You might have heard somebody say, "glass is a liquid, and old windows that are thicker at the bottom prove it". Are they right?

Well, if you look at old windows, you'll see that some of them are thicker at the bottom. But it's nothing to do with the glass flowing, and all to do with how the glass was made into a flat sheet.

The glassblower would blow some molten glass into a large sphere, and then spin it until the sphere turned into a large disc - spinning vertically. Then the molten glass would be flipped from vertical to horizontal, and plonked onto a flat surface. Even after it was polished, it was still a fairly wavy sort of glass.

When the glaziers would install it in a window, they would normally do it with the thick bits at the bottom and the thin bits at the top.

Astronomers would have noticed if glass flowed. They've been making and using telescopes, with large glass lenses, for well over a century. Lenses need a shape that is accurate to roughly 50 billionths of a metre. Not one astronomer has ever complained that their old lens is unusable because the glass has flowed.

If you want to see glass flow at room temperature, you have to wait at least 10,000 million million million times the age of the universe! It's enough to make your eyes glaze over.

© Karl S. Kruszelnicki Pty Ltd 2003.