I have always recalled back in high school in the 60’s about learning glass was a liquid and they used glass from old buildings as evidence where some how it sagged over time.
I do not know how much it sags over time, but I can tell you from experience that it is harder to cut up an old piece that has been in a window for a long time than a new piece of glass.
back in high school in the 60’s...
Some people claim that stained glass windows in old churches are thicker at the bottom than at the top because glass flows slowly like a liquid. We’ve known this isn’t true for quite some time now; these windows are thicker at the bottom owing to the production process. Back during medieval times, a lump of molten glass was rolled, expanded, and flattened before being spun into a disc and cut into panes. These sheets were thicker around the edges and installed such that the heavier side was at the bottom.
https://io9.gizmodo.com/the-glass-is-a-liquid-myth-has-finally-been-destroyed-496190894