RE: GLASS Flows?

["Hladky, John" <John_Hladky at tdytsi.com>]

I did a little research yesterday, AFTER (unfortunately) I sent the e-mail
on glass flow.  Although it is a well known scientific fact that glass does
flow if given enough time, it is apparently a WRONG fact.  Does anyone out
there have a good recipe for crow?  Does it go well with killies?
John



-----Original Message-----
From: Wright Huntley [mailto:jwwiii at pacbell_net]
Sent: Thursday, August 14, 2003 9:45 PM
To: killietalk at aka_org
Subject: Re: GLASS Flows?


There is probably some limited cold flow, under enough stress, of almost 
any amorphous material (i.e., glassy solid) but the mythology of windows 
slumping with age is just that -- mythology.

Most big store windows, and other large sheets, were made with float 
glass, on float tables that were deliberately tilted. This gave a thicker 
end that was thought to be stronger as the load-bearing base. [Ill-trained 
grad students probably applied impecable post-hoc-ergo-propter-hoc logic 
to look at old windows, as old builings were torn down, and start the 
myth. It continues, obviously, to this day.]

No big telescope has ever been refigured due to cold flow, AFAIK.

About 40 years ago, extensive studies on the stability of glass for 
optical scales and gratings was done, and the final conclusion was that 
glass is inherently more stable than our ability to know the velocity of 
light was at that time (about a part in ten to the eighth).

Acrylic (i.e., "Plexiglas") does not usually warp unless uneven moisture 
is involved. Look in your rubber book, Charles and see just how much it 
expands when exposed to moisture. It stays very flat over a terrarium if 
the humidity is the same inside and out. [Few other plastics have that 
problem, BTW.]

Wright

Charles Harrison wrote:
>> I believe the reason the covers are bending is that despite it's solid
>> appearance, plastics have some properties of liquids, i.e. they flow.  
>> Even
>> glass exhibits this behavior.  It can be observed in the windows of 
>> 200 year
>> old houses.  They are slightly thicker at the bottom than at the top.  
>> Also,
>> the big optical telescopes lose their focus after a few decades unless
>> something is done to compensate for the glass flow.  I suspect that if we
>> leave our aquariums set up long enough, in a few 1000 years the tops 
>> will be
>> so thin that they will break!
>> John
> 
> 
> It is strange from a physical chemistry point of view, considering the 
> *non-solid* nature of glass, but the windows in many of the oldest 
> churches in Florida and Cuba - put into place by the Spanish 
> missionaries back in the 1500's have been measured to determine the 
> amount of flow which can occur over a period of four centuries.
> 
> I will have to go back into the Chemical & Eng News to find the issue, 
> it has been a dozen years ago -  but, the results of the survey was that 
> there was no measurable flow. Actually, this was a complete surprise to 
> me and many scientists who have some training in glass blowing and 
> constructing glass devices.
> 
> I wonder if anyone has laid lengths of these various plastics over empty 
> aquariums or shelves or whatever to determine: does the stuff bend 
> without water involved???
> 
> Charles H
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-- 
Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514


Physics is the study of frictionless elephants whose masses may be ignored.




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