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

The notion that glass slowly flows over time, causing old windowpanes to be thicker at the bottom, is a persistent misconception. This myth likely originates from the visual evidence of wavy, uneven glass found in ancient buildings, particularly cathedrals. The seemingly melted appearance of these old panes leads many to believe that glass, being a "supercooled liquid," gradually sags under gravity over centuries. However, this popular belief misinterprets both the nature of glass and the history of its manufacturing.

Scientifically, glass is not a liquid but an amorphous solid. Unlike crystalline solids with ordered atomic structures, glass has a disordered arrangement of molecules, similar to a liquid, but these molecules are rigidly fixed in place. While glass does exhibit a "glass transition" when heated, it does not flow at ambient temperatures. Any molecular movement at room temperature would be infinitesimally slow, requiring timescales far exceeding the age of the universe to produce a noticeable change. The true explanation for uneven old windowpanes lies in historical glassmaking techniques. Methods like the "crown glass" or "cylinder glass" processes, prevalent before modern float glass production, inherently created panes of varying thickness. When installed, glaziers would naturally place the thicker, more stable edge of these imperfect panes at the bottom.

People commonly believe this myth because the visual evidence is compelling and seemingly logical. The wavy distortions and thicker bottoms of antique glass panels strongly suggest a material that has yielded to gravity over a long period. Without knowledge of the specific historical manufacturing processes or the precise scientific definition of an amorphous solid, the "slow-flowing liquid" explanation offers a simple and intuitive, though incorrect, answer to a curious observation.