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There’s an often-repeated myth that glass is a liquid. In support, people cite old church windows that bulge at the bottom, saying they’re slowly flowing downward.

But you can trust your common sense on this one. Glass is not a liquid. It’s generally considered to be a solid, though with a molecular structure that’s a bit like a liquid.

But exactly what we mean by that is complicated, and how such a weird thing happens, is still a mystery... So to start with some basics, let’s talk about the difference between a liquid and a solid. Both are essentially collections of atoms or molecules.

The difference is that in a liquid, the bits are arranged higgledy-piggledy and able to move past each other. In a conventional solid, atoms or molecules are stuck where they are and arranged in stable, repeating configurations. We can see and even predict what position an atom or molecule takes in that pattern.

In many solids, like ice or diamonds, this stable, repeating configuration is called a crystalline structure. This includes substances we refer to conventionally as “crystals”. If you zoom in on a quartz crystal, you’ll see this repeating structure.

But it also includes a wide range of other substances, including many familiar solids. Water forms a crystalline solid at atmospheric pressure and zero degrees Celsius. It’s called “ice”.

If you melted the ice and then froze it again, it would crystallize again. Glass, on the other hand, is different. Glass is mainly made of sand, which consists of silicon dioxide, also known as silica.

And the silica in sand has a crystalline structure. When you superheat sand to make glass and it becomes molten, glowy-red and drippy, it’s a proper liquid, like liquid water. But as you cool it down, it doesn’t necessarily return to its crystalline structure.

Instead, it becomes an amorphous solid, which is a solid that lacks an ordered structure. In the kind of glass we use for window panes, the atoms of silicon and oxygen, plus any extra additions, are stuck where they are, like with a conventional solid. And they’re fused into a sprawling, interconnected network.

But that interconnected structure doesn’t have a regular, repeating long-range pattern. It looks disordered, like a liquid. And yet for all intents and purposes, it acts like a solid.

As for why this happens, and why glass doesn’t crystallize when it cools down, that’s actually still a mystery. Seriously, it’s one of the big unknowns in the physical sciences. And figuring out how glass forms has practical implications.

For example, scientists and engineers want to incorporate various metals into glass because metallic glass is extremely strong and durable. So it could be used to make medical devices, electronics casings and even parts of spacecrafts. But they haven’t mastered this ability yet because we don’t fully understand glass.

We know that a key step in glass formation is to cool the glass rapidly. If silicon and oxygen are allowed to cool down super slowly, you end up with those highly ordered quartz crystals rather than glass. But we don’t know exactly what’s going on during rapid cooling that keeps the molecules from falling into place.

And there are a couple of other strange twists we have to explain. It is technically possible that the atoms in glass do keep moving, as if they were liquid, albeit extremely slowly. Like, it may take millions of years to see any real change.

One estimate suggested it’d take longer than the age of the universe for windows to completely relax into puddles. It’s also possible that glass may, given extremely long time periods, convert from an amorphous solid to a crystalline one. So, the exact nature of glass is still hotly debated.

There are multiple definitions for what glass is, including some that do call it a kind of liquid. And there isn’t a single simple answer that explains why glass happens. The good news is that scientists are investigating the question.

In 2015, for instance, researchers in the UK and Japan published a paper that tried to use a computer to show how individual particles in glass might act as the glass is cooling down. They found that one group of particles relaxes slowly and organizes into geometric patterns, and another group relaxes rapidly and forms less stable structures. So now we know that glass particles can behave multiple ways as they’re cooling down, but that clearly isn’t a whole answer yet.

Then in January 2021, another group used simulations and experimental data to investigate what was going on as glass cooled from a liquid to an amorphous solid. They put a suspension of particles under a very powerful microscope and watched how their position and orientation changed as the scientists moved the suspension around. Their research shed some more light on the behavior of different groups of particles, but there are still a lot of mysteries to solve.

So, if you’re a materials scientist, best of luck to ya. But the rest of us can consider glass a solid. Just a very strange one.

Oh, and as for why those old glass windows bulge, the answer’s pretty simple. Back in the day when they were poured, it was really hard to make perfectly flat glass. So, when the panes were installed, the workers put the thicker, heavier side on the bottom, because that was more stable.

Thanks as always for watching this episode of SciShow. Thank you to our lovely editorial staff who put together this episode that answered so many questions that I have had for so long! And thank you also to our wonderful patrons for helping us bring this information to you, and also to me.

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