Non-destructive Compositional and Structural Analysis of Early Chinese Currencies

Abstract

Non-destructive analysis of the composition and structure of various ancient Chinese currencies, including two Ming-style knife money from the Zhou Dynasty and nineteen square-holed coins from the Tang and Northern Song Dynasties are presented. The knife money are estimated to have been minted around 360 to 255 B.C., while the nineteen square-holed coins are estimated to have been minted between 621 and 1117 A.D. All currency, except for the bent knife money, was analyzed non-destructively using synchrotron radiation X-ray diffraction and scanning electron microscopy with focused ion beam and energy dispersive spectroscopy to determine their structure (phases present), microstructure, and elemental distribution. The currency was generally found to be composed of primarily a CuSn alloy with small amounts of Pb and Fe and trace amounts of other elements and oxides. The main phase present in all coins examined using synchrotron radiation X-ray diffraction is an FCC CuSn alloy along with FCC reflections from the traces of Pb. The immiscible Pb within these Cu-based alloys was found to be distributed as small globules or islands ranging between 10 and 80 μm and scattered throughout the Cu-based matrix. Scanning electron microscopy with energy dispersive spectroscopy provided some microstructural and elemental distribution information, but analysis was made difficult due to corrosion/oxidation at the surface which cannot be removed non-destructively. Synchrotron radiation X-ray diffraction was much more helpful in providing crystallographic orientation and phases as it could be used to measure the bulk material of the samples past the relatively thinly oxidized surface. While authenticity of the currency is difficult to determine, the presence of Pb is a good indicator as it is not commonly used in modern CuSn alloys.