Why Glass Is Not Actually a Liquid

A persistent myth claims that glass is technically a liquid, slowly flowing over centuries, and that old cathedral windows are thicker at the bottom because the glass gradually dripped downward over hundreds of years. This idea is false, and modern materials science explains clearly why. Glass is what's called an amorphous solid: unlike a liquid, its molecules don't flow or rearrange themselves over time under normal conditions, but unlike a typical crystalline solid such as ice or table salt, its molecules also never settled into a neat, repeating, orderly pattern in the first place. When glass is manufactured, it's cooled from a molten liquid state so quickly that its molecules get frozen in a disordered, random arrangement before they have time to organize into an orderly crystal lattice the way a slowly cooled liquid normally would. That disordered structure is locked in place once the glass hardens, and it stays that way indefinitely; the molecules in a solid pane of glass are not slowly sliding past one another the way liquid molecules do, even over an extremely long timescale. The real explanation for uneven old windowpanes has nothing to do with flow at all: centuries-old glassmaking techniques simply couldn't produce perfectly uniform sheets, so panes came out thicker on one edge than the other, and glaziers of the time typically installed the heavier, thicker edge facing downward for structural stability, which is exactly why so many surviving old windows appear thicker at the bottom.