Cork is harvested from the bark of Cork Oak (Quercus suber) trees that are most commonly found in Portugal, Spain and Algeria. The bark provides the tree with some protection against wild fires. If these fires pass through relatively quickly, burning waste material on the ground, then the natural fire resistance of cork, combined with its high insulation properties, protects the tree.

Just like the trunk of a tree, the bark grows with the seasons, thus creating growth rings. A mature tree normally produces bark that is thick enough for the production of bottle stoppers every nine years (see Figure 1). Only half the bark is removed from the trunk because the underlying cambium, which is the living part of the tree, is killed (see Figure 2). Therefore, a mature tree will be harvested for bark every four to five years. If all the bark were to be removed, then the tree would die.

The planks of harvested cork are dried in the open air for about six months. They are then boiled in water for a few hours. This cleans and sterilises the bark but also the high temperature causes the bark to expand so that it develops the soft, resilient feel that we know. The planks are then cut into strips and the stoppers are punched out by experienced operators.

Cork is ideal for sealing bottles because it is impermeable to liquids and has a very low permeability to gases, too. These two properties allow bottles of wine to be aged for decades without liquid loss but with some gas exchange to allow flavours to develop.

Another reason why cork is good for stoppers is that it has an unusually low Poisson’s ratio for a low density, organic material. What this means is that when a cork is compressed, its sides do not bulge out much (try it the next time you open a bottle). Therefore, a cork with the same diameter as the opening of a bottle can be pushed in relatively easily.

What is clear from Figure 1 is that there is a lot of solid cork waste after the stoppers have been punched out. This explains why there are so many cork composite products around, including: champagne corks, flooring tiles, underlay, damping products and many more.

Obviously, a range of particle sizes is produced when cork is ground into particles. In addition, the density of cork decreases from the outer to inner bark. Therefore, two cork particles of the same size might have very different densities. Cork processing companies use an ingenious series of inclined vibrating tables to sort the particles by size and weight simultaneously. The higher density particles tend to have better mechanical, but poorer thermal insulation, properties.

The next time you open a bottle of bubbly, take time to admire the technology in the wood based composite used to seal that bottle – in other words, stop-per and think!