Page:Popular Science Monthly Volume 81.djvu/329

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the milling is complete, which ensues after about 3,000 revolutions at the rate of 30 per minute, the mixture of water, comminuted shrub and bagasse is discharged and flows into skimming tanks. In these a portion of the bagasse, consisting of fibrous material (bast, wood) and fragments of other cell walls, sinks, while the agglomerated rubber ("worm rubber"), accompanied by a bagasse composed of minute flakes of cork, floats. This floating mixture can now be separated into practically clean worm rubber and rubber-free bagasse by boiling, and then by submitting it to a pressure of about 250 pounds under water, in order to water-log the cork flakes. A further cleansing of the worm rubber flakes takes place in a tank supplied with a paddle wheel, which beats the floating particles and so rids them of adherent particles. After this there remains but the "sheeting," which is accomplished by passing the worm rubber between corrugated steel rollers to form sheets. It may be packed in sacks in this form or pressed into solid blocks, say, of 50 pounds weight and packed in boxes. In this process it is evident that there are two critical phases, first the agglomeration of the rubber and, secondly, its later separation from the bagasse. The former is relatively easy or difficult, according to the richness of the tissues in rubber, or stated otherwise in the size of the rubber particles within the cells of the tissue. The separation of rubber from bagasse depends on the difference in their specific gravity, using this term in a loose sense. The fibrous elements, namely wood, bast and broken open cell walls, are easily waterlogged—that is, the occluded air is easily dispelled, while the cork is difficult to break up and even more so to rob of its air.

The coarse worm rubber floats rather readily; but the smaller the particles the more slowly they rise to the surface. Any means which may be used to cause a swelling of the particles, or to lessen the distance which they must travel to reach the surface, afford help in attaining to segregation of the rubber. The colloidal properties of rubber make swelling possible by means of any of its solvents. The depths of the tanks which are used for separation of rubber and bagasse is obviously important. When it is known that an extraction of 7 per cent, rubber (of which the moisture amounted to 25' per cent.) was raised during about five years to 15 per cent, by slowly improved methods, we may realize the amount of experiment and ingenuity necessary. It would have been well for the industry to-day if, during the earlier period, there had been less desire to take advantage of the ease of acquiring profit and more study and experiment. To be sure, a good deal of rubber left in the bagasse was later reclaimed, but the total lost in spite of this must have amounted to a good deal. This was, however, only one form of economic waste. By far the most serious lay in the method of gathering the shrub by pulling it up by the roots, called,