Microchemical surface analysis of historic copper-based coins by the combined use of FIB-FESEM-EDX, OM, FTIR spectroscopy and solid-state electrochemical techniques

Abstract

A multi-technique strategy, including microscopy, spectroscopic and electrochemical techniques, is proposed to study thin corrosion layers that form on the surface of historic copper-based coins. An accurate characterisation of this external corrosion layer is important for selecting a suitable conservation and/or restoration treatment. For this purpose, a series of copper-based coins from different historical periods and provenances, which mainly exhibited atmospheric corrosion, was analysed. The morphology of the corrosion layer and the upper core of coins was studied in trenches done on coin surfaces with a focused ion beam gun, coupled to a field emission scanning electron microscope-X-ray microanalysis (FIB-FESEM-EDX). The X-ray microanalysis performed with FESEM-EDX on trenches allowed elemental composition profiles on the corrosion layer to be obtained. These results were complemented with the compositional data of the corrosion products provided by the voltammetry of immobilised microparticles (VIMP) and Fourier transform infrared spectroscopy (FTIR), and by studying visual appearance by optical microscopy and colorimetry. Cuprite and tenorite (the latter increased with coin age) were the main identified corrosion products, which were accompanied, to a lesser extent, by copper trihydroxychloride polymorphs. Interestingly, metal oxalates and metalcarboxylate complexes were identified on the corrosion layer of most coins. These unusual alteration products of copper-based coins were formed from the organic matter deposited on coin surfaces given their use when placed in circulation or by further manipulations of collectors.

Funerary colors in Pre-classical Maya culture: the red pigment in the 19th tomb of Rio Azul (Peten, Guatemala)

“…guano) present in caves, shelters and less frequently in outcrops. A similar mechanism has been identified on old copper coins, where recently, the authors have found copper oxalates as result of the degradation of the thin film of organic matter adhered to the coin surface when the money was in circulation [43].…”

Solid-state electrochemical characterization of emissions and authorities producing Roman brass coins

“…Although studies on copper coins are focused on elemental analysis and microstructure [1][2][3][43][44][45][55][56][57][58], prompting the characterization of provenances and ateliers preparing forgeries [58].…”

“…Studies on metal coins are focused on the determination of the composition and microstructure of the base alloy and the identification of the corrosion products using multi-technique approaches some of which require some destructive sampling [43][44][45]. In this context, we applied the VIMP methodology in order to acquire archaeometric information based on a minimally invasive (at the nanogram level) sampling on the metal patina.…”

Chemical analysis and computed tomography of metallic inclusions in Roman glass to unveil ancient coloring methods

“…The presence of lead islands within the bronze matrix can be explained considering the very low miscibility of lead in copper: when the two metals are mixed in liquid phase, copper solidifies at a higher temperature than lead, which therefore is still in the liquid phase 19 , 20 . The solidification of lead in a Cu–Pb system could be reached only at the alloy eutectic temperature (326 °C): this induced the formation of grain boundaries that were observed in the cross sections 16 .…”