Vulcanization

Vulcanization is a kind of chemical reaction between sulphur, or another agent, and natural rubber. It makes cross-links between the molecular chains in the rubber. The reaction is not reversible. Charles Goodyear discovered it in 1839.
The purpose of this is to harden the rubber so it does not melt or soften in warm temperatures. This hard rubber can keep its shape when it is used in applications where it might be warmed or cooled.