Phenolics: catalin and bakelite
As bakelite and catalin age, UV light causes a layer of phenyl alcohol to form on its surface. Phenyl alcohol is yellow-brown, so it imparts that tint to the original color. Thus cobalt becomes "blue moon" to olive green (see photo below), bright red becomes mahogany, white becomes butterscotch. Phenyl alcohol is an excellent sunscreen, so the discoloration only penetrates a millimeter or so. These plastics are very resistant to chemicals, and will never melt or ignite. Catalin, however, will shrink over time. This can result in warping and cracking of the piece.
Celluloid
As I mention in Care of Plastics, celluloid is prone to significant changes with age. It can yellow with time, turn pinkish-brown, crumble, and implode.
Casein
Casein can lose moisture over time, and sometimes you can see this as a subtle "crinkle" on the surface. My casein Mah Jong set doen't show this, however, and so far appears to be pretty bulletproof.
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