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The Debunker: Is Window Glass Really a Slowly Flowing Liquid?

by Ken Jennings

April is the traditional month for spring cleaning: opening doors wide for the first time in months, polishing things till they gleam, possibly beating on rugs with some kind of stick or club? In honor of this season of good housekeeping, we've asked Jeopardy! mastermind Ken Jennings to help us out with a little mental spring cleaning. He'll be dusting away some persistent around-the-house myths and spraying the sweet-smelling Lysol of Truth over all your remaining brain clutter.

The Debunker: Is Window Glass Really a Slowly Flowing Liquid?

It's easy to see the appeal of this myth. The windows of medieval cathedrals, it's been noticed, are often thicker at the bottom than they are at the top. Imagine these brightly colored panes of religious scenes, seemingly frozen but imperceptibly melting over the centuries, a potent symbol of Time's wingéd chariot, undetectable in any given instant, but inevitably coming for us all!

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This legend has been circulating in high school chemistry classes as far back as 1947, and flowed downward into mainstream culture somewhat faster than the alleged phenomenon in question. By 1970 it was quoted as fact in technical journals and textbooks. Today, you'll find it in popular science books like Bill Bryson's A Short History of Nearly Everything. But it's still wrong, as Robert C. Plumb first claimed in his seminal 1989 article, "Antique Windowpanes and the Flow of Supercooled Liquids."

The tricky thing about glass is that it isn't really a solid or a liquid. It's an amorphous solid, meaning that its atoms aren't lined up in a fixed crystalline structure. Don't get me wrong, there are still strong chemical bonds holding the atoms together, which is not true of room-temperature liquids like mercury or water. But glass didn't freeze. It's a supercooled liquid that just got very, very viscous.

But that doesn't mean that it's flowing very slowly. It turns out that glass has, effectively, stopped. Scientists studying 20-million-year-old amber have found no fluid behavior over its long lifespan. A 1998 study by a Brazilian researcher calculated that, at normal temperatures, a cathedral window could go about a hundred million trillion years without any observable flow.

So, wait, why are antique windows sometimes thicker on the bottom? Dr. Plumb, way back in 1989, proposed the explanation that's still accepted today: because glass-blowing processes were less precise back then, leading to irregularities in thickness near edges. Sometimes workers installed windows with the thicker side down. Sometimes they didn't. It's a manufacturing question, not a gravitational or an existential one. (But we are all going to die.)

Quick Quiz: Today, the world's largest collection of glassblowing studios is found in Murano, an island in the lagoon north of what city?

Ken Jennings is the author of six books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.