Re: Re Thorium Oxide

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Sat, 11 May 1996 01:20:38 -0500
>From: P3D Bob Wier <wier@xxxxxxxxxxxxxxx>
>Subject: Re: Re Thorium Oxide

>...We checked on the radioactive question on gas mantles (although I thought
>I remembered something about barium as opposed to thorium - but I'm
>probably wrong). Turns out that the radioactive component in the 
>thread was eliminated (the manufacturers say) quite awhile ago. 

If they were right, I'd be very interested in finding out what they use
instead. Thorium oxide was originally selected because it has a melting point
of 3300 C (the highest of all oxides, and just about the highest of all
substances) and it isn't consumed in a flame. As of the 1984 CRC Handbook of
Chemistry and Physics, it appeared that it was still being used in Welsbach
mantles (thorium oxide with ~1% cerium oxide and other ingredients). The
Handbook seems to indicate that pure cerium oxide would have a lower melting
point and would be more brown, so that doesn't appear to be what they use.
I wouldn't think they'd have much incentive to change from thorium oxide, except
perhaps for public relations.

Thorium 232 seems to be by far the most common isotope. It is an alpha
emitter, and has a half-life of 1.41x10^10 years. (If you have to be around
a radioactive isotope for an extended time, a long half-life is generally
better.)

I believe a large number of people have gas lights outside their homes,
which use gas mantles.

The Handbook notes that "Th232 is sufficiently radioactive to expose a
photographic plate in a few hours". Perhaps if an unburned gas mantle were
set on the emulsion side of a strip of unexposed photographic film (in complete
darkness) for a few hours to a few days, it would be possible to see an effect.

John R


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