Is old window glass thicker at the bottom because glass flows?

No. Old panes are not thicker at the bottom because glass slowly flows. Penn State researchers calculated that glass at room temperature flows far too slowly to change a window's shape in centuries; uneven panes come from how glass was made back then.

What's actually true

Old windows aren't thicker at the bottom because glass flows. A popular story says glass is a super-slow liquid that sags over centuries. Penn State researchers ran the numbers using the recipe of medieval glass from Westminster Abbey and found any flow at room temperature is far too slow to change a pane's shape. The likelier culprit is old glassmaking, which produced uneven sheets.

Takeaway: Uneven antique panes tell you about the glassmaker, not about glass melting.

Why people believe it

The story looks like it has evidence: some antique panes really are thicker at one edge, and glass has no neat crystal structure, so calling it a very slow liquid sounds scientific. The step that fails is the time scale. Older glassmaking produced sheets of uneven thickness, and fitting the heavier edge at the bottom would have been a natural choice, so a real observation got a dramatic but wrong explanation that teachers kept repeating.

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