The width of the iron blade measured to the wooden back, limits the depth of the cut to which the grub-saw can be applied, and in selecting a saw for any particular piece of stone, preference is given to as narrow a blade as can be fairly applied to that thickness, as when the blade is wide, it is rather feeble sideways, and it is besides more liable to be twisted from the perpendicular, when rubbed backwards and forwards in the cut, with one or both hands applied on the back of the saw near the middle of its length.

Slabs of marble or stone that are required to have flat surfaces, after having been sawn to their respective sizes are laid upon the rubbing-bed, with that side upwards which is to be ground flat, a smaller slab of stone, with a tolerably flat surface, is then selected to be used with sand and water as the grinder, the size of the grinder or, as it is called, the runner, depends upon the size and condition of the work to be ground; if the slab be large and moderately well sawn, as large and heavy a runner is used as the workman can conveniently push backwards and forwards; if the work be rounding in the middle, a smaller runner is employed; and if the slab be hollow in the direction of its length, a long narrow runner is used, the selection depending upon the condition of the slab, and the judgment of the workman.

The kind of stone which is used for the runner is partly dependent upon the kind of stone to be ground, but, generally speaking, the runner should be the harder stone; indeed, two soft stones, such as Portland, if ground together would hang to each other to such an extent as very materially to increase the labour of grinding. Portland stone is therefore generally ground with a runner of York stone. York stone and marble are ground with runners of the same material as the slab, but it is better that the runner should be of a harder variety.

The stone used as the runner becomes itself ground flat in the process, and advantage is taken of this circumstance to grind slabs of moderate size, by using them as runners for larger slabs, the two stones being ground flat just as readily as one.

Sometimes iron rubbers or runners are employed, and these have the advantage of retaining a much greater accuracy of form; they are far more durable, and the sand and water can be applied in a more regular manner, as the iron runners are frequently made with a raised rim around their upper surface, so as to form a kind of tray, within which the mixed sand and water is placed, and the flat surface constituting the bottom of the tray is perforated with holes, through which a constant supply of sand and water is admitted to the grinding surfaces.

Of whichsoever material the runner may consist, it is provided with a handle of sufficient length to enable the workman to traverse it over the entire surface of the slab; if the runner be large and of stone it is in general held as in fig. 1088, the end of the long handle is nailed on the upper side of a board about 1 foot long and 9 inches wide, having a slip of wood about 2 inches wide nailed on the under surface, to rest against the one end of the runner, which is retained at the other end by a loose iron ring about 1 1/2 inch wide provided with a tail-piece. The loose ring called the hook is slipped up the handle until the tailpiece is stopped by the stone, when from the angular position assumed by the loose ring its edges slightly penetrate the handle and prevent its return, the runner is thus securely grasped between the wooden stop and the iron tail-piece. If the runner be of iron, the handle is generally passed through two holes cast in the projecting ends of the runner, or otherwise two upright pieces are cast on the back for the reception of the handle. For small runners the handle is sometimes fixed at an angle, and sometimes vertical, as mentioned at page 1089 under the head Rubber, Article 2.

Fig. 1088.

The Production Of Plane Surfaces By Abrasion Part  30040

In grinding the flat surface of a marble or stone slab, the runner plentifully supplied with sharp sand and water, is pushed backwards and forwards in all directions over the face of the slab, the flatness of which is frequently examined with a straight edge applied in all positions upon the surface of the slab, but principally upon the four margins and the two diagonals of the stone, and as the slab approaches a flat surface the sand is gradually changed for finer kinds, according to the quality of the surface required on the stone: for marble, the last process of smoothing prior to the commencement of polishing is in London effected with silver sand, which is generally obtained from the neighbourhood of Croydon. The smoothing should be continued until all the marks made by the saw and the coarser sand are entirely removed, and the slab presents a uniformly smooth surface, the last marks to be eradicated in the smoothing are generally those called stuns, made in sawing the marble by coarse particles of sand getting between the side of the saw blade and the saw kerf, and which are sometimes forcibly driven into the surface of the marble, and cause specks that unless removed greatly impair the appearance of the work when polished.

Single pieces of marble of a moderate size and weight, say not exceeding 18 inches square and 1 inch thick, are ground by laying them face downwards upon a slab supplied with sand and water, the marble to be ground is rubbed by the hands in all directions over the slab, but chiefly in the form of a figure of 8, to insure its being ground equally, the path in which the marble is rubbed being occasionally reversed.

When several small pieces of marble have to be ground flat, such as the squares for a tesselated pavement, it would be very tedious to grind them separately, they are therefore arranged close together and face downwards upon a large flat stone. Plaster of Paris is then poured over their upper surfaces, and a long stone a little narrower than the width of the pieces called a liner is laid over the whole, which thus become cemented together with all their faces level, notwithstanding that they may be of irregular thicknesses. The whole are then ground together as a runner upon a slab of marble, and the liner being narrower than the squares, allows of the two edges being ground as explained in the next paragraph.