Cathy Shurie, age 12, of Niagara Falls, Ontario, Canada, for her question:
How is cork made?
Cork, real cork is a gift from the plant world. The ancient Romans used it to seal bottles and made sandals and in the modern world it has countless other uses. We also have synthetic corks but none of these man made materials have all the good qualities of the real thing. For that, we must go to certain sturdy old oak trees that prefer to grow in sunny parts of Europe.
Chances are, there is a slim lining of cork under a bottle cap and maybe a layer of cork in the sole of your shoe. There is cork inside the walls of most refrigerators and often it is used to line entire rooms. For cork is used to make materials used to cover floors, insulate walls and soundproof ceilings. Some is used to make bottle stoppers and, of course, cork is the most dependable material you can use to stuff a life belt.
And all of these corky materials are made from the bark of the cork tree. The sturdy old tree is an evergreen oak that keeps its lively green leaves all year. Obviously we would like this valuable tree to grow around the world. But it does best in Spain, Portugal and the Algerian region of North Africa. There, every summer, they harvest thousands of tons of thick porous bark from the cork trees.
For the first 20 years of its life, a cultivated cork tree produces nothing more useful than shade. Meantime, it gradually grows a layer of remarkable outer bark around its trunk and branches. All trees, of course, grow a rough layer of outer bark, built from boxy cells of dead wood. But the cork tree's outer bark is something special. Its thin woody cell walls are thickened with waxy material and filled with air. There may be 100 million of these tiny cells in a cork bottle stopper. Because of the porous pockets, the total weight is only one quarter as heavy as water. Because of this porous quality, cork floats on water, keeps out moisture and tends to insulate against heat and cold.
After 20 years, cork strippers arrive with long handled knives. They make careful slices down the trunk and along the main boughs and peel back the valuable outer bark. The job must be done with great care, for if the inner bark is bruised, the next crop of outer bark will fail to cover the wound. If all goes well, the patient cork tree will spend the next ten years or so re building outer bark over the stripped area.
The best quality cork begins with the third harvest, when the tree is 40 years old. Ten years seems a long time to wait between harvests, but cultivated cork trees live and keep on yielding through three or four centuries. And usually the growers arrange to strip about one tenth of their trees every year.
The strips of corky bark are boiled to remove tannic acid and to soften the woody cells. It is sorted into different grades and the best qualities are made into bottle corks and such. Poorer qualities are ground into crumbs, pressed and remodeled to make flat rolls of insulation, floor covering and soundproof materials.