Passage II

SOCIAL SCIENCE: This passage is adapted from the book Tree Bark: A Color Guide by Hugues Vaucher, translated and edited by James E. Eckenwalder (@2003 by Timber Press, Inc.).

Bark makes up 6-22% as much of the bulk of tree trunks as the wood. In many lumber mills, bark has often been discarded as waste or burned in the open air. This poses a big problem in terms of groundwater contamination and air pollution. The exploitation of bark takes advantage of its three most important properties: its energy content, which is about the same as that of wood; its low density; and the diverse cellular contents. These physical properties and the high concentration of chemical cell contents explain the diverse potential uses for bark.

Cork oak (Quercus suber) is virtually without parallel among trees. It is the only one that is exploited industrially and exclusively for its bark: cork, a raw material whose properties are nearly impossible to duplicate artificially. This tree of the Mediterranean countries and of the adjacent Atlantic coasts is cultivated principally in Portugal, which with 330-440 million pounds per year provides 50% of world production. Portugal has more than 1.8 million acres of cork oaks under cultivation. Next come Spain, Algeria, Morocco, France, Italy, and Tunisia.

A standardized procedure is used for harvesting cork. When the cork oak reaches the age of 20-25 years and its trunk measures about 12 inches in diameter, it can be debarked (or stripped) for the first time. The virgin cork from this harvest is of low quality and is generally only used for granulated cork. After another 8-10 years, the cork will have regrown to a thickness of inches. The second harvest can then proceed, and thenceforth every 8-10 years to an age of 130-150 years if the tree holds up well and grows in favorable climatic conditions. Trees that are not subjected to this harvesting regime can live 300-400 years!

The uniqueness of the cork oak lies in the great ease with which the outer bark (cork and rhytidome) can be separated from the inner bark (bast), preserving the living part that produces the cork. The art of stripping, which is carried out from mid-June to mid-August (while the sap is rising), lies in making longitudinal and then horizontal cuts on the trunk in order to release strips or sheets without damaging the inner bark. Thus the growth of cork can resume without initiating faults in the new cork. The living bark, which is reddish yellow after debarking, gradually turns ochre thereafter, then brownish red over the course of weeks, and finally blackish gray after about a year.

Although cork is a dead bark, it should be considered a noble plant material since its intrinsic properties are numerous and irreplaceable by any single synthetic material of the same cost. The qualities of cork can be summarized as follows: lightness (density of 0.15-0.25 ), good resistance to compression and bending, good elasticity, great capacity to absorb vibration, very high coefficient of friction, low coefficient of swelling, very low heat conduction, inertness to chemical agents and to boiling in water (212°F), very good resistance to absorption, impermeability to moisture, excellent durability and freedom from decay nearly noninflammable, good coefficient of noise absorption, and handsome appearance, permitting its use for decorative articles.

The industrial and small-scale applications of cork are as varied as they are numerous. Its most familiar use is as stoppers (corks) for bottles. However, the largest volume of cork consumed by industry actually goes into the manufacture of cork aggregate soundproofing panels and cork tiles used as flooring in private homes, public buildings, factories, sports halls, etc. Significant quantities of cork are also used to make sheets of various thickness that are used in light carpentry and for interior decoration.

The list of products that are made wholly or partly of cork is impressive. We can cite, in no particular order: floats for fishnets; life preservers; antivibration blocks for machinery; insulation for appliances, including refrigerators; parts for toys; linings for carrying cases; blocks for printing on bags, fabrics, or wallpaper; tabletops, trays, and cork boards; model making by sculptors and architects; boxes and decorative objects sculpted from cork; insulating covers; polishing wheels for the glass industry; articles for sports, education, and handicrafts; protective packing material for fragile commodities; parts for footwear, clogs, and orthopedic devices; etc. One could also mention the aerospace industry, which uses cork for its qualities of lightness coupled with good thermal insulation.