CHARACTERIZATION OF TARNISH SPOTS IN CHINESE HIGH-PURITY GOLD JEWELRY

摘要

In recent years, red, brown, and black tarnish spots on high-purity gold jewelry (99.9% Au) have been widely reported, mainly in China but also in other parts of the world. Yet the cause of these spots and their possible formation mechanism are unclear. In this study, high-purity gold with typical tarnish spots was systematically investigated using differential contrast microscopy, laser scanning confocal microscopy, SEM, X-ray fluorescence, and X-ray photoelectron spectroscopy. The study detected silver and sulfur along with gold within the tarnish spots. The tested samples all had a gold purity of 99.9%, regardless of whether there was tarnish. Higher-magnification 3-D microscopic observations revealed the typical presence of an "impurity center" within the tarnish spot and a thin film of stain that varied from several nanometers to several hundred nanometers thick. XPS analysis confirmed the presence of both Ag and S in the thin film. An in situ Ar ion sputtering experiment for removing the spot revealed that the sulfur could only be detected at the surface, where the film was less than 12 nm thick. To prevent these tarnish spots, we recommend careful cleaning during the gold manufacturing process and proper care and maintenance to eliminate the possibility of silver contamination.