Raw RUBBER is sticky and plastic. It hardens in cold and softens in heat, and it is not elastic. In 1839, Charles Goodyear discovered that by combining raw rubber with sulfur and heating it, the rubber became stronger, elastic, unaffected by heat and cold, and resistant to solvents. Today almost all rubber, as well as synthetic rubber, is vulcanized. The degree of hardness varies directly with the amount of sulfur and heat used in the process.