Natural rubber is made harder and more elastic over a wide temperature range by vulcanisation, in which it is heated with

Explanation

Short chains of sulphur atoms bridge the polyisoprene chains, so they can no longer slide past one another permanently and the rubber recovers its shape instead of deforming, remaining useful when hot or cold. The more sulphur is added, the harder the product becomes, with ebonite at the extreme. Carbon black is also added to tyres but acts as a reinforcing filler rather than a cross linking agent.