Bitumen is a product derived from distillation of crude oil, the most familiar daily use being for road surfaces or roofing.
Bitumen is made from part of the residue left over after gas oil has been drawn off from the bottom of a distillation column. This residue is subject to further processes to produce different grades of bitumen.
The qualities of bitumen will vary according to the type of crude oil distilled. In the UK refineries producing bitumen use crude oil from Mexico, South America or the Middle East.
Bitumen has a complex chemical composition, consisting mainly of hydrocarbons, with a high carbon content of between 80% and 90%. At normal ambient temperatures it is solid to semi-solid but becomes progressively more liquid at high temperatures.
Bitumen is made in a number of grades according to the use to which it will be put. These grades are rated according to a "penetration test", utilising a needle under a specified load which measures the extent to which the needle sinks into a test sample.
Bitumen can be modified or given specific qualities by further processes. These include blowing air through the bitumen mix to give it higher viscosity and greater resistance to softening (particularly for road applications), and secondary processes involving additives or emulsifying with water.
The many uses for bitumen include road surfaces, pavements, roofing and roofing felt, waterproofing and cladding.