Glass talk gets thick with worry

A friend recently sent me one of those lists of odd or unusual "facts" that you find on the Web sometimes. One of the items said that the windows in some very old churches in Europe are thicker at the bottom than at the top because glass is a liquid and the glass in the windows had flowed down.

Is this true?

Is what true? That glass is a liquid or that part about the windows?

The thing about the windows is hooey.

The windows in some medieval buildings are thicker at the bottom because of the process used to make them.

It's too complicated to go into here, and besides, why would you want to know about it anyway?

So you could build medieval windows?

The part about glass being a liquid or a solid is a little more complicated.

Obviously, if you rap on a pane of glass with your knuckles, it is a solid.

However, in theory it isn't a solid and it isn't quite a liquid, either.

In a liquid, the molecules are constantly bumping around, making and breaking loose connections.

In a solid, the molecules are locked together in very rigid orderly bonds.

In glass, the molecules don't do either of those things.

OK, now don't write or call to tell me about different forms of glass, like polystyrene, for example. For the purpose of this discussion, we're just going to discuss glass like the glass in your windows, OK?

So you make glass by mixing sand with some other stuff - soda ash, limestone, etc., depending on what you're making - and getting it really, really hot and then cooling it rapidly, super-cooling it to below its freezing point.

When you super-cool this mixture of molten sand, etc., its little molecules don't have time to arrange themselves as crystals, like in an ice cube. What happens is that the molecules move very, very slowly and the viscosity of the mixture gets thicker and thicker and finally the molecules pretty much just come to a halt and you have glass.

This is called an amorphous solid. The molecules stick together enough to give the glass some rigidity, but they do not stick together in an orderly manner, like in a real solid.

So if glass is really just an extremely viscous liquid, it would flow, and I guess it does. It just doesn't flow at the pace we can comprehend. One source I read said it would take 10 million years or more for the glass in a church window to flow downward to the point that the bottom of the window was 5 percent thicker than the top.

So don't worry about it.

600450 Glass talk gets thick with worry
Date: 28 January 2004

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