It is generally considered that only low-carat golds tarnish during normal wear, with 18-carat golds tarnishing on rare occasions. The perceived wisdom is that high-carat golds do not suffer tarnishing. In India, frequent comments are made that 22-carat golds go redder in colour on exposure in shop windows and, more recently, complaints have been heard of 22-carat gold jewellery that has gone black whilst in normal use by the owner. Is this conventional tarnishing of high-carat gold or is it due to tarnishing of lower-carat material such as solders used in their manufacture? Or is there some other cause such as severe oxidation? Following a general description of the tarnishing of gold jewellery and possible causes, this presentation describes an investigation into two samples of 22-carat gold jewellery obtained from India which show such blackening in recessed areas after wear. The analytical evidence enables us to determine the nature of the blackened surface and to conclude that even 22-carat golds can tarnish under the relatively severe corrosive conditions that appear to exist in some households.

Author: Dr. Christopher W. Corti