Cork

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.

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.

The Manufacturing
Process

• 1 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.
• 2 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.
• 3 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.
• 4 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.
• 5 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

• 6 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.
• 7 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.
• 8 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.
• 9 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

• 10 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.
• 11 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.
• 12 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.
• 13 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.

Other

"Transformation Procedures for Natural Cork." Natural Cork Quality Council. http://corkqc.com/ctcor3.htm (February 1999).

Oliveira, Manuel, and Leonel Oliveira. "The Cork." http://www.portugal.org (February 1999).

"The Story of Cork." http://www.shofftackle.com/loadit.html (February 1999).

—LorettaHall

Cork, the Irish city with probably the longest urban pedigree, and for the last four centuries the most important port south of Dublin. The marshy flood‐plain of the river Lee gave the settlement its Gaelic name, Corcaigh, but the earliest urban embryo developed on rising ground along the south bank in the 7th and 8th centuries, around St Finbarre's monastery. Two nearby islands formed the lowest fording point across the Lee and they became the site for the second phase of Cork's urban beginnings. Permanent Viking settlement began on the south island before the mid‐9th century, and over the following two centuries a Hiberno‐Norse trading community evolved below what was then a prestigious monastery.

The two islands had defensive fortifications and a sea‐going fleet when Anglo‐Norman grantees of Henry II captured the town in 1177. Cork was made a royal borough and received a series of charters from 1189 which led to the emergence of Norman‐style municipal government in the following century. A simple street system similar to that of the south island emerged on the north, and the city's maritime commerce, based on wool and hides, reached new levels. The wealth of the urban community was evident in new monastic foundations, the switch from timber to stone housing, and the growth of suburbs, Shandon north of the river and the episcopal borough of Fayth to the south. The city was particularly disadvantaged by the general economic and demographic reverse of 14th‐ and early 15th‐century Ireland. The repeated destruction of the suburbs by rival Gaelic and Gaelicized warlords made the walled city even more emphatically an island fortress loyal in its protestation to the distant English crown, if in practice subservient to neighbouring Munster powers.

In the later 16th century the interlinkage of a reassertion of royal authority, the state promotion of Protestantism, and a fresh wave of English colonization acros the hinterland turned the urban elite into unlikely champions of the Counter‐Reformation. The city benefited in material terms from the trade boom generated by the Munster planatation in the following half‐century, but the continued dominance of the Old English burgher families protected its Catholic ambience. All changed midway through the Confederate War when most of these families were expelled from the town by the royalist commander in 1644. It was, however, only with the Cromwellian reconquest that a radically new municipal elite was installed from whom were derived a number of the great Protestant merchant dynasties.

The island parishes remained predominantly Protestant and New English for several generations, and New English for several generations, the subrubs culturally and religiously mixed. During this time Cork forged ahead as centre of Munster's agricultural export trade: the medievel walls were demolished, the sloblands west and east progressively reclaimed, and the suburbs transformed to become the workshop, market, and stockyard zone of a bustling city. Cork's growth curve was steepest from the 1670s (when it contained 10,000–12,000 inhabitants) to the 1770s (when it had probably reached 50,000), and this despite the physical destruction caused by the savage Williamite siege in September 1690.

Cork's competitive success in processing and supplying beef, butter, pork, hides, and tallow products to commercial and naval markets in southern Europe, the Americas, and later Britain itself made it famous round the Atlantic world and gave it 150 years of urban prosperity and physical expansion, but it was a narrowly based and inequitably distributed economic system. For all the dynamism of the 18th‐century port and the complexity of its Atlantic and European commerce, Cork failed to become a shipping or financial centre akin to Glasgow or Bordeaux.

Commercial and industrial growth faltered after the end of the revolutionary and Napoleonic wars. Cork's salted provisions trade was eclipsed. The reorientation of southern agricultural exports towards England weakened its locational advantages while its wholesale and financial functions were undermined by Anglo‐Irish market integration; only its status as centre of Irish butter exports remained. Local industrialization in brewing and distilling, flour milling, textiles, and shipbuilding lost momentum in the mid‐19th century and failed to offer employment to the pauper flood coming from its overpopulated hinterland, most starkly so during and after the Great Famine. By then Cork's outport in the harbour, Cove/Queenstown, had become the principal Irish exit point for the mass migration to America, and remained so for a century. The nearly static Victorian city became notorious for appalling standards of housing and public health.

Perhaps more than any other southern Irish city, Cork remained a religious battleground in the 19th century, as a fractured middle class competed for control over municipal government, intellectual life, and philanthropic activity. However, success in securing one of the Queen's Colleges for Cork in 1845 brought one institution to the city that cut across denominational battle‐lines. The college and the city's newspapers reinforced Cork's cultural leadership of Munster society, and the distinctively radical edge of popular politics in the city, demonstrated in every generation between 1830 and 1922, rippled outwards.

The death by hunger strike of the imprisoned lord mayor, Terence MacSwiney, in October 1920, followed by the burning of central Cork by crown forces in December, were the city's bitter rites of passage to second city in a Free State. Reindustrialization followed in the 1920s and 1930s, the first wave of investment being led by multinationals Ford and Dunlop. The new political order brought with it a housing revolution with the great expansion of local authority building and the explosive growth of the outer suburbs, bound to the centre by a proliferation of bus services. From mid‐century Cork harbour emerged as a premier location for new industry, becoming the burgeoning centre of Ireland's chemical industry by the 1970s. Belatedly the exceptional natural qualities of the outer harbour were being exploited, enhanced by the discoveries of massive reserves of natural gas off Kinsale Head. Yet even this commercial resurgence towards the end of the 20th century did not reinvest the city with the strategic international significance it had had 200 years before.

Bibliography

Clarke, Howard (ed.), Irish Cities (1995)
Cronin, Maura , Country, Class or Craft? The Politicisation of the Skilled Artisan in Nineteenth‐Century Cork (1995)
O'Flanagan, Patrick, and Buttimer, C. G. (eds.), Cork: History and society (1993)

David Dickson

Cork county (1991 pop. 410,369), 2,881 sq mi (7,462 sq km), SW Republic of Ireland. Cork is the county seat. Largest of the Irish counties, it has a rocky and much-indented coastline (Bantry, Dunmanus, Roaringwater, Courtmarsherry, Clonakilty, and Youghal bays, and Kinsale and Cork harbors). The interior has wild rugged mountains rising as high as 2,239 ft (682 m) and fertile valleys (notably of the Bride, the Blackwater, the Lee, and the Bandon). The main occupations are farming (dairying, raising livestock, and growing grains and sugar beets) and fishing. There is a growing manufacturing sector, centered around the city of Cork, which includes products as diverse as tweed cloth and electronic components. There is a large oil refinery at Whitegate. Cóbh is an important transatlantic harbor. Tourism is important, and notable attractions include prehistoric remains (dolmens and stone circles), the ruins of medieval abbeys and churches, and Blarney Castle.

Cork

Anatomically, cork is a secondary tissue formed from a specialized lateral meristem located in the stems and roots of woody gymnosperms and angiosperms . The tissue develops from a ring of meristematic cells (the cork cambium or phellogen) located beneath the outer surfaces of the tree, and to the outside of the vascular cambium. The cells that form from the cork cambium are specialized, in that their cell walls contain a high proportion of suberin, a fatty material that impedes the movement of water. As cells derived from the cork cambium continue to grow, they eventually die when mature, not unlike the development of xylem cells from the vascular cambium. The result of this process is that the stem (trunk) or root of the tree develops a waterproof covering, generally known as bark. During active phases of tree growth, bark protects the tree from excessive water loss due to the suberized cork cells it contains. Additionally, bark provides a measure of physical protection from direct damage of the tree's trunk by non-living structures (such as rocks), animals, and humans. In some trees that occur in habitats prone to frequent fires (e.g., savannas, certain coniferous forests), the bark is extremely important to protect the tree against heat damage by providing a layer of thermal insulation between the lateral meristems (vascular cambium and cork cambium) and the outside environment. The insulation properties are due to the cellular structure of cork; the spaces inside the dead cells are filled with air, and this provides resistance to heat flow through the cork.

Several other physical characteristics make cork a unique material. Cork is inherently resistant to abrasion and can withstand very high pressures of compression without suffering physical damage. When the pressure is released, the cork returns to its original shape and is seemingly unaffected by the structural changes of compression. Due to the air in its cell spaces, cork is also a lightweight buoyant material, floating easily on water and resisting waterlogging due to its suberized cell walls.

The properties of cork derived from the bark of certain trees has been used by humans for thousands of years. Specifically, the outer bark of the cork oak, Quercus suber (family Fagaceae), is the species upon which commercial cork production is dependent. The cork oak is native to the Mediterrannean region of southern Europe, and is grown commercially in Portugal and Spain. It is an evergreen oak species, and individual trees have been reported to be in cultivation and are harvested for their bark for periods of 150 years or more.

Production and Harvest

The first cutting of cork oak trees takes place when the trees are between fifteen and twenty-five years old, and produces virgin cork, which is of lesser quality than the cork that develops in the years following the initial cutting. While removing the bark/cork layer, harvesters must avoid damaging the cambial layers beneath the accumulated outer tissues. The first cutting (virgin) cork is not discarded. Some virgin cork is used in the horticultural industry as a growing substrate for epiphytic plants, such as bromeliads, orchids, and certain ferns. The waterproof nature of the virgin cork, as well as its rough surface and resistance to decay, provides a long-lasting, natural medium onto which the epiphyte's roots may attach. The virgin cork is also ground up into small pieces, mixed with fillers, adhesives, and other materials to be manufactured into a variety of materials. Subsequent strippings of cork harvests are done at eight to ten year intervals. Each successive stripping causes the production of better quality cork in the next harvest. The trees do not seem to be negatively affected by this harvesting practice when done by experienced cork cutters.

Processing

Once the cork has been removed from the trees, the material is washed in water to remove debris and to keep the cork supple for further processing. It can be flattened into sheets and is generally cut to uniform thickness. Depending upon which product is being manufactured, the order of cutting and sizing the pieces may vary. Bottle stopper corks, such as those used by the wine industry, must be of excellent quality and have the properties of uniformly small cell size, uniform suberization and water repellancy, and favorable properties of resiliency. In use, the wine cork is compressed into the neck of the bottle, where it expands and provides an airtight seal; the wine bottle must be stored on its side to keep the liquid wine in contact with the cork in order for the cork to remain moist and maintain the seal. Some wines stored in this manner are useable for over one hundred years. Certain wine experts also feel that over time, the cork imparts certain subtle and desirable characteristics to the flavor of some wines.

Uses

In addition to the familiar uses of cork to close bottles of beverages, cork has a wide range of other uses by humans. It has historically been used as soles of shoes since Grecian and Roman times. Its buoyancy characteristics have been exploited for use as floats for fishing nets, buoys, flotation ballast in small boats, decoys, life preservers, fishing lures, and bobbers for line fishing. Prior to the development of specialized plastics, cork was used in the manufacture of artificial limbs due to its favorable structural characteristics, carvability, and light weight. It also has been used extensively in the preparation of wall coverings and flooring, as cork may have favorable acoustic characteristics, such as the ability to absorb sound, thus reducing noise. In addition, the sealing and insulating properties of cork are used by the automotive and other industries for the manufacture of gaskets. Cork is also frequently found as the surface material in bulletin boards as a prepared composition veneer material made from ground cork particles (often from the first-cut virgin cork, or from lower quality cork harvests). It is used for this application because of its self-healing properties when tacks, staples, or other items are pushed through it and are then removed. Cork is a renewable plant-derived resource and despite advances in wood technology, it continues to be grown, harvested, and used in a way similar to its production and utilization hundreds of years earlier.

see also Alcoholic Beverages; Ecology, Fire; Trees; Wood Products.

Robert S. Wallace

Bibliography

Constantine, Jr., A. Know Your Woods. New York: Albert Constantine and Son, Inc., 1969.

Simpson, B. B., and M. C. Ogorzaly. Economic Botany: Plants in Our World, 2nd ed. New York: McGraw-Hill Inc., 1995.

cork / kôrk/ • n. the buoyant, light brown substance obtained from the outer layer of the bark of the cork oak: [as adj.] cork tiles. ∎  a bottle stopper, esp. one made of cork. ∎  a piece of cork used as a float for a fishing line or net. ∎  Bot. a protective layer of dead cells immediately below the bark of woody plants. • v. [tr.] (often be corked) 1. close or seal (a bottle) with a cork. ∎  [as adj.] (corked) (of wine) spoiled by tannin from the cork. 2. draw with burnt cork. 3. illicitly hollow out (a baseball bat) and fill it with cork to make it lighter. DERIVATIVES: cork·like / -ˌlīk/ adj.

cork (phellem) A protective waterproof plant tissue produced by the cork cambium. It develops in plants undergoing secondary growth and replaces the epidermis. Its cells, whose walls are impregnated with suberin, are arranged in radial rows and fit closely together except where the cork is interrupted by lenticels. Some cork cells become air-filled while others contain deposits of lignin, tannins, and fatty acids, which give the cork a particular colour. The cork oak (Quercus suber) produces cork that can be used commercially.

cork (phellem) In woody plants, a layer of protective tissue that forms below the epidermis. It comprises dead cells, derived from the cork cambium (phellogen), and coated with a waxy substance (suberin) that renders them waterproof. Cork develops abundantly in the bark layer of certain plants, e.g. Quercus suber (cork oak), and is removed for commercial use. See also QUERCUS.

cork In woody plants, a layer of protective tissue that forms below the epidermis. It comprises dead cells, derived from the cork cambium (phellogen), and coated with a waxy substance (suberin) that renders them waterproof. Cork develops abundantly in the bark layer of certain plants (e.g. Quercus suber, cork oak), and is removed for commercial use.

cork Outer, dead, waterproof layer of the bark of woody plants. The bark of the cork oak, native to Mediterranean countries, is the chief source of commercial cork. Family Fagaceae; species Quercus ruber.

Cork County and county town in Munster province, s Republic of Ireland. Largest of the Irish counties, Cork has a rugged terrain with fertile valleys and a rocky coastline. The chief occupations are farming and fishing. The Danes occupied Cork in the 9th century, but Dermot McCarthy drove them they out in 1172, and then he swore allegiance to the English throne. Oliver Cromwell occupied Cork in 1649. Many public buildings were destroyed in nationalist uprisings in 1920. The city has Catholic and Protestant cathedrals, the University College of Cork (1845), and a large harbour. Cork is renowned for its tweed and linen. Area: 7462sq km (2881sq mi). Pop. (2002 est.) 438,400.

Cork (Corcaigh), Ireland ‘Marshy Place’ from corcach. The town was developed from a church built on an island in the River Lee.

Cork (Corcaigh) Cork. ‘Swamp’.

cork •auk, baulk, Bork, caulk (US calk), chalk, cork, dork, Dundalk, Falk, fork, gawk, hawk, Hawke, nork, orc, outwalk, pork, squawk, stalk, stork, talk, torc, torque, walk, york •pitchfork • nighthawk • goshawk •mohawk • sparrowhawk • tomahawk •back talk • peptalk • beanstalk •sweet-talk • crosstalk • small talk •smooth-talk • catwalk • jaywalk •cakewalk • space walk •sheep walk, sleepwalk •skywalk • sidewalk • crosswalk •boardwalk • rope-walk