Cork is the bark of the Cork Oak tree (Quercus suber) which is harvested in sheets from the tree when the bark is thick enough. The bark is no longer a living part of the tree hence can be removed without greatly affecting the tree. Cork has been used for many centuries, and continues to have many uses, most notable of which is as a stopper or "cork" on bottles of wine

-Uses of cork

-Properties of cork

-The cork oak tree

-Harvesting of cork

-Links

-References

The use of cork can be divided into two, that of unmodified cork and that of composition or agglomerated cork. The unmodified cork pieces are punched out from a large piece of cork that has not been modified since its removal from the tree. Composition cork is formed from small pieces and trimmings of cork which are left over from the processes of using the unmodified cork. These small pieces are ground down to an acceptably small size and glued together to form various products. This results in less wastage of the cork, as very small pieces can be used, though the properties of the composition cork may be slightly different to those of unmodified cork (Cooke 1961).

Bottle Stoppers. (Corksupply) The most common use of cork is for bottle stoppers, also known as "corks", especially in the wine industry. The ability of cork to be compressed yet not be permanently distorted (discussed previously under the title "Properties of Cork") gives cork the ability to form a very good seal with the glass bottles, while the high friction surface keeps the stopper in place. Being impervious to water and other liquids has made the cork the chosen stopper for centuries, and it continues to do so (Cooke 1961).

Gaskets. These items are used extensively in the automotive industry to form a seal, most commonly between two metal surfaces. Other materials are beginning to be used more widely for this purpose, however cork has been very commonly used for this purpose in the past (Industries Assistance Commission 1981).

Flooring. Due to its ability to insulate well, and not transmit energy, cork flooring (usually sold in the form of tiles) has gained a reasonable market share. The sound and vibration insulation of cork is also far superior to that of a similar product, ceramic tiles (Industries Assistance Commission 1981).

Other less notable products that use cork include; shoes, handles of fishing rods (C&D Trading inc., G.Loomis and Sage) , bulletin boards (Magiboard and American Chalkboard) and various uses in insulation.

(all from Cooke 1961)

Most of the unique properties of cork relate to the cell structure, which is very different to other natural materials. The difference is that each cell is filled with air, is sealed and not internally connected to any adjacent cells. The individual cells are bonded together by a strong natural resinous substance, giving the cells strength.

Resiliant.Cork is very unique and useful because it can be compressed, yet it remains resilient. Cork can be compressed by exerting pressure, and when the pressure is removed it will return to its original volume. When the piece of cork is put under pressure, the air in the cells is compressed, and when the pressure is removed the air expands to fill the volume it previously occupied such that the piece of cork will also return to the volume it previously occupied. However if too much pressure is exerted the cells will collapse and the volume of the piece of cork will be reduced.

Impervious to water and other liquids, including oil. This is because individual cells are not connected to each other therefore capillary action does not occur, as it does, for example in some types of wood.

Low density because the cells are filled with air.

Low thermal conductivity, again due to the cells that do not transmit energy well because they are filled with air.

High friction surface. When the surface of a piece of cork is cut thousands of small cups are formed that create a vacuum when the surface is pressed against a smooth surface.

Vibration and sound absorbency. This is because the air filled cells compress and therefore soak up some of the energy that is being transmitted through them.

The Cork Oak (Quercus suber) is naturally found only in the western Mediterean basin and adjacent areas on the Atlantic coast. The countries where it is naturally found are Portugal, Spain, Southern France, Italy, Greece, Tunisia, Algeria and Morocco.

The Cork Oak grows to a maximum height of approximately 12 meters in height, and in rare instances as high as 18 meters, and diameter of the bole (trunk) of up to 60 cm and in rare instances up to 1 meter. The tree usually has a short bole with large branches that spread out (Byles 1930).

(See Corksupply USA for a photo essay of the process)

Harvesting begins when the tree is around 20 years old, and 20 cm in diameter and the outer, dead, layer of bark is first removed >from the tree. This cork is of relatively low standard and would be irregular, hard and have deformities (Palmgren 1947). After the first stripping the tree will have its bark harvested every 8 to 10 years and the quality of this cork will improve and be at a maximum after the third harvest (Cooke 1961). The interval between the subsequent harvests will vary depending on the growth of the cork, which is slower in the older trees. (Palmgren 1947).

The stripping of the bark is performed in late spring, but mostly during summer when the sap in the tree is flowing freely and separation of the bark from the tree is easiest, minimising damage to the tree (Cooke 1961). When stripping the bark, cuts are made around the tree, through the bark, at points just above the ground and below the first branch. Care must be exercised not to cut through the cambium, which lies below the bark, and if cut could lead to the death of the tree. A vertical cut between the two circular cuts is made, and then the bark can be levered off the tree. The cork is then put in boiling water to remove tannins, dirt and other unwanted matter that is water soluble. The outermost portion of the bark is scraped off and the cork is allowed to dry before being graded and packaged (Cooke 1961).

Corksupply (Australia)

Corksupply (USA)

American Chalkboard

Magiboard

FAO non-wood Forest Products

ANU

Byles, B.U. 193-? Report on cork oak. Commonweatlth Forestry bureau, Bulletin No. 4. Commonwealth government printer, Canberra.

Cooke, G.B. 1961. Cork and the cork tree. Pergamon press, Oxford.

Industries Assistance Commision. 1981. Report no. 279. Cork and articles of cork. Australian government publishing service. Canberra.

Palmgren, E. 1947. Cork production and international cork trade. International institute of agriculture. Rome.

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Copyright© 1998 The Australian National University

Author: Max Grimbacher

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Last modified 29^th October 1998-10-22

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