Storm in a Teacup: The Physics of Everyday Life(55)

It started with the pool of molten glass that sat in a small furnace, glowing bright orange because it was at the horrific temperature of 2,000°F. Protected with Kevlar gloves, we obediently poked long iron rods into the pool and twisted them, so that glass with the consistency of honey wound on to the iron as we twirled it. That was the easy bit. The hard bit was everything else. Glassblowing is about controlled coaxing, and there were three main forms of persuasion that we could apply. Heating the glass up makes it softer. Holding it still lets gravity conveniently pull it downward without your having to touch it. And if the iron rod is hollow, you can blow bubbles in the molten blob.

We took turns at practicing all three, and the astonishing thing about glass is how quickly its nature changes. As the molten blob comes out of the furnace, you have to keep spinning the iron because it really is liquid; stop twirling and it will just drip onto the floor. A couple of minutes after that, we could roll the blob along a metal workbench and it felt as though it had the consistency of modeling clay. Only three minutes later, you could tap it on the bench and hear it go “ting,” just as you’d expect a solid glass object to behave. The fun of glass is that you’re manipulating a liquid, playing with the smoothness and curviness that liquids offer. A solid, cold bit of glass is just a liquid that was interrupted, frozen in time like a fairy-tale character.

Glass gets its character from the way its atoms move around each other. The most common form of glass (and what we were practicing with) is soda-lime glass. It’s mostly silica (silicon dioxide, SO2, which makes up the majority of sand), but it’s also got sprinkles of sodium, calcium, and aluminum in it. What makes a glass distinctive is that instead of the atoms having specific places in a regular lattice, they’re all jumbled up. Each atom will be linked to the ones around it, and there won’t be too much free space, but it’s all quite disorderly. As the glass is heated up, the atoms jiggle about more, moving apart ever so slightly, and since they weren’t in strictly regimented positions to begin with, it’s quite easy for them to slip past each other. The molten glass that we took from the furnace was made of atoms with loads of heat energy, and they would easily slither over each other as gravity pulled them downward. But as it cooled in the air, the atoms would move a bit less, settling slightly closer to each other, and the liquid became more viscous.

The clever bit about glass is that as it cools down, there isn’t enough time for the atoms to move into an egg carton–like regular pattern. So they don’t. Glass becomes solid when the atoms are just too sluggish to move over each other anymore. It’s quite hard to say exactly where the line between liquid and solid really is.

The first task was to make a bauble each, which turned out to be a posh description for blowing a bubble of glass and then watching the teacher attach a loop of molten glass to the top. Blowing the bubble was hard work; my cheeks hurt afterward as though I’d just inflated a particularly stubborn balloon. The most delicate part of the process is right at the end, when the final piece of glass needs to be separated from the iron rod. You pull and shape the glass so that there’s a thin neck where you want it to break. Then you file that neck to introduce tiny cracks. And then you take it over to what was entertainingly called the “knock-off bench,” tap on the iron ever so gently, and the glass bubble breaks off. It all worked perfectly—until we were on the last one, when the newly introduced cracks weren’t going to wait. The final bauble dropped off the end of the rod just as it was being finished, hit the concrete floor, and bounced. Twice. The teacher quickly scooped it up, and it was fine. But this delicate membrane of glass had bounced. And apparently if it had fallen just a minute or so later when it was just a little bit cooler, it would have shattered.

This is the lesson of glass. The way its atoms behave depends on its temperature. When it’s hot, the atoms can flow freely over each other. Cool it just enough not to be sticky, and the atoms can press together and rebound so that the glass can bounce. A little bit cooler than that, and the atoms really are frozen in place. Any atom that’s pushed slightly out of place opens up a crack in a fragile brittle solid, and the glass can be smashed into sharp smithereens.

Glass is intensely satisfying because it captures the curvy beauty of a liquid without your having to worry about where the liquid is going. It has the atomic structure of a liquid—a fairly disorganized mob—but it’s definitely a solid. The bouncing is a giveaway: Elasticity is something that solids have and liquids don’t. And you can see the consequences of that structure in how the material behaves as the temperature changes.

This might be the time for a bit of myth-busting about old glass windows. It has sometimes been said that the reason that 300-year-old windows are thicker at the bottom than at the top is because the glass has flowed downward over time. This isn’t true; window glass isn’t a liquid and it isn’t flowing anywhere. It’s because these window panes were made using an incredibly ingenious method. A molten glass blob was stuck on an iron rod, and the rod was spun very quickly until the glass flowed outward into a flat disk.? This disk was cooled, and cut up to make window panes. The downside of this method is that the disk will always be thicker closer to its center. So the diamond-shaped window-pane pieces were cut with the thicker bit at one end, and when it was put into the window, the thicker end was often placed at the bottom to help rain run off. The glass didn’t move itself downward, it was put there.