⚠️This fact has been debunked
This is a widely debunked myth. Glass is an amorphous solid, not a liquid. It does not flow over time. The myth originated from observations of old medieval windows being thicker at the bottom, but this was due to manufacturing inconsistencies in the crown glass process, not flow. Scientific studies, including viscosity calculations and analysis of ancient amber, have definitively proven glass does not flow at observable timescales.
Technically speaking, crystal glass is actually a Liquid that flows very slowly.
Does Glass Flow Like a Liquid Over Time?
You've probably heard it before: glass is technically a liquid that flows very slowly. Look at old church windows—they're thicker at the bottom because the glass has been flowing downward for centuries! It's a compelling story that's been repeated in classrooms and documentaries for decades.
There's just one problem: it's completely false.
Glass Is Solid, Not Liquid
Glass is neither a liquid nor a traditional crystalline solid. It's what scientists call an amorphous solid—a state of matter somewhere between the two. When molten glass cools, its molecules don't arrange into the orderly crystal structure of most solids. Instead, they freeze in a disordered, liquid-like arrangement. But make no mistake: they are frozen. Glass does not flow.
At room temperature, glass molecules move so slowly that any "flow" would be impossible to detect—not just in a human lifetime, but over geological timescales. Scientists at Corning calculated the flow rate of medieval glass and found it's far too slow to account for any measurable change, even after centuries.
The Medieval Window Mystery
So why are old windows thicker at the bottom? The answer is refreshingly simple: manufacturing defects.
Medieval windows were made using the crown glass process. Glassmakers would blow molten glass into a hollow globe, flatten it, then spin it into a flat disk. This created panes that were naturally thicker in the center and thinner at the edges—never uniform.
When installing these imperfect panes, builders sensibly put the thicker, heavier edge at the bottom for stability. It wasn't flow; it was gravity working on an already uneven piece of glass during installation.
Scientific Proof
Modern science has demolished this myth from multiple angles:
- Viscosity calculations show glass would take longer than the age of the universe to flow measurably at room temperature
- 20-million-year-old amber (also an amorphous solid) shows no evidence of flow despite its ancient age
- Horizontal old glass panes show the same thickness variations as vertical ones—if glass flowed, only vertical panes would change
The "glass flows" myth persists because it sounds scientifically plausible and explains an observable phenomenon. But like many compelling stories, the truth is more mundane: old glass looks weird because old manufacturing methods produced weird glass.
The next time someone tells you glass is a slow-moving liquid, you can confidently correct them: glass is solid, and it's staying that way.