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How the Panda Got Its (Red) Spots
One of the mysteries of gold Panda coins is, what are those awful red spots that often turn up on them? Some people call them copper spots as copper's nickname is the red metal and it's a natural impurity in gold. Copper is also traditionally mixed with gold to create a harder, more durable alloy. Now a trio of Chinese chemists have done a study of the red spots and found they are actually made up of compounds of silver and sulfur. The presence of silver isn’t surprising as it is often mixed in with gold in nature and may not be completely removed by refining.
Sulfur, though, is the contaminant that turns your old newspapers yellow. It’s also a component of smog, spewed from power plants that run on fuel oil or coal. In one of the researchers’ tests gold coins were exposed to a sulfur-laden atmosphere. The gas reacted with silver impurities on the coin’s surface to produce red spots.* This means that every effort should be taken to reduce the contact between gold Pandas and air, particularly polluted air (which isn't so easy to do these days).
P.S. There is a lively discussion on this research at CoinTalk. Unfortunately, from my perspective, it sidetracked into what fineness gold could be purified to in ancient times.