Here's an interesting link:
http://www.bnphoto.org/bnphoto/LostS...adioactive.htm
Quote:
Radioactive Elements in Glass
Thorium is derived commercially from certain monazite sands (e.g., from India). Thorium is
radioactive itself, emitting alpha particles. The resulting "daughter" products of that radioactive
decay series also produce both alpha and beta particles. Related rare earth such as lanthanum are
often produced from the same sources, with monazite being up to 25% lanthanum.
This decay process means these thoriated glass lenses can gradually become more radioactive over
time, as the more highly radioactive decay products build up in the glass. This result is counterintuitive.
You would expect the radioactivity to decrease over time. But after chemically purifying
the thorium from its ore sources, the thorium is relatively free of these daughter products. Over
time, the thorium decays, and the levels of radioactive daughter by-products builds up. Eventually
a more highly radioactive equilibrium will be reached, as in the original radioactive ores. So over
the years, your "hot" lenses are likely to get more radioactive rather than less. Surprise!
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Therefore - the older the lens is the worse it gets.