Subject: Gypsum correspondence

From: strauss@astro.Princeton.EDU

Submitted: Tue, 21 Apr 1998 14:34:37 -0400 (EDT)

Message number: 273 (previous: 272, next: 274 up: Index)

Hello all,
  I've received the following correspondence on the gypsum question
(see apo35-general #271).  Remember that hitting "reply" to a message
sent to the APO mailers does not go to the sender of the mail, nor to
the mail exploder, but rather goes straight to me. 

			-Michael Strauss

Date: Thu, 16 Apr 1998 14:10:05 -0700
From: Bruce Balick <>
Subject: Re: Gypsum

Some thoughts:

* The gysum gets to the site by flying on the winds.  It won't take much of
a fan to blow keep it suspended in the air for a long time.  So a few tiny
$8 fans mounted above the mirror should easily do the trick.  They should
also help keep pollen away.

* If the gypsum is pure, then your arguments are on point.  However, if the
gypsum carries salts up with it, then we've got a problem.  Being
hydroscopic, salts will find their own water in the humid night air and
dissolve onto the glass.  Na+ will react with Al-Ox to brew up troubles.
Actually, any charged particles are scary.

* Do we know anything about the salinity of the gypsum salts and dust from
the beds of valley sand?  You might just try dissolving some carefully
collected dust into pure water, and see if you can get a current to go
through it.

* Pollens are generally coated in oils which helps them to adhere to the
backs of bees, etc. and to protect the pollen from rain.  Even after pollen
is shaken off, the oils can remain behind and accumulate.  But I suspect
that the oil accumulation is really unnoticable.  Oils from lady bugs and
moths are probably a more significant issue.

* One approach to studying mirror contaminants is to see what makes the
windows in the dorm become dirty.  Bring up a small microscope some time,
and/or get the dirty glass chemically analyzed.  Of course, this approach
borders on unrealistic since windows get wet and (hopefully) the mirror


Date: Thu, 16 Apr 1998 15:41:48 -0600
From: Edward Kibblewhite <>
Subject: Re: Gypsum

We had overcoated silver mirrors in ChAOS. The ones usually pointing down
are fine but the ones facing up are badly pitted and seem to be seeded by
dust. The coating is eaten away round each grain. I am having them sent
back to Denton Vacuuem who have promised to look at them before recoating.
I don't know what that proves except there MAY be some evidence that that
something is harmful, I guess its the gypsum.

From: ()
Date: Fri, 17 Apr 1998 17:43:35 -0600 (MDT)

I suspect it isn't the gypsum, specifically, but dust generically. A dust
grain should provide a condensation center when it is humid and a holding
center as a wet mirror dries out. As I mentioned, gypsum is pretty insoluble
and the Ca isn't going to grab the oxygen from the aluminum nor is the
aluminum going to take the sulphate from the calcium. If we are looking for
something reactive, I'd look at organics (e.g., pollen) first. Of course,
silver is effectively much more reactive than oxide-overcoated aluminum
(a substance you probably know as aluminIum!).

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