Many people believe that glass “flows” like a liquid, and the proof most often cited is that stained glass windows in ancient cathedrals are thicker at the bottom than at the top. The idea that glass is a highly viscous liquid at room temperature has even made its way into some textbooks.
Someone finally decided to test the idea, and it turns out to be wrong. Edgar Dutra Zanotto, a professor of materials engineering at the Federal University of Sao Carlos in Brazil, looked up the chemical composition of some 350 pieces of glass from 12th century cathedrals, calculated their viscosity, and then determined their flow rates by extrapolating the viscosity curves of hot glass to lower temperatures.
According to Zanotto’s calculations, you would have to heat a typical 12th-century piece of glass to approximately 414 degrees C to observe any significant movement in the course of 800 years. Without high heat, you would have to wait about a hundred million trillion trillion years to observe any flow, far longer than the age of the universe.