Cork, the protective outer covering of the stems and roots of woody plants. Cork also refers to the outer bark of the cork oak, an evergreen that is grown commercially. The cork oak reaches a height of 60 feet (18 m) and a diameter of up to 24 inches (60 cm). The glossy, dark green leaves have toothed edges.
The cork oak grows along the coasts of southern Europe and North Africa. It is especially abundant in Spain and Portugal, which produce most of the world's supply, and in Morocco. Harvesting and marketing the bark of the cork oak is an important industry in these countries. Some cork oaks grow in California and the southeastern United States, but they do not reach a large enough size to be of commercial importance.
Cork is a very lightweight, resilient material. Cork cells are produced in the inner bark and are pushed outward. Their cell walls contain suberin, a fatty substance that acts as waterproofing. Although all trees produce a layer of cork, only the cork oak has a layer thick enough to be commercially valuable.
Cork cells are like tiny empty boxes with flexible walls. The cells trap and hold air. This air is responsible for the buoyancy of cork and for its insulating properties. Cork weighs less than one-fourth as much as water. It is waterproof and a poor conductor of heat and sound.
The biggest use for cork is in low-temperature insulation—particularly in cold storage warehouses, in water coolers, and around pipes of mechanical refrigerating systems. Cork for insulation is ground up, pressed into molds, and baked with its own natural resins into slabs of the desired shape. Cork sheets or cork tiles are used in soundproofing and as floor coverings.
Because it is flexible and waterproof, pure cork is used to make stoppers for bottles and barrels. Cork composition is used to line metal caps for sealing bottles. Other products made from cork or compositions of cork include life preservers, floats, innersoles for shoes, gaskets, washers, fishing pole grips, bulletin boards, and cork paper for cigarette tips.
When the cork tree is about 20 years old, it is ready to have its outer bark removed for the first time. This first bark, coarse and uneven, is used principally for ground-cork products. Nine or 10 years later, the tree is ready to be stripped again. This time the cork is of better quality. The cork attains its highest quality at about the fifth stripping. Cork trees are usually stripped of their bark every 10 years or so until they are 100 to 500 years old.
Cork is harvested in July and August. The workers make cuts around the trunk and down its length, dividing the bark into suitable sections. They must be careful not to injure the inner bark or the tree will stop producing cork at the scarred spot. The sections are removed from the trunk by inserting the flat handle of the cutter's tool between the cork and the inner bark.
The cork is then boiled to remove some of the tannin and to make it flexible. The rough outer bark is scraped off, and the sheets are flattened and baled for market.
Cork is graded according to its quality and thickness. After it is received at the factory, it is steamed to soften it further, then sliced into strips to prepare it for the various mechanical operations such as cutting and tapering. Cork stoppers are made by machine, then washed and dried. There are about 20 regular grades of cork and many special grades.
Cork has been used for more than 2,000 years. In ancient times, it was used for life preservers, shoe soles, and floats for fishing nets, and was made into stoppers for wine vessels and casks. When glass bottles came into general use in the 15th century, the cork industry expanded greatly. Many cork products have been replaced by rubber and plastic substitutes.
The cork oak is Quercus suber of the family Fagaceae.