The old cellulose nitrate dice breakdown with time and generate acetic acid (vinegar) in the process. I'm not sure of the chemistry, but vinegar on another old die might cause it to decompose faster. Best to keep those that are crumbling physically apart from those that still look good. The Dice Man says you can slow the decomposition by coating the dice with clear laquer (nail polish). If you don't want to change the collectibles by painting them, you can at least put them in acrylic boxes or in poly bags to keep them separate.