OK! This idea that glass flows at room temperature is an "urban myth"
which persists because technically untrained people keep spreading it.
Lets kill this thing once and for all.
Glass is a super cooled fluid. Basically that means it is NOT composed
of discrete crystals. When molten glass cools, it stays in the fluid
(amorphous) configuration rather than crystallizing. That does not mean
that it is still moving at room temperature.
Steel, cast iron, copper and other metals crystallize as they cool.
Even metals can be formed in the super cooled fluid configuration if you
can raise the heat transfer rate sufficiently. There are companies that
produce ribbons of amorphous metal by spraying a thin stream of liquid
onto a large rotating metal drum. The metal cools so quickly that it
does not have a chance to crystallize. These ribbons have special
electrical and magnetic characteristics that are useful in specific
applications.
One of the reasons that this myth persists is the "cathedral window"
idea. It is purported that panes of glass are thicker at the bottom
than at the top in old cathedrals in Europe. That hardly implies that
the glass has sagged over several hundred years.
Panes for glass windows were made by dipping a hot metal rod into a bath
of molten glass. The bulb of glass was spun into a disk by rolling the
metal rod along a flat surface. The disk was then cut from the end of
the rod using shears and the disk was set aside to cool. (One of the
first rules of glass making is that hot glass and cold glass look the
same.) The disk was then split into individual panes to be made into
windows. (The central piece, where the metal rod had been attached, was
often discarded or remelted. The poor would collect these "roundels" to
put glass windows into their home; you couldn't see through them, but
they brought light into the home.)
If you think about this method of manufacture, it is not surprising that
one end was thicker than the other. And if I was medieval workman
putting these expensive glass panes into the Duke's castle or the
church's cathedral, I'd be sure to put the thicker end down for
stability.
During the early part of this century, glass for windows was made using
the "plate glass" process. (Hence the reason that PPG Industries used
to be called Pittsburgh Plate Glass.) Glass was poured (actually
pulled) from the furnace and rolled flat as it cooled. Even this
process resulted in thickness variations that were substantial enough to
affect the optical properties of the glass. It only takes variations on
the order of the wavelength of light to be visible by eye. You folks in
San Francisco can go look at the large glass panes in some of those old
homes up on the hill.
Most modern glass (including automotive glass) is made using the "float
glass" process. The glass is poured in a ribbon from the furnace onto a
molten bath of tin. The glass hardens as it floats across the molten
tin. This process produces glass of exceptional quality and uniform
thickness. You can thank the Limeys (Pilkington Brothers Ltd.) for this
invention.
And finally, if glass did continue to flow at room temperature, then
refractive and reflective telescope lenses that were made 100 to 150
years ago would no longer work properly. Changes on the order of a
quarter wavelength of light are detectable and no such change has ever
been observed.
If you want to learn more about the "sagging" glass myth, visit the
following site.
http://glassnotes.com/Window%20Panes.html
If you want to know more about glass making and the history of glass,
visit the following site.
http://www.glassonline.com/
Now you want to know (assuming you read this far) how I know about
glass? My old room mate from college works for PPG and it is amazing
what you can learn by asking questions.
Rick Chianese
SRL311-14035
Orlando, FL - 82F with scattered clouds but expecting afternoon
thunderstorms
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