|The Physics of .
. . Glass
Yet another mystery in our everyday life that science can't really explain very well.
By Robert Kunzig
Discover Magazine October 1999
THE HOUSE IN FRANCE I LIVE in was built in the 16th
century, and though I doubt any of the windowpanes are original, some of them are quite
old. People go all wavy as they pass my office. This tickles me: Those vertical
distortions in the panes remind me that glass -brittle, breakable glass -is really a
fluid. The windows of medieval cathedrals are thicker at the bottom, I've heard, because
the glass has pooled there; and even the little streams in my own panes seem to evoke the
transience of existence: Time is a river; glass is a river - you get the idea.
"The idea that glass is a fluid is a very widespread
myth," says Yvonne Stokes, a mathematician and spoilsport at the University of
Adelaide in Australia. "I was told it as a fact by my adviser. And once, a class of
schoolchildren came into the lab, and one of them told me the very san-re thing. If you
want to talk microscopically, then you can call glass a fluid. But people understandably
tend to think that if it's a fluid, it flows. It's that notion that's false."Stokes
has recently proved with detailed calculations that old windows could not have flowed
In fact, structurally there is no sharp line between a liquid and a glass. You form glass
by 'super cooling" a liquid below its freezing point, then cooling it some more. If
you cool it fast enough, the molecules can't organize themselves into crystals. As the
temperature drops, the liquid becomes more viscous and the molecules more sluggish. It's
like a game of molecular 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 all but
motionless, like bugs in amber.
Window glass is made
mostly of sand - with soda ash mixed in to lower melting temperature and limestone to
decrease solubility in its final form but just about any liquid can be a glass if it is
cooled quickly enough. Some liquids are easier to turn into glass than others, however.
The more viscous a liquid is at its freezing point, the more trouble the molecules will
have moving into crystalline formation, and the more likely they are to end up a glass
Take polystyrene, the stuff of coffee cups; it's a polymer
glass with a lot of bubbles in it. Each molecule, says chemist Mark Ediger of the
University of Wisconsin, is a long, awkward chain, and so liquid polystyrene is a tangled,
viscous soup. "No one has ever crystallized it, says Ediger. "Glass is the only
solid state common polystyrene can have." At the other extreme is water: Its small
symmetric molecules are so eager to form tetrahedrons of ice that the only way you can
stop them is by freezing them one by one. In between lies sugar: We put crystals of it in
coffee, but sugar is also easy to supercool, and lots of candies are glassy. Glasses are
everywhere these days, from telephone cables to Life Savers and yet researchers are still
arguing about their very nature. Is a glass simply a liquid in solid clothing? Or, as
chemical engineer Pablo Debenedetti of Princeton University puts it, is the glass around
us really a "masked version of something more profound," something called an
ideal glass? "It's called ideal because no one has seen it, says Debenedetti. But as
a theoretical concept it might explain some of the properties of real glass.
Debenedetti says real glass is puzzling because both the
temperature at which it forms and its final properties depend on the rate at which the
liquid is cooled: "In order to tell you that water boils at 212 degrees Fahrenheit or
that it freezes at 32 degrees, I don't need to specify anything about the velocity of
heating or cooling. But the glass transition is different." The slower you cool the
melt, the lower the temperature at which it will change into glass, and the more dense
that glass will be. So window glass forms at anywhere between 1,472 and 1022 degrees
An ideal glass, some theorists believe, is what you would
produce if you could cool a liquid with geologic slowness while somehow preventing it from
crystallizing. It would form at one precise temperature, just as solids, liquids, and
gases do. Like them, it would be a distinct phase of matter, and not merely - like
ordinary glass - a solidliquid hybrid. An ideal glass would be as motionless and nearly as
orderly as a crystal, but it would not be a crystal. No one knows what it would look like.
"You're asking me to engage in poetry," says
Frank Stillinger of Bell Labs. "It's like saying, 'What color is the hair of angels
dancing on the head of a pin? I can't answer that question either."
Whatever its structure, an ideal glass would be in
equilibrium, just like the other phases of matter-but unlike real glasses. "A glass
is constantly evolving, trying to get to equilibrium, says Ediger. "A crystal is
happy just to sit there; because it's already at the bottom of an energy valley. A glass
is sitting on the side of a hill. And very, very slowly it does roll down."
That's the fat kernel of truth inside the myth: Glass
really does flow- just not on a human timescale. If you could somehow preserve a cathedral
window for long enough, it might become a rigid puddle on the floor. Or it might become an
opaque crystalline solid. Conceivably, that church window might even evolve into an ideal
glass. "We can't do the experiment for long enough," says Ediger. "But it's
a very interesting question."
Yvonne Stokes has calculated how long would be long enough.
Assuming a windowpane simply flows downhill, she asked, how long would it take for the
bottom to get just 5 percent thicker? At least 10 million years, she figures -and that is
likely to be a "big underestimate." Cathedral windows may be thicker at the
bottom, she says, Simply because handmade glass varies in thickness, and medieval builders
chose to put the thick ends down. And wavy old windowpanes are not glass that has flowed;
they're glass that was flawed from the beginning.
Nowadays most windows are made of flawless "float
glass": Molten glass is poured onto a bath of molten tin and allowed to spread out
and solidify into a perfectly flat sheet. But at Blenko Glass in Milton, West Virginia,
they still make windowpanes the old-fashioned way. The artisan gets a ball of molten glass
on the end of his pipe, which he blows into a long, cylindrical wooden mold so the
malleable glass forms a hollow tube inside. After the glass cools, he removes it from the
mold, scores it lengthwise, rehears the glass, and irons it into a single 18 by 25-inch
pane. Each one contains air bubbles and large wavy bands, and sells for $27, because some
people like the aesthetics of old glass people in charge of historical landmarks, for
instance. A few years ago, recalls Blenko's Lewis Powers, the company got a large order
for replacement panes from the White House: "They wanted the wavy look."