When the rock contained but little bitumen and a large proportion of oily matter, it waa formerly crushed between mill-stones, hut this plan has also been abandoned, and a kind of mill resembling a large coffee-mill is now exclusively employed to pulverize these asphalts. Iu spite of many defeets it is the most expeditious and economical method yet devised.
The sifting of the powdered rock is ta very simple operation. When it leavea the mill, tho powder falls into cylindrical sieves, which are made to revolve twenty-five times in a minute. The sieve is made either of wire or of perforated sheet-irou ; in either case, the apetture is about in. in width.
Boding.—The powder which is colleeted from the sieve is taken immediately to the boilers in which the conversion into mastic is effected. These boilers are made of sheet iron, and are semi-eylindri cal ; they are usually 6 ft. in length and 3 ft. in diameter, and are built over an ordinary brick furnace. Each boiler is fitted N‘itli suitable agitating gear, worked by steam, and with a metal hood, which is also semi-eylindrical, and furnished with door, through which the material is intro duced. At the top of this hood, there is a pips to convey away the vapours of steam which are dis engaged abundantly throughout the operation. The bottom of the boiler is generally made of two sheets, one a.bove the other, and fitting closely together ; the bottom one is much thicker than the other, and may be removed, iu case of any damage done to the boiler, without necessitating the removal of the contents.
The process is conducted as follows :—About 3 cwt. of pure bitumen are thrown into the boiler, where it immediately begins to melt. When it has become sufficiently liquid, the agitator is set in motion, and the powdered asphalt is then added, little by little, each separate addition being made when the last hss become well mixed with the liquid bitumen. This is continued until the mixture becomes thick and pasty, and begins to adhere to the arm of the agitator, when it is ready to be cast into bloeks. At this time, about 2 tons of the asphalt will have been added, and about 3 cwt. of essential oils will have been disengaged ; 2 tons of mastic therefore remain in the
boiler.
Casting into Blocks.—The mastic is ladled out in vessels, containing about 56 lb., into iron cylinders of about 6 in. in height, and 14 io. in diameter. These moulds are soaped in the interior in order to prevent the adherence of the mastic to the iron. In about four or flve hours' time, the moulds become cool, and the mikado solidifies; this is hastened by sprinkling cold water upon them. The blocks may be easily detached by striking a light blow upon the side of the mould, when it instantly becomes detached, and may be turned out whole. Each block weighs about 56 lb.
Besides its me for paving purposes, asphaltic mastic bets numerous other applications. A patent was taken out in 1837 by Claridge, who applied it to paving roads and terraces, kitchens and stables, and to preserving buildings from damp. He also published a pamphlet, entitled `Instructions for the Use of Seyssel Asphaltic Mastic,' in which he gives a very full and complete description of the various applications for which this substance is fitted.
If the mastic, be required for roofing or other cement, it is reduced to powder and mixed with mineral pitch ; but if required for paving purposes, it is necessary to mix with it clean gravel or sea-grit. The ingredients are strongly heated for some hours in large cauldrons, and stirred by machinery. The mastic is afterwards run into moulds and formed into blocks weighing about 1 cwt. In this state, it may be easily transported from place to place, and melted down when required for use.
Asphaltic mastic is ductile and elastic, and consequently very durable. It is used in France as a lining for the walls of stables; for, owing to the toughness and elasticity of the surface, no injury is sustained by the horses from kicking, nor is any damage done to the wall. An asphaltic surface is easily laid down, and easily repaired ; weather has no effect upon it whatever.