First thing to do is smell 'em. If they have any 'vinegar' aroma, this will NOT work. (If you aren't sure, close them into a small plastic box for several days. When you open the box, you'll know.)

Then look at 'em. If the sides have started to 'implode' (cave in) this will NOT work. (I tried it once, just for the heck of it. All I got was sticky dice!)

If crystallizing is the only problem, use a coat of clear nail laquer on them. It's best to do just one side at a time, then let it dry from 12-24 hours before coating another side. Just kind of dab the logo areas, as opposed to long brush strokes. This will usually slow or stop further crystallization.

p.s. if you do notice a vinegar smell, or they have started to implode, get them away from other dice...apparently deteriorating dice can be contageous.