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I was told that it's not true, by someone who usually knows such things. Ah, here we go:
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Posted by stone on 2009-02-20 03:49:36

In Reply to: Nate heard it from his stoner friends in high school, so it could use some confirmation, lol posted by nate on 2009-02-20 02:30:14

"Glass windowpanes in old houses are thicker at the bottom because glass is a liquid that which flows slowly under the action of gravity.

This multiply mistaken unfounded assertion has even crept into textbooks and physics lectures. Windowpanes in old houses have varying thickness due to the glass manufacturing process used, and were usually installed thicker side down.

First, the assertion that the panes are thicker at the bottom isn't quite true. Some old buildings have window panes thicker at the bottom, and when that is so, it was probably custom or preference on the part of the builder. Many old buildings have panes where the thick sides are oriented in various ways.

Second is the misleading assertion that glass is a liquid that flows slowly. There's some validity to this, for glass does sometimes flow, but so slowly and so little that in the time since the old glass was made, you'd not get anywhere near this much thickness difference, or even a noticable difference.

If the assertions were true, the amount of thickness observed in houses of the 18th century should be, on average much less than that observed in buildings of the 15th century. But that is not the case.

The thickness variation is observed in glass made by a process in which a layer of glass is flattented by rotation on a flat surface. The thickness varies from center to rim of the wheel. When cut into pieces, each piece has a thicker edge. A professional window-maker will usually choose to orient the thick edges the same way, for esthetic reasons. Also, we have an intuitive feeling that a solid object is more stable if its heavy side is down.

Modern glass for windowpanes is made by floating molten glass on a liquid surface, which produces a very flat surface, and very uniform thickness.

You can learn more about it at these links.

* Antique windowpanes and the flow of supercooled liquids by Robert C. Plumb.
* Glass flow myth by Ed Wynn.
* The Physics of . . . Glass Supercooled Sand by Robert Kunzig, from Discover Magazine October 1999. "


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