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
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.
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
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
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.
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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Update: June 2012