Cork is a waterproof and floating material. It comes from the phellem layer of bark, which is mainly taken from the cork oak tree (Quercus suber). This tree grows in southwest Europe and northwest Africa. Cork contains suberin, a substance that repels water. Because it is waterproof, floats, is elastic, and resists fire, cork is used in many products, most commonly as wine bottle stoppers.

The montado landscape in Portugal produces about half of the cork collected worldwide each year. Corticeira Amorim is the largest company involved in cork production. Scientist Robert Hooke studied cork under a microscope, which helped him discover and name the cell.

The makeup of cork changes based on where it is grown, the climate, soil, the tree’s genetic background, size, age (whether it is from a young tree or a tree that has regrown after harvesting), and growing conditions. Generally, cork is made of suberin (about 40%), lignin (22%), polysaccharides (such as cellulose and hemicellulose) (18%), extractables (15%), and other substances.

History

Cork is a natural material that humans have used for more than 5,000 years. People have known about its uses since ancient times, especially for making floating devices and stoppers for drinks, mostly wine. The wine stopper market grew a lot in the early 20th century because of the creation of cork-based mixtures.

In China, Egypt, Babylon, and Persia around 3000 BC, cork was used to seal containers, make fishing tools, and for everyday purposes. In ancient Greece (1600 to 1100 BC), cork was used in sandals. These sandals had straps made of leather and soles made of cork or leather.

In the second century AD, a Greek doctor named Dioscorides wrote about using cork for medical purposes, such as treating hair loss. Today, most people know cork for its use as stoppers in wine bottles. The idea of using cork as a wine stopper began in the late 17th century, credited to Dom Pierre Pérignon. Cork stoppers were first used by Ruinart in 1729 and later by Moët et Chandon in 1973.

Structure

Cork has a special cell structure where the cells are usually shaped like five-sided or six-sided figures. Each cell wall has three layers: a thin middle layer rich in lignin (called the internal primary wall), a thick secondary wall made of alternating layers of suberin and wax, and a thin outer layer made of polysaccharides (called the tertiary wall). Some studies suggest the secondary wall may contain lignin, meaning it might not be made only of suberin and wax. The cells inside cork are filled with a gas similar to air, which helps them act like real pads. This feature allows cork to return to its original shape after being pressed.

Harvesting

Cork is taken from trees only between early May and late August. During this time, the cork can be removed without harming the tree permanently. The first time cork is removed, the tree is usually 25 to 30 years old and has a trunk that is about 24 inches (60 cm) around. However, cork from this first harvest is often low quality and is called virgin cork (Portuguese cortiça virgem; Spanish corcho bornizo or corcho virgen).

People who remove cork are called extractors. They use a sharp axe to make two types of cuts on the tree: a horizontal cut around the trunk, called a crown or necklace, placed at a height equal to two to three times the tree’s circumference, and several vertical cuts called rulers or openings. This step is very careful because, even though cutting requires strength, the extractor must avoid harming the layer of tissue beneath the cork, called the phellogen, or the tree will be damaged.

To remove the cork, the extractor pushes the axe handle into the rulers. A skilled extractor uses firm but precise movements to take off large pieces of cork without damaging the tree or the product.

The pieces of cork that are removed are called planks. These are usually carried by hand because cork forests are rarely accessible to vehicles. The cork is then stacked in piles in the forest or at a factory and left to dry naturally. After drying, it is loaded onto trucks and sent to a processor.

Cork from the first harvest can be used to make flooring, shoes, insulation, and other industrial products. Later harvests usually happen every nine years, though it may take up to thirteen years for the cork to grow to a usable size. High-quality cork is called gentle cork (Portuguese cortiça amadia, but also cortiça secundeira only if it is the second harvest; Spanish corcho segundero, also only for the second harvest) and is ideally used to make stoppers for wine and champagne bottles.

Properties and uses

Cork’s ability to stretch and return to its original shape, along with its near-impermeability, makes it a good material for bottle stoppers, especially for wine bottles. Cork stoppers account for about 60% of all cork-based products. Cork has a nearly zero Poisson’s ratio, meaning its width does not change much when squeezed or pulled.

Cork is a good material for gaskets. Some carburetor float bowl gaskets are made from cork.

Cork is also an important part of making badminton shuttlecocks.

Cork’s bubble-like structure and natural fire resistance make it useful for sound and heat insulation in walls, floors, ceilings, and building exteriors. Corkboard, a by-product of stopper production, is becoming a popular alternative to petrochemical-based insulation because it is non-allergenic, easy to handle, and safe.

Cork is used to make vinyl record slipmats because it does not attract dust. It also helps reduce static and vibrations.

Cork sheets, often leftover from stopper production, are used to make bulletin boards and floor or wall tiles.

Cork’s low density makes it a good material for fishing floats, buoys, and fishing rod handles (as an alternative to neoprene).

Cork granules can be mixed into concrete. Concrete made with cork granules and cement has lower heat transfer, lower density, and good energy absorption. Some properties of these materials include density (400–1500 kg/m³), compressive strength (1–26 MPa), and flexural strength (0.5–4.0 MPa).

As late as the mid-17th century, French wine makers did not use cork stoppers. Instead, they used oil-soaked rags placed inside bottle necks.

Wine corks can be made from a single piece of cork or from particles, as in champagne corks. Corks made from particles are called "agglomerated corks."

Natural cork closures are used for about 80% of the 20 billion wine bottles produced each year. After a decline in use due to synthetic alternatives, cork stoppers are returning and now make up about 60% of wine stoppers in 2016.

Cork’s cellular structure allows it to be compressed when inserted into a bottle and expand to form a tight seal. The inside of glass bottle necks often has uneven sizes, so this ability to adjust is important. However, natural flaws in cork, such as cracks or channels, can cause inconsistencies. In a 2005 study, 45% of corks leaked gas during pressure tests, either from the sides or through the cork itself.

Since the mid-1990s, some wine brands have used alternatives like plastic stoppers, screw caps, or other closures. In 1972, more than half of Australian bottled wine spoiled due to corking. This led to suspicion that Portuguese and Spanish cork suppliers intentionally provided poor-quality cork to non-EEC wine makers to block cheap imports. Cheaper wine producers later developed the aluminum "Stelvin" cap with a polypropylene stopper. More expensive and carbonated wines continued to use cork, but with closer quality checks. Some high-end producers prefer Stelvin caps because they ensure wine quality over long periods. Some consumers think screw caps indicate lower-quality wine, but in Australia, many non-sparkling wines now use Stelvin caps. Some producers have recently switched back to cork due to issues with screw caps.

Alternatives to cork have both benefits and drawbacks. For example, screw caps prevent a chemical called TCA from forming, which causes cork taint in wine. However, they also stop oxygen from entering the bottle, which can reduce wine quality over time. Natural cork allows oxygen to interact with wine, which is important for aging.

Stoppers that look like natural cork can be made by separating the suberin component of cork from lignin, mixing it with a material used in contact lenses and an adhesive, and molding it into a product free of TCA or other harmful substances. Composite corks with real cork layers are used in cheaper wines. James Halliday, a well-known Australian wine critic, has written that cork is 350-year-old technology and that modern methods of preserving wine should be explored.

A study by PricewaterhouseCoopers, commissioned by a major cork manufacturer, found that cork is the most environmentally friendly stopper compared to plastic and aluminum closures in a one-year life cycle analysis.

• On November 28, 2007, the Portuguese postal service CTT released the world’s first cork postage stamp.
• In musical instruments, especially woodwinds, cork is used to join parts and make seams airtight. Low-quality baton handles are sometimes made from cork.
• In shoes, cork is used in welt construction to improve comfort and climate control.
• Because it is moisture-resistant, cork is used as a leather alternative in handbags, wallets, and other fashion items.
• In Portugal’s pavilion at Expo 2000, cork was used to make bricks for house walls.
• Cork is the core of baseballs and cricket balls. A "corked" baseball bat has cork inside, a practice once used to cheat in baseball.
• Cork is used in spacecraft heat shields and fairings.
• Cork is used in the paper pickup mechanisms of inkjet and laser printers.
• Cork is used to make later-model pith helmets.
• Cork is hung from hats to keep insects away (see "cork hat").
• Cork is used as a core material in sandwich composites.
• Cork is used as a friction lining in automatic transmission clutches in some mopeds.
• Cork is used as an alternative to wood or aluminum in car interiors.
• Cork slabs are sometimes used by orchid growers to mount plants.
• Cork paddles are used by glassblowers to shape hot glass.
• Racing bicycles often have handlebars wrapped in cork-based tape in various colors.
• Cork is used to make architectural models.
• Cork is used to make figurines, known as corkmen.