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Cork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from Quercus suber (the Cork Oak), which is endemic to southwest Europe and northwest Africa. Cork is composed of suberin, a hydrophobic substance, and because of its impermeability, buoyancy, elasticity and fire resistance, it is used in a variety of products, the most common of which is for wine stoppers. The montado landscape of Portugal produces approximately 50% of cork harvested annually worldwide, with Corticeira Amorim being the leading company in the industry. Cork was examined microscopically by Robert Hooke, which led to his discovery and naming of the cell.
Once the trees are about 25 years old the cork is traditionally stripped from the trunks every nine years, with the first two harvests generally producing lower quality cork. The trees live for about 200 years.
The cork industry is generally regarded as environmentally friendly. Cork production is generally considered sustainable due to the fact that the entire cork tree is not harvested; merely its bark. The tree continues to live and grow. The sustainability of production and the easy recycling of cork products and by-products are two of its most distinctive aspects. Cork Oak forests also prevent desertification and are a particular habitat in the Iberian Peninsula and the refuge of various endangered species.
Carbon footprint studies committed by Corticeira Amorim, Oeneo Bouchage of France and the Cork Supply Group of Portugal concluded that cork is the most environmentally friendly wine stopper in comparison to other alternatives. The Corticeira Amorim’s study, in particular ("Analysis of the life cycle of Cork, Aluminum and Plastic Wine Closures"), was developed by PricewaterhouseCoopers, according to ISO 14040. Results concluded that, concerning the emission of greenhouse gases, each plastic stopper released 10 times more CO2, whilst an aluminium stopper releases 26 times more CO2 than does a cork stopper.
The Cork Oak is unrelated to the "cork trees" (Phellodendron), which have corky bark but are not used for cork production.
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Cork is extracted only from early May to late August, when the cork can be separated from the tree without causing permanent damage. When the tree reaches 25–30 years of age and about 24in (60 cm) in circumference, the cork can be removed for the first time. However, this first harvest almost always produces poor quality or "male" cork. Subsequent extractions usually occur at intervals of 9 years, though it can take up to 13 for the cork to reach an acceptable size. If the product is of high quality it is known as "gentle" cork, and, ideally, is used to make stoppers for wine and champagne bottles.
The workers who specialize in removing the cork are known as extractors. Extractors use a very sharp axe to make two types of cuts on the tree: one horizontal cut around the plant, called a crown or necklace, at a height of about 2-3 times the circumference of the tree, and several vertical cuts called rulers or openings. This is the most delicate phase of the work because, even though cutting the cork requires quite a bit of strength, the extractor must not damage the underlying phellogen or the tree will die.
To free the cork from the tree, the extractor pushes the handle of the axe into the rulers. A good extractor needs to use a firm but precise touch in order to free a large amount of cork without damaging the product or tree.
These freed portions of the cork are called planks. The planks usually have to be carried off by hand since cork forests are rarely accessible to vehicles. Finally, the cork is stacked and, traditionally, left to dry, after which it can be loaded onto a truck and shipped to a processor.
Cork's elasticity combined with its near-impermeability makes it suitable as a material for bottle stoppers, especially for wine bottles. Cork stoppers represent about 60% of all cork based production.
Cork's bubble-form structure and natural fire resistance make it suitable for acoustic and thermal insulation in house walls, floors, ceilings and facades. The by-product of more lucrative stopper production, corkboard is gaining popularity as a non-allergenic, easy-to-handle and safe alternative to petrochemical-based insulation products which are flammable and emit highly toxic fumes when burned.
Granules of cork can also be mixed into concrete. The composites made by mixing cork granules and cement have lower thermal conductivity, lower density and good energy absorption. Some of the property ranges of the composites are density (400–1500 kg/m³), compressive strength (1–26 MPa) and flexural strength (0.5–4.0 MPa).
Wine corks can be made of either a single piece of cork, or composed of particles, as in champagne corks; corks made of granular particles are called "agglomerated corks".
Natural cork closures are used for about 80% of the 20 billion bottles of wine produced each year. After a decline in use as wine-stoppers due to the increase in the use of cheaper synthetic alternatives, cork wine-stoppers are making a comeback and currently represent approximately 60% of wine-stoppers today.
Because of the cellular structure of cork, it is easily compressed upon insertion into a bottle and will expand to form a tight seal. The interior diameter of the neck of glass bottles tends to be inconsistent, making this ability to seal through variable contraction and expansion an important attribute. However, unavoidable natural flaws, channels, and cracks in the bark make the cork itself highly inconsistent. In a 2005 closure study 45% of corks showed gas leakage during pressure testing both from the sides of the cork as well as through the cork body itself.
Since the mid-1990s, a number of wine brands have switched to alternative wine closures such as synthetic plastic stoppers, screw caps, or other closures. In some countries, screw caps are often seen as a cheap alternative destined only for the low grade wines; however, in Australia, for example, the majority of non-sparkling wine production now uses these caps as a cork alternative. These alternatives to real cork have their own properties, some advantageous and others controversial. For example, while screwtops are generally considered to offer a trichloroanisole (TCA) free seal they reduce the oxygen transfer rate to almost zero, which can lead to reductive qualities in the wine. TCA is one of the primary causes of cork taint in wine. However, in recent years major cork producers (Amorim, Álvaro Coelho & Irmãos, Ganau, Cork Supply Group, and Oeneo) have developed methods that remove most TCA from natural wine corks. Natural cork stoppers are important because they allow oxygen to interact with wine for proper aging, and are best suited for bold red wines purchased with the intent to age.
The study "Analysis of the life cycle of Cork, Aluminum and Plastic Wine Closures," commissioned by cork manufacturer Amorim and made public in December 2008, concluded that cork is the most environmentally responsible stopper, in a one-year life cycle analysis comparison with the plastic stoppers and aluminum screw caps.
Cork is used in musical instruments, particularly woodwind instruments, where it is used to fasten together segments of the instrument, making the seams airtight. Conducting baton handles are also often made out of cork.
It is also used in shoes, especially those using Goodyear Welt Construction.
Cork is used as the core of both baseballs and cricket balls. A corked bat is made by replacing the interior of a baseball bat with cork—a practice known as "corking". It was historically a method of cheating at baseball; the efficacy of the practice is now discredited.
Cork is also used inside footwear to improve climate control and comfort.
Corks are also hung from hats to keep insects away. See cork hat.
Cork has been used as a core material in sandwich composite construction.
Cork can be used instead of wood or aluminium in automotive interiors.
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