Corking plays a critical role in guaranteeing the quality of the finished wine. The cork must be inserted into the bottle smoothly and evenly to ensure an airtight seal. It should be designed for easy extraction with only minimum effort, expanding once pulled to assume the famous mushroom shape. Only natural cork has the properties required to achieve this sealing capability— exclusively used by all Champagne Houses for the production of their cork stoppers. .
It takes about 45 years for a cork oak tree to provide its first useful crop of oak – a layer of corky bark with the right thickness for production of a wine stopper. While the tree is ready for harvesting from 35 years onwards, these first layers of cork are too fissured to be of any use.
This is true for conventional wine corks and even more so for Champagne corks, which are unusual in that they have to withstand a tremendous amount of pressure— around 5-6kg per square centimeter, or nearly three times the pressure inside a typical car tyre. To retain that pressure, Champagne corks start off with a diameter of 31mm, which is 13mm wider than the internal diameter of the bottle neck! And that’s where it gets difficult, because it is practically impossible to cut a cylinder shape as wide as this out of a layer of cork from a 45-year old tree.
Older trees do of course have thicker bark, but with increased size comes a coarser, more fissured trunk.
The only solution was an entirely new production process. To make a cylinder with a 31mm diameter,the cork is ground into fine particles using a precisely calculated granulation process. These particles are then bonded together with a specially-developed glue — which had to be water and pressure resistant and able to withstand any amount of pulling and twisting! It was this glue that led to the adoption of the agglomerated (composite) cork used for Champagne. The bottom section of the cylinder, the end in contact with the wine, is formed of two stacked and glued discs of pristine cork with a diameter of 31mm and a thickness of 4mm.
"The number and size of the discs are specifically designed to ensure an appropriate distribution of the load throughout the cork", stresses Marc Sabaté. Not content to rest on its laurels, cork stopper production is the focus of continuous quality improvement. Five years ago, for instance, the practice was to soak corks in hot water for about an hour to make them easier to insert into the bottle. Today’s manufacturers use food-grade silicone instead — for its easy-sliding property and also for its adhesive quality once the cork is in place.
Switching to silicone is particularly clever since, on the one hand, it speeds up the rate of corking and on the other hand, it reduces the loss of carbon dioxide and liquid by promoting good adhesion between glass and cork.
The cork must not exceed a length of 48mm and a diameter of 31mm, with a maximum tensile strength of no more than 25kgF. The top or main body of the cork, called the manche, is composed of agglomerated cork granules. The bottom portion, in contact with the wine, is called the miroir and consists of one, two or three, 6mm-wide discs of supple, natural cork .
The part of the cork that will be inside the bottleneck must display the name of the Champagne appellation and in some cases the vintage too. The name of the producer or a five-pointed star with a long tail, usually appears underneath — symbol of the 1811 Comet Vintage that coincided with the appearance of the great comet of 1811. The star is also displayed at Christmas time when sparkling Champagne really comes into its own.
Natural products like cork have their risks, in this case the risk of cork taint — a fault that threatens all wines and sparkling Champagne is no exception. Some cuvees are particularly vulnerable, requiring each batch of wine to be approved separately following statistical quality control. There is hope however that new technologies may make cork taint a thing of the past .
One particularly interesting avenue of research focuses on the use of silicone-based technology to create a coating on the wine-facing end of the cork — so preventing cork taint and also leakage. Given the working title préserveur de déviations sensorielles, the project is the brainchild of French Company, Cortex and though still in the experimental stage is already showing encouraging results.
Silicone-based materials have made great strides in recent years and are now able to deliver the same performance as cork in terms of impermeability (an airtight seal) and also breathability (the micro-oxygenation essential for proper aging). Synthetic corks made from polyethylene now fulfill exactly the same function as cork in this respect, with no difference in quality.
Another innovation is the process used to soften the suberin — the resinous, heat-sensitive constituent of cork. Steeping in tanks is increasingly replaced by dry heat methods (bouchage à sec) such as hot air blowers and microwave ovens that make the cork more pliable.
Once softened, the cork is hammered halfway into the Champagne bottle, compressed to almost half its diameter (around 17mm) and forced into a mushroom shape. It s now ready for muselage: the fitting of the wire cage or muselet that holds the cork in place.
NB: EU regulations covering sparkling wines and Champagne wines alike state that "bottles must be closed with a mushroom-stopper made of cork or any other material permitted to come into contact with foodstuffs".
Compulsory or not, with the exception of certain small quarter bottles, all Champagne bottles are sealed with natural cork.
The muselet consists of a wire cage with a protective metal cap displaying the name of the producer and sometimes the House logo. It fits snugly over the cork, holding it securely in the bottle neck against the force of the pressure. The fitting procedure is fully automated and carried out by a pneumatic machine called a museleteuse.
 It was Champagne, indeed, that led to the birth of the cork stopper. It all goes back to the 18the Century when a now famous French Benedictine monk called Dom Pérignon noticed that the bottle stopper in use at the time — a wooden bung wrapped in oiled hemp — kept popping out under pressure. Having discovered the superb elasticity of cork while on a trip to Catalonia, he decided to use a cork stopper instead. An invention that marked a new era in bottling and corking.
 Cork Quality Control
Producers and suppliers of Champagne corks are required to submit their corks for laboratory testing by the CTCPA. All corks must be formally approved by the Comité Champagne before they can be sold to Growers and Champagne Houses