“Gold corrosion”: An alternative source of red stains on gold coins

Abstract

Abstract Recently, the phenomena of red stains defects was found on historical and modern golden coins around the world. This “Gold corrosion” can be the reason for reduction of coin value, especially of modern high-premium coins. In such modern investment coins, the contamination of the surface by microscopic particles of metallic silver is reported to be a common cause of such stains. Corrosion of these particles (or even back reduction) is associated with a distinct change of colour. In the case of historical coins, this relatively simple explanation does not respond in some cases. The submitted study is dealing with analysis of red stains on of Austro-Hungarian gold ten crowns coins, which served as a common part of circulating currency. They were therefore minted in thousands/million pieces by standard procedures each year. It is therefore quite logical that these coins can have different mechanisms of red spots formation than modern high-premium coins. Red stains on Austro-Hungarian coins were analysed by the means of light (LM) and digital microscopy (DM), energy-dispersive X-ray spectroscopy (EDS), energy dispersive X-ray fluorescence spectroscopy (XRF), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy, X-ray powder micro-diffraction (µ-XRPD) and Raman microscopy. The EDS, XRF and XPS analysis points to the presence of iron and oxygen in red stains, no silver or its corrosion products were detected. The exact determination of red stains was done by µ-XRPD and Raman microscopy where an iron(III) oxide-hydroxide (Goethite) was found as main product.