Slate as mentioned at page 165 of Vol. I. is sawn and sometimes planed with cutting tools very similar to those used for wood, except that they are stronger and are applied by machinery, the action being partly cutting, and partly forcing off the flakes of slate, as if the tools are allowed merely to scrape over the surface their edges become rapidly worn away. But the various sandstones, limestones and marbles are too compact to be thus treated, and they are consequently worked almost exclusively by the chipping chisel and various abrasive processes; the chisel being used for such parts of the material as are in excess, as in sculptured works, and the abrasive processes being employed for dividing the blocks into slabs and small pieces, which are subsequently ground to the required forms with sand and water. In the case of marble the pieces are finally polished with abrasive powders applied on rubbers of various materials as mentioned at pages 1076 to 1078 of the present volume.
The ordinary saw used for dividing blocks of stone and marble into flat slabs, is shown in figure 1086. It consists of a parallel blade of soft iron from 5 to 10 feet long, from 4 to 5 inches wide, and from one-eighth to one-sixth of an inch thick, the blade is perforated near each end with a hole about three quarters of an inch diameter, for the reception of an iron pin, by which the saw is strained in a rectangular wooden frame. The blade is inserted in the saw kerfs in the upright sides of the frame, called the heads, and the pins rest in two notches near the lower extremities of the heads, which serve as the handles of the saw, and are kept distended by the wooden stretcher called the pole, placed about a foot from the upper ends of the heads, and rested at each end against a loose block of wood called the bolster.
Instead of a coil of string twisted with a short lever being employed for drawing the upper ends of the frame together, as in the saws for wood, this object is effected by the use of a kind of chain made of looped iron rods, with intermediate C-shaped links, for adjusting the total length of the chain, which is furnished with iron loops that embrace the upper ends of the heads. The tension is given by a right and left hand screw fitted to two looped nuts, attached to the iron rod by C links, the double screw has holes for a lever, by which it is twisted so as to draw the upper ends of the heads of the frame together with great force, and thereby stretch the saw in a most effectual manner. The top view of the tightening apparatus is shown separately at a.
The depth to which the saw can penetrate, is limited by the distance from the edge of the blade to the under side of the pole, the nearer the pole is to the saw the greater is the stability of the blade, and all the parts of the frame are made detached, so as to allow of their being combined and adjusted to suit the different sizes of blocks of stone. The same pair of heads are used with poles and saws of various lengths, and the pole is placed at different heights from the blade, according to the depths of the blocks of stone. When the latter are very deep, a longer pair of heads are substituted, but long heads are avoided as much as possible, as the stability of the saw frame is thereby much reduced.
The blade of the stone-saw, like the metal-laps used for grinding generally, does not itself cut the stone, but simply serves as the vehicle for the application of the sand, which acts as the teeth of the saw, and performs the cutting process. The coarseness of the sand that is employed depends upon the hardness of the stone to be cut, for moderately soft stone a coarse sharp sand is employed, and for the harder varieties of marble a fine sand is used; the sand or grit generally employed in London for cutting stone is obtained from the scrapings of roads paved with flint. The scrapings are sifted and washed through perforated copper sieves, much the same as emery, as it is of great import-ance that the sand should be clean and quite free from small pieces of stone, or any other extraneous matters. Should a small piece of wood or a bit of coarse gravel by any accident get into the kerf beneath the saw blade, the little piece would roll over backwards and forwards, and materially impede the cutting of the block, and it then becomes necessary to remove the saw and wash away the obstacle, by pouring water down the saw kerf.
The cutting action of the sand is assisted by a small stream of water, supplied from a barrel placed a little above the block of stone. A small hole is made near the bottom of the barrel, to which is fitted a spigot and faucit, or more commonly a loose wooden peg grooved up the one side, which allows of the escape of a minute stream of water, that trickles down a sloping board placed so as to lead the water into the saw kerf. A little heap of sand is placed near the path of the water, and the workman is provided with a wooden stick with an iron hook at the end, or more commonly an old knife blade placed at right angles to the stick near its end. This tool is called a drip stick, and is used occasionally to draw forward a small quantity of sand into the running water, which thus carries down the necessary supply of sand for the cut, and the water flows away at the ends of the kerf, carrying with it the worn-out sand and the particles of stone removed in the cutting; the drip stick is also used for tapping the wooden peg, so as to increase or diminish the flow of water according to circumstances.
The weight of the saw and frame supplies the necessary pressure for causing the penetration of the sand, so that the workman has only to guide the saw, and push it backwards and forwards for the cut, and when the pressure is so great as to render the work too laborious, a counterpoise weight is hung from a pulley placed over the saw frame, to which a cord is attached, so as to reduce the pressure to the required amount. Under this arrangement the saw works more easily, but it does not cut so rapidly.