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Not to digress but in the watch world when a watch is marked say 200 meters it is certified for that. They test it by putting the case sans mvmt in a vessel and pressurizing it and then check for...

Newer celluloid (and I mean really recent stuff) has a stabilizer in it which retards the breakdown. Of course under the right conditions.... It's like a water resistant watch. It ain't waterproof.

There are several factors that can cause cell rot. Maybe heat maybe humidity maybe light maybe the particular formulation or maybe evil spirits. Take your pick.