The lightweight elastic outer bark of the cork oak, used especially for bottle closures, insulation, floats, and crafts.
Something made of cork, especially a bottle stopper.
A bottle stopper made of other material, such as plastic.
A small float used on a fishing line or net to buoy up the line or net or to indicate when a fish bites.
Botany. A nonliving, water-resistant protective tissue that is formed on the outside of the cork cambium in the woody stems and roots of many seed plants. Also called phellem.
tr.v., corked, cork·ing, corks.
To stop or seal with or as if with a cork.
To restrain or check; hold back: tried to cork my anger.
To blacken with burnt cork.
[Middle English, from Dutch kurk or Low German korck, both from Spanish alcorque, cork-soled shoe, probably from Arabic dialectal al-qūrq : al-, the + qūrq (from Latin quercus, oak).]
(click to enlarge) Cork oak (Quercus suber) with sections of cork removed (credit: Eric Carle/Shostal Associates)
Outer bark of the evergreen cork oak (Quercus suber), native to the Mediterranean. In its broad sense, cork consists of the irregularly shaped, thin-walled, wax-coated cells that make up the peeling bark of many trees, but commercially only cork-oak bark is called cork. Cork is obtained from the new outer sheath of bark that forms after the original rough outer bark has been removed. This outer sheath can be stripped repeatedly without hurting the tree. Cork is unique because it is made of air-filled, watertight cells that are a remarkably effective insulating medium. The air pockets make cork very light in weight. Though specialized plastics and other artificial substances have replaced cork in some of its former uses, it has retained its traditional importance as a stopper for bottles of wine and other alcoholic beverages.
Cork has been used since antiquity as a stopper for bottles because of its compressive abilities. During the Renaissance, cork stoppers were commonplace, and cork-oak trees were grown and processed in the Pyrenees Mountains especially for this purpose. Wine bottles were commonly sealed with oiled hemp. When Pierre Perignon (1638-1715) invented champagne in 1688, he found that the gaseous pressure inside his bottles blew out the hemp stoppers. To solve the problem, he invented corks held in place by wire.
The modern metal bottle cap was developed by the prolific Maryland inventor William Painter, who patented his first stopper in 1885. By 1891, his definitive design, a cork-lined metal cap with a corrugated edge that is crimped around the bottle lip, appeared. Painter called his invention the "crown cap," founded the Crown Cork and Seal Company to market it, and became very wealthy from it.
The crown cap was the industry standard for nearly 80 years. In 1955, the crown cap's cork liner was replaced by plastic, and a high-speed machine to inspect crown seals was introduced in 1958. In the 1960s, the Coca-Cola company offered lift-top crown caps. The push-on, twist-off cap was first developed for baby food. Screw caps for carbonated beverages appeared in the 1960s and 1970s and are the standard today.
An incredibly versatile natural material, cork is harvested from living cork oak trees somewhat like wool is gathered from sheep. The trees are unharmed by the process, and they continue producing cork for an average of 150 years.
Background
Cork is composed of dead cells that accumulate on the outer surface of the cork oak tree. Because of its honeycomb-like structure, cork consists largely of empty space; its density (weight per unit volume) is one-fourth that of water. Unlike a honeycomb, however, cork consists of irregularly shaped and spaced cells having an average of 14 sides. With 625 million of these empty cells per cubic inch (40 million per cubic centimeter), cork is like many layers of microscopic Bubble Wrap, making it an effective cushioning material. Its low density makes cork useful in products like life preservers and buoys. The large amount of dead-air space makes cork an effective insulation material for both temperature and noise. Furthermore, it is fire retardant; flames will only char the surface, and no toxic fumes are generated. Cutting the surface of cork turns many of the microscopic cells into tiny suction cups, creating an effective non-slip surface. In addition to being flexible, cork is highly resilient. After being crushed under a pressure of 14,000 lbs/in2 (96,000 kPa), cork will regain 90% of its original size in 24 hours. Cork absorbs neither dust nor moisture, and it resists both rot and insects. Highly resistant to wear, it is used for polishing diamonds.
Among the many products made from cork are flooring materials (e.g., linoleum), shoe insoles, roofing panels, gaskets, safety helmet liners, bottle stoppers, dartboards, bulletin boards, and cores for golf balls and baseballs. Numerous artificial materials have been developed to substitute for cork in specific applications (e.g., a synthetic pea in a referee's whistle, foam insoles for shoes, or Styrofoam life preservers). However, no general substitute has been developed for cork that can be used in diverse applications.
History
Cork bottle stoppers have been found in Egyptian tombs dating back thousands of years. Ancient Greeks used cork to make fishing net floats, sandals, and bottle stoppers. Two thousand years ago, Romans widely used cork in variety of ways, including life jackets for fishermen. For hundreds of years, Mediterranean cottages have been built with cork roofs and floors to keep out summer heat and winter cold—and to provide a soft walking surface.
Glass bottles were invented in the fifteenth century, but their use did not become widespread until the seventeenth century. The popularity of cork as a stopper led to deliberate cultivation of cork trees, which prior to about 1760 had simply been harvested wherever they happened to grow. The revolutionary crown cap—a metal lid lined with a disk of natural cork commonly known as a bottle cap—was invented in 1892.
A great deal of the cork harvest was wasted until around 1890, when a German company developed a process for adding a clay binder to cork particles and producing sheets of agglomerated (composite) cork for use as insulation. The following year, an American named John Smith developed a technique for producing pure-cork agglomeration out of waste material by subjecting cork particles to heat and pressure without adding any binder. The next major development occurred in 1909 when Charles McManus invented a type of agglomerated cork that could be used to line crown caps. Since then, many other techniques have been developed to produce cork compounds with a variety of properties and uses.
Raw Materials
The raw material for cork products is harvested from the cork oak tree (either the evergreen Quercus suber or the deciduous-Quercus occidentalis). The trees typically reach a height of 40-60 ft (12-18 m) and a trunk circumference of 6-10 ft (2-3 m). Virtually all of the world's commercial cork trees grow in the western Mediterranean region and the Iberian Peninsula. Portugal's cork forests are the most productive. Accounting for 30% of the existing trees, they produce half of the world's harvested cork.
A cork tree is ready for its first harvest when it is about 20 years old. The first harvest is of poor quality, and can only be used to make agglomerated cork products. Subsequent harvests occur at nine-year intervals, when the cork layer reaches a thickness of 1-2 in (2-5 cm). The harvest from a young tree yields about 35 lb (16 kg) of cork, while the yield for an older tree may be 500 lb (225 kg). Each tree has a productive life of about 150 years.
During the production of bottle stoppers, chemical baths are used to condition the corks. Among the more popular are a chlorinated lime bath followed by a neutralizing bath of oxalic acid, a hypochlorite bath neutralized by sodium oxalate, and a peroxide bath neutralized with citric acid.
Production of compound agglomerated cork involves adding a binder or adhesive agent to cork granules. Different binders are chosen, depending on the qualities desired in the ultimate product (e.g., flexibility, softness, resistance to wear). Among those frequently used are asphalt, rubber, gypsum, glue, and plastic.
The Manufacturing Process
Using a specially designed hatchet, the harvester slices through the cork layer on the trunk of the tree, taking care not to cut deep enough to damage the living portion of the trunk. Horizontal cuts are made at the base of the trunk and just below the lowest branches. A few vertical cuts separate the circumferential cork ring into sections of an appropriate size. Using the wedge-shaped handle of the hatchet, the harvester strips each panel of cork from the tree. On some large trees, cork is also stripped from the lower branches.
The cork planks are stacked outdoors and left to cure for a time ranging from a few weeks to six months. The fresh air, sun, and rain encourage chemical changes that improve the quality of the cork. By the end of the curing process, the planks have flattened out and lost about 20% of their original moisture content.
The planks are then treated with heat and water to remove dirt and water-soluble components like tannin, and to make the cork softer and more flexible. This process typically involves lowering stacks of cork planks into large copper vats filled with boiling water containing a fungicide. Heavy weights are placed on top of the cork to keep it submerged for 30-75 minutes.
When the planks are removed from the vat, a hoe-shaped knife is used to scrape off the poor-quality outer layer of cork, which amounts to about 2% of the volume of the plank but 20% of its weight. The planks are stacked in a dark cellar and allowed to dry and cure under controlled humidity for a few more weeks.
The cork planks are trimmed to a uniform, rectangular shape and are sorted by quality. The finest quality material will be used to make natural cork products like wine bottle stoppers. Poorer quality material will be ground and used to make composition or agglomerated cork.
Boffle corks
Cork slabs of the desired thickness are placed in a steam chamber for 20 minutes to soften them. The slabs are then cut into strips whose width corresponds to the intended length of the bottle stoppers. The strips are fed through a machine that punches hollow metal tubes through them, removing cylinders of cork.
Although some beverage bottlers want cylindrical corks, others want tapered ones. To achieve this shape, the cylinders are arranged on a slanted conveyor that carries them past a rapidly rotating circular knife. As they pass the blade, the corks are also revolving on the conveyor, so they are trimmed to a taper.
Both cylindrical and tapered corks are washed, bleached, and sterilized in large vats. Rotating wooden paddles continually push the corks down into first a washing solution and then a neutralizing solution.
After being dried in a large centrifugal dryer, the corks may be marked with an identifying label (with ink or a hot-metal stamp). Some are also coated with a sealant such as paraffin or silicone. Then, they are packed in airtight bags in quantities of 1,000 or 1,500; the air is removed from the bags and replaced with sulfur dioxide (SO2) to keep the corks sterile.
Agglomerated cork
Waste cork is passed through a machine that breaks it into small pieces. The pieces are washed and dried, and then sent through two successive grinders to further reduce the particle size. After another washing and drying process, the particles are screened for uniform size.
Pure agglomerated cork is formed by packing cork particles into a mold and covering it securely. Superheated steam (600° F or 315° C) is passed through the mold. Alternatively, the mold is baked at 500° F (260° C) for four to six hours. Either process binds the cork particles into a solid block by activating their natural resins.
Compound agglomerated, or composition, cork is made by uniformly coating the cork granules with a thin layer of an additional adhesive agent. The coated granules are pressed into a mold and slowly heated (the temperature varies, depending on the adhesive used). When removed from the mold and cooled, the blocks are stacked to allow air circulation and are allowed to season.
The agglomerated cork is cut for its intended use. For example, sheets may be cut from rectangular blocks. Or if a tubular mold was used, the resulting cork rod may be sliced into discs. A large, cylindrical block might by revolved against a knife blade to shave it into a long, continuous sheet that is rewound into a roll.
Byproducts/Waste
Cork waste generated during the manufacturing process is ground and used to make agglomerated cork products. Cork powder that is generated by the grinding process is collected and burned to help fuel the factory. Chemical components removed from cork during its processing can be recovered as useful byproducts and include tannin (used for curing leather), hard wax (used in products like paraffin, paint, and soap), resinous gum (helps vanish adhere to copper and aluminum), and phonic acid (used to make plastics and musk-scented toiletries).
Where to Learn More
Books
Cooke, Giles B. Cork and the Cork Tree. New York: Pergamon Press, 1961.
The belief that cork keeps cramp at bay has been recorded a number of times since the 1850s, and is still current. Sufferers place the substance under their pillow or between the sheets, or even wear home-made cork garters or cork between their toes (Folk-Lore 62 (1951), 268). Another letter published in N&Q (12s: 3 (1917), 449) asks why it is believed that wearing cork will keep off a heart attack, but no other report has been found to corroborate this idea. A completely different notion leads to the keeping of champagne corks, popped at some significant time, ‘for luck’, often with a slit cut in the cork and a coin placed in it. Opie and Tatem list examples back to the 1950s, and the custom is still widely observed at weddings, major birthday parties, and so on.
Bibliography The full bibliography list is available here.
cork, protective, waterproof outer covering of the stems and roots of woody plants. Cork is a specialized secondary tissue produced by the cork cambium of the plant (see meristem, bark). The regularly arranged walls of cork cells are impregnated with a waxy material, called suberin, that is almost impermeable to water or gases. Commercial cork, obtained from the cork oak, is buoyant in water because of the presence of trapped air in the cavities of the waterproof dead cells. It is also resilient, light, chemically inert, and, because of the suction cup action of the cut cells, adhesive. These qualities make cork valuable for bottle stoppers, insulating materials, linoleum, and many household and industrial items.
In these champagne corks, the base is a single piece, while the crown is composed of granules.
Varnished cork tiles can be used for flooring, as a substitute for linoleum or tiles.
Simple cork
Cork material is an impermeable, buoyant material, a prime-subset of generic cork tissue that is harvested for commercial use primarily from Quercus suber (cork oak) that 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. Portugal produces approximately 50% of cork harvested annually worldwide.[1]
There are about 2,200,000 hectares of cork forest worldwide; 33% in Portugal, and 23% in Spain. Annual production is about 340,000 tons; 52% from Portugal, 32% from Spain, 6% Italy.
Once the trees are about 25 years old the cork is stripped from the trunks every nine years. The trees live for about 200 years. The first two harvests produce poorer quality cork.
The cork industry is generally regarded as environmentally friendly.[2] 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 the home of various endangered species.[3]
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 winestopper. 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 and ISO 14044 standards.[4]
Cork oak forests comprise the majority of habitat for the Iberian lynx, one of the most endangered cat species in the world. Its decline has been a function of a loss of habitat due to development of housing as well as the introduction of a virus to control the rabbit population which was the lynx's primary prey.
Properties and uses
Cork's elasticity combined with its near-impermeability makes it suitable as a material for bottlestoppers, especially for wine bottles. Cork stoppers represent about 60% of all cork based production.
Sheets of cork, often the by-product of more lucrative stopper production, are used to make bulletin boards, and when used as floor tiles and other building materials, because it is a good insulator, it can reduce the energy required for heating.
Granules of cork can also be mixed into concrete. The composites made by mixing cork granules and cement have low thermal conductivity, low 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).[5]
Use for wine bottle closures
As late as the mid-1600s, French vintners did not use cork stoppers, using oil-soaked rags stuffed into the necks of bottles instead.[6]
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 "technical corks".
Natural cork closures are used for about 80% of the 20 billion bottles of wine produced each year.[citation needed] 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.
Cork is a suitable material for use as a bottle stopper. 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.[7]
Since the mid-1990s, a number of wine brands have switched to alternative wine closures such as synthetic plastic stoppers, screwcaps, or other closures. In some countries, screwcaps 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, 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 screwcaps.[8]
Other uses
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.
Cork can be used as bricks for the outer walls of houses, as in Portugal's pavilion at Expo 2000.
On November 28, 2007, the Portuguese national postal service CTT issued the world's first postage stamp made of cork.[9][10]
Cork is used as the core of a baseball. Replacing the interior of a baseball bat with cork—a practice known as corking—was historically a method of cheating at baseball; the efficacy of the practice is now discredited.
Miscellaneous
The Cork Oak is unrelated to the "cork trees" (Phellodendron), which have corky bark but are not used for cork production.
