Many terms and definitions of terms are used to describe bitumens. Within this class of materials there are many substances, the most common of which are tars, pitches and asphalts. Tars are dark brown condensates produced by the destructive distillation of materials such as wood, peat, shale, bone and coal. Fractional distillation or partial evaporation of tar results in a solid or semi-solid residue known as pitch. Coal-tar pitch is the most common material of this type that is used in construction.
Asphalts are dark brown to black solids or semi-solids that gradually liquefy when heated.
They are found in the natural state but may also be obtained from petroleum. Essentially, the origin of natural asphalt is the same as that of asphalt produced by the refining of petroleum, except that the latter process is accomplished at higher temperatures in a much shorter time. The largest commercially exploited natural asphalt is a lake over 300 feet deep and covering an area of 100 acres on the island of Trinidad. Other natural asphalts occur as rock asphalt in Kentucky and as very hard asphalts such as Gilsonite in Utah and Colorado. The removal of gasolines, oils and other volatile products from crude oil results in a residual asphalt often called a straight-run asphalt.