Asphalt is a highly viscous liquid that occurs naturally in most crude petroleums. Asphalt can be separated from the other components in crude oil (such as naphtha, gasoline and diesel) by the process of fractional distillation, usually under vacuum conditions. A better separation can be achieved by further processing of the heavier fractions of the crude oil in a de-asphalting unit which uses either propane or butane in a supercritical phase to dissolve the lighter molecules which are then separated.

Further processing is possible by "blowing" the product: namely reacting it with oxygen. This makes the product harder (and more viscous) where the end use requires it.

It is sometimes confused with tar, which is an artificial material produced by the destructive distillation of organic matter. Both tars and asphalts are classified as bitumens, a classification that includes all materials entirely soluble in carbon disulphide.

Asphalt is commonly used in roofing shingles, and combined with mineral aggregate to make asphalt concrete (also known as macadam) for making roads. The asphalt used for asphalt concrete is called bitumen in many countries and is blended up from various components in an oil refinery to meet the desired hardness.

Asphalt is rather hard to transport in bulk (it hardens unless kept very hot) so it is sometimes mixed with diesel or kerosene before shipping. Upon delivery, these lighter materials are separated out of the mixture. This mixture is often called bitumen feedstock, or BFS.

Gilsonite is a natural form of asphalt.

External link

• Black Stuff (http://www.biffvernon.freeserve.co.uk/black_stuff.htm)