Popular Science Monthly/Volume 19/August 1881/Origin and Uses of Asphalt
BITUMEN appears in nature as an accidental mineralogical product, under the most diverse and often most inexplicable conditions. It is found sometimes in the native state, sometimes mixed with clays, sometimes as the cement of conglomerates, sometimes impregnating limestones. The last combination produces the mineral commonly called asphalt. When the bitumen contained in any of these substances is chemically isolated, it appears always a nearly identical substance, in composition, consistency, and appearance, except that the empyreumatic odor that characterizes it may become alliaceous in volcanic countries. Asphalt is doubtless one of the most considerable and valuable of the forms in which bitumen appears. It is a soft limestone, naturally and closely impregnated with that substance. When a specimen of it is examined under the microscope, each grain of it appears to be immersed in a pellicle of pure bitumen, by which it is cemented to the adjoining particles. It is thus a species of very fine-grained bituminous conglomerate. When a lump of this rock is heated to a temperature rising from 176º to 212º, the pellicle of bitumen is melted, the cohesion of the asphalt is destroyed, and it crumbles into dust. If it is taken while it is still hot, or if it is heated again after it has become cool, and strongly compressed, the particles will adhere again, and the stone will recover, after cooling, precisely the consistency and appearance it had originally. The employment of compressed asphalt for pavements is founded on this property.
Asphalt, or bituminous limestone, is generally found in the Jurassic strata, in regular beds of a lenticular shape, which are uniformly cut in two by a stream of water. Sometimes the bed is single, at other times it is multiple; there are formations containing seven beds, one above the other, and distinctly separated by strata of white limestone.
Different views prevail respecting the origin of asphalt and the circumstances under which it is formed. Some believe that the bitumen was already in existence when the calcareous formation took place, and that the particles of limestone were deposited in a bituminous sea. Others consider that the bituminous matter is derived from the organic matter associated with the shells that have furnished the carbonate of lime; and other more hazardous hypotheses have been advanced. A careful observation of asphaltic formations has led me to adopt what appears to me to be a more plausible theory.
It is permitted to suppose, from indications furnished by the study of bituminous districts, that in some geological epochs, which have yet been only imperfectly determined, accumulations of organic matter, buried under enormous masses of Jurassic limestone, and heated by the central fire, became vaporized, and in that condition sought a passage through the crust of the earth (Fig. 1). In time the crust cracked, and a fissure was formed. The bituminous vapors, compressed by incalculable pressure, forced themselves through the way that was opened to them, and passed by such strata as were too compact to be penetrated; but, when they reached the oölite, they found on either side of the fissure beds of a limestone soft enough to admit of their impregnating it (Fig. 2). As long as the pressure lasted, the bitumen continued to insinuate itself through the pores of the limestone, and to fill its infinitesimal cavities.
Mineral asphalt is relatively a soft stone. It becomes more compact as the temperature diminishes, but yields under the influence of
heat to such a degree that an exposure of a few days to the summer sun will sometimes cause it to crumble. This property has induced the application of the compressed material to the making of pavements. Its fitness for this purpose seems to have been suggested by accident. When the mineral was first quarried, the pieces which fell along the road from the wagons carrying it were ground up by the wheels, and were finally compressed again by the continued passage of the wagons over the dust, so as to form a kind of spontaneous pavement. A Swiss engineer, M. Mérian, acting upon the suggestion of this incident, asphalted a part of the road from Travers to Pontarlier, in a rough way, but with a satisfactory result. In the next year (1850), M. Darcy, inspector-general of bridges and highways, recommended asphalt as a material for pavements in a report to the Minister of Public Works. The first asphaltic pavement was laid in Paris in 1854.
The mineral appears in industry, under a still more useful form than the compressed form, as asphaltic mastic. This is made by throwing the powdered mineral into a bath of seven or eight per cent, of its weight of liquid bitumen, and mixing the whole thoroughly while it is cooked for live or six hours. The substance produced, although
chemically the same, except for the difference in the relative proportions of bitumen and limestone, is physically entirely different from asphalt. It can not be pulverized by heating, but forms a paste in which the two ingredients seem to be perfectly combined, and which may be molded into desired forms. The manufacture of the mastic has become an important industry. The annual production of the French shops alone must amount to fifteen or twenty thousand tons.
In "La Nature," of April 9th, Mr. A. Woeikofen, of St. Petersburg, describes the asphaltic beds of Russia, which occur on the grand curve of the Volga, or the arc of Samara, a short distance above the city of Syzran. They are not deposited in the Jurassic formation, as are those in France, Switzerland, and Germany, but in a dolomitic limestone of the lower Carboniferous series. The mineral is rich in bitumen, of less fusible quality than the bitumens of France and Switzerland, and has not been produced in the compressed form. It is extensively made into mastic, the fabrication of which already amounts to ten thousand tons a year, and is rapidly increasing.—La Nature.