“Glass is a slow-moving liquid.”

Many people have heard the idea that glass is actually a very slow-moving liquid, and that over centuries, it can gradually flow downwards, causing old windowpanes to become thicker at the bottom. This widespread belief often stems from observing antique windows, where the glass does indeed appear noticeably thicker at the base than at the top. This visual evidence seems to support the idea of glass "slumping" under gravity over long periods.

However, scientific evidence firmly busts this myth. Glass is, in fact, an amorphous solid, not a liquid. While it lacks the crystalline structure of typical solids, its atomic arrangement is fixed and does not allow for flow at ambient temperatures. The atoms in glass are bonded together in a rigid, albeit disordered, network (Review). If glass were truly a liquid, even a very viscous one, it would not fracture in the way it does, and its properties would change significantly over time, which they do not.

The uneven thickness observed in very old windowpanes is not due to the glass flowing, but rather a fascinating testament to historical manufacturing techniques. Centuries ago, glass production methods were far less refined than today. Panes were often made by blowing a cylinder of glass and then flattening it, or by spinning molten glass into a disc. These processes inherently created imperfections and variations in thickness across a single sheet. Glaziers, when installing these imperfect panes, would naturally orient them with the thicker, heavier edge at the bottom for greater stability, giving rise to the illusion of flow.

So, while the sight of wavy, thick-bottomed antique glass might make the "slow-moving liquid" theory seem plausible, it's a misconception rooted in historical craftsmanship rather than the physical properties of the material. Glass remains a solid, steadfastly holding its shape over millennia, a true testament to its unique structure.