Glass Is Not A Liquid

The wavy, distorted view through an ancient church window seems to offer physical proof of a persistent myth: that glass is a very slow-moving liquid, gradually slumping under gravity's pull. While it’s a compelling idea, the real explanation is found not in modern physics, but in historical manufacturing. Glass is properly classified as an amorphous solid. Its molecules are frozen (Review) in a disordered, jumbled state, unlike the neat, repeating lattice of a crystalline solid like a diamond. This "frozen liquid" structure is what makes glass transparent, but its atoms are locked rigidly in place at room temperature.

The viscosity of glass is so incredibly high that scientific models predict it would take trillions of years—far longer than the age of the universe—for it to deform measurably under its own weight. The true reason for the unevenness in old windows lies with early glassmaking techniques. Methods like the "crown glass" process involved spinning a blob of molten glass into a large, circular disk. This naturally resulted in a pane of glass that was uneven, often with a thick lump in the middle (Review) and thinner, wavier sections toward the edges.

When glaziers cut rectangular panes from these imperfect discs, it was simply common sense and good practice to install the pane with the thicker, heavier edge at the bottom of the window frame. This provided more stability and a better fit. Over centuries, this practical decision has created the widespread illusion of a material slowly melting downwards, a testament more to the craftsmanship of the past than to the fluid nature of glass.