Older dice from early 50s and back will most likely be cellulose nitrate plastic. They will break down and strat to crystalize. The breakdown byproducts are acidic and will cause other dice they come in contact with to deteriorate. Heat , Water, and UV light will speed up deterioration. Nitrate dice need to be properly cleaned and then sealed with a "clearcoat" to protect them.
from the 50s on casino dice are made from cellulose acetate plastic. Heat, moisture, and sunlight are also the enemies of these dice. Decaying acetate dice will discolor (usualy darken) get britle and warp(usualy imploding) they will also have a vinegar smell. Unfortunetly once acetate dice start to go there is nothing you can do other than keep them from heat, and light. The byproducts of acetate degradation are also bad for any other dice. hope this helps.
I am working on an article to submit to the club magazine on the storage and preservation of dice and will also be adding that info to my website.