1902 Encyclopedia > Asphalt

Asphalt




ASPHALT, or ASPHALTUM (____, Aristotle, Bitu-men, Pliny), the German Bergpech, or mineral pitch, so called from the Lacus Asphaltites or Dead Sea, where it was found in ancient times, is a product of the decomposition of vegetable and animal substances. It is usually found of a black or brownish-black colour, externally not unlike coal, but it varies in consistency from a bright pitchy condition, with a sharp conchoidal fracture, to thick viscid masses of mineral tar. Asphalt melts at or a little below the boiling point of water, and it burns with a rather smoky flame. It is regarded as the ultimate result of a series of changes which take place, under certain conditions, in organised mat-ter, producing—1st, naphtha; 2d, petroleum; 3d, mineral tar; and 4th, asphalt or hard bitumen. The whole of these substances merge into each other by insensible degrees, so that it is impossible to say at what point mineral tar ends and asphalt begins. Naphtha, which is the first of the series, is in some localities found flowing out of the earth as a clear, limpid, and colourless liquidf As such it is a mixture of hydrocarbons, some of which are very volatile and evaporate on exposure; it takes up oxygen from the air, becomes brown and thick, and in this condition it is called petroleum. A continuation of the same process of evaporation and oxidation gradually transforms the material into mineral tar, and still later into solid glassy asphalt. Asphalts are very variable in composition, and their proximate constituents have not been subjected to a thorough examination. Traces of naphtha or light oils are usually found in them, and they always contain a per-centage of the heavier hydrocarbons not vaporisable below boiling point. Resins soluble in alcohol, and solids, some soluble in ether, and some resisting the solvent action of both ether and alcohol, are found in varying porportions. According to Dana " asphalt may consist of either—(1), unoxygenated, or (2), partly unoxygenated and partly oxygenated (the usual fact), or (3), solely of oxygenated hydrocarbons (very rarely, if ever, true in nature). The state of solidity is not proof that any part of the bitumen is oxygenated." Asphaltic deposits exist widely diffused throughout the world, more especially in tropical and sub-tropical regions. It is found in a state of great purity in the interstices of the older rocks, but its occurrence is not characteristic of any particular formation or period. The most remarkable deposit of asphalt exists in Trinidad, where it forms a lake 99 acres in extent, and of unknown depth, intersected with rivulets of water. At two or three places on the surface of the lake an emission of semi-fluid tar may still be seen in progress, accompanied with an evolution of sulphuretted hydrogen. At these points the substance is still soft and viscid, but by exposure it gra-dually obtains the consistency of the rest of the mass. In addition to the lake deposit, asphalt occurs in the surrounding country—named La Brea, on account of this peculiarity—in detached patches, or in sheets of consider-able size, at one point protruding into the sea, and pieces of asphalt are frequently cast up on the neighbouring shore. A considerable quantity of a fine asphalt is also derived from Cuba under the name " Chapapote," or Mexican asphalt; and from Caxatambo in Peru, a very pure variety of high lustre is exported. The asphalt of the Dead Sea is more a tradition than a reality, it being now found there in very 3mail quantities; but the source of the supply of ancient Babylon, the fountains of Is, on a tributary of the Euphrates, still yields asphalt. It occurs in many localities throughout Europe, but not to any considerable extent. The following table gives the ultimate composition of specimens from several localities :—
Carbon. Hydrogen. Oxygen. Nitrogen. Ash.
Auvergne, France ...77'64 7'86 8"35 1-02 5-13
Cuba 8234 9-10 6-25 1-91 0-40
Caxatambo, Peru....88-66 969 1-65
Of greater importance industrially than simple asphalt is asphalt stone,—a limestone impregnated with bituminous matter, which occurs in large quantities at several European localities. The most valuable deposits are in the Val do Travers, canton of Neufchatel; in the neighbourhood of Seyssel, department of Ain; at Bechelbronn in Alsace, Limmer near the city of Hanover, and Holle in Ditmarschen; in Holstein, &c. These bituminous stones con-tain from 7 or 8 to about 20 per cent, of asphalt in their composition, that from Val de Travers being richest in bituminous matter and of most value in its industrial applications. The asphalt beds of the Val de Travers were first discovered and utilised by Eirinus, a Greek physician, in 1712, who recommended the material as being " peculiarly suitable for covering all kinds of constructions ; to protect wood and stone work against decay, worms, and the ravages of time, rendering them almost indestructible even when exposed to wind, wet, and extreme variations of temperature." Eirinus was aware that asphaltic mortar had been used in the building of ancient Babylon, and he himself succeeded in using it with great effect for the lining of cisterns and walls as a cementing material, and for the flooring of warehouses, &c. After some time the material fell into disuse; the quarries of Val de Travers were even forgotten, and it was not till the year 1832 that the material was again prominently and successfully re-introduced, the credit on this occasion being due to the Count Sassenay. Under his direction asphaltic stone came to be extensively used in France for pavements and roadways, and for protecting floors and walls from the effects of damp. From France the application of the material for such purposes extended to other countries, and there is now a wide-spread demand for asphaltic pave-ments. Two principal methods are adopted in laying asphalt pavements,—-1st, the mastic process; 2d, the hot compressed process. The mastic process is essentially as follows :—The bed of the road-way is prepared with a smooth level foundation of concrete, which must be thoroughly dry before the application of the asphalt. The mastic is prepared for application by heating the asphaltic stone and breaking it into small pieces, which are then melted up with a quantity of mineral tar, to which some sand is added, The molten mass is then poured over a section of the prepared concrete uniformly to the requisite depth ; the surface is smoothed, and covered with a coating of fine sand which is stamped into the asphalt. The proportions of tar and sand used vary with the composition of the asphaltic stone employed and the position occupied by the pavement The other mode of laying pavements, now extensively adopted, consists in spreading hot powdered asphaltic stone on the prepared surface, which is then heavily pressed till it forms a homogeneous elastic coating. Roadways so prepared are very durable, smooth, cleanly, and noiseless, but the material is not well adapted for other than level streets on account of the difficulty of foothold. Complaints are also made against such pave-ment, to the effect that accidents to horses from slipping and falling are much more frequent than is the case on ordinary stone pavement. An artificial asphalt is pre-pared by boiling up the pitch of gas-tar with chalk and sand, such a substitute, though much cheaper, has not the durability of the natural compound. Gas-tar asphalt is also applied for other purposes in which the natural product is used. Asphalt was used by the ancient Egyptians in their process of embalming bodies. It is the principal ingredient in black Japan varnish. It is distilled in large quantities for the illuminating and lubricating oil which it yields, but the bituminous shales from which paraffin and paraffin oils are distilled must not be con-founded with asphaltic deposits. It is also used for preparing roofing felts ; paper water-proofed with asphalt has been used, and drain-pipes of compressed asphalted paper are manufactured.








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