With a few exceptions most of your pre 60s casino dice will be cellulose nitrate. From the mid 60s on it is all cellulose acetate. Cellulose acetate will also start to decompose with time and exsposure to heat and ultraviolet light. You might take a look at a page called the care and feeding of dice:
http://dice.onaraonline.org/care.php
The page contains some good info on early and modern plastics.