Cork | Research & Encyclopedia Articles

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 thatoccur 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.

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 forthis 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.

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

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.

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