Are Old Windows Slowly Melting?

Old cathedral windows are sometimes thicker at the bottom than at the top. This observation produced a popular explanation: glass is really a liquid, and centuries of gravity have caused it to flow downward.

The explanation is memorable, but it is wrong.

Glass begins as a liquid and cools without forming the orderly crystal structure found in materials such as salt or quartz. Its atoms remain arranged irregularly, somewhat like those in a liquid. This is why glass is described as an amorphous solid.

At room temperature, however, ordinary window glass does not flow at a rate capable of changing a window within any historical period. Calculations suggest that visible movement would require a span of time vastly longer than the age of the universe.

The unevenness of old windows comes from how they were manufactured. Before modern float glass, craftspeople produced panes by spinning or blowing molten glass and then cutting it. The resulting sheets were not uniformly thick. When installing them, glaziers often placed the heavier edge at the bottom because it was more stable, although old panes with thick sides or tops also exist.

Glass can flow when heated enough. Its transition from rigid to workable happens gradually rather than at a single sharp melting point. This unusual behaviour helps explain why people describe it as being somewhere between familiar categories, but it does not turn a cold window into a slowly moving liquid.

The ripples in an ancient pane are genuine traces of history. They record the hands and tools that made the glass, not centuries of glass dripping toward the floor.