Don’t go with the flow
Stories from Physics for 11-14 14-16
It is often reported that glass is a super-cooled liquid and that glasses gradually flow over time. This claim is linked to the observation that old windows, for example those in cathedrals, are thicker at the bottom than at the top. This notion is actually incorrect — once solidified, glass no longer flows.
The wedge shape of old windows has an alternative explanation. Historical glass-making techniques were unsophisticated and it was difficult to produce panes that were of uniform thickness. To prevent fractures, glaziers typically chose to fit the panes with the thickest part at the bottom, to reduce the stress on the thinner material.
Glass can flow if heated, but medieval window glass would have to experience temperatures of over 400°C for over a period of 800 years for significant flow to occur.