The unique physical properties of natural cork have made it an ideal way to seal wine bottles since the Roman days. Let's look at why cork is so useful and then where it comes from.
Cork has a unique honeycomb cell structure. Each cell is sealed, filled with air and not connected to any other cell.
Cork is elastic. Cork can be compressed and when released will return to its original volume. This comes from the cell structure. As the cork is compressed, the air in the cells is compressed. When pressure is removed the cork expands back to its original volume.
Cork is waterproof. This again comes back to the wonderful cell structure. Individual cells are sealed so they're not connected to each other. No capillary action or wicking can happen in cork.
Individual cells filled with air are very poor conductors of heat and vibration. This makes cork ideal for flooring and sound proofing. Not that we care about this in wine bottle corks but a huge number of wine bottle corks are re-cycled into floats, gaskets and flooring each year.
Cork is lightweight. It has a low density because the cells are all full of air. Cork has been used for fishing floats almost as long as it's been used for wine bottle stoppers.
Cork is naturally fire resistant. That doesn't mean it's fireproof. It will burn if heated long enough.
And finally, cork has a high friction surface. That is it stays in the bottle. As the cork is cut, millions of cells are ruptured. This forms rough domes that can seal onto a smooth surface such as a glass bottle neck.
But where does cork come from and how is it made?
Natural Cork is harvested from the living bark of the Cork Oak (Quercus Suber L.). It is harvested in a regular cycle over the lifetime of the tree. Most of the worlds cork comes from the Mediterranean countries. Cork is grown in Asia as well but only cork from Mediterranean countries is considered of high enough quality to be used in wine bottle stoppers.
The first harvest of virgin cork comes when the tree is about 25 years old. More 'reproduction' cork can be harvested in the next 9 to 12 years. Then another 9 to 12 years later comes the first harvest suitable for wine bottle corks. From the time you've planted the tree till the first wine bottle cork is harvested is around 40 years so it's not a get rich quick scheme.
Cork bark is stripped from the trees during spring or early summer. At this time there's plenty of sap flowing and bark comes away easily. There are laws regarding when each tree can next be harvested. Portugal allows harvests every 9 years while the island of Sardinia stipulates 12 years between harvests. Big white numbers are painted onto the bark to know when to next harvest the cork.
The oldest and most productive cork tree on record is the Whistler Tree in Portugal. The countless birds living in the tree's branches led to the name; the Whistler Tree. The tree is over 213 years old and has been producing cork since 1820. Each harvest produces cork for over 100,000 wine bottle corks. Not bad when you consider the average tree accounts for around 40,000.
Cork is big business. The industry employs around 30,000 people worldwide and is worth around $AUSD 3billion each year. Wine bottle corks account for only 15% of cork production by weight, but almost 65% of value. This is spread over an estimated 13 billion wine bottle stoppers per year.
* Reproduced with permission from Peter Svans at The Gurdies Winery