“Glass is a slow-moving liquid”

Many people have heard the intriguing idea that glass in old windows is actually a very slow-moving liquid, gradually flowing downwards over centuries, causing the bottom to become thicker. This notion often sparks curiosity, picturing ancient cathedrals with their panes subtly deforming under the relentless pull of gravity. It's a compelling image that suggests glass possesses a hidden, fluid nature.

However, despite its widespread appeal, this claim is a myth. Scientifically, glass is classified as an amorphous solid, not a liquid. While it lacks the crystalline structure of typical solids, its atomic arrangement is fixed and does not allow for flow at ambient temperatures. The bonds between its molecules are far too strong to permit any noticeable movement, even over geological timescales. If glass were truly a liquid, even a very viscous one, other properties like its refractive index would also change over time, which they do not.

The visual "evidence" for this myth comes from antique windowpanes, which indeed often appear thicker at the bottom. This unevenness is not due to glass flowing, but rather a testament to the less refined manufacturing processes of the past. Before modern float glass techniques, glass was often made by blowing cylinders and then flattening them, or by spinning molten glass into disks. Both methods inherently produced sheets of varying thickness. When glaziers installed these imperfect panes, they naturally placed the thicker, heavier edge at the bottom for greater stability. This practical installation choice, rather than any liquid property of glass, explains the perceived phenomenon.