Glass Is Not a Slow-Flowing Liquid

One of the most persistent myths about glass is that it's a 'supercooled liquid' that flows very slowly over centuries, supposedly explaining why old windows are thicker at the bottom. This is false. Glass is an amorphous solid - it has a disordered atomic structure like a liquid but behaves mechanically like a solid. Medieval and early modern glass makers created panes using the 'crown glass' method, where molten glass was spun into large discs. The outer edges were naturally thicker and when cut into window panes, glaziers often installed them with the thick edge at the bottom for stability. Calculations show that if glass flowed as claimed, windows would need millions of billions of years to show noticeable thickening. Modern measurements of ancient Roman glass show no flow at all. Glass molecules are essentially frozen in place at room temperature.