Forest Preserve District of Cook County, Illinois
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Cork
Nature Bulletin No 626   January 28, 1971
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist

CORK
The first of these weekly nature bulletins was published on February 1, 1945. As a rule, during those 16 years, we have avoided general statements about groups of plants or of animals because, almost invariably, there are exceptions.

For example, consider the oaks. Several hundred kinds are found in North America, Europe and Asia. Most of them are slow growing, long-lived, and when mature are stately sturdy trees with wood that is strong, tough, durable, and valuable for many purposes.

The Cork Oak, native in Mediterranean countries, is a notable exception. It, too, becomes a large majestic tree but the wood, although dense and hard, is of little use as lumber because it tends to check and crack. The bark, however, is unique and from it we obtain cork, a material with innumerable uses. So far, few substitutes for it have been found.

All kinds of trees build a new layer of inner bark each year but on a cork oak these additions gradually form a homogeneous mass of soft spongy material -- several inches thick -- with a remarkable combination of desirable qualities.

Each layer is composed of rows of brick-shaped, air-filled cells so tiny that a one-inch cube of cork contains approximately 200 millions of them. More than 50 percent of it is air. The thin walls of those cells are saturated with a fatty waxy substance, called suberin, which makes cork almost impervious to water and air. Cork is also very light, buoyant, elastic, compressible, and a poor conductor of heat.

Cork oaks become from 30 to 60 feet tall, with trunk diameters of 4 feet or more and thick wide-spreading branches. The oval leaves, about 3 inches long, are evergreen. About half of the world's annual crop of cork comes from forests in Portugal but the tree is also abundant in Spain, Morocco, Algeria, Tunisia, Corsica and southern France. The highway from Bordeaux to the Pyrenees is lined with gnarled old cork oaks with deeply furrowed bark. Some trees live to be 500 years old. Cork oaks are cultivated on plantations in India and, since 1940, in California, Arizona and New Mexico.

Young trees are about 20 years old when they yield their first crop but that "virgin" cork, coarse and woody, is of little value except for insulation. As this is replaced by new annual layers, subsequent sheaths of outer bark -- from l/2 to 2-1|2 inches thick -- are removed every 8 or 10 years. The quality of the crop continues to improve until the fifth or sixth stripping, after which it remains stable.

The stripping is usually done in July or August. Using a sharp hatchet, with great care not to injure the inner bark, a cut is made around the trunk near the base and another just below the first branch. These are joined by two or more vertical cuts and the workman then pries off the slabs of cork. On large trees the lower limbs yield thin sheets of high quality.

Until about 1900 the principal uses of cork were for stoppers in bottles and jugs, life preservers, floats, tropical helmets, and in linoleum. Artificial limbs, baseball centers, and badminton "birds" were made of cork. Then it was discovered that by grinding the poorer grades and combining them, under heat and pressure, with other materials, a great variety of products could be manufactured.

For example: corkboard for soundproofing and as insulation in refrigerators and cold storage plants; gaskets and washers in engines and motors; pipe coverings; polishing wheels; floor and wall coverings, beverage bottle caps; etc.

Irishmen say that the best Cork in the world is on the Emerald Isle.


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