Fun_People Archive
12 Oct
Glass Flow - Not Just An Urban Legend Anymore

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From: Peter Langston <psl>
Date: Sun, 12 Oct 97 12:31:35 -0700
To: Fun_People
Subject: Glass Flow - Not Just An Urban Legend Anymore

Excerpted-from: Earth & Sky Scripts for the week beginning 10/13/97
Forwarded-by: Chris Luther <cluther@EARTHSKY.COM>

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Tuesday,  October 14, 1997
DOES GLASS FLOW?
At 1100 degrees Centigrade, glass can be blown and shaped into beautiful
objects.

JB:  This is Earth and Sky, on whether -- under normal conditions -- glass
flows.

DB:  A year ago, we aired a show in which we mentioned that, like a liquid,
glass does flow.  We noted that evidence of this could be seen in the
sagging window panes of old buildings.  Since then we've learned we were
wrong.  Although glass has many of the same properties as a liquid --
especially its random arrangement of molecules -- glass does not flow at
normal temperatures.   The puzzle of sagging window panes in old buildings
has another solution.

JB:  Some scientists call glass a solid, because of its rigidity and
resistance to flow.  Others would say that it's a separate state of matter
-- neither liquid nor solid -- but that it has properties of both. Experts
will agree, however, that one property of liquid that glass does not have
is the capacity to flow at normal temperatures.

DB: The "myth" that glass flows at normal temperatures has been perpetuated
in chemistry text books. The evidence usually offered is that antique window
panes sometimes do appear thicker at the bottom.  But some old windows are
thicker on the sides -- and some on top.  These variations were present when
these old window panes were manufactured.

JB:  One evidence that glass does not flow can be found in objects unearthed
in Egyptian tombs.  They're much older than any window panes -- yet they've
held their shapes well. That's our show for today.  With thanks to the
National Science Foundation, we're Block and Byrd for Earth and Sky.

http://www.earthsky.com/1997/es971014.html

© 1997 Peter Langston