Page:Popular Science Monthly Volume 31.djvu/655

Rh winds and insolations, has the additional advantage of expediting by a year the time when the next crop of cork will be fit to gather in.

Before being put into the market, the bark has to be subjected to the operations of steeping, scraping, sorting, and packing. The object of the steeping, which is performed in large boilers of water, heated by means of chips of the bark, is to swell the cork and increase its elasticity, and it has the further effect of enabling curved pieces to be made straight. Scraping, for the removal of the woody parts, is done with iron scrapers, or with a machine in which the rotating chisels make nine hundred turns in a minute. It involves a loss of twenty-eight per cent of the crude bark, but is not so necessary when the formation of the young cork has been protected in the manner that we have described. In England, these two primary operations are replaced by a process of scorching and brushing the bark. The sorting is done with reference to five degrees of thickness, after which the bark is packed in bales containing about two hundred pounds each. At the market it undergoes another testing for quality, in which there is a wide range, and a corresponding diversity of prices. According to M. Lamey, in his book on "The Cork-Oak in Algeria," the bark should never be gathered till it is seven eighths of an inch thick, and that is preferred in commerce which is from one and an eighth to one and a quarter inch thick. To produce such thickness, from six to nine years of growth are needed.

The density of cork varies with its quality and age. Thin corks are usually heavier than those of the same volume that have grown more rapidly, and, in corks of the same class, the density increases with the age. M. Brisson gives 0·240 as an average maximum, and the ordinary density of a ten-years-old cork may be taken at 0·2. With extreme lightness are associated other valuable qualities: that of being a poor conductor of heat and sound; impermeability to liquids; imperfect combustibility, and non-liability to decay, by reason of which it is susceptible of very numerous applications in industry. The most important use of the substance is for bottle-corks. The bark which is intended to be used in this form is kept in a damp cellar. When taken to the shop, it is cut by the first workman into strips, the width of which corresponds with the length of the future cork. A second workman cuts these strips into squares suited in size to its diameter. The squares, strung, are plunged into boiling water to make them swell out. They are then stored in a cool place, and kept constantly moist by sprinkling, till they pass into the hands of the cork-maker. He applies them in succession, giving them a rotary motion, to the edge of a wide-bladed knife, drawing them at the same time slowly along its length, and by skillful manipulation transforms the square into a round cork. This is the method usually practiced in France. Workmen in other countries handle the knife in different manners. It is essential, to obtain a good and solid cork, to take care