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03.07.2024

Birkopedia

In each issue going forward, this is where we intend to tell you about interesting raw materials we use to manufacture our products. This column is sponsored by Lutz Glahn, who manages our raw materials procurement in his capacity as Vice President Global Sourcing.

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Cork: a natural material full of wonders

We kick this column off with – what else? – our most significant natural raw material in terms of volume: cork. We process approximately 2,000 tons of it every year. It is delivered in sacks of up to 20 kg apiece. Were our entire annual needs to be delivered in a single convoy of trucks, this would equal the length of approximately 320 trucks. A large proportion of the cork granulate is a by-product of wine cork manufacture. Bark which is not suitable for use for wine corks because it is too thin or has too many holes is likewise turned into granules. The punching scraps that accrue during wine cork manufacture are ground in a mill, mixed in two different grades with natural latex milk, and baked to create our footbeds.

Cork is a natural, renewable and recyclable raw material with unique and unparalleled properties. No one has ever managed to replicate or improve on these.

Materially speaking, cork is the regenerative bark of the cork oak (Latin: Quercus suber), which belongs to the oak genus. The cork oak grows slowly and can be over 200 years old. The world’s oldest cork oak, known as the Whistler Tree, is 236 years old and can be found in Portugal.

The bark of the cork oak is harvested for the first time after approximately 25 years – by hand, without damaging the tree. From then on, the bark is harvested every nine years, depending on the weather conditions and the region. The quality only becomes really good after the third harvest, in other words after 43 years.

Incidentally, the cork oak is the only tree in the world with a bark that can be peeled without doing the tree any harm. The harvesting process requires a great deal of skill and practice. The bark is harvested between May and August. It is hard work, but it is also the best-paid agricultural work – peelers earn €100–€150 a day. Forest owners are nevertheless finding it increasingly difficult to find migrant laborers who are willing to move from forest to forest.

Cork is a textbook example of a regenerative raw material. It is a natural raw material that man has been using for thousands of years and which is highly valued due to its wondrous properties. The cork oak’s natural habitat is around the Mediterranean – Portugal is comfortably the biggest exporter of cork.

The secret to the properties of cork lies in its cell structure. The cork bark consists of a honeycomb structure with small cells – 40 million per cm³. It is this bark that protects the tree from the hot summer temperatures – as well as from the cold. The cork oak has its resilient bark to thank for the fact that it survived the ice age in the Mediterranean unscathed 25 million years ago. Every cell acts as a small, natural thermal insulator, and additionally boasts sound- and shock-absorbing properties.

This explains why cork is used for flooring. Cork floors offer natural soundproofing and, with their insulation properties, also create a good indoor climate. The elastic cell structures also absorb steps taken on the surface, thereby protecting the joints – an effect we are familiar with from our footbeds. The key component of the cell walls is a chemical substance called suberin, which lends cork its elasticity.

Cork oak forests play an important part in sequestration of the greenhouse gas carbon dioxide. Covering an area of around 2.3 million hectares in -Portugal, Spain, Algeria, Morocco, Tunisia and France, the Mediterranean cork oak forests capture some 14 million tons of carbon dioxide a year. According to expert calculations, Portugal’s cork oaks alone capture close to five percent of the country’s carbon emissions. A cork oak which has its bark harvested regularly even captures three times as much carbon as an unused cork oak.

Cork oaks additionally offer protection from soil erosion and therefore help to preserve the soil. They contribute to improved rainwater absorption in the soil and see to it that the aquifers are refilled. In this way, the cork oak offers natural protection from desertification, in particular in countries in North Africa. Thanks to cork’s resilience to fire, cork oaks also form a natural defense against forest fires. Cork oak forests serve a number of endangered species as retreats, such as the Iberian lynx, the Spanish imperial eagle and the Eurasian black vulture.

This raw material is so versatile that cork has even recently been used in the infinite expanse of space, featuring in the Mars probe’s heat insulation shields. But grounded as it is, the majority of cork continues to be put to use back down on earth – connecting people to our planet via the footbed.

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