1869.] Plastic Slate for Roofing. 535 PLASTIC SLATE FOE ROOFING. TIIIS fire and weather-proof material is so interesting, that we cannot re- frain from penning a few paragraphs upon it, for the information of our readers. It is the result of the comminution and re-formation of slate, the only other material required, for the re-aggregation of the mass of almost impalpable slate flour, or meal, being coal gas tar. Im- mediate^ 7 associated in situ as part of the general group of stratified rocks — and perhaps originally combined — when brought into intimate mechanical con- tact, the first as a fine powder, and the last in its semi-fluid condition, slate and coal tar— the latter representing the natural bitumen — form a union which once effected, is absolute^ indissoluble. And the artificial compound, from its very slowly diminishing plasticity, and quickly drying quality, is specially adapted to roofing. In forming a roof of this kind, the wooden sheathing must be of the same general nature, as that prepared for the gravel roof; but the compound, being both very flexible and very tenacious, does not require the same extreme care and exactness. It is not necessary to match, or tongue and groove, the sheathing-boards, but the more solid, level, and close they are laid, the better and more durable the roof. A course of patent water-proof, double slate-felt, is laid across the boards, the edges lapping two inches. Each edge is fastened down hy cleats, securely nailed, which are also covered with a four-inch strip of the same kind of felt, pasted down with plastic slate. The cleats may be elevated, so as to make a ribbed roof, or be flat, so as to present a smooth, unbroken surface. This lining is designed to insulate the mastic from the boards, lest their shrink- ing and swelling should injure the final covering. After the process of felting or lining the roof is completed, the plastic slate, mixed to the consistency of a thick mor- tar, which is more than two-thirds slate, is then spread upon the felt, with a trowel, to the thickness of a scant eighth of an inch. This makes a com- plete, flexible slate roof, if the work is properly done. The Ingredients are — First: A fine- grained, hard, solid Clay-dote. No other kind of stone will make the article desired. It must be kept free from dirt, bits of wood, leaves, straw, and, in fact, any substance not in affinity with coal- tar ; or it is useless to expect a fine and perfect mastic. It must be ground as fine as wheat flour, and sifted through a fine sieve, or bolted, to remove such par- ticles as may pass between the stones unpulverized. Second : Coal-gas Tar. This is a mix- ture of various substances, more or less volatile, which can be separated by heat, or distillation, from a residuum com- monly known as coal-tar, gas-tar, and sometimes, coal-tar pitch. Coal-tar, proper, is a thick and viscid semi-fluid. The powdered slate should be tho- roughly mixed with the tar, to about the consistency of plasterers' mortar; and, after being mixed, should stand at least twenty-four hours before being used. Within any moderate period, the longer the materials have been mixed, the better will be the roof. In cold weather, the coal-tar must be heated, but not to the boiling point, while mixing in the slate ; and the mastic itself must be warmed, but not made very hot, while spreading. A close observance of these directions will insure success. The covering material is peculiarly lasting ; and, if the roof leaks, it will be owing to other causes than deca}', or
