Glass Is Not Actually a Solid

Although it feels perfectly firm and stable, the inner world of glass is a scene of molecular chaos. In a true solid, like a salt crystal or a diamond, atoms are locked into a neat, repeating, and highly ordered structure called a crystal lattice. Glass, however, is formed by cooling a molten liquid so rapidly that its molecules are frozen (Review) in their jumbled, liquid-like state before they have a chance to organize. This unique state of matter, caught between a liquid and a solid, is the reason scientists classify glass as an amorphous solid, from the Greek for "without form."

This unusual structure is the source of a persistent myth that the glass in old cathedral windows is thicker at the bottom because it has slowly flowed downwards over centuries. The truth is far less dramatic. Medieval glass manufacturing techniques were simply imperfect, resulting in panes of uneven thickness. For stability, glaziers would prudently install the thicker, heavier edge at the bottom. In reality, the viscosity of glass at room temperature is so immense that scientific calculations show it would take longer than the age of the universe for any noticeable flow to occur.