Surface defects on collection coins of precious metals

The presence of various defects of different colours and sizes on precious collection coins is the cause not only of the rejection of defective coins but also of an increase in the production costs. In this work, the surface defects on the Austrian collection coins have been investigated by using the techniques of XPS, AES and SEM/EDS. Multi-technique surface analysis revealed the presence of silver and sulfur in the stains on the golden coins. Auger spectra and chemical images proved that both Ag and S are located in the coloured surface areas. From the XPS results, it was concluded that the coloured stains consist of Ag2S. XPS depth profiling revealed the difference in the thickness of the dark-grey and red stains. In both cases, the topmost layer of silver sulfide has a thickness of a few nm, whereas the inner layer of silver is about 300 nm thick in the dark stains and only about 2-3 nm in the red ones. The origin of these stains has been attributed to foreign particles of silver embedded onto the coins. The chemical composition of the visible stains on silver coins is quite different. AES results revealed that some defects contain chlorine, potassium and oxygen; the others include iron and oxygen. In all these cases, the surface contaminants were attributed to occasional problems that occurred during the manufacture of the coin. Copyright © 2004 John Wiley & Sons, Ltd