Am I seeing things, or is this rust on a sestertius? If it is, is it something to be avoided, or is it a simple fix? And since I'm in a questions mood, how does a bronze coin rust?
You can get rust on bronze coins as the alloys can contain trace amounts of iron, sometimes enough for the coin to be slightly magnetic. I have a page on the metallurgy of Constantinian bronzes, if anyone is interested. http://www.constantinethegreatcoins.com/metallurgy/