Cork (Span. corcho, from Lat. cortex) is the extraordinarily developed corky layer (see below, and article BARK) of the bark of the Cork-tree or Cork-oak (Quercus suber) of the Mediterranean. Spain and Portugal chiefly supply the world with cork. The cork-tree is not of great size, generally 20 to 60 feet high, the trunk often 3 feet in diameter, much branched, with ovate-oblong evergreen leaves, entire or serrate. The acorns are edible, and resemble chestnuts in taste. The tree is usually twenty or twenty-five years old before it yields a gathering of cork, and attains an age of 150 years. The first produced (Virgin Cork) is of little value, but is removed in order that the next production may be better, for every successive formation is superior to that which preceded it, and it is not till the third gathering that cork of the highest quality is obtained. About every eight or ten years a crop is taken; but the improvement is being introduced of leaving the loosened cork-layer for some months as a protective jacket upon the tree. The cork-cambium is thus protected from the sun and the attacks of insects, and the new growth is thus both more rapid and of finer quality. In stripping off the cork, longitudinal and transverse incisions are made to the proper depth, and each piece is then cut away from the tree by a curved knife with two handles, like that of a cooper. The pieces are soaked in water, pressed flat, dried, and superficially charred, to remove decayed parts and conceal blemishes, before they are packed in bales for the market. Besides the use of cork for stopping bottles, casks, &c., it is much used, on account of its lightness, for floats of nets, life-belts, &c.; and on account of its impermeability to water, and its being a slow conductor of heat, inner soles of shoes are made of it. All these uses are mentioned by Pliny; but the general employment of corks for glass bottles appears to date only from the 15th century. The Spanish Black used by painters is made by burning cork-parings in closed vessels. Cork-waste is also utilised for many objects, and most largely in the manufacture of linoleum.
The cork-tree is occasionally planted in England; it has been found to do well in some parts of the United States.
The wood of some trees possesses the cellular sponginess, lightness, and elasticity of cork in such a degree as to be sometimes substituted for it in many of its uses, as that of the Anona palustris (Corkwood
Formation of Cork in a branch of Black Currant, one year old (mag. 350 diameters): e, epidermis; b, bast-cells; pr, cortical parenchyma; k, the cork-cells formed from c, cork-cambium. (Sachs.) or Alligator Apple) in the West Indies, &c.
As already mentioned under Bark (q.v.), cork is by no means botanically the exceptional product it seems from the practical or popular point of view. The epidermis being usually only a single layer of definitely formed cells, it is insufficient either for covering or protection in almost all conditions of continuous growth and exposure to climate (the mistletoe being rather a less developed type than a true exception among woody plants, since its deeper ordinary epidermic cells never lose the power of multiplication). In sections of the young shoots of dicotyledonous trees or shrubs this provision for replacing the epidermis can be beautifully traced; a layer of cells just outside the cellular envelope, the so-called cork-cambium, is seen to have already formed an appreciable thickness of cork-cells, easily recognised by their prismatic shape, tolerably regular vertical series, and thin walls. As the stem grows older the epidermis wears off and the cork-layer thickens, while the access of air to the living cellular envelope below is permitted by the occurrence of spaces empty or filled only with looser tissue, the Lenticels (q.v.), which, of course, become represented by the deep air-channels so obvious in the inferior quality of cork.
CORK-CUTTING.—The bark, after being cut into square pieces or sheets, is pressed to remove its natural curvature and flatten it. If it is found that simple pressure has not flattened it sufficiently, it is heated on the convex side, and the contraction thus produced straightens it. It is then cut into slips, and these slips into squares, according to the required size of the corks. These are rounded by the cork-cutter by means of a broad sharp knife; the cork is rested against a block of wood, and the knife pushed forward, its edge at the same time being made to describe a circular curve. The knife requires continual sharpening; the workman has a board before him on which the knife is rubbed on each side after every cut. But cork-cutting is now largely done by machinery; in the United States the process has been carried to great perfection. Corking-machines are ingeniously contrived to force the cork into the neck of the bottle, and, if necessary, to wire it down. For rock cork, see ASBESTOS.