Amazing Bark That Serves Man
HAVE you ever taken a cork stopper from a bottle? Did you know that the cork came from the bark of an oak tree? That bark serves man in a fascinating variety of ways.
The cork oak grows almost exclusively in areas near the Mediterranean Sea. It is an evergreen tree the thick outer bark of which is natural cork. Most trees have bark that contains cork cells. But the cork oak tree is the only one that has them in layers of sufficient thickness to be of commercial importance.
The bark of the cork oak indeed has amazing properties. Its outer layer is a compact mass of dead cells the thin walls of which have become thickened and waxy. This bark is soft and spongy, yet it does not readily absorb water and is practically airtight. It can be compressed a great deal, yet it springs back when released. Even after ten years in a bottle, cork will regain 75 percent of its volume when removed.
Besides this amazing resilience, cork is practically as light as a feather. A one-inch cube of it contains some two hundred million tiny air-filled cells. Thus more than half of cork’s volume is air. No wonder it is so light and resilient!
When a cork oak reaches about twenty years of age its outer bark can first be removed. This first or virgin bark is of inferior quality. Every eight to ten years thereafter the bark is stripped. Its quality continues to improve until the fifth or sixth stripping, after which it remains fairly stable. A cork oak will produce for 150 years or so, and will usually reach no more than fifty feet in height.
Care must be exercised when stripping that the inner bark is not damaged. If it is, cork will never again grow on the bruised spot. Workers use a long-handled hatchet, and carefully remove the bark in long, oblong sections. The bark is taken from only the trunk of young trees. However, from larger, older trees it is also stripped from the lower limbs.
After the bark is removed from the tree, it is stacked in piles and permitted to dry. Then the strips are boiled. This loosens the grit and dirt, allowing it to be scraped away. Boiling also dissolves the tannic acid, and improves the cork’s elasticity and softness.
The bark is now ready to be cut into natural cork products, one of special importance being bottle stoppers. Cork is viewed by many as the best stopper to close bottles of fine wines and liquors. This is because cork is not deteriorated by alcohol, even after many years. And it will not permit air to penetrate the bottle to damage the contents. Although cork was used as early as 400 B.C.E., it was actually not until the sixteenth century C.E. that it came into general use. The introduction of glass bottles contributed to this.
Until the turn of the present century, the cork industry was a relatively simple cork-cutting trade. Besides bottle stoppers, the articles produced included life preservers, floats, buoys, hat linings and shoe soles. While these items are still made, the discovery of a way to mold ground cork under heat and pressure has brought a great variety of new uses for cork.
For example, the development of corkboard provides an excellent insulating material for both low and normal temperatures. Its introduction completely revolutionized the cold-storage industry. Cork replaced makeshift insulating materials such as sawdust, hay, cinders, and so forth.
Isolation corkboard is ideal for stopping vibration of machinery. Acoustical corkboard reduces noise in schools, hospitals, restaurants, broadcasting studios and other places. Composition cork gaskets are important in the automobile industry. Cork powder is mixed with linseed oil and spread as a paste over canvas or burlap to make linoleum, a popular waterproof floor covering. Cork tile, cork carpeting and cork brick also are widely used for floor and wall covering.
To meet the demand for cork, Portugal, Spain, Algeria, Morocco, France and Tunisia produce over 800 million pounds of it a year. The ways this amazing bark serves man are indeed many.