Artificial Asphalt. This is prepared from coal tar by distilling off the volatile oils which hold the tar in solution, the result being that a kind of fatty pitch is left, which must be boiled until a sample, when cooled, becomes nearly solid. The operation may be accomplished in the open air, but if this means of evaporation be adopted, the process is attended with a very unpleasant odour, and the volatile oils are dissipated. These volatile oils are used for the preparation of varnish, for lubricating machinery, and for the manufacture of a superior kind of lampblack. They have also been employed to increase the illuminating power of coal-gas, which purpose they accomplish by imparting their vapours to gas passed over them when they are placed
in shallow vessels. Various forms of patent apparatus have been designed for this purpose.
When it is required to collect the oils, the coal tar is placed in a retort made of sheet iron, with a convex bottom, which is placed immediately over a fire. The products of the combustion after striking the bottom of the retort circulate round it, then proceed under a second boiler to heat the tar contained in it, and from which the retort is replenished when necessary. This vessel, when three quarters full, contains nearly 24 cwts. of tar; it should be perfectly embedded in masonry; the capital itself by which the volatile products escape should be surrounded with materials that are bad conductors of heat, such as ashes. But for this precaution the volatile oils would become condensed, and fall back into the evaporating vessel.
The volatile oils are collected by being made to pass through a tube cooled by a current of water, this tube running in a direction the reverse of that pursued by the vapours, and terminating in a closed vessel, which acts as the receptacle for the oils. A tube branching from the boiler conducts the uncondensed products outside the building in which the distillation is conducted.
When the tar has been boiled sufficiently long to give it the requisite consistence, it is removed by means of a pipe into a third hemispherical boiler of cast-iron. To prepare the bituminous mastic directly from this fatty pitch, the latter is kept in a state of fusion, and chalk in sufficient quantity is then added. If the chalk be previously heated, ground to a coarse powder, and sifted, the mixture is effected more rapidly and satisfactorily.
The asphalt becomes the more solid the greater the proportion of chalk added; on the other hand, it becomes less elastic and more brittle. The asphalt is moulded as follows:—A long table is covered with cast-iron plates, surrounded with a framework, which is subdivided into eight or ten equal compartments by means of rules of about six inches in height, introduced vertically into grooves formed at equal intervals in the long sides of the frame. The eight or ten moulds obtained by this means are coated internally with a paste composed of sixty parts of water and forty of chalk. This compound prevents the mastic adhering to the sides of the mould, and ensures its being easily detached.
Two barrels, or 9 cwts. of tar, lose by distillation one fourth of their weight, the loss consisting of 1 cwt. 3 qrs. 15 lbs. of volatile oils, and 1 qr. 13 lbs. of water.
Sometimes ground or fine sand enters into the composition of asphalt in proportions equal to the chalk; but in some cases only half as much sand as chalk is used.
In the manufacture of asphalt it is very important that the contents of the cauldron should be stirred during fusion, not only to prevent the tar adhering to the bottom, and so getting burnt, but to ensure the ingredients being brought into intimate combination, and a homogeneous and smooth compound being produced.
As soon as the whole is thoroughly incorporated, the proper consistence attained, and the vapours of the volatile oils and water come off in very minute quantities, the asphalt is run off into the moulds before described, and when sufficiently set may be removed, and is ready for use.