In the industrial working of indiarubber, the various impurities present in the crude " wild rubber (bark, dirt and the principal impurities derived from the latex, except resin) are removed by the following process: The lumps of crude caoutchouc are first softened by the prolonged action of hot water, and then cut into slices by means of a sharp knife - generally by hand, as thus any large stones or other foreign substances can be removed.
The late Sir Benjamin Baker, F.R.S., suggested that the stresses might be measured by experiments with elastic models, and among others, experiments were carried out by Messrs Wilson and Gore a with indiarubber models of plane sections of dams (including the foundations) who applied forces to represent the gravity and water pressures in such a manner that the virtual density of the rubber was increased many times without interfering with the proper ratio between gravity and water pressure, and by this means the strains produced were of sufficient magnitude to be easily measured.
The stretching of the cell wall by the hydrostatic pressure is fixed by a secretion of new particles and their deposition upon the original wall, which as it becomes slightly thicker is capable of still greater extension, much in the same way as a thick band of indiarubber is capable of undergoing greater stretching than a thin one.
It must, however, be distinctly understood that it is not the mere admixture but the actual combination of sulphur with indiarubber that causes vulcanization.
Cut sheets, or articles made from them, may be saturated by being laid in powdered sulphur maintained for some hours at about 110° C. Sheets sulphured in this way can be made up into articles and joined together either by warming the parts to be united, or by means of indiarubber solution; after which the true vulcanization, or " curing," as it is termed, can be brought about in the usual way.