From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu Sep 23 1999 - 12:01:19 PDT
Message-Id: <l03110726b41025a74976@[192.174.2.173]> Date: Thu, 23 Sep 1999 11:01:19 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole what IS glass
Hi Paraluman
Here is what the Corning Museum of glass says:
http://www.cmog.org/Education/edrungl.htm
One can occasionally hear in art history courses the erroneous idea that
"Because glass is a liquid, it
flows very slowing. Evidence of this can be seen in Medieval stained
glass windows which are thicker
at the bottom than they are at the top." This is not true, and the
notion behind it is unfounded.
Glass does not flow to a measurable extent at room temperature even
over very long periods of time.
Thickness at the bottom, the sides and the top of the segments that
make an entire Medieval stained glass
window happened by chance in the cutting, and as a result of the
manufacture of the disk from which
they were cut.
The term "viscosity" is usually applied to liquids, and means, in a
qualitative sense, the resistance that a
liquid offers to flow. A liquid with a high viscosity such as
molasses flows slowly, compared with
water with a lower viscosity which flows much faster.
Viscosities are expressed in a unit called the poise. The viscosity
of water at room temperature is .010
poise; that of SAE30 motor oil is about 1.0 poise. The viscosity of
most glass at room temperature is
theoretically about
1019 - 1022 poises.
As the temperature of glass increases, the inflexible molecular
network breaks down into smaller units.
Viscosity drops to about 107 poises and may start to deform under
its own weight. At higher
temperatures, viscosity drops to 103-4 poises when the glass will
flow into a mold, or can be blown
with ease.
When heated to very high temperatures, the viscosities of glasses
may be so low that they become as
liquid as thin motor oil.
Therefore, glasses, instead of having sharp melting points, soften
gradually as the temperature is raised
(as the weaker chemical bonds, with their individual melting points,
break), until at high temperatures
they finally become quite liquid. It is this gradual softening over
a range of several hundred degrees
centigrade which makes it convenient to describe glasses in terms of
viscosities.
Paul Doherty
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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