“Glass is a slow-flowing liquid”

The widespread belief that glass is a slow-flowing liquid often originates from observations of antique windowpanes, particularly those found in medieval cathedrals. These old windows frequently appear thicker at the bottom than at the top, leading many to conclude that the glass has gradually sagged over centuries due to gravity. This misconception is further fueled by the fact that glass lacks the ordered, crystalline structure typical of most solids, making its molecular arrangement seem more akin to that of a liquid.

However, scientific consensus firmly establishes glass as an amorphous solid, not a liquid. While its molecular structure is indeed disordered, the atoms within glass are held rigidly in place by strong chemical bonds, preventing any measurable flow at ambient temperatures. Calculations have demonstrated that even ancient glass would exhibit negligible flow, on the scale of nanometers over billions of years, making any visible deformation impossible within human timescales. The varying thickness observed in old windowpanes is, in fact, a relic of historical manufacturing techniques. Early glassmaking methods, such as the crown glass process or blowing glass cylinders and then flattening them, inherently produced sheets of glass with inconsistent thickness. Glaziers installing these panes would naturally orient the thicker, heavier side downwards for greater stability within the window frame.

The enduring nature of this myth can be attributed to the compelling visual evidence in aged structures and the intuitive, though incorrect, explanation that a material without a defined crystalline structure might flow. The term "supercooled liquid," sometimes used to describe glass, has also contributed to this confusion, despite scientists differentiating between a supercooled liquid state and the rigid, amorphous solid state of glass. Ultimately, the unevenness in historical glass serves as a testament to past craftsmanship, rather than a demonstration of glass flowing like a very slow liquid.