Funny thing I was just wondering about that. I have some dice that I "saved" in a poly bag in the back of a filing cabinet. I didn't look at them for a good five years. Opened recently and they are stinking of vinegar, imploding and the spots are falling off. Not to leave out, crystallizing. Interesting is, some that were in a felt box, in a closet, same dice, same age, are not turning bad. I'll assume I made a big mistake and they shouldn't be sealed in anything enclosed that can't let them breath. Wonderful article that explains pretty much everything, thanks for the link!
Was a collection of 30 Stockmen's dice from Elko.
Upper left was two reacting dice, that were touching.