Re: ATM Mercury flats

[Kurt Hillig <khillig@Chem.LSA.UMich.Edu>]

On Wed, 30 Apr 1997, Robert  C. Pfaff wrote:

> I would like to hear from anybody that has had experience with mercury
> flats. From my reading it is recommended that a very high viscosity oil
> be used to cover the mercury.  This is to damp vibrations and prevent
> oxide.  Anybody know what kind of oil to use and where to find it? 

I haven't made one - at least, not yet, though I expect I will one of
these days - but here's what I know about 'em: 

*Don't* use an aluminum pan.  It's not exactly easy to get the mercury to
react with the aluminum, due to the strong surface oxide layer on the Al;
but if it does, you'll end up eating holes through the pan and the mercury
will leak out (leaving behind a pile of gray Hg-Al amalgam powder). 

Hg has a very high surface tension, curvature at the edges is significant. 
I'm told if you use a copper (or brass?) pan, and polish the edge where it
contacts the mercury (if you can do this without getting any oil on the
surface), then you can get the Hg to wet the metal surface and edge
effects will be greatly reduced.  

As for oil, pretty much any oil will do, though higher viscosity is better
at damping vibrations than low viscosity.  I expect clear, colorless
mineral oil from the drugstore would work quite well.  

Depending on your environment, you may still need to do some vibration
isolation of the pan from the floor; or work at 3 AM when there's no
traffic on the street outside.

> I am planning to use this mercury flat to figure the secondary
> for my 13" SCT by autocollimation.

Sounds like an interesting project; let us know how it turns out.

> PS
> Please no posts telling me that after I open the mercury
> bottle that my life span will be 24 hours <G>

The equilibrium vapor pressure of Hg at room temperature exceeds OSHA
limits; but in a well ventilated area, it's hard to build up that high a
concentration.  The danger comes from the stuff that gets spilled on the
floor, and that you can't recover again - it will continue evaportaing for
years. The oil-covered pan shouldn't pose much of a hazard; but keep it
covered when not in use, preferably in a ventilated cabinet.

You might want to pick up a mercury spill clean-up kit - they've got some
stuff that will react with the liquid, to form a solid salt that can be
vacuumed up. 

I don't know what your local laws are regarding the disposal of mercury,
but it's on the federal TSCA list and it *is* regulated.  I don't know
what the federal regs are, you'd have to talk to someone in the business;
but don't flush it down the sink/toilet; don't put it in trash that gets
incinerated! 

                            Dr. Kurt Hillig
   Dept. of Chemistry      I always tell the    fax (313)647-4865
 University of Michigan     absolute truth     phone (313)647-2867
Ann Arbor, MI  48109-1055    as I see it.    X.500 khillig@umich.edu
   Computers were invented to help people waste more time faster.