Old church windows are not thicker at the bottom because glass flows.
It is a favourite classroom fact: glass is really a slow liquid, and medieval windows have sagged over the centuries. It is not true. Glass is a solid, and calculations suggest medieval glass would flow about a nanometre in a billion years. The unevenness was there on the day it was made.
Glass is unusual because it is an amorphous solid. In a crystal, such as quartz or ice, atoms sit in a neat, repeating pattern. Glass forms when a melt is cooled too quickly for the atoms to arrange themselves, so their disordered, liquid-like arrangement is frozen in place. At the atomic scale it resembles a supercooled liquid, which is where the myth begins. Mechanically, though, it behaves in every way as a solid.
The popular story points to old windows that are thicker at the bottom than the top, supposedly because the glass has slowly run downwards. But the ripples and uneven thickness in old panes come from how they were made. Earlier techniques produced sheets with wavy surfaces and varying thickness. Perfectly flat, even glass only became widespread with the float glass process, developed by Alastair Pilkington and Kenneth Bickerstaff between 1953 and 1957, which floats molten glass on a bath of molten tin.
Physics settles the question. A 2017 study calculated the flow rate of medieval glass of the kind used at Westminster Abbey from 1268. Even using an estimate of its viscosity far lower than earlier ones, the maximum flow came out at about one nanometre per billion years, far too slow ever to see. Measured viscosities of ordinary glass are so enormous that it would not visibly change shape over any span of human history.
So the thicker bottoms are a clue to medieval craftsmanship, not to hidden liquidity. Glass remains strange enough without the myth: a true solid whose atoms are still arranged like a frozen snapshot of a liquid. Heat it and it passes through a glass transition range, which does not fit neatly into the textbook categories of phase change.
Source: Wikipedia — Glass · Text summarised from Wikipedia (CC BY-SA 4.0)
