ASPHALT. According to the standard definition of the American society for testing materials, asphalts are "black to dark brown solid or semi-solid cementitious materials which gradually liquefy when heated, in which the predominating con stituents are bitumens all of which occur in the solid or semi-solid form in nature or are obtained by refining petroleum, or which are combinations of the bitumens mentioned with each other or with petroleum derivatives thereof." In more popular terms asphalt may be defined as a semi-solid sticky residue formed by the par tial evaporation or distillation of certain petroleums. This is as true of native asphalts as of those obtained by refining petroleum.
The solid or semi-solid native bitumens were termed by the Greeks. Only native asphalts were known, but late in the 19th century it was found that asphalt was a constituent of certain petroleums and could be recovered by distilling off the volatile oils which held it in solution. In 1928 over 8o% of the world's supply of asphalt was produced at petroleum refineries.
Sandstones and limestones naturally impregnated with asphalt —commonly known as rock asphalt or bituminous rock—are found in various parts of the world, the most widely known de posits being at Val de Travers in Switzerland ; Seyssel in France and Mons in Belgium ; San Valentino and Cesi in Italy ; Ragusa in Sicily; Limmer, Lobsann and Vorwohle in Alsace; and in Texas, Oklahoma, Alabama and Kentucky in the United States. The largest and best known deposit of relatively pure asphalt occurs as an asphalt or pitch lake on the Island of Trinidad, British West Indies, and covers an area of about i oo acres. A sample of crude contains approximately 39% of pure asphalt or bitumen, the bal ance being gas, water, light oils and organic and mineral matter. The surface of the lake forms a crust which may be removed by means of picks. The plastic material from beneath flows into the excavations and rapidly hardens into a new crust. A more exten sive but shallower deposit is found at Bermudez lake in Venezuela. This asphalt is softer than that found at Trinidad and contains less mineral matter. An exceptionally pure asphalt of very brittle nature known as Gilsonite, is obtained in Colorado and Utah. Other varieties are known as manjak, glance pitch, wurtzelite, etc.
Practically all native asphalt is too hard for direct use and must be heated until water, gas and other volatile materials are driven off and then fluxed or softened to the desired consistency by mixing with it the proper amount of residual petroleum. On the other hand asphalt recovered directly from petroleum (q.v.) by distillation does not require fluxing, as the process is stopped when the product reaches the desired consistency. When a high melting-point is desired, the asphalt is heated to fluidity and air is blown through it, producing what is known as "blown" or oxidized asphalt.
Asphalt Cements.—The hardness of asphalt is determined by measuring the distance that a standard needle, under a load of i oog. applied for five seconds, will penetrate the asphalt at 77°. This "penetration" test classifies asphalts into grades, and when a consistency suitable for some commercial use is reached, the products are known as "asphalt cements." These are used primarily as water and weather resisting binders. Their value for these purposes was utilized in ancient times for preserving mummies and in some cases for coating and binding masonry.
Asphalt is extensively used in paving, roofing and water-proof ing. Over 75% of the total production is employed in the con struction of highways in which it is used to bind together the stone, sand and mineral dust which constitute the bulk of the wearing surface. Approximately r 4o,0Oo,000sq.yd, of asphalt pave ments are constructed annually in the United States alone. Rela tively fluid asphalt products known as "cut-backs" and "road oils," are used for the surface treatment of macadam and gravel roads.
Asphalt is also employed in the manufacture of mats for under water revetment, wall boards, flooring, floor coverings, sheathing, moisture-proof wrapping paper, paints, varnishes, enamels, japans, acid-proof coatings, pipe dips, sealing compounds, insulating prod ucts, emulsions and moulding composition. The roofing industry consumes about 15% of the total production in the manufacture of asphalt-impregnated roofing and shingles.
The total annual production and importation of asphalt into the United States amounts to over 6,000,000 tons of which some