When it was first invented, ebonite was made of rubber and sulphur, and even to-day nothing has been found to better that combination of two simple ingredients. But it is essential that nothing but the best qualities of rubber and sulphur be used if good quality ebonite is to be the result.

Thus we find that the utmost attention is paid to the selection of raw materials-nothing but the finest latex rubber and the purest flowers of sulphur obtainable go to the making of high grade ebonite. As a further precaution, the sulphur is passed through hair seives to prevent any possible trace of impurities or foreign bodies getting through to the mixing and grinding machines.

These machines perform the first operation in the transformation of the rubber and sulphur into the familiar ebonite, and at this stage the panel material is still of a bright yellow colour. For purposes other than panels, however (such as rods, tubes, and so on), a small proportion of ebonite dust is mixed, this being necessary to ensure that the true shape of the material is retained during the vulcanising process.

In the grinding machines the raw ebonite is passed between two steel rollers under pressure, one of the rollers being steam heated and the other water-cooled. Heat plays an important part in the manufacture of ebonite, and all the machines and tables on which the material in its initial stages is worked are kept heated at a constant and uniform temperature.

After the grinding and mixing process the ebonite has attained the consistency of well-kneaded dough, and its colour varies from yellow to brown, according to the purposes for which it is ultimately intended, as has been already mentioned. Then it goes to one of two types of machine-the calendering or the extruding (or forcing) machine.

The calender rolls the ebonite into sheets, and, contrary to popular belief, only one thickness is rolled (about 1/10 in.), from which sheets of any thickness are built up. It was explained that if thick sheets were rolled there would be a chance of air bubbles forming in the material during the operation. As it is, the sheets are built up ply upon ply, by hand, and made into a homogeneous whole by the subsequent vulcanising process. But we shall return to this later.