Your guess
Why are old panes often thicker at the bottom?
Uneven from the start
Flat glass used to be blown by hand, for example as cylinders. Such panes were wavy and of uneven thickness. Truly flat float glass only became widespread in the 1960s.
Worked out
1nanometerper billion years – at most
For medieval glass from Westminster Abbey dating from 1268, a team calculated in 2017 how fast it could flow. Even in the worst case: one nanometer in a billion years. No human could ever observe that.
