There's a persistent quasiscientific myth that glass is actually a supercooled, superviscous liquid or fluid. Tour guides in historic churches and other old buildings will often point out that the panes in the windows are thicker at the bottom than the top, "because they've flowed ever-so-slowly downwards over the years." But it turns out this isn't quite true.

Glass is actually an "amorphous solid". It has no crystalline structure, and it does not undergo a normal phase change as most materials (such as water, and metals) do. (Another amorphous solid is rubber.) Nevertheless, the molecules in room-temperature glass are rigidly bound to each other, and there's no evidence that they flow, even after hundreds or thousands of years.

So what about those old windowpanes that are thicker at the bottom? Well, until recently, there wasn't a good way of making perfectly flat, smooth glass for windows. For example, the "crown glass" technique (used since medieval times) involved blowing, flattening, and then spinning what became a large disc of glass, slightly thicker at the outer edge due to centrifugal force. These irregularities obviously persisted even after you cut smaller square panes out of the large discs. Windowmakers tended to mount panes with the thicker edge down -- they looked better that way -- but what this means is that the bottoms had always been thicker; they didn't sag over the years. And sometimes you can find old panes that are thicker at one side, or at the top, because they happened to be installed that way, and this is a pretty good proof that the thick-at-the-bottom panes aren't that way due to "flow".


(P.S. There is at least one example of a superviscous "liquid" that takes years to flow -- pitch. If you do a web search for "pitch drop experiment" you can find pictures of a funnel full of pitch at the University of Queensland that has dribbled out just eight drops in over seventy years.)

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