Why glass is a frozen liquid that doesn't actually flow

HostIf you look through a hand-blown window from the eighteen hundreds, the world looks a little bit wavy. It's almost like you're looking through water that has been frozen right in the middle of a ripple. I always grew up hearing that this is because glass is actually a very slow-moving liquid, and over a hundred years, it just starts to slump. Is that what's actually happening?

GuestThat's one of those science stories that sounds right because it matches what we see, but the real story is much stranger. To understand glass, you first have to look at what it's not. If you look at a true solid, like a grain of salt or a diamond, the atoms inside are lined up in a perfect, repeating grid. Scientists call that a crystal lattice. It's like a huge room where everyone is sitting in their own assigned seat in neat rows. A liquid is the total opposite. It's a messy, moving crowd where everyone is just bumping into each other. Glass is a weird hybrid. It has the messy inner build of a liquid, but the stiffness of a solid. When glass is made, the hot, melted material is cooled down so fast that the atoms don't have enough time to find those neat seats in the grid. They get caught in a permanent state of mid-motion chaos. Scientists call this an amorphous solid.

HostSo it's like the music stopped in a game of musical chairs and nobody actually got to a seat?

GuestThat's a great way to put it. They're just stuck wherever they were standing. Most materials have a very clear melting point. If you have an ice cube, it stays solid until it hits thirty-two degrees, and then it turns to water. But glass doesn't do that. Instead, it goes through what we call a glass transition. As the liquid glass cools, its viscosity—which is just its resistance to flow—shoots up at a crazy rate. It becomes trillions of times thicker than water. This glass transition isn't a change in the state of the matter like ice turning to water. It's more like a kinetic freeze. The material becomes too thick and heavy to move before it ever has the chance to turn into a crystal. It's technically a non-equilibrium material. That just means it's always trying to become a crystal, but it's stuck in time because the atoms are moving too slowly to ever get there.

HostOkay, I think I follow the science of the atoms, but I have to push back on the window thing. If you go to an old house or a cathedral, you can see that the glass is thicker at the bottom. If it's not a liquid flowing down over the years, why is it so uneven?

GuestThe math just doesn't back up the flow theory. If you look at how thick and stiff glass is at room temperature, it would take much longer than the current age of the universe for a window pane to flow enough for us to even measure it with a ruler. If you sat and watched a window for a billion years, it might shift by the width of a single atom. You would be waiting a long time for a very small change. The reason those old windows look like they have pooled at the bottom is actually because of how they were made. Before we had modern ways to make glass, like floating it on a bed of melted tin, people used something called the crown glass method. A glassblower would take a big glob of hot glass and spin it into a large, flat disk.

HostLike a pizza chef spinning dough to get it flat?

GuestExactly like that. But because of the force of the spin, the edges of that disk were always naturally thicker than the center. It was impossible to make the whole thing the same thickness. When it came time to cut that disk into squares for windows, the panes were always uneven. One side was thin and the other side was thick. The workers who put those windows in, the glaziers, would almost always install the panes with the thick edge at the bottom. They did it for sturdiness and to make the window sit better in the frame. It was a choice made by the builder, not the glass moving on its own. It just creates a perfect illusion of a liquid that has settled over the centuries. We have actually tested pieces of glass from ancient Rome that are two thousand years old, and they show no signs of thickening at the bottom at all.

HostSo the waves I see aren't from the glass moving over time, they were born that way.

GuestThose waves and ripples are basically a frozen map of the moment that glass was spun and cooled hundreds of years ago.

HostThe world looks watery through those old panes because we're looking through a spinning disk that got stuck in a traffic jam before the atoms could ever find their seats.