A cork tree must grow for 25 years before it can be stripped for the first time. So as not to damage the tree’s ability to regenerate its bark, cork is extracted during the tree’s active growth season. Cork trees are only harvested during the summer, between May and August. This is when the tree is going through its most active growth phase, which makes the bark much easier to strip without damaging the cork tree trunk.
How often can cork be harvested?
A cork tree can be harvested every 9 years after its first 25-year growth period. A cork oak can be harvested 12 to 15 times during its lifespan of approximately 150 years.
It is only after the third harvest of cork, known as Amadea, that the Amadea Akes have the high standard of quality required to produce cork stoppers. The first two harvests are used to make flooring, insulation and other cork products.
How is cork made?
After harvesting, the cork bark is stacked outdoors to age for at least half a year. A combination of air, sunlight, wind and rain alters some of the chemical properties of the cork, thereby improving the quality of the cork. The aging process removes moisture from the cork bark, changing it to be level and lighter.
The cork boards are then steamed in a boiler, killing any contaminants or bugs. This process also separates the outer layers of the bark and softens the cork bark, making it easier to work with.
3 different stages of cork harvesting
The first harvest is carried out on trees between 25 and 30 years old and about 24in (60 cm) in circumference. The cork oak must be about 20–25 years old before its bark, called “virgin” is removed for the first time. Because it is particularly hard and irregular, it is used for insulation purposes or for flooring, since its quality is far from that required to manufacture stoppers
The second harvest takes place nine years later. This harvest produces the first breeding cork. A second extraction of cork is called "secundeira" (The Many Uses of Cork). Although not as hard as cork from the first harvest, its quality is still not enough to be used to produce closures.
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 exterior and interior.
This is the so-called "amadia" or breeding cork. From then on, cork oak will supply good quality cork every nine years for about a century and a half, producing an average of 15 bark harvests throughout its life.
Finally, not all cork can be used as raw material for the production of cork closures. Only female cork is of a high enough standard. In the same tree, quality improves over time.
The Cork Harvest
To become the raw material we are familiar with, cork must be extracted from cork oak. The process involves removing the cork from the tree without having to fall. This process is known as cork harvesting or bark removal.
Harvesting cork relies on a time-honored set of skills that must comply with many rules. To harvest the crotches methodically follow six steps：
A vertical cut is made into the cork, choosing the deepest crack in the cork bark. At the same time, the edge of the axe is twisted to separate the outer and inner bark. The degree of difficulty of extraction can be measured by 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. Step 1.Opening
The plant is then separated from 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.
A horizontal cut defines the size of the cork plant to be removed and what remains on the tree. During division, the inner bark is frequently marked and these mutilations can sometimes alter the geometry of the trunk.
The plant is carefully removed from the tree so that it does not split. The larger the planks extracted, the greater their commercial value. The removal of all planks depends on the skill of the workers. After the first plank has been stripped, the operation is repeated over the entire trunk.
After stripping the planks, some fragments of cork remain attached to the base of the trunk. To remove any parasites from these "wedges", the decorticator gives them a few taps with his axe.Step 5.Removing
Finally, the tree is marked with the last number of the year in which it was harvested. Step 6.Marking