CHINESE COINS made of copper based alloys: elemental and microstructural characterization

Project developed in the framework of a PhD in Conservation and Restoration by Maria João Furtado

2013

FCT studentship SFRH/BD/29736/2006

The Chinese cast coin collection from the CCCM’s Museum in Lisbon (Portugal) was accessible for analytical and metallographic studies. 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 SEMEDS 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 byproducts 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.

Publications:

M.J.Furtado. Moedas Imperiais Chinesas da colecção do Museu do Centro Científico e Cultural de Macau. Lisbon (Portugal), August 2011 Oral presentation and published with the same title and author in Numismática, 114, (38/4).

M.J. Furtado, R.J.C.Silva, M.F. Araújo and F.M. Braz Fernandes, “Composition and microstructures of imperial brass Chinese coins”, Mater. Sci. Forum 636-637 (2010) 531-537.

MJ. Furtado, R.J.C. Silva, M F .Araújo, F.M. Braz Fernandes, “Influence of Minor Elements in Ancient Chinese Brass Coins Microstructures”, TOFA-2010 (Discussion Meeting on Thermodynamics of Alloys), 12-15 September, 2010, Porto, Portugal, (submited for publication) (ISBN: 978-972-752-126-5).

M.J.Furtado, R.J.C. Silva, M.F. Araújo, F.M. Braz Fernandes, "Some Corrosion Issues in Ancient Chinese Brass Coins: Influence of Minor Elements in Corrosion Susceptibility", Proceedings of Eurocorr 2010 (European Corrosion Congress)September 13-17, 2010,Moscow,Russia(Published in CD-ROM Conference Proceedings, ISBN 9781617824029), pp 2163-2171.

M.J. Furtado, R.J.C .Silva, M.F.Araújo, F.M.Braz Fernandes - "Multiphase microstructures on late imperial Chinese brass coins", Proceedings of theVIIConference on the Beginnings of the Use of Metals and Alloys (BUMA VII), September 13-17, 2009, Bangalore, India, (ISBN No. 978-93-83566-11-2), (2015)pp. 129-136.