How Do They Put The Cork In A Champagne/Wine Bottle?

A champagne cork starts out as a straight cylinder roughly 31 mm in diameter — much wider than the ~18 mm bottle neck. A corking machine softens and compresses it to about half its diameter, drives it into the bottle, then locks it down with a wire cage (muselet) to hold back the 5–6 bar of pressure inside. Once trapped, only the part that never entered the neck keeps its full width — which is why a popped cork looks like a mushroom.

Uncorking a champagne bottle is more than the simple act of removing a bottle cap; it is, in fact, a tradition of symbolic significance.  Whether it is a sporting ceremony, a company celebration, or a social gathering, it is fairly common for the host to uncork a champagne bottle to mark an important event.

If you have ever been physically present at the uncorking of a champagne bottle, you might have noticed that the cork comes out of the bottle with an impressive pop; sometimes, it even flies out with great force, potentially injuring someone in its rapid flight. The question is, why does the cork jump out? More importantly, how do they cork the bottle at all?

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Why Do Champagne Corks Pop?

You may already know that champagne/sparkling wine bottles are under a significant amount of pressure, which makes the cork pop out once you loosen it or remove the wireframe (called a muselet) keeping it in place. However, where does this pressure come from? As you may suspect, it has everything to do with the contents of the bottle. In the méthode traditionnelle, a measured dose of sugar and yeast is added before the bottle is sealed, kicking off a second fermentation right inside the glass. The carbon dioxide produced has nowhere to escape, so it dissolves into the wine under pressure.

Since these are special yeasts that are highly tolerant to alcohol, they keep cranking out CO₂ until the sugar runs out. With no outlet, the pressure inside the bottle climbs to roughly 5–6 bar (about 75–90 psi) — close to three times the pressure in a car tyre. That’s why the cork has to be wired down, and why, when the bottle is uncorked, the pressure is released with enough force to launch the cork across the room with that iconic pop.

How Do They Get The Cork In The Champagne Bottle In The First Place?

There is an enormous pressure built up in the bottle, ready to shoot past anything that tries to prevent it from erupting, so how do they cork the bottle with so much force that works against it? Also, you have certainly observed that the corks in normal wine bottles seem to be larger than the holes they fill. So how do they get there?

The corks of champagne bottles are famous for their elasticity and compressibility. They’re made from the bark of Quercus suber, the cork oak — a Mediterranean evergreen with unusually thick, spongy bark that is harvested without killing the tree. At the microscopic level, cork is mostly air: each cell is a tiny sealed pocket inside a flexible suberin wall, which is why the material can be squeezed down to a fraction of its size and still spring back.

You may be wondering how to get a cork with a larger diameter into the opening of a bottle with a much smaller diameter.

Here’s the trick: before insertion, a champagne cork is a straight cylinder roughly 31 mm wide — well over the ~18 mm bore of the bottle neck. The corking machine first softens the cork (with dry heat or a brief microwave pulse), then four mechanical jaws clamp around it and squeeze it down to about half its diameter — small enough to slip into the neck. A plunger immediately drives it into place, and the wire muselet is twisted on top to hold it against the pressure inside.

Once trapped, the lower portion of the cork can only expand back as far as the neck allows, so it stays compressed at ~18 mm. The upper portion — the “head” — never enters the neck at all, and keeps its original ~31 mm width. After months or years under that constant squeeze, the cork only partially rebounds when pulled, which is why a popped champagne cork comes out shaped like a mushroom: a wide cap on top and a narrower stem below. The cork itself is also a two-piece affair: a body of agglomerated cork granules with one to three discs of natural cork at the bottom, the only part that actually touches the wine.

Since uncorking a bottle releases enormous pressure, it is best to point the bottle away from everyone present (including yourself, obviously!).