Slumping stained glass

I worked with stained glass for years – both the copper foil method and the lead came method that was used in medieval times –and never heard that supposedly glass is a liquid at room temperature. The "proof" of this, according to tour guides and high school chemistry teachers, is that the glass in cathedral windows is thicker at the bottom than at the top. In fact, glass is an amorphous solid, something in structure between a liquid and solid, but for it to flow slowly downward would take not centuries but billions of years. "A mathematical model shows it would take longer than the universe has existed for room temperature cathedral glass to rearrange itself to appear melted." The disparity in the thickness of the glass is due to the way it was made by the European glassblowers of the Middle Ages. They crafted glass cylinders which they then flattened into panes. The thicker parts of the uneven panes were installed at the bottom to avoid the windows being top-heavy and less stable.

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