Cork—Tiny Cells That Serve You Well
SCIENCE took a great step forward in the 1660’s, when Robert Hooke in England took a piece of cork and looked at it through the microscope he had specially constructed. He found that the material was not homogeneous, but consisted of a large number of tiny air-filled units. He called them cells, from the Latin word cella, meaning “small room.”
The cork cells are indeed small. No material made of cells, whether natural or synthetic, has as many cells per volume unit as does cork. There are, on the average, an estimated 20,000 of them per cubic millimeter! So tiny are they that it is not even possible to see the detailed structure of the cell with an ordinary microscope. Using electron microscopes, researchers at the universities of Cambridge, England, and Luleå, Sweden, have revealed the intricate construction of the cork cell. And it is this structure—a six-sided prism with corrugated walls, such as in the bellows of an accordion—that gives cork its unusual and most useful qualities.
Cork is light in weight, buoyant, strong, durable, and stable. It has resilience and compressibility. It resists air, oil, and water penetration. It absorbs vibration, has good frictional qualities, and has low thermal conductivity. Synthetic materials have not replaced it as the material of choice for bottle stoppers that are easy to insert and remove, and that seal well. Because of their chemical stability and elasticity, they can preserve wine for years without contamination. Cork has also had widespread use in insulation, soundproofing, floor coverings, bulletin boards, gaskets, shoe soles, and fishing floats and buoys—to name just a few.
The Cork Oak
While a thin layer of cork is found in the bark of all trees, it is from the cork oak of the Mediterranean area—principally Portugal, Spain, and Algeria—that most of the world’s commercial cork is obtained. The cork oak is an evergreen. The bark of the cork oak can safely be removed, and new cork will even form again!
The bark of the cork tree has two general layers. The thick outer layer, composed of dead cells, acts as a protective covering, insulating the tree from heat, mechanical injury, or loss of water. It is this layer that is harvested by a process called stripping. However, care must be exercised that the living inner layer is not damaged, or no new cork will form.
Stripping can be done when the tree has matured and its outer bark has thickened—usually taking from 20 to 25 years. After the cork has been pried off the tree, it is first allowed to dry for a few days. Then it is boiled to remove the tannic acid and sap. This also increases its elasticity and softens the cork so that it can be straightened out and packed in bundles for shipping. The rough, outer layer is also loosened by this process and is scraped off. The tree is left to regenerate its outer bark, from eight to ten years, when it can be harvested again. The best cork comes after the second stripping, and a tree can be productive for over a hundred years.
Cork production now exceeds a half million tons a year—equal in volume to 28 million tons of steel. Every year some 20 thousand million stoppers are made for wine bottles alone. Many of the applications of cork have been known for over 2,000 years. “Few materials have such a long history or have survived so well the competition from man-made substitutes,” says a study made at Cambridge University. Its secret? The uniquely constructed tiny cork cell—a marvel of creation.