What is glass?

What is the stuff in our windows?
24 September 2019

Interview with 

Paddy Royall, University of Bristol


a view of a glass skyscraper shot from ground level


Chemically speaking, what is glass, and why is it so special? Chris Smith was joined in studio by Paddy Royall from the University of Bristol, to take us through the looking glass...

Paddy - Glass is an amorphous solid, and can be produced from many different materials, in the way that ice is a crystalline solid of water and sodium chloride is a crystalline solid and these are chemically different. Glasses can also be chemically different and are solids but they are amorphous.

Chris - But what actually chemically is the stuff that we call window glass and what atoms are in there?

Paddy - Silicon and oxygen. So it's two oxygens for each silicon, and that is one example of a glass. But there are many other materials that can also be made into glass. Examples include many organic molecules like ortho-2-phenol. You can even make a glass out of water. You have to try quite hard and cool it very very quickly. To make a glass, essentially what one needs to do is cool it down before the material has a chance to crystallise. So in principle more or less anything can be turned into a glass assuming you can call it down fast enough. Essentially what we call a glass is when you've taken a liquid, and cooled it down until the viscosity has become a million billion times greater than that of water, and one could do that in principle for any liquid.

Chris - Is it true then Paddy, that when you look at windows of old buildings, they're thicker at the bottom of the pane than at the top of the pane, because the glass is a liquid and over hundreds of years it has gently sagged and flowed downhill.

Paddy - I'm sorry. No it isn't. That is an old wives tale. The glass did indeed flow, but it flowed as the material cooled. So as we just heard glass is a liquid at 1,100 degrees Celsius, and it is during the cooling process from that temperature down to room temperature that the flow occurred. At ambient temperatures glass is every bit as solid as any other solid that one might encounter. So there is no meaningful flow or deformation of the material on the timescales of centuries.

Chris - From you as a scientist, what is the interesting glasses for you? How are you trying to study them and understand them better?

Paddy - The real question about glass as far as we're concerned in glass physics, is we don't really know what glass is. That might sound strange because one can simply look at it, but fundamentally we don't understand why these liquids become so viscous, they become a million billion times more viscous than water, as I said, but why does this happen? And the truth is that we just don't know. It may be that it happens because there is an underlying material which is known as an ideal glass, and that the transition to this underlying material may result in the liquid becoming very very viscous and us terming it a glass. On the other hand, it may simply be that the mobility, the amount the particles move, or molecules move in the liquid, simply becomes less and less and less. And so essentially there are two competing ideas. There are very many different theories but they boil down to these two competing ideas, that either the molecules somehow move less and less and less, or that there is some underlying transition to something called an ideal glass.


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