“Glass is a slow-flowing liquid”

Many people have observed the wavy, often thicker-bottomed panes in very old windows, especially in historic buildings and cathedrals. This visual evidence has led to a widespread belief that glass is actually a super-slow-flowing liquid that sags under gravity over centuries. This intriguing idea, often shared by tour guides, has become a persistent urban myth.

However, scientific evidence firmly busts this claim. Glass is not a liquid at all; it is an amorphous solid. Unlike true liquids, whose molecules move freely, the atoms in glass are held tightly in a rigid, albeit disordered, structure by strong chemical bonds, preventing any observable flow at room temperature. Calculations show it would take millions of years for a window pane to thicken by even a tiny percentage, far beyond any human observation.

The uneven thickness seen in ancient windowpanes is a fascinating relic of early glass manufacturing techniques. Methods like the "crown glass" process involved blowing molten glass into a sphere and then spinning it into a flat disc, resulting in panes that were naturally thicker towards the center and thinner at the edges. Similarly, the "cylinder method" also produced uneven sheets. When glaziers installed these imperfect panes, they wisely oriented the thicker, sturdier side at the bottom for stability, creating the illusion of flow that continues to fuel this common misconception.