For marking upon the block of stone or marble the lines upon which it is to be sawn, as for cutting it into slabs of one or two inches thickness, the block is first shifted upon rollers into the position in which it is to be sawn; it is then mounted upon square pieces of wood called skids, with that side of the block upwards which is to constitute the edges of the desired slabs, and as the blocks are frequently of very irregular forms, it is necessary to make one line around the top, and two ends of the block, to serve as the basis from which the other lines are set off, much the same as in setting out round timber described in pages 703 to 707 of Vol. II.

The position of the first line having been determined, so as to allow of the greatest number of parallel slabs being cut from the block, two marks are made on the top of the stone close to the ends, with a piece of soft black slate found amongst coal, and called black, a line is then drawn under the guidance of a straight edge to connect these two marks, and the line is continued down one end, also with the straight edge. An equal distance is then set off at the bottom of the opposite end, and a line is drawn to serve as a temporary guide; two straight edges, each from two to three feet longer than the depth of the block, are applied to the two end lines, and the workman looks along the line of the two straight edges, to see whether they are parallel to each other, or out of winding, in much the same manner as in the application of the winding sticks to narrow works in wood, explained at page 500 of Vol. II., except that for setting out the blocks of stone, the straight edges are placed perpendicular instead of horizontal. Should the straight edges not appear parallel to each other, the one at the second end of the stone is shifted at the bottom until the two straight edges are in one plane; the permanent line at the second end of the block is then drawn in the corrected position of the straight edge, and if the work have been correctly performed, all the three lines will be in the same plane. The thicknesses of the required slabs are then gaged off from this foundation line, and the lines on the top of the stone are chased, or cut in about one-eighth of an inch deep with a narrow chisel, to form a groove in which the edge of the saw is placed for the commencement of the cut. The end lines are also chased, as the water and sand would wash out the black lines.

Before commencing the sawing, the workman examines with a plumb line whether the end lines are vertical, and if not, wedges are driven under one side of the block, to bring the end lines exactly upright, the saw is then inserted in the groove, and the sawing is proceeded with, care being taken in the first entry to keep the saw quite upright, which is greatly assisted by the height of the saw frame. Should the saw make the cut a little oblique to the lines, the position of the saw is slightly twisted in the saw kerfs of the wooden heads, by blows of a hammer applied on one side of the pins which retain the blade in the frame, and which causes the saw to cut in the reverse direction. The necessity for changing the direction of the cut is, however, avoided as much as possible, as it makes the surface of the slabs irregular from the hollows thus produced, and which are called galls. The necessity for grinding out these galls, much increases the labour of producing a flat surface on the slabs, and the thickness of which is also lessened; this it is sometimes an important object to avoid with valuable marbles, which are occasionally cut into veneers for inlaying, not exceeding one-eighth of an inch in thickness.

The length of the traverse of the saw is generally about 20 inches, and a saw is therefore chosen that is about 2 feet longer than the block to be cut, as the shorter the saw that can be efficiently used, the more firmly the blade is held. When two small blocks have to be cut, they are frequently placed end to end with the intended cuts in the same plane; and to prevent the sand and water, called the feed, from flowing out between the stones, the interval is filled up with straw rammed in firmly between the two blocks; in the case of light-coloured marbles clean shavings are used for this purpose, as the straw would stain the surfaces, unless the slabs were washed immediately afterwards.

After the marble has been cut into slabs with the stone saw, if it is required to be reduced into smaller pieces, or narrow slips, such as shelves, or the sides of chimney-pieces, the slab is laid on a bench, having a flat surface of hard stone, or marble, called a rubbing-bed. The lines indicating the margins of the required pieces are marked with the straight edge, and black, and the lines are chased with a narrow chisel, as for the entry of the stone saw, but the cutting is effected with smaller blades, called grub-saws, shown in fig. 1087; they consist of plates of iron from one-twentieth to one-tenth of an inch thick, from 6 inches to 4 feet long, and 6 to 8 inches wide when new. These blades are not stretched in a frame, but are stiffened by having their upper edges clamped between two pieces of wood extending their whole length, and measuring about 2 inches wide and 1 inch thick, the whole being held together by means of ordinary wood screws, passing through holes in the plate, so as to form a wooden back something like those of the dovetail saws, and which serves as the handle by which the grub-saw is used.

Fig. 1087.

The Production Of Plane Surfaces By Abrasion Part  30039

The blade should always be shorter than the length of the cut to be made, as should the blade be longer than the cut, it would be worn hollow from the greater amount of rubbing to which the middle would be exposed; but when the grub-saw is much shorter than the cut, it is liable to be worn rounding in its length. To counteract this tendency, the grub-saws are sometimes filed at every 4 or 5 inches, with angular notches about 3/4 of an inch deep, and which also allow the feed, or the sand and water, to reach the bottom of the cut with greater facility, and the grub-saws are consequently considered to cut rather faster for the notches.