Rubber gum is the sap of a south american tree. Unfortunately in it's early days of use it was found that rubber would melt in heat and became quite brittle in the cold. Simply said to vulcanize rubber is to weatherproof it. Somewhere in the 1830's the inventor Charles Goodyear, after many failed attempts accidentaly discovered that crossing sulphur with rubber stabilized it. Eventually this process became known as vulcanization of rubber and is still a method used today to strenghten rubber.