Do windows get thicker at the bottom over years because glass is really a slow-moving liquid?

Before we refute if windows get thicker at the bottom over years, let’s look at why some people have considered the idea plausible enough to make it the pernicious urban myth it has become.

Their reasoning goes something like this: Solids and liquids have different molecular structures—the molecules of solids occur in regular patterns, while the molecules in liquids are bonded in patterns that are irregular and haphazard.

The structure of glass is more irregular, like liquid, so some have surmised that maybe it’s just a very, very thick liquid. Casting around for evidence that would fit this theory, researchers found that antique windows from ancient buildings usually had glass that was thicker at the bottom than it was at the top.

Aha! they said, here’s evidence that glass slowly seeps downward with time, just like a really thick syrup would.

Well, after a lot of shouting and bickering, it looks like the “glass is solid” side won this one. Yes, glass has a different molecular structure than most solids, but it doesn’t necessarily follow that it’s a liquid. In fact, although scientists once called glass a “supercooled liquid,” most now believe it is really an “amorphous solid,” which means that it solidifies without crystals forming.

Examining other ancient glass provides the evidence. For example, scientists have examined glass bottles from ancient Rome and found no sign of seeping.

They looked at telescope lenses that were precision-ground centuries ago and found no changes in shape. And they’ve studied arrowheads from prehistoric times made of obsidian (a naturally occurring glass) and found them to be symmetrical and razor-sharp—something that presumably wouldn’t be the case if glass could droop, seep, and weep over time.

Okay, so how do they explain the bottom-heavy windows on ancient buildings? Simple. In the old days, glassmakers used a technique that all but guaranteed that each pane of glass would have a varying thickness (see below).

If you were putting a fragile, heavy pane of glass into a window frame, would you place the thicker edge or the thinner edge at the bottom, where all the weight of the pane would be resting?

Right, the thicker side. So rather than provide evidence of glass seepage, old windows merely prove that glassmakers (called glaziers) in the past weren’t fools.

We can see through glass because it’s not a very dense solid. There’s enough space between the molecules to let light shine through.