How Glass Can Flow, But Not the Way You’ve Heard

Some medieval cathedral windows do have panes that are thicker at the bottom, and you might have heard this is because glass is a slow-moving liquid that sags over centuries. The windows truly are thicker at the base, but the explanation people usually give for this has things backward. Glass itself isn’t quite what most people think it is.

On the surface, glass looks and feels like a solid. Under a microscope, though, its atoms are arranged more like a liquid that's stopped in its tracks. Scientists call this an "amorphous solid", it doesn’t have a neat crystal pattern like quartz does. Its molecules are jumbled together, similar to chilled syrup that's gone rigid. The story about glass "flowing" in old windows probably comes from someone reasoning that if it's not fully solid, maybe it just creeps downward without anyone noticing.

Actual measurements, though, show that ordinary glass won’t budge at room temperature for any time a person could care about. Scientists have heated up glass to see how fast it changes shape, then calculated how long a pane would take to noticeably sag while cool. The estimate: normal window glass would need septillions of years, a million times the universe’s age, to slump. The thicker bottom on medieval panes likely comes from the way glaziers installed them for stability, using methods that came long before modern manufacturing.

Of course, glass does flow readily at high heat. If you bring the temperature up several hundred degrees, it softens and moves just like honey. This is why glassblowing works. The windows that survived from the Middle Ages didn’t ooze down for 600 years; they’re frozen in the middle of what was once molten sand, cooled so fast that order never set in. Even obsidian, the shiny volcanic glass, formed from lava that cooled before its atoms could lock into place.

So every window you look through is in a state closer to a liquid than a crystal, locked solid but not about to drip, even if you waited for another dozen ice ages to come and go. The glass legend sticks around because it feels so plausible, no matter what the molecules are actually doing.