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General Characteristics

asphalt, odor, tar, cent, sometimes and limestone

GENERAL CHARACTERISTICS. As usually found asphalt is of a dark brown or glistening black color. It varies in hardness from a viscous liquid to about 34 on the Dana scale. The streak is almost uniformly brown, sometimes brownish-black; and the fracture is dull and conchoidal. When rubbed or freshly broken, it emits a peculiar bituminous odor, which is not disagreeable although a little sour smelling. Before the blow-pipe, the solid varieties are quickly melted; and all are readily evaporated and burned, and leave as ash, the organic and inorganic impurities, which are usually found in it in smaller or larger quantities. Its specific gravity in the natural state varies from 0.96 to 1.68 according to its porosity and the amount and the character of the impurities present. It is insoluble in water, but is more or less soluble in carbon bisulphide, alcohol, turpentine, ether, naptha, and petroleum.

Coal-tar, or gas-tar, has an appearance somewhat like asphalt, and is sometimes used as a substitute for it or as an adulterant. Tar is not so valuable for paving purposes as asphalt, since it more easily loses its cementing qualities by vaporization and oxidation. The principal method of distinguishing asphalt and coal-tar, available to the layman, is the odor. The tar emits a sharp, acrid odor; while both the crude and the refined asphalt when cold give a weak clay like odor, and must be rubbed to obtain the distinctive bituminous odor. If tar is mixed with asphalt, the presence of 25 per cent will be revealed by the odor. This is the proportion of tar in the vulcanite or coal-tar pavements laid in Washington from 1877 to 1887. When being laid, tar gives off a bluish vapor, while asphalt emits a white vapor. Expert analysis is necessary to detect the presence of tar when mixed with asphalt in small quantities. The following method will certainly detect 5 to 7 per cent of tar. "Extract the bitumen with carbon disulphide, filter, evaporate to dryness, and heat the residue till it can be ground to a dry powder; 0.1 gram is

treated with 5 c.c. of fuming sulphuric acid for 24 hours and is then mixed by continuous stirring with 10 c.c. of water. If coal-tar be present, the solution will be of a dark brown or blackish tint; if not, the solution will be of a light yellowish color." Asphalt limestone varies in color from chocolate brown to black when freshly broken, the color being darker as the proportion of asphalt is greater. The percentage of asphalt permeating the limestone varies in different deposits and in different parts of the same mine, usually ranging from 1 to 20 per cent. The fracture of asphaltic limestone is irregular, and the grain is very fine. Under the microscope the smallest particle is coated with asphalt. If cut by a sharp blow of an axe, it appears grayish white along the cut, due to the forcing out of the asphalt and the leaving of the particles of limestone exposed. If a piece contains about 10 per cent of asphalt, it can be warmed and broken in the hands, and a piece heated over the fire falls apart; and if heated in a pan and held for an hour at a high temperature, the asphalt is driven off, and a gray, powdered limestone remains. If a sample contains more than about 4 per cent of asphalt, a bituminous odor is perceptible when the piece is freshly broken.

Asphaltic sandstone contains from 1 to 70 per cent of asphalt. The grain is sometimes dense and sometimes porous, sometimes vety fine and sometimes coarse. Small particles of clay, little shells, and various other substances are often present. The color is almost invariably black. A piece heated upon a stove quickly falls in pieces if it contains more than 6 per cent of asphalt. Asphaltic sandstone often contains considerable quantities of maltha and petroleum which injure it for paving purposes.