Fact or Fiction?: Glass Is a (Supercooled) Liquid
Are medieval windows melting?
Feb 22, 2007 |By Ciara Curtin
http://www.scientificamerican.com/article/fact-fiction-glass-liquid/
In medieval European cathedrals, the glass sometimes looks odd. Some
panes are thicker at the bottom than they are at the top. The seemingly solid
glass appears to have melted. This is evidence, say tour guides, Internet
rumors and even high school chemistry teachers, that glass is actually a liquid. And, because glass is hard, it must be
a supercooled liquid.
Glass, however, is actually neither a liquid—supercooled or otherwise—nor a solid. It is an amorphous solid—a state
somewhere between those two states of matter. And yet glass's liquid-like properties are not enough to explain the
thicker-bottomed windows, because glass atoms move too slowly for changes to be visible.
Solids are highly organized structures. They include crystals, like sugar and salt, with their millions of atoms lined
up in a row, explains Mark Ediger, a chemistry professor at the University of Wisconsin, Madison. "Liquids and
glasses don't have that order," he notes. Glasses, though more organized than liquids, do not attain the rigid order of
crystals. "Amorphous means it doesn't have that long-range order," Ediger says. With a "solid—if you grab it, it holds
its shape," he adds.
When glass is made, the material (often containing silica, SO2) is quickly cooled from its liquid state but does not
solidify when its temperature drops below its melting point. At this stage, the material is a supercooled liquid, an
intermediate state between liquid and glass. To become an amorphous solid, the material is cooled further, below
the glass-transition temperature (the temperature when a glass changes from a hard, glassy material to a softer,
liquid-like material). Past this point, the molecular movement of the material's atoms has slowed to nearly a stop
and the material is now a glass. This new structure is not as organized as a crystal, because it did not freeze, but it is
more organized than a liquid. For practical purposes, such as holding a drink, glass is like a solid, Ediger says,
although a disorganized one.
Like liquids, these disorganized solids can flow, albeit very slowly. Over long periods of time, the molecules making
up the glass shift themselves to settle into a more stable, crystallike formation, explains Ediger. The closer the glass
is to its glass-transition temperature, the more it shifts; the further away from that changeover point, the slower its
molecules move and the more solid it seems.
Whatever flow glass manages, however, does not explain why some antique windows are thicker at the bottom.
Other glasses that are even older do not share the same melted look. In fact, ancient Egyptian vessels have none of
this sagging, says Robert Brill, an antique glass researcher at the Corning Museum of Glass in Corning, N.Y.
Furthermore, cathedral glass should not flow because it is hundreds of degrees below its glass-transition
temperature, Ediger adds. A mathematical model shows it would take longer than the universe has existed for room
temperature cathedral glass to rearrange itself to appear melted.
Why old European glass is thicker at one end probably depends on how the glass was made. At that time,
glassblowers created glass cylinders that were then flattened to make panes of glass. The resulting pieces may never
have been uniformly flat and workers installing the windows preferred, for one reason or another, to put the thicker
sides of the pane at the bottom. This gives them a melted look, but does not mean glass is a true liquid.
Please answer the questions below with COMPLETE sentences:
1) How are amorphous solids different from “regular” solids? (List which paragraph you found your answer in; i.e.
paragraph 2 or paragraph 5).
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2) Explain how glass is like a liquid and at the same time like a solid:
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3) If you had the materials and equipment to do experiments with glass, what would you do to make glass more
like a solid?
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4) What would you have to do to glass to make it more like a liquid?
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5) Why does the author argue that glass must be a solid and not a liquid?
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6) Some people believe the glass in the cathedrals is thicker at the bottom of the pane because glass is actually a
supercooled liquid. But, this article says that is wrong. What is the reason the author gives as to why the glass looks
thicker at the bottom?
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