Mythconception: Is glass a liquid?

20 November 2015

Interview with

Dr Kat Arney, Naked Scientists

This week Kat Arney has been reflecting on the claim that glass is a liquid...View out a window from a darkened room. Fort Sam Houston, San Antonio Texas (December 2006).

Kat - If you're lucky enough to take a tour round one of the many grand medieval cathedrals of Europe, take a careful look at the windows. You'll notice that some of the panes of ancient glass are thicker at the bottom than they are at the top, looking for all the world like the glass is very slowly melting downwards in the frame. Tour guides and teachers alike will explain that the reason this happens is because glass is actually a liquid rather than a solid, although it's a very slow-moving one.

On the surface this explanation makes a lot of sense. Many materials can exist in different states: think of water, which is solid below zero degrees Celsius, liquid up to a hundred degrees, and then becomes a gas as things heat up further. Glass is mostly made of silica - silicon dioxide as chemists call it, or sand as most of us know it. Heated up with certain other chemicals to an impressive 1,700 degrees, it becomes molten, liquid glass. From this state it can be poured into moulds to set - which is how bottles and glasses are made -  or floated on top of vats of molten tin to make perfectly flat glass panes.

And when molten glass is cooled down slightly it can be carefully manipulated, like super-heated Plasticine  - you've probably seen footage of traditional glassblowers at work, puffing down long tubes and twisting the hot, toffee-like material into all kinds of beautiful and useful shapes.

Based on this, it seems reasonable to think that the stuff inside our window frames may not be as solid as it seems. Indeed, when scientists have looked closely at the organisation of silica molecules within glass, they look jumbled and disordered - similar to molecules in a liquid like water - rather than the ordered, crystalline structures seen in other solids like metals. This irregular organisation is what's known as an 'amorphous solid', which is seen in other liquids that cool very fast into solids, such as certain types of ice that formed when very hot water is cooled down very fast - you can almost think of it as a 'solid liquid'.

So is the glass in those medieval windows actually on the move? Well, a research paper published in April 2013 by researchers in Texas came pretty close to shattering that myth. By studying a piece of 20 million-year-old amber - which is also a form of glass - and subjecting it to changes in temperature, they found that the molecules inside it didn't reorganise themselves in the kind of way that might be expected if it had any sort of liquid properties.

Then at the beginning of 2015, a team led by scientists at the University of Bristol took a closer look at glass using computer simulations, calculating all the different ways in which the molecules in glass might be interacting with each other, and seeing which outcome best reflects reality. They found that over time, molecules of silica in the glass settle down with their neighbours, forming more organised local structures over thousands and millions of years. It's not the same as saying the disorganised structure is a liquid, only that the organisation of the molecules in glass seems to get more regular over (a very long) time.

This still leaves the question of why the bottoms of those medieval windows are thicker than the tops. But there's no mystery here either. It's easily explained by the way they're made: back in the old days, glassblowers would spin a blob of molten glass into a disc and cut it into panes. This process meant that some edges could end up thicker than the others, and when the fitters put them into the frames, they tended to pop them in heavy side down, for stability and safety's sake.

So there you have it - glass is a solid, although it's a disorganised one. Which is lucky, because the only liquid I want in my glass is wine.

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