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The hard rubber (also called ebonite or vulcanite in some old advertisements) is a material invented[1] in 1843 and produced by a rubber vulcanization process in which the natural rubber is mixed with a variable percentage (from 20 to 50%) of sulfur, and hardened by keeping it at high temperature for a prolonged time (a few hours around 150°C). Hard rubber is generally produced in sheets, bars or slabs, which must be subsequently processed; in fact, it is not possible to make it with a mould.
  1. see the italian and english entries of Wikipedia, which, however, present some discrepancies, in particular on the attribution of the invention to O. Meyer and T. Hancock for the first and Charles Goodyear for the second.