Myth 1: Is glass liquid?

by Stephanie Chasteen on September 7, 2007

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.

{ 6 comments… read them below or add one }

Pann October 17, 2007 at 2:09 pm

Hey SGG,

That is great that you posted about this. I clearly remember my big brother telling me that glass is a liquid that flows really, really slowly. Everything my BB told me was true, no fact checking required, when I was a kid.

Now I have just one more bone to pick with him. (kidding of course).

I’m enjoying catching up on your blog.

ryjoe November 11, 2008 at 9:32 pm

Man!!
just this weekend I was told by a science major that his geology professor showed everyone old glass in an old house to be thicker at the bottom as proof for the movement of glass. STOOOOOOOOPs

sciencegeekgirl November 11, 2008 at 9:53 pm

It’s been misrepresented in the literature for so long that a lot of people think that it’s true. It’s much easier to get a wrong idea into people’s heads than to get it out, especially if it’s a cool idea like this one. It’s sort of viral.

steve May 3, 2012 at 1:08 am

Actually, there may be some truth to this “myth”. I recall being in a very old (100+ years) house in central ca, and was shown that ALL the old windows were thicker at the bottom. Some were so thin at the top, there were places where the glass had “melted” and there were actual gaps at the top big enough to stick a pencil through. I doubt this would be caused by how the glass was poured, because even back then, they poured glass horizontally on a flat surface or liquid. So, based on the fact that glass has been proven to have limited viscosity, plus my own observations, I’d say, yes, glass can flow, but it takes a very long time and it helps if it is in a very hot environment for long periods of time. Believe it or not!

Abhilash August 11, 2014 at 3:30 pm

Really glad that i could get to the bottom of this myth…i have been told by several people that Glass Actually a liquid and an almost equal number have told me that this is false…
Thanks!

Simoncaneplz June 13, 2015 at 7:17 am

Its, true that glass arent really liquid, it was only liquid when it was melted, so Thank you!

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