Preliminary Analysis and Research on a Wuzhu Coin Unearthed from Lijiaba Site

Abstract. Wuzhu coins coin occupies a very important position in the history of Chinese coin. In this paper， the alloy composition and corrosion of a grey Wuzhu coin unearthed from Yunyang’s Lijiaba Site using microscopic X-ray fluorescence （micro-XRF）， and scanning electron microscope with an energy dispersive spectrometer （SEM-EDS）. The result shows that it is a copper-tin-lead coin， and the element of its bright grey surface is mainly copper’s sulfide. Thus， it is preliminarily judged that the formation of the bright grey surface might be a result from the combined actions of （α+δ） eutectoids left by corrosion product Cu2S and selective corrosion.

Keywords： Wuzhu coin； Lijiaba； Micro-XRF； SEM-EDS

1 Introduction

Yunyang’s Lijiaba is a large site in the region of China’s Three Gorges， with area of about 600000 square meters. It was a dwelling place and also a tombs area in the periods of pre-Qin， and Han and Jin dynasties.

Wuzhu coin， which was originated in the fifth ruling year of Emperor Wu in Han Dynasty （118 BC） and ended in the fourth ruling year of Emperor Tang （AD 621）， developed for 739 years. It was the currency with the most formats， the longest circulation period， and the largest quantity. It has been studied by many experts and scholars from different aspects such as archaeology， history， ancient writing， composition， and lawsuit [1-6]， but the coins unearthed from Yunyang’s Lijiaba Site have been not studied yet by now.

In this study， a Wuzhu coin， which was produced in the early ruling period of the Eastern Han Dynasty， is chosen as shown in figure 1. Its basic information is as shown in table 1. The surface of this coin looks bright grey （like lead at first view）. The alloy composition and corrosion of this coin are analyzed using micro-XRF and SEM-EDS， and also the compositions and the structural forms of the alloy substrate and surface are compared. Through the analysis， the causes for the formation of the bright grey are understood as much as possible， so as to provide a theoretical basis for its further protection.

2 Experimental methods and materials

2.1 Experimental materials

The sample chosen in this experiment is an Eastern Han Dynasty’s coin unearthed from Yunyang’s Lijiaba， as shown in figure 1.

2.2 Characterization methods

2.2.1 Micro-XRF analysis

The alloy composition of the sample is analyzed using micro-XRF. The surface of the coin’s part without characters on the back is slightly scrapped using a file for letting the substrate part of about 5mm diameter as much as possible. The test conditions of Japanese HORIBA XGT-7000 fluorescence analyzer are as follows： voltage 50kV， and current 0.013mA； unless special instructions， light spot 1.2 mm and the scanning average value of two different parts’ spectrum surfaces are used as the result of the composition analysis.

2.2.2 Scanning electron microscopy―energy spectrum analysis

The structural form and composition of the sample are analyzed using Japan‘s Hitachi cold field emission scanning electron microscope （SEM）. In order to obtain more realistic information about surface， low-vacuum conditions are applied in the process of analysis， and the sample is directly analyzed without coating film. The test conditions of SEM are as follows： high voltage 20kV， magnification times 100X， working distance 10 mm， and energy disperse spectroscopy Oxford instrument.

3 The result and discussion

The alloy composition of the sample’s substrate and surface is analyzed using micro-XRF， and the result is as shown in table 2. Seen from the test result， it is known that the sample is a copper-tin-lead coin， and its basic composition is as follows： copper （about 79%）， tin （about 6%）， lead （12%）， arsenic （nearly 0.5%）， iron （about 0.6%）， and silver （higher than 0.3%）. Compared with the sample’s substrate， the copper content on the surface of the coin declines slightly， tin and lead contents increases slightly， antimony content is higher than that of the substrate， sulfur content is close to 6%， and arsenic and silver contents change little. This indicates that the impurity elements such as antimony and sulfur might be foreign， while arsenic， silver， and iron may be produced by the alloy composition.

In order to further confirm the composition of the light gray surface， the sample is microscopically observed and analyzed using SEM.

The grey part is amplified using SEM to 5000 times （as shown in figure 2 and figure 3）， from which it is found that the bright grey area is not continuous， and there are a lot of rust on the surface and different sizes of holes and cracks. The non-silvery white area is more rough and unsmooth.

The micro-area compositions of flat bright grey surface， fracture， and non-bright grey surface are analyzed， and the result is as shown in table 3. From table 3， it is seen that the composition of the bright grey surface includes mainly copper and sulfur， as well as a small amount of tin， lead， and iron. Cu2S is commonly known as molybdenite， which looks black or ash black with metallic luster. The compositions of the uneven part and non-bright grey part in the middle fracture are complicated， including corrosion products such as copper carbonate， SiO2 and phosphate from the soil. The photo and result of canning the sample’s bright grey surface to the non-bright grey surface are as shown in figure 4.

From figure 4， it is seen that the sulfur content declines until it disappears from the bright grey surface.

HAN Rufen et al propose that the surface of the ancient Chinese bronze ware looks silvery white， and a layer rich in tin is found using XRF and XRD [7]. Oddy and Meeks also explain that the surface of bronze ware is richer in copper α solid solution than the corrosion surface in a proper burial environment because of the selective corrosion； the surface is presented to be silvery white as a lot of （α+δ） eutectoids are reserved [8， 9]. In order to further investigate the conditions of the surface and substrate， the cross section of the sample is observed and analyzed with SEM， and the result of the cross-section composition analysis result is as shown in figure 5. From figure 5， it is seen that the thickness of the bright grey surface is about 20μm； from the surface to the substrate， copper content decreases slightly and then largely increases， tin and antimony increase and then decrease， and iron， lead， and arsenic contents change little； in the substrate， element S is not found， but S rapidly declines until it is almost not detected from the surface to the substrate after the cross-section is scanned； carbon content increases first and decreases to almost no， and oxygen gradually decreases to no.

4 Conclusion

The following conclusion is made after the Wuzhu coin with a bright grey surface is scientifically analyzed：

（1）The sample is a copper-tin-lead coin and its substrate is casting organization， and there are many bright white （α+δ） eutectoids left on the edge as the selective corrosion copper migrates into the soil environment at the corrosion interface.

（2）The part with bright grey surface has been broken into small pieces or corrosion holes in the burial environment due to corrosion.

（3）The bright grey presented on the surface might a result from the combined actions of （α+δ） eutectoids left by corrosion product Cu2S and selective corrosion.

Acknowledgement

This paper is supported by the Science and Technology Support Project of Sichuan Province （2013FZ0076） and the Central University Basic Scientific Research Fund of Sichuan University. Also， it is greatly supported by Chongqing Three Gorges’ Office. In the collection of the sample， great assistance is received from relevant units. Here， the authors sincerely thank all of them.

References

[1]Zhiqiang DAI， Weirong ZHOU， et al. Discussion on Chinese Successive Dynasties’ Copper COINS Alloy Composition [M]. Numismatics and the History of Smelting and Casting. Beijing： Zhonghua Press， 2002： 57～77.

[2]Weirong ZHOU. Study on Chinese Coin Alloy Composition [M]. Beijing： Zhonghua Press， 2004： 278～280.

[3]Han Dynasty Tombs in Mancheng County and the Discussion on Relevant Problems [J]. Chinese COINS， 1991， 2 （33）： 12～19.

[4]Ling He， Junyan Liang， Xiang Zhao Baolian Jiang. Corrosion Behavior and Morphological Features of Archeological Bronze Coins from Ancient China. Micro-chemical Journal. 2011， 99： 203～212.

[5]Huilan LIU. Study on the Currency Problems of Western Han and Eastern Han Dynasties. Jiangxi Normal University， Master's Thesis， 2002.

[6]Yi Mu， Wugan LUO， Chongchan ZHANG， et al. The Composition of the Coins Unearthed from Han Dynasty Tombs in Shenmingpu Site and Its Metallographic Analysis [J]. Chinese COINS， 2011， 3（114）： 22～27.

[7]Rufen HAN， Shuyun SUN， Xiuhui LI， Wei QIAN. The Microstructure of Ancient Chinese Bronze Brass or Copper Ware [J]. Journal of Beijing University of Science and Technology， 2002 （24）： 219～230.

[8]Meeks N D. Tin-rich Surfaces on Bronzes Some Experimental and Archaeological Consideration. Archaeometry， 1986， 28（2）：133～162.

[9]Odd W A， Bimson M. Tinned Bronze in Antiquity. Institution and Conservation Occasional Paper， 1986 （3）： 169.