Since ancient times, gold has been synonymous with wealth. One of the reasons gold is so valued as a precious metal is that it does not tarnish, corrode, or otherwise deteriorate with time, being nonreactive towards air, moisture and most common oxidative reagents. To get a quick idea of this, think of any gold and silver jewelry you have: the silver has a distressing tendency to tarnish relatively quickly, needing regular polishing to stay bright, but the gold will only ever need a light wipe to stay shiny.

Leaving aside my inner magpie, gold’s long track record of association with wealth has given it a dominant role throughout human civilization as the foundation of currency. Coins struck in past times give us a record of history that has global reach, raising their value beyond the mere weight and purity of the metal. And even though currency has now moved on from being backed by gold standards, bullion coins are still minted as a form of investment, examples being the South African Krugerrand, the U.S. Eagle, and the Chinese Panda. Such coins, both historical and recent, are often placed on display in museums and exhibitions, and in some cases people are starting to find that the coins are tarnishing. This is as undesirable as it is odd, given gold’s aforementioned resistance to oxidation, and is the focus of a new study reported by Chenghao Liang and a team from Dalian and Kunming in China. They took a set of Panda coins minted in 2001, which have begun to tarnish, and conducted a surface analysis using various different techniques including optical microscopy, inductively coupled mass spectrometry, and elemental analysis. Close inspection of the surface ruled out microbial activity, and ascertained that the tarnish was most developed along stress zones created during the striking process. Chemical composition tests showed that the impurities in the metal itself were present in only trace amounts – no help there. However, analysis of the elements present on the surface showed that where the coin was tarnished, there were high concentrations of silver and sulfur. So, somehow, silver was getting deposited on the coin surface, then reacting with sulfur compounds in the environment to form silver sulfide, and then degrading to silver sulfate, tarnishing the coin.

Where does the silver come from? It comes from the minting machines themselves, where the tools are used to strike both gold and silver coins, so tiny particles of silver can end up stamped onto the coin surface. The polishing process then creates a chemical environment conducive to reaction of the silver, making it possible for parts of the coin to slowly discolor over time, even if they’re kept in a glass case in a museum. Fortunately, it looks like this is a problem that is relatively straightforward to solve. By keeping separate tools for minting coins in separate metals, and employing chemical cleaning procedures once polishing is complete, it should be possible to keep those bullion coins in mint condition.

C. Liang et al., Surf. Interface Anal. ; DOI: 10.1002/sia.3597