65% Fe and 35% Sn, which is determined as Fe3SnC (with 0.01% error) by X-ray diffraction. We believe that this coin is so-called one of the 'tinned coins'. The reasons are as follow.
(1) The appearance of this coin accords with the regulations of the History of Song Dynasty.
(2) Such a high tin content can't be considered as an impurity and should be deliberately added.
(3) this kind of iron alloy is brittle and the tin of it is difficult to be lost by smelting. This characteristic is consistent with the aim of the Song's casting the 'tinned coins' to prevent the Liao and Xixia from smelting those into good steel for weapons.

the emergence of 'tinned coins' reflected that people in the Song Dynasty knew iron alloy very well.

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