windowHave you been told that glass is a liquid? I remember back in 10th grade and my teacher told me that old windows were thicker at the bottom than at the top, showing that glass flows, veeerrrry slowwwwly.

While I was at the Exploratorium, this myth was debunked for me by my mentor Paul Doherty. It is true that many old windows are thicker at the bottom than at the top, but it’s not because glass flowed over time and puddled at the bottom. Old windows were made by spinning the molten glass and then cutting it into panes, resulting in glass that was thicker at one end than the other. In fact, later observations noted that some ancient glass is thicker at the top than at the bottom. It just depends on how the window was placed.

Paul notes on his website that: Room temperature glass has a viscosity of 1022 poise. The viscosity of a liquid controls how fast it flows under gravity. (SAE 30 motor oil has a viscosity of about 1 poise, water is 0.01 poise.) The viscosity of glass is so high that you could wait the entire age of the universe and see no measurable thickening of the glass under earth gravity.

Note that the definition of a solid is a material with a viscousity greater than 13 poise.

Of course, as Paul likes to say, “it’s more complicated than that.” Some people say that glass is both liquid and solid because when you look at the underlying structure of it, it has properties of both. But in terms of its material properties (does it flow!?) it can be classified as a solid. But the answer really isn’t that cut and dried.

Here is a link with more information than you’ll probably want to know.