Gummi caoutchouc goma gomma guma gummi rubber rubber λάστιχο גומי Kautschuk caucho A natural hydrocarbon polymer formed from the resinous latex of trees of the Hevea and Parthenium genera. To prepare natural rubber, the latex is collected from a cut in the bark, precipitated with acid, then washed and dried. Rubber is very elastic and was used for bowls, shoe soles, adhesives, and bouncy balls. However, when cooled, rubber becomes brittle and when warmed it becomes sweaty and tacky. Prior to the development of synthetic resins, unvulcanized rubber was used for adhesive tapes and crepe shoe soles. In 1839, Charles Goodyear discovered that rubber can be hardened with the vulcanization process in which sulfur is used to initiate crosslinking of the hydrocarbon strands. Higher sulfur content produces a harder, denser material. Vulcanized rubber is used to make rubber bands, foams, fabric coatings, small objects, combs, pens, and musical instruments. Vulcanized rubber, however, will emit sulfur when exposed to light or heat causing the rubber to degrade and become brittle. Since the 19th century, small amounts of wax have been added to the rubber during vulcanization. The wax slowly migrates to the surface and provides a thin layer of protection from oxidation (AAT).