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.
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.
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.
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.
- 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.
- 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.
- 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.
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
Cooke, Giles B. Cork and the Cork Tree. New York: Pergamon Press, 1961.
"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).
Corcach Mór Mumhan—the great marsh of Munster—was the ancient name of the modern Corcaigh, anglicized as Cork. The marsh, a term still used colloquially to describe the heart of the old city, was the area where the river Lee became estuarial, threading itself through various islands. Old Cork was bounded by the two main channels (north and south) of the river, famously described by Edmund Spenser: "The spreading Lee that like an island fayre / Encloseth Cork with his divided flood." The seminal urban settlement was the seventh-century monastery and school associated with Saint Finbarr (or Bairre). It was situated on a ridge overlooking the river from the south side, not far from where the modern (Church of Ireland) Saint Fin Barre's Cathedral stands on the site of its predecessors.
The Scandinavian settlement of the "south island," the present South Main Street area, dates from the mid-ninth century. Native Irish as well as foreigners figured in this early urban development. With the arrival of the Anglo-Normans in the later twelfth century, the physical layout of the city was established in a form that lasted essentially until the late eighteenth century—one main street running from south gate to north gate with a separating strip of water midway, along what later became the filled-in Castle Street and Liberty Street.
Cork's earliest surviving charter was granted by King John in 1185. The city was primarily dependent on agricultural produce from the hinterland and therefore on commercial contacts with the Gaelic Irish who stood in uneasy relationship with the burgesses, particularly so during the native resurgence from the late fourteenth century. The sense of a city under siege is well documented at that period. Moreover, the small population (between 1,300 and 2,000) was ravaged by the Black Death in 1349. Nevertheless, the city prospered in the thirteenth and fourteenth centuries, the important port greatly facilitating its development. Trade was conducted with England (notably with Bristol), Scotland, and France; the exports included skins, hides, beef, grain, and wool, and the main imports were wine, cloth, and spices.
The great political, religious, and plantation upheavals from the mid-sixteenth century saw the Old English ruling class (loyal in politics but Catholic in religion) eventually supplanted in Cork by a New English/Protestant elite. The period from the middle of the eighteenth century to the end of the Napoleonic wars in 1815 was a golden age for Cork's economy. The population grew rapidly—to 41,000 in 1750, 57,000 in 1796, and a remarkable 80,000 by 1821. Cork remained Ireland's second city (after Dublin) until 1841; thereafter, industrial Belfast pulled ahead.
The Cork butter market handled nearly half of all Irish butter exports by 1789. Beef and pork exports were similarly impressive—provisioning British navy and army supply ships was a thriving business—and prosperous textile and tanning industries provided substantial employment and goods for export as well as the domestic market.
Cork has always identified itself with its harbor. The motto on the city's crest is statio bene fida carinis—"a trustworthy anchorage for ships." The harbor was important for British naval supremacy but was even more vital for commercial life. Cork was always a commercial rather than an industrial city, but a general decline in both sectors set in after the Napoleonic wars. There was a sharp decrease in agricultural prices and a falling-off in the provisioning trade, a result of the adverse impact of Anglo-Irish market integration following the Act of Union.
Meanwhile, the city was undergoing significant physical expansion beginning in the late eighteenth century. Channels were filled in and numerous bridges, including the imposing Saint Patrick's Bridge, were built, with Saint Patrick's Street becoming Cork's main thoroughfare. Throughout the nineteenth century suburban residence became the norm for the middle classes, and the areas of Sunday's Well, Tivoli, and Blackrock were variously favored by the merchant princes. Beginning in 1898 the tramline system offered citizens a reliable and economic means of enjoying residence in the suburbs while working and shopping in the city.
Queen's College (from 1908, University College) opened in 1849, making Cork a university town as well as a port and harbor city. In terms of nineteenth-century suburban growth the college was the catalyst for the development of the striking western approaches to the city. The handsome buildings and riverside grounds have given a distinctive and elegant appearance to that neighborhood over a hundred and fifty years.
Over 70 percent of all Cork families were living in slums during the second half of the nineteenth century. Leaders of the working classes were conservatives, socially speaking, concerned with preserving the aristocracy of the artisans against the unskilled workers. The lower classes were encouraged by the churches and the media to accept their "station in life" and they were diverted from socialist objectives by the lure of nationalist aspirations. Meanwhile, the professional and merchant classes were divided along sectarian lines. Catholics were envious of Protestant Ascendancy in municipal politics until something of a level playing field for the religious majority was eventually created by such measures as the Municipal Corporations Act of 1840 and the Local Government Act of 1898, and the city council gradually became more representative.
Cork nationalist politics were exciting and turbulent in the faction-ridden years after the death of Charles Stewart Parnell, MP for the city from 1880 to 1891. The most colorful and volatile figure at the turn of the century was journalist and politician William O'Brien. Later, the radical nationalist tradition in Cork found strong expression in the resurgence of Sinn Féin after 1916. The momentous highlights of this period were the murder of Lord Mayor Tomás MacCurtain in March 1920; the death from hunger strike in October of his mayoral successor, Terence MacSwiney; and the burning of the city center by Crown forces in December. Since then, the office of lord mayor has had particular prestige in Cork.
Notwithstanding urban growth and spread throughout the twentieth century, there is an immemorial charm about old Cork that was once described (doubtless with the genteel grandeur of Sunday's Well and Montenotte in mind) as "a city of tattered grace." The winding channels of the Lee and its numerous bridges make for a variety of Italianate vistas, glimpsed by the walker from midstream bridges or through narrow lanes. These views often feature Cork's symbolic and most famous landmark, the clock tower of Saint Anne's, Shandon, with its nostalgic bells "that sound so grand on / The pleasant waters of the river Lee" (O'Mahony, "The Bells of Shandon").
Industrialization in Cork in the decades after independence (1922) was dominated by such plants as Ford's and Dunlop's, which afforded steady employment for decades to great numbers of Cork workers. The Sunbeam textiles factory was also important in the Cork economy. Under native government there were great advances in public housing, and in Cork vast local-authority estates were built on the south side at Ballyphehane and on the steep slopes above the North Cathedral, siphoning the population away from the decayed "marsh" area in the city center. Meanwhile, the outer suburbs continued to proliferate.
When the staple employment industries of car assembly and textiles collapsed in the 1970s under Common Market pressure, they were replaced in time by chemical plants, electronic businesses, and high-tech industries with a new wave of inward investment from multinationals. Cork shared in the remarkable "Celtic Tiger" prosperity of the 1990s and was worried by the signs of slowdown in 2001.
In the last decades of the millennium, enlightened municipal management arrested and reversed innercity dereliction. Mean alleys have been transformed into settings for continental-style bistros and boutiques, fine plazas have been created, and there has been much imaginative pedestrianization. A land-use and transportation study (LUTS) was gradually implemented to deal with ever-growing traffic problems.
The harbor, so crucial to the Cork economy for centuries, has continued to play a central role in greatly changed circumstances. In earlier years, grain, coal, fruit, and timber imports brought about storage and workhouse facilities in the dock areas. Various harbor activities, as well as the vital business of dredging, came under the auspices of the Harbour Commissioners, whose splendid headquarters is a notable architectural landmark in respect of both facade and interior. The political significance of the harbor was underlined in 1938 when the British handover of naval bases, in Cork harbor as elsewhere, completed the process of sovereignty transfer that had begun in 1922. Today, large cross-channel and continental ferries constitute another facet of harbor business, as do the numerous industrial and chemical sites from Little Island to the lower harbor in the Ringaskiddy area. Meanwhile, international travel in and out of the city was transformed and intensified by the development of the thriving Cork Airport (opened in 1961), which combines efficiency with a warm and distinctive local flavor. According to the latest census figures, there were 127,000 people living within the municipal limits in 1996, with a further 53,000 in the suburbs.
Finally, we may observe that traditional rivalry between north side and south side is subsumed in a general Cork personality, recognized as distinctive by natives and outsiders alike. Apart from their renowned singsong-accented speech, Cork people tend to be perceived elsewhere in Ireland as wily, opinionated, self-confident to the point of hubris, ambitious, with a penchant for taking over the top jobs nationally, able, witty, garrulous, and ostensibly friendly and charming but clannish to a degree!
Cork Corporation Millennium Year Book. 2000.
Journal of the Cork Historical and Archaeological Society. 1891–.
O'Flanagan, Patrick, and Cornelius G. Buttimer, eds. Cork: History and Society. 1993.
O'Mahony, Francis Sylvester (Father Prout). "The Bells of Shandon." In The Cork Anthology, edited by Seán Dunne. 1993.
John A. Murphy
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.
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.
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
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 is the outer, regenerative bark of the cork oak tree, Quercus suber, family Fagaceae. Unlike other oak species, the cork oak is an evergreen tree and dates from the Oligocene epoch of the Tertiary period. The oldest cork fossil, dating 10 million years old, was found in Portugal and is identical to modern cork. Today cork oak trees grow exclusively around the edge of the Mediterranean, primarily in Portugal, Spain, and Algeria, and to a lesser extent in Morocco, Tunisia, Italy, Sicily, and France.
A cross section of the tree trunk or large branch of Q. suber reveals three distinct layers: (1) the inner and largest area, the xylem (woody tissue), (2) a thin layer covering the xylem called the inner bark, and (3) the outer cork layer, also known as phellogen or cork-cambium. When the tree is about ten years old, it is stripped of its outer bark (the cork) for the first time. Cork from the first stripping is called virgin cork. Strippers make vertical and horizontal cuts into the bark and lift the planks off. Care is taken not to damage the living inner bark, which cannot replenish the cork if it is damaged. Stripping takes place during spring or summer when new cork cells are developing near the inner bark, making the cork easier to strip off. Within three months after stripping, growth of the cork layer resumes. The cork layer stops growing when cold weather begins.
A cork oak is stripped about every 10 years. The second and subsequent strippings are called reproduction cork and are of better quality than virgin cork. A healthy tree can live for 150 years. An old tree with a large girth and branches can yield more than 1, 000 lb (455 kg) of cork in a single harvest. Cork oaks aged between 35 and 45 years typically yield about 200 lb (91 kg) per year, and trees aged 50 or 60 years can yield 330 lb (150 kg) of cork.
The cork planks are seasoned for about six months in the open air. Exposure to rain, wind, and sun during seasoning cause chemical transformations in the cork, improving its quality. After the planks are seasoned, they are boiled to remove tannic acid and resins, to soften the cork, and to make the rough outermost surface easier to remove. The cork planks are sorted according to quality. The highest-quality cork is used for bottle stoppers. The rest is either cut into sections or granulated to make agglomerated cork, which consists of small pieces (granules) of cork that are agglutinated (glued or adhered together) with either the cork oak’s own resin, or with products such as rubber, asphalt, or synthetic resins, and then heat treated under pressure to solidify the composite material.
The cork consists of suberose tissue formed by the phellogen, a tissue between the inner bark and cork, from which cork develops. Suberin is a waxy, waterproof substance in the cell wall (suberose tissue), made of fatty acids and organic alcohols, and making cork impermeable to liquids and gases. Although cork is the bark of a living tree, it is actually a conglomeration of dead cells. Each cell is a multisided polyhedron, only 30-40 microns in diameter, and is filled with a gas almost identical to the normal atmosphere. One cubic centimeter of cork contains about 40 million cells. Cork is made up of more gas (90%) than solid material, making its density very low. Cork floats and does not rot. It is also fire resistant, compressible, and does not conduct heat or sound very well. These characteristics make cork a useful material for diverse purposes. Cork is used, for example, in the fishing industry, the electrical and building industries, automobile and aeronautical industries, the manufacture of sporting goods and home furnishings, shoes, and musical instruments. Some examples of cork products are floor and wall tiles, expansion or compression joints in concrete structures, insulation, safety helmets, several types of sporting good balls, heat shields, gaskets, shoe soles, and fishing tackle.
Today, Portugal is the leading producer of cork. In Portugal, the cork oak tree is protected by stringent laws regarding the growing, pruning, and stripping of the tree. Cork cultivators are given technical and financial assistance, and cork products are highly regulated to maintain high standards of quality.
Christine Miner Minderovic
Cork is the outer, regenerative bark of the cork oak tree , Quercus suber, family Fagaceae. Unlike other oak species , the cork oak is an evergreen tree and dates from the Oligocene epoch of the Tertiary period. The oldest cork fossil, dating 10 million years old, was found in Portugal and is identical to modern cork. Today cork oak trees grow exclusively around the edge of the Mediterranean, primarily in Portugal, Spain, and Algeria, and to a lesser extent in Morocco, Tunisia, Italy, Sicily, and France.
A cross section of the tree trunk or large branch of Q. suber reveals three distinct layers: (1) the inner and largest area, the xylem (woody tissue ), (2) a thin layer covering the xylem called the inner bark, and (3) the outer cork layer, also known as phellogen or cork-cambium. When the tree is about 10 years old, it is stripped of its outer bark (the cork) for the first time. Cork from the first stripping is called virgin cork. Strippers make vertical and horizontal cuts into the bark and lift the planks off. Care is taken not to damage the living, inner bark, which cannot replenish the cork if it is damaged. Stripping takes place during spring or summer when new cork cells are developing near the inner bark, making the cork easier to strip off. Within three months after stripping, growth of the cork layer resumes. The cork layer stops growing when cold weather begins.
A cork oak is stripped about every 10 years. The second and subsequent strippings are called reproduction cork and are of better quality than virgin cork. A healthy tree can live for 150 years. An old tree with a large girth and branches can yield more than 1,000 lb (455 kg) of cork in a single harvest. Cork oaks aged between 35 and 45 years typically yield about 200 lb (91 kg) per year, and trees aged 50 or 60 years can yield 330 lb (150 kg) of cork.
The cork planks are seasoned for about six months in the open air. Exposure to rain, wind , and sun during seasoning cause chemical transformations in the cork, improving its quality. After the planks are seasoned, they are boiled to remove tannic acid and resins , to soften the cork, and to make the outermost, rough surface easier to remove. The cork planks are sorted according to quality. The highest quality cork is used for bottle stoppers. The rest of the cork is either cut into sections or granulated to make agglomerated cork. Agglomerated cork consists of small pieces (granules) of cork that are agglutinated (glued or adhered together) with either the cork oak's own resin, or with products such as rubber, asphalt, or synthetic resins, and then heat treated under pressure to solidify the composite material.
The cork consists of suberose tissue formed by the phellogen, a tissue between the inner bark and cork, from which cork develops. Suberin is a waxy, waterproof substance in the cell wall (suberose tissue), made of fatty acids and organic alcohols, and making cork impermeable to liquids and gases. Although cork is the bark of a living tree, it is actually a conglomeration of dead cells. Each cell is a multi-sided polyhedron , only 30–40 microns in diameter, and is filled with a gas almost identical to the normal atmosphere. One cubic centimeter of cork contains about 40 million cells. Cork is made up of more gas (90%) than solid material, making its density very low. Cork floats and does not rot. It is also fire resistant, compressible, and does not conduct heat or sound very well. These characteristics make cork a useful material for diverse purposes. Cork is used, for example, in the fishing industry, the electrical and building industries, automobile and aeronautic industries, the manufacture of sporting goods and home furnishings, shoes, and musical instruments. Some examples of cork products are floor and wall tiles, expansion or compression joints in concrete structures, insulation, safety helmets, several types of sporting good balls, heat shields, gaskets, shoe soles, and fishing tackle.
Today, Portugal is the leading producer of cork. In Portugal, the cork oak tree is protected by stringent laws regarding the growing, pruning, and stripping of the tree. Cork cultivators are given technical and financial assistance, and cork products are highly regulated to maintain high standards of quality.
Christine Miner Minderovic
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.