The term "bitumen" is of Celtic origin, and means mineral pitch or "earth resin". This flammable liquid of brown-yellow to black color shows tough, viscoelastic properties at room temperature. It is composed of thousands of high- and low-molecular-weight compounds, including hydrocarbons, resins, paraffins, waxes, fats, heavy oils, lignins, proteins and asphaltenes. In contrast to asphalt, bitumen is free of solids (e.g. minerals). Natural bitumen consists of natural organic materials.
Bitumen serves foremost as an asphalt binder for road pavement mixtures, in which it binds solid mineral additives such as sand or small stones. For this purpose, it needs to remain stable against demixing at high temperatures. On the other hand, it must not tend towards brittle fracture at low temperatures.