Glass is a slow-flowing liquid
Glass is an amorphous solid, not a liquid. The observation that old medieval windows are thicker at the bottom is explained by manufacturing techniques of the era, not by glass flowing over centuries. Calculations confirm that room-temperature glass cannot flow on any meaningful timescale.
What we know
The claim that glass is a supercooled liquid that slowly flows over time gained popular currency as an explanation for why some old European cathedral windows appear thicker at the bottom than at the top. The reasoning was that glass, being a liquid in an arrested state, would gradually flow downward under gravity.
Materials scientist Edgar Zanotto of the Federal University of Sao Carlos calculated in a 1998 paper in the American Journal of Physics that even given the specific chemical compositions of medieval glasses (which are more fluid than modern glasses), the time required for visible sagging at room temperature would be far longer than the age of the universe, on the order of 10 to the power of 33 years. This definitively rules out flow as an explanation for the observations.
The actual explanation for uneven medieval windows is manufacturing. Early glassmakers produced panes using the cylinder process: blowing glass into a cylinder, cutting it, and flattening it. These handmade panes were never perfectly uniform and when installed, workers placed the thicker end at the bottom for structural stability. Later panes made with the crown glass technique have a characteristic bullseye from the glassblower's rod at the center.
Glass is technically classified as an amorphous solid, sometimes described as being between a solid and a liquid in terms of atomic organization, but it behaves entirely as a solid under practical conditions including any temperature encountered on Earth's surface.
Common claims
- Old windows are thicker at the bottom because glass flowsFalse - manufacturing explains thickness variation
- Glass is technically a liquidPartly misleading - it is an amorphous solid
- Glass would visibly flow given enough timeFalse at room temperature - timeframe exceeds age of universe