Helen Czerski
This month, Helen takes a closer look at the shape of champagne corks.
T here was never a conscious decision to start a collection. But I’m a bubble physicist, so I notice champagne (usually as a toy first and a drink second), and some years ago I started picking up the corks. I put a couple of them in a bowl in my living room, with the matches and tealights and other such miscellany, and every so often another one would come along. Eventually, the corks outnumbered everything else, and the rest got booted out. I looked at them properly for the first time this week, and wished it hadn’t taken me so long to appreciate them. Form and function are so beautifully intertwined, and it all starts with an obvious question: why do they have that peculiar mushroom shape?
Champagne corks are distinctive, and it’s all down to the bubbles in the drink. The pressure inside a bottle of champagne is around six times atmospheric pressure, and the small region of gas at the top of the bottle is entirely filled with carbon dioxide at that pressure. But there’s even more carbon dioxide dissolved in the champagne itself – you would need a five-litre balloon to hold it all – and the high pressure is essential to keep it there. You can’t afford to let gas leak through the cork, because if the pressure drops, you’ll have flat champagne. And the cork also has to stay put even though there’s all that pressure pushing outwards on it.
The solution is a brilliant natural material from the outer layer of the cork oak. It’s got a honeycomb-like structure made of tiny empty cells with zigzag walls. If you squish it, the walls will compress like a concertina and then recover their shape when you let go. Champagne manufacturers take a cylinder with a diameter about 1cm greater than the neck of the bottle, squash it inwards until it’s slender enough to fit, and then let go. The section sticking out from the top of the bottle springs back to its original diameter, and the bit inside the neck springs back as much as it can, pushing strongly outwards and sealing the bottle. This does a pretty good job of sealing the gas in, although a little does escape – an older champagne has about 25 per cent less gas in it than one 15 years younger. You can recover the cork’s original shape if you put it in boiling water.
But a champagne cork that’s been removed from the bottle has a waist – it’s wider at the base than in the middle. Why?
When I looked at my corks, I noticed that they all have the same structure. They’re made from lots of tiny bits of cork all compressed together to form a conglomerate, but they have two whole disks of cork at the bottom, each about 6mm thick. These are the highest quality natural cork, and you can see dark channels running along the sides of the disks, showing themselves as dots at the bottom. These are lenticels, and they run from the inside of the tree to the outside to let it breathe. When you cut the disks this way, they are extremely springy sideways – fabulous for sealing the wine in. But the channels are a problem. So the manufacturers stack two disks on top of each other. This means they keep the springiness, but the channels don’t line up so gas can’t escape. And this is the reason for the mushroom shape. When the cork escapes from the bottle, the disks return to pretty much exactly their original diameter. But the conglomerate cork above isn’t as elastic, and stays compressed.
So if you hear a ‘pop!’ anywhere near you this winter, go and find the cork. The pop and the fizz are exciting, but the cork itself has a story to tell too. Happy 2018!