Red spots on modern gold coins: an analytical investigation on nature and origin

A systematic investigation was carried out to understand the nature and the origin of red spots, which sometimes are observed in modern gold coins also of high purity. The defects on coins of gold 99.999% have been examined by stereo microscopy, optical microscopy, electron scanning microscopy (SEM), energy dispersion spectroscopy (EDS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results presented in the first part of this paper indicate that the spots are due to silver contamination of the surface, likely of mechanical origin. In presence of an environment containing sulphur compounds, silver oxidates forming silver sulphide (Ag2S). The second part illustrates the characteristics of the analytical techniques employed in this study .