Is glass a liquid? No — old church windows don't flow

Glass is an amorphous solid, not a slow-moving liquid: at room temperature, window glass would need far longer than the age of the Universe to flow noticeably. The claim that medieval church panes are thicker at the bottom because the glass has crept downward over centuries is a myth, one repeated especially by tour guides.

Glass has no sharp freezing point. As it cools, its atoms lock rigidly in place without forming the repeating pattern of a crystal. Calling it a “supercooled liquid” persists, but many physicists regard that label as a misnomer.

Does glass flow over time?

Not measurably. In 1998, Brazilian glass scientist Edgar Zanotto calculated room-temperature flow for medieval and modern glass compositions. With P.K. Gupta he later put the relaxation time, roughly how long you would wait to see flow, at about 10²³ years. The Universe is about 10¹⁰ years old.

In 2017, Corning researchers Ozgur Gulbiten and John Mauro revisited the question using glass from Westminster Abbey dating from 1268. Medieval glass turned out to be 16 orders of magnitude less viscous than earlier estimates based on modern soda-lime glass, and still far too stiff: at most about 1 nanometre of flow in a billion years. Roman soda-lime glassware such as the Portland Vase has held its shape for nearly 2,000 years, and Stone Age obsidian arrowheads remain razor sharp, though obsidian is a tougher natural glass than window panes.

Why are old church windows thicker at the bottom?

Because of how the panes were made. Before modern float glass, panes were spun into discs (crown glass) or blown as cylinders and flattened by hand, and both methods gave sheets of varying thickness. Glaziers tended to set the heavier edge at the bottom, unaware they were starting an urban myth that would spread around the world centuries later.