THE CORK HARVEST
To become the raw material we are familiar with,cork must be extracted from the cork oak.The process involves removing the cork from the tree without having to fell it.This process is known as the cork harvest or bark removal.
Harvesting cork relies on a time-honoured set of skills that must comply with many rules.
When is the cork harvested?
So as not to damage the tree’s ability to regenerate its bark,or even kill it if the wrong techniques are used,cork is extracted during the tree’s active growth season.Hence,the cork harvest takes place between mid-May and the end of August，when the cork can be separated from the tree without causing permanent damage.
A latent period lasting at least nine years between harvests is required to that the bark reaches the right thickness.
It takes 25 years for a cork oak trunk to start to produce cork and be profitable.Each trunk has to reach a circumference of about 70 cm when measured 1.3 metres above ground level.From then on,As a cork oak lives for approximately 150 years,this means that each tree can be harvested twelve or up to fifteen times.
How is the cork harvested?
In addition to the season and the requisite rest time,the cork harvest requires a high level of expertise.Skilfulness is of the essence.
The workers who specialize in removing the cork are known as extractors who use a very sharp axe to make the cuts on the tree.They therefore handle their axes with a lot of care and precision.They often have substantial experience in carrying out this tricky procedure.
To harvest the cork,they methodically follow four steps:
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.
The plank 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 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.
The plank is removed from the tree with care so that it does not split.The larger the planks extracted,the greater their commercial value.The removal of entire planks depends on the skill of the workers.After the first plank has been stripped,the operation is repeated over the whole trunk.
After the stripping of the planks,some fragments of cork remain attached at the base of the trunk.To remove any parasites in these "wedges",the decorticator gives them a few taps with his axe.
Finally,the tree is marked,with the last number of the year in which it was harvested.
Rest period.After the harvest,the cork planks are stacked in piles either in the forest or in yards at a factory.There they remain exposed to sun,wind and rain.All the piles are constructed taking into account strict specific rules defined by the International Code of Cork Stopper Manufacturing Practice,so as to allow the cork to stabilise.The piles should be stacked on materials that do not contaminate the cork and prevent contact with the soil.Wood,for example,is expressly prohibited because it can transmit fungi.During this seasoning period,the raw material matures and the cork stabilises.
The different types of cork oak harvests
The life cycle of cork as a raw material begins with extraction of the cork oak bark.Throughout its life,a cork oak is harvested several times.
Depending on the age of the tree,the cork does not have the same characteristics.The thickness of cork is generally 4 to 5 cm,and high-quality cork can be 8 to 9 cm thick.Cork can be harvested from a single tree 12 times in its lifetime,from which is periodically removed without harming the tree,usually every 9–12 years (depending on the culture region),to assure the cork layer reached the min required thickness.
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 cork,” is removed for the first time.As it is particularly hard and irregular,it is used for insulation purposes or for flooring,since its quality is far from that necessary to manufacture stoppers.
The second harvest takes places nine years later.This harvest produces the first reproduction cork.a second extraction of the cork is called “secundeira” (The many uses of cork).Although not as hard as cork from the first harvest,its quality is still not sufficient for it 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 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 bark harvests throughout its life.
The cork harvest therefore requires patience and dexterity.Seasoned cork harvesters capable of handling an axe in such a way that it does not damage the cork oak are needed.As the cork can only be harvested every nine years,it is essential that the trees be properly cared for.Lastly,not all cork can be used as the raw material for producing cork closures.Only the female cork is of a high enough standard.
In the same tree,the quality improves along the time.