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Charles Goodyear was determined to invent a way to make natural rubber withstand extremes of heat and cold. Eventually he succeeded – we now know the process he devised as vulcanisation. When John Boyd Dunlop later figured out how to make pneumatic tyres from rubber, it sparked a boom in demand that had horrific consequences in the Congo Free State, a colony ruled by Belgium’s King Leopold. Tim Harford tells how natural rubber still goes mostly into tyres, and its production still causes a degree of controversy. Producer: Ben Crighton Editor: Richard Vadon