A while ago I bought a trove of Lincoln wheaties in various albums and folders. A couple of the albums were the no-no Harco ones, the notorious chemically toxic ones with vinyl and plastic holders that corrode and blacken copper coins.
And sure enough, most of the Lincolns were toned to a dull gray-black or worse. But a few, a very few of the uncirculated or near uncirculated ones were toned to a very nice deep cherry red. The vinyl actually improved their condition.
I'm wonder why. What random set of circumstances causes cherry red toning?