Glass is a Solid, Not a Slow Liquid

Many people have heard the intriguing idea that glass in very old windows appears thicker at the bottom because it has slowly flowed downwards over centuries, behaving like an extremely viscous liquid. This popular notion, however, is a persistent myth. Scientifically, glass is not a supercooled liquid but rather an amorphous solid. Unlike crystalline solids, which have a highly ordered atomic structure, amorphous solids like glass possess a disordered arrangement of atoms, similar to a liquid. Yet, crucially, these atoms are fixed in place and do not flow, giving glass its rigidity.

The observable differences in thickness in ancient window panes stem entirely from historical glassmaking techniques, not from gravitational flow. Early methods, such as the crown glass process or the broadsheet method, produced glass with inherent variations in thickness. When glaziers installed these panes, they often placed the thicker, heavier edge at the bottom for stability, inadvertently creating the optical illusion of slumping. Modern scientific instruments can detect no measurable flow in glass, even over vast stretches of time, firmly classifying it as a solid with an incredibly high viscosity, effectively preventing any perceptible movement.