Page:The New International Encyclopædia 1st ed. v. 02.djvu/169

ASPHALT. known in the Archæan. The deposits of asphalt in the United States are insufficient to meet the demand, and large quantities are imported, chiefly from Trinidad and Venezuela.

The uses of asphalt are numerous. It is employed as an ingredient of varnish and also as an insulating material. Buildings and other objects are often coated with it, to protect them from dampness, and vessels are painted with asphalt to protect them from the Teredo, or ship-boring worm. Asphalt alone is not used for medicine, but its natural solution in naphtha — viz. petroleum — is of medicinal value, used either internally or externally. About 1840 asphalt became of importance as a material for paving, and for this purpose it was mixed with powdered rock when hot, and then spread on the surface to be paved. Asphalt pavements are smooth and clean, but less durable than stone, and in moist climates are often slippery.

, in the case of rock asphalt, is usually a simple process, in which blasting is employed to loosen the rock. In the Seyssel Mines, in France, the bituminous limestone is blasted from between layers of ordinary white limestone, and the work done for the most part underground. In the United States, the Kentucky bituminous sandstone and the bituminous limestone of Texas are worked from the surface, after stripping off the overlying material. The same general process seems to be employed elsewhere in the United States. In Santa Barbara County, Cal., a deposit of sand impregnated with bitumen is mined by first using a powerful hydraulic jet, supplied by a steam-pump and water-pipe. Some 6 to 8 feet of loam is thus washed into the ocean; and after a thin layer of clay is spaded off, the bituminous sand is then thrown into cable cars by men with hot spades, and hauled up an incline to the refinery. By far the most interesting and expensive asphalt mining and refining plant in the world is that which produces the California Alcatraz product, and which is also located in Santa Barbara County. The asphaltic sandstone is mined at Sisquac, by open-face blasting, and the rock loaded on to a cable car by means of an electric crane and hauled to crushing rolls. Its subsequent treatment to reduce it to a liquid condition for piping to the refining plant, 30 miles distant, is described below, under Asphalt Refining.

The lake asphalts of Trinidad and of Bermudez are mined in a simple but interesting manner. The Trinidad asphalt is dug by means of picks before daylight, when brittle. It is loaded into buckets, placed on flat cars which run on a cable railway extending out onto the asphalt deposit. This railway is supported on palm branches, to prevent its settling into the asphalt, and at its shore end the buckets of asphalt are hoisted by hydraulic power and then conveyed on an aërial cableway to a pier 1700 feet in length, with its head in 30 feet of salt water. The contents of the buckets are dumped directly into the holds of vessels, and the empty buckets are sent back up to the lake as the heavy ones come down. The aërial cableway is about 3400 feet in length, half over the land and half over the water, and had in 1900 a capacity of 70 tons per hour. (See, for description of this method of transportation.) Adjoining the Trinidad lake-asphalt deposit is found the so-called land asphalt. This is described as an overflow from the lake, and is harder and more brittle than the lake product. It occurs in pockets and other isolated deposits, from which it is dug out, carted to the beach, placed on lighters, and conveyed to and loaded on ocean vessels.

At the Bermudez Lake the asphalt is excavated in practically the same way as at Trinidad; but it is loaded into hand-cars running on a portable track. On shore the cars are dumped into boxes placed on flat cars, which are hauled by a locomotive about 5 miles to a wharf on the Guanoco River. If a vessel is at hand, the asphalt is dumped directly into its hold; otherwise it is stored on shore. The Bermudez product is softer than that from Trinidad. It is so soft that the holds of the transports must be divided into compartments to prevent flowing to one side and giving the ships a permanent list. There is a small refining plant on the shore at the Trinidad Lake, and in 1900 one was being erected at the Bermudez shipping wharf. Most of the asphalt from these two sources is refined after ocean shipment.

varies with both the natural product and the uses to which it is to be put. Generally speaking, the object of the process is to remove water and volatile and other foreign matter from the crude material; but for some purposes it is highly desirable to leave the mineral matter mixed with the asphalt. This is particularly true of asphalt that is to be used for paving and similar purposes, since for such uses the asphalt really serves merely as a cementing and waterproofing medium, the bulk of the paving material consisting of sand, or powdered limestone or sandstone. The rock asphalts, when used for pavements direct, are not refined; but are simply crushed, reduced to powder by heat, and then compressed in place, as described under. The crushing is done by means of rolls, or by toothed cylinders, and the heating is effected in revolving cylinders, at a temperature of some 300° F.

One of the simplest methods yet attempted for abstracting asphalt from the rock products is by boiling the rock in water. This is possible in the case of the lighter asphalts; for, with a specific gravity less than water, the asphalt will rise to the top of the kettle, whence it may readily be removed. Either calcium or sodium chloride (common salt) may be substituted for the water in the boiling kettles. The heat may be applied directly or indirectly, steam being used in the latter case. The asphalt thus separated must be refined by evaporation and sedimentation. Carbon bisulphide may be used as a solvent to extract asphalt from the rock, without the use of heat. This material, however, costs more than naphtha, the solvent most commonly applied, and a smaller proportion of it can be recovered. The same general process is followed in the case of the bisulphide as with naphtha, except for the absence of heat and the introduction of the solvent at the bottom instead of the top of the vessel.

The Alcatraz sandstone rock asphalt already described is treated as follows: The rock is dumped from the cars coming from the mines into a hopper, from which it passes through a succession of fluted crushing-rolls, each nearer