Re: Swelling pot-metal

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 19 Feb 1997 17:56:26 -0600
>From: P3D bob wier <wier@xxxxxxxxxxxxxxx>
>Subject: Swelling pot-metal

>I don't normally cross post from other lists, but from time to time
>there are interesting discussions which are germane here. One currently
>going on in one of the "antique radio" lists concerns the use of
>"pot metal" and how it degrades over time. Since this might also be
>found on older cameras, it's worthwhile knowing about...

Do you know what "pot metal" is? Some people say that it's just cast zinc,
others aren't so sure. I've had to throw out several tire pumps because the
metal tab on the nozzle snaps off when I try to fasten onto a tire valve.
As the cross-post described, this brittleness seems to occur with age.
I had assumed the parts were made of cast zinc.

I recently discussed with a person on P3D the possible pitfalls in the choice
of metal for a stereo bar. So far I've used galvanized steel, and there
don't seem to have been any adverse interactions. Others seem to have good
luck with aluminum. Don't build a bar out of laminated strips of copper
and zinc, then try to use it for undersea photography! :-)

I think some parts are plated with cadmium - I don't know whether there are
any long-term health concerns in using cadmium for something you'll be holding
in your hands for hours at a time.

For those who hope to save weight in their stereo cameras by using titanium,
be careful. I believe the people who designed the SR-71 discovered that the
metal was so sensitive to chromium contamination that if the parts were
put together using chromium-plated tools (fairly common), the parts would
eventually fail!

John R


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