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In a search through the great number of papers in the scientific journals dealing with plantation and wild rubbers, one comes to the conclusion that the rubber technologists concerned with manufacturing operations would much prefer to purchase their natural rubber on the basis of specifications set up by scientific laboratory tests than from the results of the past and present-day methods of evaluation solely by superficial appearance. Ever since the beginning of the rubber industry, the natural-rubber broker has purchased crude rubber from the producer and sold it to the manufacturer on the basis of cleanliness and a few other rule-of-thumb tests or observations. This is a very peculiar situation because practically all of the other ingredients used in the manufacture of rubber products are purchased on the basis of specifications which are dependent on scientific laboratory tests. Also the final rubber products are usually sold on a performance or quality basis rather than on appearance alone. Except for cleanliness, the superficial appearance of a crude rubber has very little in common either with the intrinsic quality of the crude rubber or with the quality of the product made from it. Practically all uses for rubber are for products in the vulcanized state, so why should not the manufacturer be interested in purchasing his crude rubber on the basis of tests made on its vulcanizate? The present method of evaluating rubber on superficial appearance alone is, however, not altogether unwarranted or unsound. It involves an extremely simple and rapid procedure and does not require an expensive laboratory and scientifically-trained personnel. It operates on the assumption that if the crude rubber is clean, uniform in color, bubble-free, “rust”-free, and free from fungus formations, it probably has had extreme care in its preparation and will therefore exhibit its optimum physical and chemical properties. This assumption, however, obviously does not always hold true.
National Bureau of Standards, Washington, D. C.,