The Physics of... Glass

Yet another mystery in our everyday life that science can't really explain very well

By Robert Kunzig|Friday, October 01, 1999
The house in France Ilive 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 same 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 perceptibly.

If the myth survives, it will be because it contains a kernel of truth--and because glass is a confusing kind of matter, quite unlike the three ordinary kinds. A gas is an anarchy of molecules going every which way; a liquid is a tighter but still disorderly society in which molecules constantly dissolve and reestablish weak bonds; a solid is a molecular army in rigid formation. But glass is . . . none of the above. It is rigid like a solid, but its molecules are not arranged in repeating crystals. It is amorphous like a liquid.

In fact, structurally there is no sharp line between a liquid and a glass. You form glass by "supercooling" 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 instead.

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 1,022 degrees Fahrenheit.

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 solid-liquid 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, ŒWhat 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, reheats 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."
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