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https://en.wikipedia.org/wiki/Ebonite
Ebonite is a brand name for a material generically known as hard rubber, and is obtained via vulcanizing natural rubber for prolonged periods. Ebonite may contain from 25% to 80% sulfur and linseed oil.[1][2] Its name comes from its intended use as an artificial substitute for ebony wood. The material has also been called vulcanite,[3] although that name formally refers to the mineral vulcanite.