Cork is a surprisingly interesting and useful substance. Cork is the outer bark of the cork oak tree Quercus suber L, which is native to the west Mediterranean basin. Cork oak plantations cover approximately 2.2 million hectares in Portugal, Spain, France, Italy, Morocco, Tunisia, and Algeria, and about 360,000 tons of cork are stripped from those trees each year. The world's largest producer of cork is Portugal, which produces over 50% of the world's cork. In 1999, the cork industry accounted for 3% of Portugal's gross domestic product (GDP)!

If performed properly, the stripping of cork from cork oaks is nondestructive to the trees. When a cork oak reaches about 25 years old, it is stripped for the first time, producing "virgin cork." After 10 years or so, the oak has regrown its cork layer. This "reproductive cork" is extracted from the tree and the tree is allowed to regrow its bark yet again. Finally, after another 10 years, commercial-quality cork can be harvested. Commercial-grade cork can be stripped from the oak for the remainder of its lifetime, which is typically between 150 and 170 years.

Cork is of one of many examples of nature's incredible engineering prowess. Cork consists of 14-sided, vacuous polyhedral cells (nonliving) arranged in a hexagonal pattern. The cells are filled with atmospheric gas (minus carbon dioxide). The structure and properties of cork are dominated by these gas-filled cells. The cork cells are composed mainly of suberin, a complex compound of fatty acids and organic alcohols. Suberin is impermeable to gases and liquids and is resistant to fire and insects. These factors helped contribute to the evolutionary benefit of cork bark to the cork oak tree.

Interesting properties of cork

  • The gaseous cells make cork very low-density. Therefore cork is lightweight and buoyant, and useful in buoys, lifejackets, etc.
  • Stress on a chunk of cork causes only local compression of trapped gases, and not overall shape deformation. Cork has a Poisson's ratio close to 0. This property and cork's impermeability are the reasons for its use in the wine (and Chimay beer) industry.
  • Cork is a very poor thermal/acoustic conductor. These properties make it useful in the construction industry. Cork flooring is one prominent application of cork's low acoustic conductivity.
  • Cork is a renewable resource. The stripping of cork does not harm the cork oak trees.