30Cal,
That is interesting, I looked up the decay chain to see if I could spot the daughter isotope that would be responsible for the browning.
Thorium-232 (14 billion years) alpha
Radium-228 (5.6 years) (most likely culprit) beta
Actinium-228* (6.1 hours) beta
Thorium-228 (1.9 years) beta
Radium-224 (3.7 days) alpha
Radon-220 (56 seconds) alpha
Polonium-216 (.15 seconds) alpha
Lead-212* (11 hours) beta
Bismuth-212* (61 minutes) alpha
Thallium-208* (3.1 minutes) beta
Lead-208 (stable)
Looking into the Radium, we find that discoloration of glass is common.
Quote:
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When first prepared, nearly all radium compounds are white, but they discolor on standing because of intense radiation. Radiation causes a purple or brown coloration in glass on long contact with radium compounds. Eventually the glass crystallizes and becomes crazed.
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What is happening is the Beta and Gamma radiation are knocking atoms around in the compound, and they are ending up in a more ordered state (crystallized) which is creating some sort of change in the index of refraction (as best I can tell).
Jimro