Is glass a liquid or a solid?

There's something strange about the stained-glass windows of medieval buildings: They tend to be thicker at the bottom than at the top. Many believe this is because glass is actually a liquid that has flowed down the pane over the centuries. In contrast, others argue that glass does not flow, because it's a solid — or maybe an amorphous solid or a supercooled liquid.

So what is glass, really — a solid or a liquid, or something in between?

"It's neither a true liquid nor a true solid — it has properties of both, but it's its own distinct state of matter," John Mauro, a materials scientist at Penn State, told Live Science. "The technical definition is that glass is a nonequilibrium, noncrystalline state of matter that appears solid on a short timescale but continuously relaxes towards the liquid state."

This may sound complicated, but looking at each component of this explanation separately reveals how glass sits between solid and liquid form.

First, it's important to remember that glass isn't simply the transparent material we use for windows. Rather, it's a family of materials, some of which aren't even clear. 

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"There are all types of different natural and human-made glasses," Mauro said. "Most everyday glass is what's called soda-lime silicate glass, which is made out of sand, limestone and soda ash melted together at around 1,500 degrees Celsius [2,732 degrees Fahrenheit]. But there are other types, like heat-resistant Pyrex or strong and bendable Gorilla Glass, which contain other chemicals to give them specialized properties."

Although the types of glass are chemically different, the structures of the individual materials have a lot in common. It's this structure — the arrangement of the atoms and molecules inside the material — that causes the confusion around the structure of glass and whether it is a solid or a liquid.

"What makes a solid is that the atoms are fixed in position in the material," John Parker, a materials scientist at the University of Sheffield Centre for Glass Research in the U.K., told Live Science. "On the other hand, in a liquid, the atoms are all over the place and can rearrange easily."

Usually, when we think of a solid, we actually mean a crystalline material — something like salt, which has a regular and ordered structure. Below the melting point, this solid crystalline structure is the most stable form, and above the melting point, the liquid form becomes more stable. All materials want to adopt their most stable form, which is known as being in equilibrium. However, glass is unusual because it doesn't exist in this stable state.

Glass is made by melting various components to get a very viscous liquid, like syrup, which doesn't flow easily, Parker said. "Then, it's cooled quickly, but because it's so viscous, the atoms can't move around easily to rearrange into a more orderly solid structure, and they get trapped into disorderly arrangements," he explained. "The material is mechanically a solid but with a liquid-like disordered structure."

To reach equilibrium, the glass would need to "relax" back toward a liquid state. But at room temperature, the particles move so slowly that this just isn't practical. "This is what's called a metastable equilibrium," Mauro said. "It appears solid because when we're interacting with it, it's on a timescale that's much shorter than the relaxation behavior back towards a liquid."

For all intents and purposes, this means that although there may be incredibly slow changes occurring in the material — measured on a timescale of billions of years — glass behaves more like a solid than a liquid.

"In a practical sense, I would define glass as being a rigid solid, measured on any sensible timescale," Parker said.

Even the timescales of Europe's oldest cathedrals are insufficient to see panes of glass behave like a liquid. Indeed, there's a far simpler explanation for warped window glass, Parker said: "Some panes are thicker at the bottom than the top because 1,000 years ago, they couldn't make glass that was uniformly thick, and it either had to go that way up in the frame or the other way."