Chinese Coins In Copper Based Alloys: Elemental And Microstructural Characterization

Abstract

The Chinese cast coin collection from the CCCM’s Museum in Lisbon (Portugal) was accessible for analytical and metallographic characterization. In spite of the considerable number of similar items that populate museums and private collectors in Europe and North America, studies carried out in the western world are very scarce. Firstly, elemental characterization by EDXRF spectrometry was performed on 380 copper-based coins, revealing different types of alloys (coppers, bronzes and brasses) with variable alloy elemental composition. In bronze coins one of the main constituents was lead, which sometimes presented considerable content variations between the obverse and reverse of the same coin. Brass coins were found to be rich in zinc (<63 wt.%), but often presented traces or small contents of lead, tin, antimony, iron and arsenic. Metallographic techniques used in the study of 109 coins microstructures could confirm the production process consistency (sand casting), and establish a parallel between the elemental contents (obtained by micro-EDXRF) and the present metallic phases (observed by OM and SEM, and analyzed by SEM-EDS and micro-XRD), resorting to equilibrium phase diagrams. A comparison between the corrosion susceptibility of metallic phases in bronze coins showed preferential transglobular corrosion of lead-rich globules, and also that lead and/or tin corrosion by-products are often deposited at the surface of these coins. When considering brass coins, the most common corrosion process was dezincification. Also, the presence of some minor elements was found to greatly influence the corrosion susceptibility, demoting (e.g., Fe) or promoting (e.g., Sn, Sb and As) the corrosion resistance of the phases in which these elements are in higher concentration. The obtained results are an added value to the characterization of similar alloys, namely through the co-relation between their elemental contents and the current micro-constituents. Also, the corrosion susceptibilities in bronze and brass metallic phases can contribute to an improved macroscopic diagnose for conservation purposes in similar items. All the results obtained during this study support the coins authenticity.