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.