The cork oak (Quercus Suber L) is the tree that provides cork, which is its bark. It is the only tree whose bark regenerates. Harvested after 25 years’ growth in cycles of 9 years, the cork oak presents us with a natural raw material with unique characteristics, cork.
What makes it unique is the honeycomb structure of its microscopic cells filled with a air-like gas and coated mainly with suberin and lignin.
It’s chemical composition as a material is the following:
Suberin, being the main component of cork, is a mixture of organic acids that coat the walls of the cork cells, preventing the passage of water and of gases. The properties of suberin are notable: it is practically infusible and is insoluble in water, alcohol, ether, chloroform, concentrated sulphuric acid, etc.
The cells grouped in a characteristic alveolar structure are the essence that defines cork. A cubic centimeter of cork contains nearly 40 million cells, arranged in rows perpendicular to the cork oak trunk, giving it its capacity to float, insulate and re-expand after compression.
Each cell is shaped like a tiny pentagonal or hexagonal prism, the height of which is no more than 40 to 50 micrometres (=thousandths of a millimetres). The smallest cells measuring 20 or as little as 10 micrometres. All these cells are filled with a mixture of gases similar to air. A plank of cork contains nearly 60% gaseous elements, which explains its extraordinary lightness. These small cushions of air are what make cork so remarkably compressible. At the same time, suberin makes the walls of the cork cells impermeable and therefore airtight. The gas they contain cannot escape, which is the reason for the elasticity of the tissue and also its low thermal conductivity.
Cork is natural and recyclable, and the cork oak tree grows without the need of chemical substances or irrigation, although recent research confirms that irrigation can allow a short time between harvests.
The cork oak, grown in forests or groves, lives up to 200 years. Cork oak bark grows in around of 1.5 mm each year retaining huge amount of CO2 from the atmosphere during his growth. A forest of cork oak trees is usually called a Montado.
The cork is harvested by specialized professional with traditional methods during the spring and summer period, usually May up to august.
Cork oak trees are nowadays common in western Mediterranean region: Portugal, Spain, Italy, France and in North Africa (Morocco, Algeria and Tunisia). Portugal, however is the largest producer and processor of cork in the world.
Cork History goes back in time to 3000 BC cork being used in fishing tackle in China, Egypt, Babylon and Persia. In Italy remains dating from the 4th century BC, having been found artefacts such as floats, stoppers for casks, women’s footwear and roofing materials. Also dating from that period is one of the first references to the cork oak, by the Greek philosopher Theophrastus who, in his botanical treatises, referred in wonder to “the ability that this tree has to renew its bark after it has been removed”.
Later, in the 1st century CE, the Roman naturalist Pliny the Elder made extensive reference to cork oaks in his celebrated Natural History. He explained that in Greece the trees were adored as symbols of liberty and honour, for which reason only priests were allowed to cut them down. In the same work, we can read that cork oaks were consecrated to the god of Olympus, Jupiter, and their leaves and branches were used to crown victorious athletes. In Pompeii, the Roman city destroyed by the brutal eruption of Mount Vesuvius, wine amphorae sealed with cork have also been found.
Additionally, in Portugal is documented in environmental legislation the protection to cork in 1209. Later, during the Age of Discoveries, the builders of the Portuguese ships and caravels that set sail in search of new worlds used cork oak wood for the parts that were most exposed to inclement weather. They claimed that the “sôvaro”, as it was called then, was the best wood for masts and yards: besides being exceptionally strong.
In the 18th century, was obtained in England the first microscopic images of cork using a microscope that he himself had designed, and in France, the monk Dom Pierre Pérignon, began to use cork to seal bottles of his famous Dom Pérignon champagne. A choice that continued over the years and is still maintained.
However the systematic cultivation of the cork oak forest dates only from 18th century, when the production of cork stoppers became the main objective. This was also when the first studies were made of its chemical composition by the Italian chemist Brugnatelli, and the first compendium on cork oak culture, “Azinheiras, Sovereiras e Carvalhos da Província de Além-Tejo”, was published in Portugal in 1790 by Joaquim Sequeira.
In the 19th century, France, Italy and Tunisia invested in the systematic planting of cork forests and countries as different as Russia or the United States also started planting cork oaks. In the United Kingdom the first cork stopper manufacturing machine was patented, and auxiliary equipment was invented, such as cork stopper counting and calibrating machines. For the first time, new industrial applications for cork were used, such as simple or white agglomerate for flooring discovered by the Americans. In the last years of the century, in Reims, France, the first two piece glued natural cork stoppers began to be manufactured.
In 1903, cork stoppers with natural cork discs and a body of agglomerate first appeared. Some years later, patents were registered for the use of cork in transmission belts and tires and during the Second World War, this material was used in many pieces of military equipment. In the 1950s, an American company produced the first agglomerated cork tiles with a vinyl film covering.
In recent decades, various initiatives have emerged aimed at research and the definition of international standards for the cork industry, including the Confédération Européenne du Liège (C.E. Liège), founded in 1987. Formed by cork federations from various countries, this organisation presented in 1996 the International Code of Cork Stopper Manufacturing Practice, a key document for quality control in the production of cork stoppers. This document describes and regulates the corresponding manufacturing processes and is still being permanently revised and updated in accordance with increasingly demanding levels of quality.
Finally, all the signs are that in the 21st century, environmental concerns have become a constant, the use of an ecological, recyclable and biodegradable material such as cork has increased, particularly in innovative areas such as Design for Sustainability and Eco-Design. Increasingly, new generations of artists seek to create everyday objects – articles for the table, kitchen, leisure, furniture – from the “fruits of the earth”, materials that are one hundred per cent natural and contribute to environmental sustainability.
Recently, the market was presented with an absolute innovation: a car seat with a base made from cork which halved its volume and made it three times lighter than traditional seats. The extraordinary thing about this invention is that each of these new seats can reduce the weight of a normal car by 45 kilos, thus helping to resolve two of the major problems of the automotive industry, weight and volume. Made from 60% ground cork, this cushion, besides offering the same comfort with half the volume, has the added benefit of being recyclable. The seat is a national project conceived entirely by Portuguese know-how – design, technical and scientific support –, and although it is still at a prototype stage, it has already captivated Magna, the world leader in automotive parts which has made orders worth more than 300 million euros.