Cork is the outer, regenerative bark of the cork oak tree, Quercus suber, family Fagaceae. Unlike other oak species, the cork oak is an evergreen tree and dates from the Oligocene epoch of the Tertiary period. The oldest cork fossil, dating 10 million years old, was found in Portugal and is identical to modern cork. Today cork oak trees grow exclusively around the edge of the Mediterranean, primarily in Portugal, Spain, and Algeria, and to a lesser extent in Morocco, Tunisia, Italy, Sicily, and France.
A cross section of the tree trunk or large branch of Q. suber reveals three distinct layers: (1) the inner and largest area, the xylem (woody tissue), (2) a thin layer covering the xylem called the inner bark, and (3) the outer cork layer, also known as phellogen or cork-cambium. When the tree is about 10 years old, it is stripped of its outer bark (the cork) for the first time. Cork from the first stripping is called virgin cork. Strippers make vertical and horizontal cuts into the bark and lift the planks off. Care is taken not to damage the living, inner bark, which cannot replenish the cork if it is damaged. Stripping takes place during spring or summer when new cork cells are developing near the inner bark, making the cork easier to strip off. Within three months after stripping, growth of the cork layer resumes. The cork layer stops growing when cold weather begins.
A cork oak is stripped about every 10 years. The second and subsequent strippings are called reproduction cork and are of better quality than virgin cork. A healthy tree can live for 150 years. An old tree with a large girth and branches can yield more than 1,000 lb (455 kg) of cork in a single harvest. Cork oaks aged between 35 and 45 years typically yield about 200 lb (91 kg) per year, and trees aged 50 or 60 years can yield 330 lb (150 kg) of cork.
The cork planks are seasoned for about six months in the open air. Exposure to rain, wind, and sun during seasoning cause chemical transformations in the cork, improving its quality. After the planks are seasoned, they are boiled to remove tannic acid and resins, to soften the cork, and to make the outermost, rough surface easier to remove. The cork planks are sorted according to quality. The highest quality cork is used for bottle stoppers. The rest of the cork is either cut into sections or granulated to make agglomerated cork. Agglomerated cork consists of small pieces (granules) of cork that are agglutinated (glued or adhered together) with either the cork oak's own resin, or with products such as rubber, asphalt, or synthetic resins, and then heat treated under pressure to solidify the composite material.
The cork consists of suberose tissue formed by the phellogen, a tissue between the inner bark and cork, from which cork develops. Suberin is a waxy, waterproof substance in the cell wall (suberose tissue), made of fatty acids and organic alcohols, and making cork impermeable to liquids and gases. Although cork is the bark of a living tree, it is actually a conglomeration of dead cells. Each cell is a multi-sided polyhedron, only 30–40 microns in diameter, and is filled with a gas almost identical to the normal atmosphere. One cubic centimeter of cork contains about 40 million cells. Cork is made up of more gas (90%) than solid material, making its density very low. Cork floats and does not rot. It is also fire resistant, compressible, and does not conduct heat or sound very well. These characteristics make cork a useful material for diverse purposes. Cork is used, for example, in the fishing industry, the electrical and building industries, automobile and aeronautic industries, the manufacture of sporting goods and home furnishings, shoes, and musical instruments. Some examples of cork products are floor and wall tiles, expansion or compression joints in concrete structures, insulation, safety helmets, several types of sporting good balls, heat shields, gaskets, shoe soles, and fishing tackle.
Today, Portugal is the leading producer of cork. In Portugal, the cork oak tree is protected by stringent laws regarding the growing, pruning, and stripping of the tree. Cork cultivators are given technical and financial assistance, and cork products are highly regulated to maintain high standards of quality.
Christine Miner Minderovic