“Glass is a liquid that flows very slowly over time”

Many people have observed old windowpanes, particularly in historic buildings, and noticed that they often appear thicker at the bottom than at the top. This observation has led to the widespread belief that glass is actually a supercooled liquid that flows incredibly slowly over centuries, causing it to pool at the bottom of the pane. This idea, while intriguing, is a persistent myth that misrepresents the true nature of glass.

Scientifically, glass is classified as an amorphous solid, not a liquid. Unlike crystalline solids with their ordered atomic structures, glass has a disordered arrangement of molecules, similar to a liquid, but these molecules are rigidly fixed in place. The perceived "flow" in old windows isn't due to glass slowly dripping downwards. Instead, it's a direct result of the manufacturing processes used hundreds of years ago. Early glassmaking techniques, such as the crown glass or broadsheet methods, were unable to produce perfectly uniform sheets. Panes often had slight variations in thickness, and glaziers would typically install them with the thicker edge at the bottom for greater stability.

The reason this myth endures is partly due to the visual evidence of those old windows and partly because the concept of a "supercooled liquid" sounds scientifically plausible to many. While glass does technically lack a true melting point and transitions gradually from a viscous liquid to a rigid solid upon cooling, its viscosity at room temperature is so extraordinarily high that any flow would be imperceptible even over geological timescales, far beyond the lifespan of any building. The myth provides a simple, albeit incorrect, explanation for a common observation, making it easy to believe and pass along.