How Everything Works
How Everything Works How Everything Works

Question 1305

I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn't that show that glass flows? — MJ
While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant glass bubble on the end of a blowpipe or "punty" and this bubble was cut open at the end and spun into a huge disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were usually put in thick end down.

Modern studies of glass show that below the glass transition temperature, which is well above room temperature, molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.


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