Re: Viscosity of Glass

[John Cooper <jcooper@BUCKNELL.EDU>]

  From the Chemed-list:

 From hharris@umsl.eduMon Jan 27 13:13:57 1997
Date: Mon, 27 Jan 1997 11:09:57 -0600
From: Hal Harris <hharris@umsl.edu>
To: chemed-l@atlantis.cc.uwf.edu
Subject: Re: Glass - does it flow?

At 12:23 AM 1/25/97 -0500, lcarroll wrote:
> ..................  In my Materials class the other day, we
> were discussing polymers, and the discussion turned to glass transition
> temperatures, and, as interesting discussions are wont to do, it further
> turned to the structure of glass - silicate glass (I know it is not a
> polymer in most views, but this is a good class!!).  Anyway, the professor
> (who shall remain nameless) said that glass was above its glass transition
> temperature at room temperature, and therefore could be expected to flow,
> albeit slowly.  He suggested that this was in fact the case, as very old
> glass was thicker at the bottom than at the top.  Most of you are familiar
> with this argument.  I brought up that this has been suggested as an
> artifact of production methods, but I do not happen to remember where I
> read this.  I believe it was in a broad JCE article.  Could someone who has
> that handy little electronic index check this out for me?  Meanwhile, if
> anyone has references, either for the flow, or against it, please give them
> to me, as i would like to be able to possibly continue this discussion on
> the resumption of class, or perhaps to present this to the class (part of
> our grade is based on two presentations - actually, almost all of it).
>
> Thanks for all the help.
>
> The most amazing thing about rock-hounding and    | Lloyd Carroll
> mineral collecting is that you could - even right | lcarroll@unity.ncsu.edu
> now!! - be standing, sitting or walking over the  | NC State University
> most beautiful thing you have ever seen - or,     | Raleigh, NC
> more correctly, that no one has ever seen.        |

 This question was very decisively addressed in a paper in the Journal of
Chemical Education back in 1962 (vol 39, p. 84-85), by David Dingledy.
The reason so many people "know" about this is that it is a common
textbook error, and Dingledy's paper was in the "Textbook Error" series of
the Journal.
 In it, he says that it is theoretically impossible for glass to flow
under its own weight at room temperature.  Experiments done to measure
this "flow" have been done, and have shown that there is none. An easy way
to convince yourself that this must be true is to consider the giant glass
mirror at Mt. Palomar.  If glass flowed under its own weight, that
telescope would have become uselessly distorted by now. Older methods of
producing plate glass for windows made panes that were thicker on one side
than the other.  They were commonly installed uniformly with the thick
side down, so that windows with many panes would have similar distorting
refraction in all the panes.

Hal Harris
Dr. Hal Harris   hharris@umsl.edu     |Nothing is too wonderful
Department of Chemistry               |to be true.
University of Missouri-St. Louis      |-Michael Faraday
St. Louis, Missouri 63121
(314)516-5344   Fax:(314)516-5342

    -- Challenge opens the vista of capability --

 John N. Cooper, Chemistry
 Bucknell University
 Lewisburg PA 17837-2005
 jcooper@bucknell.edu
 http://www.facstaff.bucknell.edu/jcooper
 VOX 570-577-3673    FAX 570-577-1739

On Wed, 15 Dec 1999, SSHS KPHOX wrote:

> We viewed the MU film on Resonance yesterday. An issue was made of the
> viscosity of glass. Two of my students said they had read  that the
> variation in glass thickness in very old buildings was not a sign of glass
> flowing, but rather a manufacturing event.
>
> Can anyone give me information on this?
>
> Thanks,
>
> Ken Fox
> AP/IB Physics Teacher
> Smoky Hill High School