Asphalt and asphaltite

Varieties of naturally occurring bitumen. Asphalt is also produced as a petroleum by-product. Both substances are black and largely soluble in carbon disulfide. Asphalts are of variable consistency, ranging from a highly viscous fluid to a solid, whereas asphaltites are all solid. Asphalts fuse readily, but asphaltites fuse only with difficulty. Asphalts may, moreover, occur with or without appreciable percentages of mineral matter, but asphaltites usually have little or no associated mineral matter. See also Bitumen; Impsonite; Wurtzilite.

Many asphalts occur as viscous impregnations in sandstones, siltstones, and limestones. Most such deposits are thought to be petroleum reservoirs from which volatile constituents have been stripped by exposure of the rock. Relatively pure asphalt occurs in Kern, San Luis Obispo, and Santa Barbara counties, California. Occurrences of asphalt are also known in Kentucky and Oklahoma. Although asphalt seeps have long been known in France, Greece, Russia, Cuba, and other countries, the best known and largest are those of Venezuela and Trinidad.

The asphaltites (gilsonite, grahamite, and glance pitch) were probably derived from a saline lacustrine sapropel and owe their variable properties to differences in environment of deposition. These substances occur on a large scale in the Uinta Basin of northeastern Utah, where they are derived from upper Eocene Green River sediments, most of which are oil shales high in carbonate content. See also Oil shale; Sapropel.

Asphalt is derived from petroleum in commercial quantities by removal of volatile components. It is an inexpensive construction material used primarily as a cementing and waterproofing agent. See also Petroleum products.

Asphalt is composed of hydrocarbons and heterocyclic compounds containing nitrogen, sulfur, and oxygen; its components vary in molecular weight from about 400 to 5000. It is thermoplastic and viscoelastic; at high temperatures or over long loading times it behaves as a viscous fluid, while at low temperatures or short loading times it behaves as an elastic body.

The three distinct types of asphalt made from petroleum residues are straight-run, air-blown, and cracked. Straight-run asphalt, characterized by a nearly viscous flow, is used in the construction of pavement surfaces for roads and airport runways. Air-blown asphalt is resilient and has a viscosity that is less susceptible to temperature change than that of straight-run asphalt. It is used mainly for roofing, pipe coating, paints, underbody coatings, and paper laminates. Cracked asphalt, with limited applications such as dust laying or as an insulation board saturant, has a nearly viscous flow, and its viscosity is more susceptible to temperature change than straight-run asphalt.