How is cork harvested?
The harvesting of cork consists of stripping the outer bark of cork oaks. The best time for bark stripping is at the most active phase of the cork oak's annual growth: from May to August.
The first harvest produces cork of a very irregular structure. This is called “virgin cork”.
Nine years later the second harvest brings “reproduction cork” – a material with a more regular structure, less hard, but still not suitable for cork stoppers. Reproduction cork is usually granulated for use in products such as flooring.
It is from the third and subsequent harvests that the cork with the best properties is obtained - the “amadia cork” - and from this time, the tree will provide good quality cork, harvested at a minimum of every nine years for about 150 years.
The stripped area, known as the “mother,” changes from a rose color to red ochre, then a reddish brown, and the following year to a grey, crust-like formation. Loggers use a special axe (machado) for the harvest. The blade is used to make the incision while the end of the handle is shaped to detach the cork.
How is the harvest regulated?
To keep the trees in good health, government laws regulate the harvesting of cork oaks. In Portugal, trees are harvested in cycles of not less than nine years. Calendar years are painted on the bark to monitor when a tree was last stripped.
How is cork stripped from a tree?
The delicate operation of stripping cork has been performed in the same way for decades. Today, cork stripping with a special axe continues to be the quickest, cleanest and most tree friendly method available.
The stripping process consists of five steps:
What happens after harvest?
After harvest, the cork planks must stabilize. They are sorted according to their future use as natural cork stoppers, discs, or agglomerated cork products, depending on their quality.
The selected planks are then stacked in piles to be exposed to sun, wind and rain for six months or more. During this seasoning period, the elements purge most of the sap from the cork, the polyphenols are oxidized and the cork texture stabilizes.
After stabilization, the cork planks are boiled in clean water for at least one hour. All cork must be boiled before it is worked to make it more pliable, and to fully expand the lenticels.
The cork cells are collapsed and wrinkled before boiling, but after boiling, the gas in the cells expands and creates a very tight, more uniform cell structure. This hot water process makes the cork increase its volume by about 20 per cent, and become flatter and smoother.
The boiling operation - a standard procedure defined by the International Code of Cork Stopper Manufacturing – extracts polyphenols and also ensures that microflora is significantly reduced. Once the boiling is finished, the cork planks are dried and left to rest in warehouses with controlled humidity and temperature for three weeks. Cutters then trim the edges to make the planks rectangular.
The trimmed cork planks are sorted into various thicknesses and qualities, depending on a number of criteria, including porosity.