Cork is composed of dead cells that accumulate on the outer surface of the cork oak tree. It has a honeycomb like structure consisting largely of empty space; its density (weight per unit volume) is one-fourth that of water. The low density nature cork makes it useful in products like life preservers and buoys. The large amount of dead-air space makes cork an effective insulation material for both temperature and noise. Cork is fire retardant; flames will only char the surface, and no toxic fumes are generated. Cork has a very high threshold for wear that it is actually used to polish diamonds.
If you cut the surface of cork turns many of the microscopic cells into tiny suction cups, creating an effective non-slip surface. In addition to being flexible, cork is highly resilient. After being crushed under a pressure of 14,000 lbs/in2 (96,000 kPa), cork will regain 90% of its original size in 24 hours. Cork absorbs neither dust nor moisture, and it resists both rot and insects.
Among the many products made from cork are flooring materials e.g. linoleum, shoe insoles, roofing panels, gaskets, safety helmet liners, bottle stoppers, dartboards, bulletin boards, and cores for golf balls and baseballs. Numerous artificial materials have been developed to substitute for cork in specific applications such as synthetic pea in a referee’s whistle, foam insoles for shoes, or Styrofoam life preservers. However, no general substitute has been developed for cork that can be used in diverse applications.
The raw material for cork products is harvested from the cork oak tree (either the evergreen Quercus suber or the deciduous-Quercus occidentalis). The trees typically reach a height of 12-18 m and a trunk circumference of 2-3 m. The vast majority of the world’s commercial cork trees grow in the western Mediterranean region and the Iberian Peninsula. Portugal’s cork forests are the most productive accounting for 30% of the existing trees; they produce half of the world’s harvested cork.
A cork tree is ready for its first harvest when it is about 20 years old and each tree has a productive life of about 150 years. The first harvest is of poor quality, and can only be used to make agglomerated cork products. Subsequent harvests occur at nine-year intervals, when the cork layer reaches a thickness of 2-5 cm. The harvest from a young tree yields about 16 kg of cork, while the yield for an older tree may be 225 kg.
Production of compound agglomerated cork involves adding a binder or adhesive agent to cork granules. Different binders are chosen, depending on the qualities desired in the ultimate product (flexibility, softness, resistance to wear). Among those frequently used are asphalt, rubber, gypsum, glue, and plastic.
When Pierre Perignon (1638-1715) invented champagne in 1688, he found that the gaseous pressure inside his bottles blew out the hemp stoppers. To solve the problem, he invented corks held in place by wire.
The modern metal bottle cap was developed by the prolific Maryland inventor William Painter, who patented his first stopper in 1885. By 1891, his definitive design, a cork-lined metal cap with a corrugated edge that is crimped around the bottle lip, appeared. Painter called his invention the “crown cap,” founded the Crown Cork and Seal Company to market it.
Using a specially designed hatchet, the harvester slices through the cork layer on the trunk of the tree, taking care not to cut deep enough to damage the living portion of the trunk. Horizontal cuts are made at the base of the trunk and just below the lowest branches. A few vertical cuts separate the circumferential cork ring into sections of an appropriate size. Using the wedge-shaped handle of the hatchet, the harvester strips each panel of cork from the tree. On some large trees, cork is also stripped from the lower branches.
The cork planks are trimmed to a uniform, rectangular shape and are sorted by quality. The finest quality material will be used to make natural cork products like wine bottle stoppers. Poorer quality material will be ground and used to make composition or agglomerated cork.
Cork waste generated during the manufacturing process is ground and used to make agglomerated cork products. Cork powder that is generated by the grinding process is collected and burned to help fuel the factory. Chemical components removed from cork during its processing can be recovered as useful by-products and include tannin (curing leather), hard wax (used in products like paraffin, paint, and soap), resinous gum (helps vanish adhere to copper and aluminium), and phonic acid (used to make plastics and musk-scented toiletries).
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