Cork is animpermeablebuoyant material. It is thephellem layer ofbark tissue which is harvested for commercial use primarily fromQuercus suber (the cork oak), which is native to southwestEurope and northwestAfrica. Cork is composed ofsuberin, ahydrophobic substance. Because of its impermeable, buoyant, elastic, and fire retardant properties, it is used in a variety of products, the most common of which iswine stoppers.
Cork composition varies depending ongeographic origin,climate andsoil conditions,genetic origin, tree dimensions, age (virgin or reproduction), and growth conditions. However, in general, cork is made up ofsuberin (average of about 40%),lignin (22%),polysaccharides (cellulose andhemicellulose) (18%), extractables (15%) and others.[3]
Harvesting of cork from the forests of Algeria, 1930
Cork is a natural material used by humans for over 5,000 years. It is a material whose applications have been known sinceantiquity, especially in floating devices and as stopper forbeverages, mainlywine, whose market, from the early twentieth century, had a massive expansion, particularly due to the development of several cork-based agglomerates.[4]
InChina,Egypt,Babylon, andPersia from about 3000 BC, cork was already used for sealingcontainers, fishing equipment, and domestic applications. Inancient Greece (1600 to 1100 years BC) cork was used infootwear, to manufacture a type ofsandals attached to the foot by straps, generallyleather and with a sole in cork or leather.[5][6]
In the second century AD, a Greek physician,Dioscorides, noted several medical applications of cork, mainly for hair loss treatment.[5] Nowadays, the majority of people know cork for its use as stoppers inwinebottles. The innovation of using cork as stopper can be traced back to the late 17th century, attributed toDom Pierre Pérignon.[7] Cork stoppers were adopted in 1729 byRuinart and in 1973 byMoët et Chandon.[3]
Cork presents a characteristiccellular structure in which thecells have usually a pentagonal or hexagonal shape. The cellular wall consists of a thin, lignin-rich middlelamella (internal primary wall), a thick secondary wall made up from alternating suberin and wax lamella, and a thin tertiary wall of polysaccharides. Some studies suggest that the secondary wall is lignified, and therefore, may not consist exclusively of suberin and waxes. The cells of cork are filled with agasmixture similar to air, making them behave as authentic "pads," which contributes to the capability of cork to recover after compression.[3]
There are about 2,200,000 hectares of cork oak(Quercus suber) forest in theMediterranean basin, the native area of the species. The most extensively managed habitats are inPortugal (34%) and inSpain (27%).Annual production is about 300,000 tons; 49.6% from Portugal, 30.5% from Spain, 5.8% fromMorocco, 4.9% fromAlgeria, 3.5% fromTunisia, 3.1% fromItaly, and 2.6% fromFrance.[8]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 (male cork orvirgin cork). The trees live for about 300 years.
The cork industry is generally regarded as environmentally friendly.[9] Cork production is generally consideredsustainable because the cork tree is not cut down to obtain cork; only the bark is stripped to harvest the cork.[10] The tree continues to live and grow. Thesustainability of production and the easyrecycling of cork products and by-products are two of its most distinctive aspects.Cork oak forests also preventdesertification and are a particular habitat in theIberian Peninsula and the refuge of variousendangered species.[11]
Carbon footprint studies conducted byCorticeira Amorim, Oeneo Bouchage of France and the Cork Supply Group of Portugal concluded that cork is the most environmentally friendlywinestopper in comparison to other alternatives. TheCorticeira Amorim's study, in particular ("Analysis of the life cycle of Cork, Aluminum and Plastic Wine Closures"), was developed byPricewaterhouseCoopers, according toISO 14040.[12][13] Results concluded that, concerning the emission of greenhouse gases, each plastic stopper released 10 times moreCO2, whilst an aluminium screw cap releases 26 times more CO2 than does a cork stopper. For example, to produce 1,000 cork stoppers 1.5 kg CO2 are emitted, but to produce the same amount of plastic stoppers 14 kg of CO2 are emitted and for the same amount of aluminium screw caps 37 kg CO2 are emitted.[4]
TheChinese cork oak is native to East Asia and is cultivated in a limited extent in China; the cork produced is considered inferior toQ. suber and are used to produce agglomerated cork products.[14]
The so-called "cork trees"(Phellodendron) are unrelated to the cork oak, they have corky bark but not thick enough for cork production.[citation needed]
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 24 in (60 cm) in circumference, the cork can be removed for the first time. However, this first harvest almost always produces poor quality orvirgin cork (Portuguesecortiça virgem; Spanishcorcho bornizo orcorcho virgen[15]).
The workers who specialize in removing the cork are known asextractors. An extractor uses a very sharp axe to make two types of cuts on the tree: one horizontal cut around the plant, called acrown ornecklace, at a height of about two to three times the circumference of the tree, and several vertical cuts calledrulers oropenings. This is the most delicate phase of the work because, even though cutting the cork requires significant force, the extractor must not damage the underlyingphellogen or the tree will be harmed.[citation needed]
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.[citation needed]
These freed portions of the cork are calledplanks. The planks are usually carried off by hand since cork forests are rarely accessible to vehicles. The cork is stacked in piles in the forest or in yards at a factory and traditionally left to dry, after which it can be loaded onto a truck and shipped to a processor.
Bark from initial harvests can be used to make flooring, shoes, insulation and other industrial products. Subsequent extractions usually occur at intervals of nine years, though it can take up to thirteen for the cork to reach an acceptable size. If the product is of high quality it is known asgentle cork (Portuguesecortiça amadia,[16] but alsocortiça secundeira only if it is the second time; Spanishcorcho segundero, also restricted to the second time[15]), and, ideally, is used to make stoppers for wine and champagne bottles.[17]
Cork'selasticity combined with its near-impermeability makes it suitable as a material forbottlestoppers, especially forwine bottles. Cork stoppers represent about 60% of all cork based production. Cork has an almost zeroPoisson's ratio, which means the radius of a cork does not change significantly when squeezed or pulled.[18]
Cork's bubble-form structure and natural fire retardant make it suitable for acoustic andthermal 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.
Cork is also used to make vinyl record slipmats, due to its ability to not attract dust. They also dampen static and vibrations.[20]
Granules of cork can also be mixed intoconcrete. 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).[21]
As late as the mid-17th century, Frenchvintners did not use cork stoppers, using instead oil-soaked rags stuffed into the necks of bottles.[22]
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".[23]
Natural corkclosures are used for about 80% of the 20 billion bottles ofwine produced each year. After a decline in use as wine-stoppers due to the increase in the use of synthetic alternatives, cork wine-stoppers are making a comeback and currently represent approximately 60% of wine-stoppers in 2016.[24]
High-speedair-gap flash image of a champagne bottle being uncorked
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.[25]
Import value of cork manufactures to Australia since 1988 ($million/month), showing a peak in 2002
Since the mid-1990s, a number of wine brands have switched toalternative wine closures such asplastic stoppers,screw caps, or other closures. During 1972 more than half of the Australian bottled wine went bad due to corking. A great deal of anger and suspicion was directed at Portuguese and Spanish cork suppliers who were suspected of deliberately supplying bad cork to non-EEC wine makers to help prevent cheap imports. Cheaper wine makers developed the aluminium "Stelvin" cap with a polypropylene stopper wad. More expensive wines and carbonated varieties continued to use cork, although much closer attention was paid to the quality. Even so, some high premium makers prefer the Stelvin as it is a guarantee that the wine will be good even after many decades of ageing. Some consumers may have conceptions about screw caps being representative of lower quality wines, due to their cheaper price; however, in Australia, for example, much of the non-sparkling wine production now uses these Stelvin caps as a cork alternative, although some have recently switched back to cork citing issues using screw caps.[26]
The alternatives to cork have both advantages and disadvantages. For example, screwtops are generally considered to offer atrichloroanisole (TCA) free seal, but they also reduce the oxygen transfer rate between the bottle and the atmosphere to almost zero, which can lead to a reduction in the quality of the wine.[citation needed] TCA is the main documented cause ofcork taint in wine. However, some in the wine industry say natural cork stoppers are important because they allow oxygen to interact with wine for proper aging, and are best suited for wines purchased with the intent to age.[27]
Stoppers which resemble natural cork very closely can be made by isolating the suberin component of the cork from the undesirablelignin, mixing it with the same substance used for contact lenses and an adhesive, and molding it into a standardized product, free of TCA or other undesirable substances.[28] Composite corks with real cork veneers are used in cheaper wines.[29] Celebrated Australian wine writer and criticJames Halliday has written that since a cork placed inside the neck of a wine bottle is 350-year-old technology, it is logical to explore other more modern and precise methods of keeping wine safe.[30]
The study "Analysis of the life cycle of Cork, Aluminum and Plastic Wine Closures," conducted byPricewaterhouseCoopers and commissioned by a major cork manufacturer,Amorim, concluded that cork is the most environmentally responsible stopper, in a one-yearlife cycle analysis comparison with plastic stoppers and aluminum screw caps.[31][32]
The famous early (1665) micrograph of cork, drawn byRobert Hooke, and described inMicrographia, book written by him. Fig. A (Right) isTS and Fig. B (Left) isLS of cork.
On 28 November 2007, the Portuguese national postal serviceCTT issued the world's firstpostage stamp made of cork.[33][34]
In musical instruments, particularlywoodwind instruments, where it is used to fasten together segments of the instrument, making the seams airtight. Low quality conducting baton handles are also often made out of cork.
In shoes, especially those usingwelt construction to improve climate control and comfort.
Because it is impermeable and moisture-resistant, cork is often used as an alternative to leather inhandbags,wallets, and other fashion items.
To makebricks for the outer walls of houses, as in Portugal's pavilion atExpo 2000.
As the core of bothbaseballs andcricket balls. Acorked bat is made by replacing the interior of abaseball bat with cork – a practice known as "corking". It was historically a method of cheating atbaseball; theefficacy of the practice is now discredited.
^ab"Cork: culture, nature, future".Santa Maria de Lamas: Press Release. APCOR Cork Information Bureau (Facebook: CorkInWorld). 2010. Retrieved10 December 2022.
