Why do some cellos turn brown?

[allanb3369]

Originally Posted by T.G

I once turned a pile of used but still clear cello to a dark amber resembling cello found on cigars 10-15 years old in a matter of just a few weeks just by sealing them in a tupperware with a ramekin of ammonia and setting the tupperware where it would be exposed to some heat.

Only you, Adam! I learned something new today! Had no clue that could be done.