The physics community seems to be divided
about what exactly glass
is, but for the mean time
it has been labeled an
amorphous solid.
Essentially it is any
material that has been
super cooled such that it
has the properties of a
glass, primarily that it appears
to have many of the
properties of a
solid, but
microscopically it
looks exactly like a
liquid (as
demonstrated in the
image to the right).
When I say it looks like a
liquid I mean that the
molecules are disorderly.
Traditional solids form
crystalline structures as they
harden (think glass or
stalactites), but glass does
not. Glass is formed when you
cool a liquid quickly and the
molecules loose energy to fast
to form proper crystalline
latices. As Robert Kunzig
poetically puts it in his
discover magazine article The
Physics of...Glass:
"As the
temperature drops, the
liquid becomes more
viscous and the molecules
more sluggish. Its like a
game of musical
chairs in which the
music never stops
and the players
never sit down;
instead they seem to
move through honey,
then tar, until they
are but motionless,
like bugs in amber."
Generally the slower you cool
something (that still results
in glass) the more dense (and
the more break resistant) the
resulting glass. Although
since the less viscous a
liquid is at its cooling point
the less like it is to become
glass depending on the
material this slower cooling
rate may just mean it becomes
a proper solid instead.
The really
intriguing thing about glass,
and another part of the reason
it can't quite be considered a
new state of matter, is that
once formed glass is never in
equilibrium, while all other
states of matter are.
Note: glass was initially
formed into things like
windows by being stretched
into a long string and then
beaten until it was mostly
flat by glassblowers. This
resulted in wavy windowpanes
with thicker edges (this is
where the myth that glass is a
liquid comes from, since the
thicker edges give the
appearance that the glass has
dripped down the windowpane
slightly). Nowadays though
typical flat ordinary glass,
and some of the glass that
will become tempered and or
laminated glass is formed by a
process called floating, where
it is poured over a vat of
molten tin. This produces much
more uniform and flat sheets
of glass.
