Phisherman wrote:
....
Most glass is a liquid and sags. In an old house you may see the
bottom of the windowpane is thicker than the top portion. Quartz
glass is a solid--probably less sag over time and transmits more
lightwaves than regular glass.

This is commonly heard but...

Corning Museum of Glass Research Scientist's discussion is at--
http://www.cmog.org/dynamic.aspx?id=294

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