Cork intended for the market is generally stripped off a year or two before it would natu rally conic away. and the process is repeated at intervals of twelve or fourteen years, according to the vigor of the tree. The cork of the first barking, which is yenta\ ed usually when the tree is about twenty-five years old. is known as the virgin bark, and is valueless. The removal of the cork does not require the removal of the whole bark, hut only of external layers of spongy cel lular tissue, all or the greater part of which has ceased to have any true vitality, and has become an incmnbranee to the trees. Consequently, in stead of being injurious, the taking of the hark, when done with proper care, rather promotes the health of the tree, which continues to yield crops for 150 years. In stripping off the cork longitudinal and transverse incisions are made to the proper depth, and each pieee is then cut away from the tree. The average yield for one tree is about 45 pounds. The slabs are placed in boiling water for one hour, a process which increases their bulk one - fourth and also their elasticity, and dissolves the tan nin and other substances. Steam is sometimes employed instead of hot water. .The cork is then scraped either by hand or by scraping ma chines, in which process it loses from 20 to 30 per cent. of its weight. The cork is then cut
crosswise into strips of the proper length, then lengthwise, and finally into pieces of the proper shape. in many districts this industry is car ried on in the homes by hand, but there are also cork-cutting machines, especially in the United States, of great efficiency. The cork is then washed in oxalic salt, or some other solution, and then dried. It is now ready to be sorted and packed for the market. In Spain the corks are packed in lots of 30,000 in bagging cloth, and called bales.
Besides its use for stopping bottles, casks, etc., cork is largely used, on account of its light ness, for floats of nets, swimming-belts, etc.: and, on account of its impermeability to water, and its being a slow conductor of heat, inner soles of shoes are made of it. All these uses are mentioned by Pliny; but the general employment of corks for glass bottles appears to date only from the fifteenth century. It is also used for bicycle-handles, smokers' mouthpieces, and many other modern devices. The bits that are left in the process of manufacture are ground to powder and used in the manufacture of linoleum and for many other purposes. The Spanish black used by painters is made by burning cork in close vessels.