“Glass is a slow-moving liquid.”

It is a widely held belief that glass, over long periods, flows like a very slow-moving liquid, causing old window panes to become thicker at the bottom. This intriguing idea often stems from observations of historic buildings where the glass in ancient windows does indeed appear uneven, sometimes noticeably thicker at the base. This perceived change over time has led many to conclude that glass must be slowly succumbing to gravity.

However, scientific evidence firmly busts this myth. Glass is not a liquid; it is classified as 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. They lack the kinetic energy required to flow, even over centuries. If glass were a liquid, albeit a very viscous one, other amorphous solids like obsidian would also show signs of flow, which they do not.

The unevenness observed in antique window panes is not a result of glass flowing, but rather a testament to historical manufacturing limitations. Centuries ago, glass production methods, such as the crown or cylinder process, were not capable of producing perfectly flat and uniform sheets of glass. These early techniques inherently resulted in variations in thickness across a single pane. Glaziers would then install these imperfect sheets with the thicker edge placed at the bottom for perceived stability or simply because it was the most practical way to fit the pane.

Therefore, the charming irregularities in old windows are a fascinating glimpse into past craftsmanship, not a demonstration of glass behaving as a super-slow liquid. The molecules within a pane of glass remain as rigidly fixed today as they were the day it was made.