Here is a fact you may have been taught as true: glass is really a very slow liquid, and old cathedral windows are the proof, because the panes are thicker at the bottom where the glass has gradually oozed downward over the centuries. It is a lovely idea. It is also wrong, and physicists have put numbers on exactly how wrong.
Glass is what scientists call an amorphous solid. Its atoms are jumbled like a liquid’s rather than stacked in a neat crystal, which is what gives glass its odd in-between reputation. But a jumbled arrangement is not the same as flowing. At room temperature the atoms in a pane of glass are locked in place for all practical purposes, and «all practical purposes» here is a wild understatement.
When researchers actually calculated the flow, using the measured stiffness of medieval glass, the timescales came out absurd. For a window to sag by any amount you could see, it would need to sit undisturbed for far longer than the current age of the universe. One analysis of the stained glass at Westminster Abbey found the popular explanation was off by about sixteen orders of magnitude, a gap too large to picture. The glass in those windows has not visibly moved since it was installed, and it will not move on any human or geological timescale.
So why are the old panes uneven? Because of how they were made. Before modern float glass, panes were produced by spinning or blowing molten glass and letting it settle, which left sheets slightly thicker at one edge than the other. Glaziers fitting these irregular panes into a frame had a sensible habit: put the heavy edge at the bottom, where it sat more stably. Sometimes they set it at the top or the side instead, which is why plenty of old windows are thickest somewhere other than the bottom.
The window never dripped. It was cut unevenly, hung thick-side down by a careful workman, and then it simply stayed there, a solid pretending, for one very persistent myth, to be a liquid.
