Cork is essentially a piece of bark from an oak tree known as cork oak, Quercus suber. The cork tree grows naturally in a region bordering the western Mediterranean Sea. The major cork producing countries include Portugal, Spain, France, and Italy in Europe; and Morocco, Algeria, and Tunisia in Africa. Several efforts have been made to grow this species in other parts of the world; but, so far, the results have not been encouraging. Worldwide production of cork is estimated to be 3,075,000 tons. Of this amount, Portugal produces the lion’s share (about 55%).
Why use Cork as a Wine closure
Cork is a unique substance and a long proven closure for wines. The cork possesses many remarkable qualities which make it an ideal stopper for wine bottles. Some of the important qualities include: compressibility, resilience, impermeability to liquids, low density, little tendency to rot, and a high coefficient of friction.
The life cycle of cork as a raw material starts with the extraction of the bark from cork oaks, the so-called harvesting or stripping which is carried out during the most active stage in the annual growth of the cork, from mid-May or early June to the end of August.
However not many people know that it takes 25 years for cork oak trunk to start to produce cork and be profitable. Each trunk has to reach a circumference of 70 cm when measured at 1.5 metres from the ground. From then on, the cork can be harvested from the tree for on average 150 years.
The first stripping, which is known as “desbóia“, produces cork of a very irregular structure which is too hard to be easily handled. This is the so-called virgin cork which will be used for applications other than cork stoppers (flooring, insulation etc.), since its quality is far from that necessary to manufacture stoppers.
Nine years later, the second harvest produces material with a regular structure, less hard, but still not suitable for cork stoppers – this is known as secondary cork.
It is from the third and subsequent harvests that the cork with the best properties is obtained, suitable for the production of quality corks, since its structure is regular with a smooth outside and inside. This is the so-called “amadia” or reproduction cork. From then on, the cork oak will supply good quality cork every nine years for around a century and a half, producing, on average, 15 to 16 bark strippings throughout its life.
The stripping of the cork oak is an ancient process that can only (and should only) be done by specialists, the debarkers, since much manual skill and experience is required in order not to harm the tree. The stripping process consists of five steps:
1. Opening. A vertical cut is made in the cork, choosing the deepest crack in the cork bark. At the same time, the edge of the axe is twisted so as to separate the outer from the inner bark. The degree of difficulty of extraction can be gauged from the ‘feel’ of the axe. When the edge of the axe is applied to the strip, a hollow sound of tearing is heard if the cork is going to come off easily. If it is going to be difficult, the axe gives off a short, firm, dry sound.
2. Separating. The plank is then prised off the tree, by inserting the edge of the axe between the strip and the inner bark. The axe is twisted between the trunk and the cork strip to be extracted.
3. Dividing. A horizontal cut defines the size of the cork plank to be removed and what is to remain on the tree. During dividing, the inner bark is frequently marked and these mutilations can sometimes alter the geometry of the trunk.