Myth or not?
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Claim: Glass is a liquid, not a solid. Evidence of this is seen in that Medieval windows are sometimes thicker at the bottom than at the top.
Rating: MOSTLY FALSE/MAIN CLAIM IS INCORRECT
Explanation: Glass is not a liquid or a solid, but an "amorphous solid", a state of matter that has properties of both liquids and solids.
When glass is made, the material (often containing silica) is quickly cooled from its liquid state but does not solidify when its temperature drops below its melting point. At this stage, the material is a supercooled liquid, an intermediate state between liquid and glass. To become an amorphous solid, the material is cooled further, below the glass-transition temperature. Past this point, the molecular movement of the material's atoms has slowed to nearly a stop and the material is now a glass. This new structure is not as organized as a crystal, because it did not freeze, but it is more organized than a liquid. For practical purposes, such as holding a drink, glass is like a solid, [chemistry professor Mark] Ediger says, although a disorganized one.
Like liquids, these disorganized solids can flow, albeit very slowly. Over long periods of time, the molecules making up the glass shift themselves to settle into a more stable, crystallike [sic] formation, explains Ediger. The closer the glass is to its glass-transition temperature, the more it shifts; the further away from that changeover point, the slower its molecules move and the more solid it seems.
—Scientific American
But this doesn't explain why old windows are thicker at the bottom. Amorphous solids only flow at higher temperatures than would happen in churches. And Medieval, even ancient, glasses and cups aren't thicker lower down.
Actually, the windows are thicker lower down because of the uneven way they were made, and the builders then put them into the frames with the thicker part down for structural & balance reasons.
So glass is not a liquid. It's an amorphous solid.
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