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Cork comes from a variety of Cork Oak which regrows its bark as soon as it is removed. Each year, the tree produces a layer of cork which varies in thickness from 1 to 10 mm depending on the age of the bark itself, the age of the tree bearing it, its health and its growing conditions. Each annual growth is separated from the preceeding growth by a clearly defined brown line.

Alveolar structure of cork Cork consists of cells with thin, slightly flat, cellulose walls from about 20 to 30 mm in size, filled with gas (mainly nitrogen), the walls being 1 to 2 thousandths of a millimetre thick. The transfer of liquids and the diffusion of gases through the walls of these cells is extremely slow, to the extent that a thickness of a single millimetre of cork, consisting of about 30 layers of cells, is almost totally impermeable. The percentage volume of gas contained in these cells is 90%. This considerable proportion of gas, hermetically compressed by flexible walls, explains the mechanical and physical qualities of cork. Its value as a stopper lies in this cellular composition. It makes it a unique material. Cork is the only natural or manufactured product which offers : - low density, which makes it very light, - high elasticity (responsiveness) combined with excellent “compressibility” (flexibility), - remarkable adherence allied to a high degree of impermeability, - resistance to rotting, making it long-lasting. The conjugation of these characteristics means that the most delicate, the longest and the most difficult corking operations can be achieved. The material's elasticity enables it to take on the shape of the bottleneck, whatever it may be, even when irregular, in conformity with the established standards, with a spring force which keeps the cork firmly pressed against the glass sides. Its impermeability is due not only to the counterpressure exerted by the gas in the cells, but also to the presence in the cork's membranes of a high proportion of fatty substances (suberin, cerin). Its adherence to the glass is firstly the result of the force of expansion of an elastic material against a rigid barrier, but, and this is very important, it is also due to the numerous cells which were cut when the cork was cut, which form microscopic suction cups. Its adhesive properties are further increased by the presence in corkwood of resinous substances. Manufacturing process Recommendations for using corks