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Bitumen is the residual (bottom) fraction obtained by fractional distillation of crude oil. It is the heaviest fraction and the one with the highest boiling point, boiling at 525 degrees Celsius.
In British English, the word 'asphalt' refers to a mixture of mineral aggregate and bitumen (or tarmac in common parlance). The word 'tar' refers to the black viscous material obtained from the destructive distillation of coal and is chemically distinct from bitumen. In American English, bitumen is referred to as 'asphalt' or 'asphalt cement' in engineering jargon. In Australian English, bitumen is sometimes used as the generic term for road surfaces.
Most bitumens contain sulphur and several heavy metals such as nickel, vanadium, lead, chromium, mercury and also arsenic, selenium, and other toxic elements. Bitumens can provide good preservation of plants and animal fossils.
Bitumen is primarily used for paving roads. Its other uses are for general waterproofing products, including the use of bitumen in the production of roofing felt and for sealing flat roofs. It is also the prime feed stock for petroleum production from tar sands currently under development in Alberta, Canada. Bitumen from tar sands is projected to account for 80% of Canadian oil production by 2020.
In the past, bitumen was used to waterproof boats, and even as a coating for buildings with some additives. The Greek historian Herodotus said hot bitumen was used as mortar in the walls of Babylon.  It is also possible that the city of Carthage was easily burnt due to extensive use of bitumen in construction.
Vessels for the heating of bitumen or bituminous compounds are usually excluded from public liability insurance policies.
Most geologists believe that naturally occurring deposits of bitumen are formed from the remains of ancient, microscopic algae and other once-living things. These organisms died and their remains were deposited in the mud on the bottom of the ocean or lake where they lived. Under the heat and pressure of burial deep in the earth, the remains were transformed into materials such as bitumen, kerogen, or petroleum. Bitumens are found also in meteorites, archean rocks, copper, zinc mineralizations, and caves. It is possible that bitumens are primordial material formed during accretion of the earth and reworked by bacteria that consume hydrocarbons.
Bitumen was also used in early photographic technology. It was most notably used by French scientist Joseph Nicéphore Niépce in the first picture ever taken. The bitumen used in his experiments were smeared on pewter plates and then exposed to light, thus making a black and white image.
The world has become increasingly concerned over the global climate change thought to be caused by greenhouse gases, chief among them anthropogenic carbon dioxide which is released into the atmosphere from burning fossil fuels. This has led to the introduction of bitumen alternatives that are more environmentally friendly and non toxic. Bitumen can now be made from non-petroleum based renewable resources such as sugar, molasses and rice, corn and potato starches. Bitumen can also be made from waste material by fractional distillation of used motor oils, which is sometimes disposed by burning or dumping into land fills .
Non-petroleum based bitumen binders can be made light-colored. Roads made with lighter-colored pitch absorb less heat from solar radiation, and become less hot than darker surfaces, reducing their contribution to the urban heat island effect. 
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Bitumen". A list of authors is available in Wikipedia.|