CORK HAS 1000 USES

Press, Volume CXIV, Issue 33442, 25 January 1974, Page 15

CORK HAS 1000 USES

[By

David Guntston

Just as human beings and animals are covered with skin, one of the main functions of which is to keep out diseases, young trees- are covered by “tree skin,” a compact layer of cells, for the same reason.

As the tree grows and increases in girth, this skin is often replaced by another layer, formed from a row of cells beneath the surface. This is cork, one of nature’s most wonderful materials and used for many purposes by man for at least 2300 years.

Water cannot pass through cork, so all the cells outside it die off to form the rough outer layer of a tree we call bark. However, some water and also air must pass through if the inner cores of wood are to remain alive, so small gaps of very loosely-packed cells, called" “lenticels,” are dotted here and there in the corky layer, through which air and water can pass freely. These lenticels can clearly be seen as the dark streaks on an ordinary bottle cork.

Each year a new layer of cork is formed just beneath the old, which means that cork oaks can go on producing fine cork for cutting for at least 100 years. Beneath the ever-thicken-ing layer of dead bark, the work of cork formation goes on. This explains why when initials are cut in the outer

bark of a tree,, they become more and more indented year by year. For such a wound comes to be “healed” by successive layers of cork around it.

Cork in fact is a seven-fold natural wonder, and no completely satisfactory artificial substitute has yet been produced by scientists. Its seven unique properties are:

(1) Each individual cork cell has a geometric pattern of 14 faces. This divides all space effectively, without gaps, so work will not let air, water,

through. (2) Each cork' cell is more than half filled with air, which explains wT.y it is one of the lightest solid substances known. (3) This “dead” air, separated in the finelydivided honeycomb pattern of cork ceils, is, after a vacuum, the best insulator of heat or cold known.

(4) For the same reason it is also a near-perfect in-

sulator against the vibrations that cause sounds.

(5) The light yet tough walls of the cork cell, holding in the “dead” air, form a chemically inert material that will last almost indefinitely. In fact, only tunnelling insects normally destroy cork, not age or decay.

(6) Cork cell walls are also tougher and more resilient than rubber, even when enormously compressed.

(7) Slice a section of raw cork cleanly with a sharp blade, and you expose myriads of the hexagonal open cells on every square inch, each one acting as a tiny vacuum cup. This gives cork tremendous shock-absorbing, non-slip-ping and also polishing properties that add innumerably to its 1000 of so recorded uses.

Cork-producing oak trees are grown commercially in many parts of the world, with Spain and Portugal leading other countries like Algeria, France, Tunisia, Italy and Morocco.

Harvesting cork is an expert job, requiring great knowledge of each tree’s development and capabilities of further production, plus the fine skill of a surgeon. Then careful cuts are made just above the roots, linking them with long vertical cuts that follow the bark’s deepest natural cracks. The outer bark is pried off, and a not-too-deep layer of cork removed without injury to the tree’s living inner core, or “phellogen,” for the idea is for more cork to grow. These first crops of cork are low-grade material, often used only to make granulated cork for packing or soundproofing purposes.

It is another nine or 10 years before the same tree is harvested again, but this time the new’ growth of cork is fine-grained and much more valuable. The quality improves until the cork oak is about 40 years old, after which it will produce steadilyincreasing amounts until it dies, some time before its 150th birthday. The curved slabs of cork are boiled in large vats. This softens the roughlv-creviced outer layer so that it can be removed, and also enables the layers to be flattended before grading and trimming. The corks we pull from bottles come from the best quality product; but they represent only a tiny fraction of the 1000 or so uses in industry, science, medicine, building and manufacture.