Because of its remarkable qualities, cork is used in high-tech applications including car engines, dam mechanisms and airport runways. The aeronautics has used cork as a thermal insulator in space shuttles.
The use of cork as a raw material dates back to Phoenician and Greek times. Cork began to become known all over the world as an effective bottle stopper for wine. In fact, cork is the only material that makes a perfect seal during the ageing of the wine.
The cork oak
Today, cork is a valuable resource for Portugal, representing one of its most important export products.
Cork oak forests cover approximately 2.5 million hectares across the Mediterranean region and most of them are located in seven countries: Portugal, Algeria, Spain, Morocco, France, Italy and Tunisia.
The tree has a life span of 250-350 years. Each cork tree must be 20 to 25 years old before it can provide its first harvest of cork bark. This cork is known as virgin and has a hard and irregular structure. After the virgin cork has been stripped, a new layer of cork begins to grow.
The first of these layers, harvested after nine years, is called secondary cork; cork harvested after this second stripping is known by the Portuguese word: amadia.
A typical tree produces several hundred kilograms of cork at each harvesting and will survive for many generations. The bark is stripped off the tree in sections by highly skilled men using special axes, a traditional manual skill that dates back many hundreds of years.
Cork is harvested on a sustainable basis and the stripping of the bark does not harm the tree in any way. The bark grows back completely, taking on a smoother texture after each harvest. A cork oak tree can be safely harvested up to 20 times during its life cycle, making cork a truly inexhaustible natural resource.
New plantations of cork oak trees are planted each year to ensure the level of cork production is maintained. Cork oak trees cannot be felled or removed without government authorization, which is rarely granted.
Portugal, which produces more than 50% of the world's cork, has been particularly careful to safeguard this valuable resource. The first Portuguese laws protecting cork oak trees date back to the 14th century. At the beginning of the 20th century, it became illegal to cut down cork oak trees, except for essential thinning or the removal of old, non-productive trees.
In a context of increasing concern for the environment, cork remains the only tree whose bark can regenerate itself after each harvest — leaving the tree unharmed. It is truly a renewable, environment.
The bark of the cork oak tree has a unique honeycomb structure composed of tiny cells. Each cell has the form of a 14-sided polyhedron and the inner cell space is entirely filled with an air-like gaseous mixture. The properties of cork derive naturally from the structure and chemical composition of its extremely strong, flexible cell membranes, which are waterproof and airtight.
Because about 89% of the tissue of the bark consists of gaseous matter, the density of cork is extremely low, in the order of 0.12 to 0.20, a fact that bears witness to the huge disproportion between the volume and the weight of the material.
Cork is light and floats on water. For many thousands of years, this has been its most evident and most celebrated characteristic. Since ancient times, cork has been used in fishing equipment.
Elasticity and resiliency
The cell membranes of cork are highly flexible, making it both compressible and elastic. This means it returns to its original shape after being subjected to pressure. This and other characteristics explain why cork has become an indispensable material for making bottle stoppers.
These physical qualities mean that cork can be fitted perfectly against the walls of the bottleneck. When cork is subjected to strong pressure, the gas in the cells is compressed and considerably reduced in volume. When the pressure is released, the cork immediately recovers its original shape and volume, showing no trace of having been subjected to any appreciable deformation.
The presence of suberin (a complex mixture of fatty acids and heavy organic alcohol) renders cork impermeable to both liquids and gases. As a result, it does not rot, making it one of the best seals available.
Insulation and fire retardant qualities
The value of cork is further enhanced by its low conductivity of heat, sound and vibration. This is because the gaseous elements it contains are sealed in tiny, impermeable compartments, insulated from each other by a moisture-resistant material. This endows cork with one of the best insulating capacities, both thermal and acoustic, of any natural substance.
Cork is also a natural fire retardant
It neither spreads flames nor releases toxic gases during combustion.
Resistance to wear
Cork is remarkably resistant to wear and has a high friction coefficient. Thanks to its honeycomb structure, it is less affected by impact or friction than other hard surfaces.
Because cork does not absorb dust, it helps protect against allergies and does not pose a risk to asthma sufferers. It also has an unchangeable constitution that guarantees efficiency.
More information about cork
Cork is the bark of an oak tree known botanically as quercus suber. The tree is an evergreen that grows only in areas bordering the Mediterranean. Portugal represents approximately 50% of the world's cork output, Spain accounts for 25% and the remainder comes from Algeria, France, Morocco, Italy and Tunisia.
As early as 2500 BC, cork was being used for fishing floats in ancient Egypt. The ancient Greeks also used cork to make fishing buoys, sandals and stoppers for vessels containing wine and olive oil.
The conqueror Alexander the Great is reported to have been one of the many historic figures to have used cork. It is said that a piece of cork once saved him from drowning when crossing a turbulent river.
The Romans used cork in the construction of house roofs, beehives, ships and women's shoes.
In the 1600s, a French monk called Dom Pérignon took a giant step towards the modern, widespread use of cork as a stopper for bottles of wine.
In the 20th century, cork was the only organic substance to travel to Mars when it was used for thermal insulation in the Viking space probes.
A natural 'green'
In these times of increased concern for the environment, cork remains the only tree which can regenerate itself after each harvest. The cork bark is stripped off two-thirds of the tree. The first harvest of cork is not stripped until the tree is approximately 20 years old. A thin layer of protective inner bark gives the cork oak its unique ability to survive and regenerate itself after the debarking process. Stripping the bark requires great skill, as the inner bark must not be damaged. A specially designed hatchet is used for the stripping process.
The first bark taken from a tree is called virgin bark. It has a very irregular exterior surface and is grayish in color. This bark is suitable for grinding into various minute sizes ideal for cork insulation and composition cork. Interestingly, it has also become a very popular material for the manufacture of decorative items.
After the first harvest of cork, the bark is stripped from the tree once every nine years, until the cork oak is about 150 years old. The tree is then replaced by a much younger one. The bark which grows after the virgin bark has been stripped is called refugo bark. It looks entirely different, having a much smoother surface, which is brown in color. The first crop of refugo is used mostly for grinding. Subsequent strippings yield better quality cork that has fewer and more tightly closed pores (grains). Most of the cork from these subsequent harvests is used for the production of cork stoppers or other items requiring cork with a finer appearance.
After being stripped, the bark is left in the forest for some days to dry and, possibly, to be inspected by potential buyers. Purchasing cork bark is not an easy task as the quality not only varies from forest to forest, but also from tree to tree. Even the same tree may produce cork of varying degrees of quality depending on its exposure to sunlight.
Once in the factory area, the refugo bark is boiled to make it easier to remove the woody outer layer and to make the bark more elastic so that it can be flatten it out more easily. The bark is then sorted into various thicknesses which are, in turn, sorted into different qualities. These different qualities determine the sale price and/or the suitability of the cork for different manufacturing uses.
Cork is a natural product with remarkable and unique qualities that are unmatched by any other natural material. One cubic inch of cork is composed of no less than 200 million tightly enclosed air cells, each measuring l/1000" in diameter. Each minute cell is 14-sided, which virtually eliminates any empty space between the cells.
This quality is what gives cork its remarkable elasticity and ability to regain its original shape after being compressed.
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