RE: [ATM] Does glass retain moisture?

["Richard Schwartz" <richas@earthlink.net>]

> why do some people with mirrors of very high quality claim that the star
test is different at warm temperatures than at cold temperatures?? (Of
course, I'm talking about AFTER the mirror has equilibrated to the ambient
temp. We are fully aware that the star test is horrible AS the mirror
equilibrates.) 

There are two causes:
(1) The observer has read so many books that he is brainwashed into a
certain state of belief.
(2) For larger solid mirror blanks, the glass may be poorly mixed before it
is poured; this results in a non-uniform composition and non-uniform
thermal expansion coefficient.

> So, I note that warm temperatures are usually encountered in summer, when
there is more moisture in the air (higher dew point). Normally, the air is
relatively very dry when cold temperatures are encountered (most of us only
bring our scopes out when it's clear). So the question: Does glass retain
moisture from the atmosphere into the gaps in its molecular structure? 

That is a damn good question.   I do know that glass is slightly soluble in
water.   In high school we did an experiment where we  put a wad of
fiberglass into a jar of water.  The water turned ping with
phenylphthalein, indicating that it was alkaline.   But this is a case of
extremely large wetted surface area.   

> For example, I recall a thread where it was pointed out that "waterproof"
dental stone does not "absorb" water, but water molecules do get into the
material. The water molecules do not bind to the material, but they can get
into the gaps between the molecular structure of a material. So, if
moisture can get into a piece of glass, does it push apart the molecules of
the material, thereby causing the piece of glass as a whole to expand
slightly? 

That is another damn good question.   And if the water can get in, can it
also get out, or is it trapped forever?   One way to find out might be to
use extra-heavy water tritium oxide.   If it gets inside the glass, the
radiation would be detected.   Also, this method could be used to assess
the depth of penetration.   (If water was expanding the glass and it
penetrated perfectly through the whole piece, the expansion would be
uniform and the figure would not change.)

> If a piece of glass changes size slightly depending on different amounts
of moisture in the air, shouldn't that affect the figure enough so that
someone with a very finely figured mirror can detect the difference in a
star test? The edge of the mirror is thicker than the center, so it seems
reasonable (to me) to assume that the edge would contract more than the
center in the low humidity of winter, giving the impression of a tiny bit
of over-correction. The tiny bit of overcorrection is what is observed in
the cold, if perfect correction is observed in the warmth. 

Another experiment could use a pair of long glass fibers to determine if
moisture is expanding the glass.   One fiber would be in a dry tube, the
other in an adjacent tube that starts out dry, but is then wetted.   The
slight differences in length would be easy to measure by interferometry.  
Also, a similar experiment could measure the flow of glass under tension at
room temperature that some claim to have observed in ancient glass.

> If perfect correction is observed in the cold (winter, low humidity),
then undercorrection is observed in the summer (warmth, more humidity). As
the mirror retains moisture it expands slightly, and the edges expand most
- deepening the curve. Of course, someone with a quarter-wave mirror is not
likely to see any of this, it requires a very well-made mirror. 
>
> Does any of this make sense to the 
> physicists out there? 

OK, I was a physics major in college, and they never, ever taught us
anything about glass expanding with moisture.   Actually, when you get down
to it, it is apalling how little we know after many years of making optics
out of glass.   Nobody has designed and done real experiments, and except
for a few skeptics, most of us rely on urban myth as a substitute for
science.

. . . Richard


_______________________________________________
ATM mailing list http://www.atmlist.net/