(image credit: by Sindre Fs )
Old window glass looks warped and thicker at the bottom—but is it because it flows like a slow-moving liquid? The real reason is far more fascinating and rooted in centuries-old craftsmanship.
Have you ever walked through an old church, castle, or colonial building and noticed that the window glass looks strangely uneven—often thicker at the bottom and rippled like water? This has led to a persistent myth that glass flows over time, albeit extremely slowly, like a viscous liquid. The idea is intriguing: imagine windows imperceptibly sagging under the pull of gravity over hundreds of years.
But here’s the science: glass does not flow at room temperature. It’s actually classified as an amorphous solid—a material that’s solid but has a disordered atomic structure, unlike the neatly arranged atoms in crystals.
So why does old glass look uneven?
The answer lies in the way glass was made before the 20th century. Before the invention of modern float glass manufacturing (introduced in the 1950s), glass was crafted using methods like crown glass and cylinder glass. Both involved blowing molten glass and flattening it into sheets, often by spinning or cutting cylinders. The process was labor-intensive and resulted in inconsistent thickness.
When glaziers installed the glass, they typically put the thicker edge at the bottom for better stability. So what we now interpret as glass “flowing” is really just a reflection of pre-industrial glassmaking techniques.
Fun Fact: The float glass method, developed by Sir Alastair Pilkington, revolutionized the industry by allowing molten glass to “float” on a bed of molten tin, creating perfectly flat, uniform panes. This is how most window glass is made today.
