Com­ment & Anal­y­sis

“THE POP AND FIZZ OF CHAM­PAGNE ARE EX­CIT­ING, BUT THE CORK IT­SELF HAS A STORY TO TELL TOO”

BBC Earth (Asia) - - Contents - Dr He­len Cz­er­ski is a physi­cist and BBC pre­sen­ter. Her lat­est book Bub­bles will be avail­able later this year

He­len Cz­er­ski on cham­pagne corks

There 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, go and find the cork. The pop and the fizz are ex­cit­ing, but the cork it­self has a story to tell too.

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