"Gold corrosion": red stains on a gold Austrian Ducat

Stains of different colours have been observed on historic and modem gold coins in several countries. An Austrian Ducat at the Kunsthistorisches Museum in Vienna has developed some red spots on its surface over the years. The same defects have also been observed in modem coins of higher gold purity. The spots have been examined by OM, SEM, EDS, XPS and AES. Optical microscopy showed that "red" defects exhibit in fact a nuance of colours. The surface analysis put in evidence the presence in the stains, in addition to gold, of silver and sulphur. The values of the modified Auger parameter alpha' of silver correspond to those of A(g2)S; thus, it can be assumed that the stains are composed of silver sulphide (A(g2)S). It was not possible to determine whether the presence of silver on the surface is due to segregation towards the surface or to external particles of silver embedded in the matrix. Depth profiling performed on modem coins suffering from the same problem allowed us to demonstrate that the nuance of colours is due to the inhomogeneous thickness of the spots. Moreover, it was demonstrated that spots are formed by two layers: an outer layer of silver sulphide and an inner layer of silver.