What do you think of this fact?
Maybe you’ve heard it too, maybe even in school: glass is secretly an extremely thick liquid. The proof? Ancient church windows are thicker at the bottom than at the top, because the glass has been sagging for centuries. Sounds logical. It’s also wrong.
What’s really behind those thick panes
Here’s the true part: some old windowpanes really are thicker at the bottom. But that comes from how they were made. Back then, nobody could produce perfectly even panes. Glassmakers worked by hand, and the result had imperfect surfaces and uneven thickness.
According to Scientific American, whoever installed a pane like that usually put the thicker side at the bottom. It seemed sturdier. Centuries later, it looked as if the glass had melted.
Truly smooth panes with an even thickness only arrived with the float glass process, which according to Wikipedia became widely available starting in the 1960s.
So how fast does glass flow?
Practically not at all. A 2017 study looked at medieval glass from Westminster Abbey in London, dated to the year 1268. The result, per Wikipedia: at room temperature, glass like that flows at most about 1 nanometer per billion years. That’s not measurable, not visible, not relevant.
Scientific American puts it even more bluntly: for cathedral glass at room temperature to rearrange itself enough to look melted, it would take longer than the universe has existed.
Solid, but messy
So what is glass? Scientists call it an amorphous solid. In a crystal, like salt or ice, atoms are lined up in a regular grid. Glass lacks that long-range order. Its atoms are jumbled up like in a liquid, but they don’t go anywhere anymore.
Ancient Egyptian glass vessels show no sagging at all, according to Scientific American, even though they’re far older than any church window.
So glass isn’t honey in slow motion. It’s just a solid with very untidy atoms.