The cork oak tree (Quercus Suber) is an ancient variation of the oak tree that has withstood the test of time. Fossils discovered in Portugal date back to more than 10 million years ago; however, some scholars argue that cork trees in the greater Mediterranean have existed for closer to 30 million years. This means that cork oak trees have survived ice ages, inter-glacial ages, glacial ages, and, more recently, human-caused climate change, proving their resilience. While the cork oak tree does share many similarities with the other more than 600 subspecies of oak tree, specific characteristics make it a wholly unique and remarkable plant.
The most remarkable characteristic is that the outer and middle bark can be harvested from the cork oak tree without harming the inner bark or the tree. This allows the bark to be harvested many times over the 200+ year lifespan of the cork oak tree, with the bark regrowing every time. This harvested bark is what most people today think of as "cork."
What is cork bark? Cork bark is made up of three layers on the tree. This is a dark brown, rougher layer of bark that serves to protect the subsequent two layers. The middle layer of the bark is a lighter tan color you would recognize from seeing bulletin boards and wine corks. This layer is more flexible than the outer layer and is used to make most cork products. The final layer is the inner layer. This layer is not removed from the cork oak tree. It serves as a protective layer for the tree itself and has the ability to regrow the outer two layers.
After the initial harvest of a cork tree, there is a change in the outer bark layers. This change is usually completed after the third harvest. The bark produced during the first harvest is called virgin cork bark, while the bark produced in subsequent harvests is known as mature cork bark. Virgin cork bark has a thicker and rougher outer layer that helps protect the tree when it is still small and thin. The middle bark layer of virgin cork bark is also thinner and less consistent than mature cork bark.
On the other hand, mature cork bark has an outer layer that is much thinner and less rough because the larger cork tree no longer needs the extra protection. The middle layer of the mature cork bark is thicker and more consistent, making it suitable for applications such as natural punched wine corks, gaskets, and others.
Mature Cork Bark on the Left, Virgin Cork Bark on the Right
Cork trees have been historically grown in the Mediterranean for a host of geopolitical and environmental reasons, leading Portugal to be the world’s largest producer of cork bark. Outside of Portugal, cork oak trees grow well in other Mediterranean countries, including Spain, Italy, Greece, and Turkey, as well as in Northern African countries, such as Algeria, Morocco, and Tunisia.
Over the last several decades, the range of the cork oak tree has been extended into new regions with similar climatic conditions. These new areas include the American West, parts of Ukraine, and India. There is also a specific type of cork oak tree (Quercus Variabilis), which is native to a wide swath of eastern Asia and is cultivated in parts of China. The ideal climate for cork trees is a traditionally hostile climate for most trees that, includes acidic sandy soils low in nitrogen and phosphorous. This makes cork trees a compelling option for traditionally arid climates, especially those prone to forest fires. In the United States, cork is rated for hardiness zones 8-10 and heat zones 7-9, and the trees are drought-resistant once established.
Cork Oak Tree in Georgia’s Veterans State Park.
In the Mediterranean, cork bark is harvested in the dry, hot summer months of July and August. The first harvest occurs when the tree is 25 years old, and subsequent harvests ensue every 7 to 9 years after that. Each harvested tree is spray-painted with the last digit of the harvest year, ensuring the proper time elapses between harvests.
As harvesting the bark does not harm the tree, cork is a completely sustainable material. The cork tree continues to sequester carbon and produce oxygen, all while regrowing the bark, and some research even shows an increase in carbon sequestration by a tree that has been recently harvested.
After the bark is harvested, it is left to dry in the sun and then boiled before it is processed into the familiar cork goods you see as bulletin boards in schools, yoga mats, or wine corks. First, wine corks, stoppers, and cork balls are punched out of the cork bark [insert bark punch photo]. These highest-grade cork products are removed first; however, cork is a zero-waste product, so the rest of the cork bark will not be trash!
Next, the cork bark punches can be sliced into thin strips to make decorative veneers used in fabric, flooring, and wall tiles, or they can be ground into cork granules. These cork granules are then pressed into molds with a binder to create agglomerated cork products, from bulletin boards to furniture.
Any remaining cork, either grain or cut-off "waste" pieces, are used in this next processing method. Any cork not already ground is ground up. These cork granules are then subjected to high heat and pressure, where they undergo a molecular change, turning the cork from a light tan to a dark chocolatey brown. During this process, a natural binder is released from the cork granules as they expand to make semi-rigid insulation cork. This low-density cork has many uses but is especially effective as insulation - both for acoustics and temperature.
Want to learn more about cork? Click here to find out more about what a cork tree is!