Cork is an impermeable and buoyant material that is most often used to make wine stoppers. It can also be used to make woodwind instruments, baseballs and bulletin boards. But what is it, and what tree does cork come from?

What tree does cork come from?

Cork is actually the bark from the cork oak (Quercus suber), an evergreen tree native to the Mediterranean area (southwestern Europe and northwestern Africa). While it can grow to be up to 66 feet tall (20 meters), it is usually shorter than that. The thick bark evolved to protect the tree from forest fires.

Cork oaks generally live for two centuries. People generally start harvesting cork from the tree, when it is around 25 years old. Virgin or male cork is cork taken during that first harvest. Since it takes time for the bark to grow back, people will only harvest cork from a given tree once every 9 – 12 years, and they will mark the tree with a number to record the year of harvest.

Similarly, people will harvest cork from the same tree about a dozen times during its lifetime. An 80-year-old tree, on average, can produce 40 to 60 kilograms (88 to 132 pounds) of cork.

How is cork oak harvested?

Harvesting cork is an environmentally sustainable process, for no trees are cut down. In fact, it is illegal in Portugal to cut down cork oak trees without a permit from the Ministry of Agriculture. Farmers will need such permission to chop down unproductive and old trees. Cork harvesting is typically done in the late spring or summer when the bark can be most easily stripped off the tree.

Cork strippers or tiradors earn between €80 and €120 ($92.92 and $139.39) per day, making them among the highest paid agriculture workers in the world. They also receive worker’s compensation and health care benefits. Cork strippers receive fixed wages rather than commissions that pay by the amount of cork they strip.

Harvesting cork is an art, and the cork strippers can’t rush through the process. They need to be careful, for if they cut too deeply into the inner bark, they risk injuring or even killing the tree. Cork strippers are taught their trade by more experienced cork strippers. They use hand axes; no machines have been developed that do their job as effectively.

Cork strippers work in teams; one will stay on the ground, while the other climbs the tree. They will harvest the cork from the trunk from the ground up to where it starts branching. The cork strippers make circular cuts at the top and bottom of the truck. They then make two long vertical slits dividing the bark into halves.

They use a wedge-shaped tool to work the halves off the tree and peel it like an orange. The halves come off in the form of curved planks that the cork strippers then take back to the factory.

How is cork processed?

Unprocessed cork planks are weighed and measured in “arroba,” a unit of measurement that equals 15 kilograms (33 pounds). The workers then set them out to dry for several months. The better companies will place the planks on concrete floors rather than bare earth to make them less vulnerable to contaminants.

Afterwards, the workers will boil the cork in order to make it more flexible and kill all of the bacteria and insects. During this process, the workers will replace, filter and replenish the water to avoid contamination. Boiling also makes the planks flatter.

The workers will then grade the cork and cut the planks into smaller, more manageable pieces. During the grading process, the workers will decide what the cork can be used for. For example, only the best cork will be used to make stoppers for expensive wine.

Workers making such stoppers will use hand tools to punch the cork out of the bark; the procedure is similar to coring an apple. Workers making stoppers for cheaper wines will use machinery to punch the cork.

After the workers have finished punching corks, they will grind up the leftover bark and mix it with glue to make agglomerated cork. They will also sort and grade the wine stoppers.

Grades of cork

Virgin cork, which is produced by the first stripping, isn’t corky at all. It is thin, hard with an irregular structure and makes a good tan bark. As such, it can be used to tan leather. People also use it for insulation or flooring.

The bark that grows back has the spongy structure characteristic of cork. It is still not good enough to be used as wine stoppers, but it can be used as flotation devices for fishermen’s nets.

Cork oak trees don’t start producing cork good enough to be used for wine stoppers until they are at least 50 years old. They undergo their third stripping around this time. The Portuguese call the best cork “amadia,” and it has an even cellular structure.

For the next 50 years or so, a cork oak tree will produce amadia cork. Then the cork’s quality starts to decline. Eventually, the farmer will chop the tree down and sell its bark to tanners. They can use the wood for fuel or make it into charcoal.

What makes cork so special?

Cork cells were first observed and described by the English natural philosopher Robert Hooke (1635-1703). In the 1665 book, “Micrographia,” he described some of the observations he had made with microscopes and telescopes. He thought plant cells resembled the cells of monasteries and named them accordingly.

Cork cells originally develop within the inner bark. As they mature, they migrate to the outer bark and die and their interiors become filled with air that makes them light and buoyant. A cork cell’s wall is made from a substance called suberin that repels water. It is impermeable to both gases and liquids and thus resists rot. Such traits make it useful as wine stoppers and coasters. Cork is also very elastic and can withstand a wide range of temperatures and pressures.

Also see:

Where Does Cork Come From?
What Are Cork Cells?
Is Cork a Type of Wood?
How to Make Cork?
Are Cork Trees Endangered?
What Country is the Leading Producer of Cork?
How is Cork Harvested?