“Glass is a slow-moving liquid.”

The idea that glass is a slow-moving liquid, slowly flowing downwards over centuries, is a persistent misconception. This myth often originates from observing very old window panes, which frequently appear thicker at the bottom than at the top. It's easy to assume this unevenness is evidence of the glass gradually deforming under gravity over extended periods, much like a very viscous liquid.

However, scientific evidence clearly indicates that glass is not a liquid at room temperature; it is classified as an amorphous solid. While its atomic structure lacks the long-range order of a crystalline solid, resembling the disordered arrangement found in liquids, its molecules are held firmly in place by strong chemical bonds. This makes glass rigid and stable at typical temperatures, preventing any noticeable flow. The apparent thickening at the bottom of ancient windowpanes is a fascinating historical artifact, not a physical phenomenon of glass flow.

The true reason for the uneven thickness in old glass lies in the manufacturing processes of earlier eras. Before modern techniques, glass production was less precise, often resulting in sheets with varying thickness. Glaziers would typically install these imperfect panes with the thicker, heavier edge at the bottom for greater stability and ease of fitting. This historical practice, rather than any slow movement of the glass itself, is responsible for the visual evidence that fuels the enduring myth. In reality, scientific calculations confirm that any molecular movement in glass at room temperature would be infinitesimally slow, imperceptible even over billions of years.