The work is now laid face upwards and a second coat of plaster of Paris, and a second liner is applied to the face of the work, which is thus cemented between the two pieces of stone, the first of which is then removed. To effect this the block is laid with the first liner upwards, a rim of clay is made on the surface of the second liner, and boiling water is poured in, which soon destroys the cohesion of the first coat of plaster, which is removed together with the liner, thus again exposing the backs of the pieces constituting the pattern, which are then cemented as one piece in the recess, previously prepared of the exact size, in the slab of marble in which the pattern is to be inserted. If the work is not intended to be exposed to the weather, plaster of Paris is used as the cement, but if the work is required to resist moisture or frost, the slab and pattern are both heated, and cemented together with the soft cement used for marble and stone which will be hereafter described.

The cement made of rosin and bees-wax is melted in a pipkin, sculptured works, as it corrodes the surface and greatly depreciates the artistic character of the work.

Granite and Porphyry may be sawn and worked after the same general manner as marble, but from the greater hardness and compactness of these substances the saw cuts more rapidly when made of copper, and supplied with emery and water. The grinding is best effected with a block or rubber of lead also supplied with emery, and which is generally used like a muller for grinding paint. For further particulars on working granite and porphyry the reader is referred to Vol. I. pages 169 to 172.

Marble has of late years been extensively worked by machinery driven by steam power, the processes are closely analogous in principle to those pursued by hand, but with various modifica-ions of the apparatus, and it is now proposed to explain briefly some of the peculiarities of the machine processes.

In the simplest application of machinery to sawing marble, as for making one or two cuts in a large block, the construction of the ordinary stone saw, fig. 1086, is closely followed, but the frame is made much stronger, of squared timber firmly bolted together, and stayed with chains; to constitute three sides of a rectangular frame; the place of the pole and tightening chain of the saw, fig. 1086, is occupied by two fixed beams, and the saw is held and stretched by means of two clamps with screws passing through the ends of the frame, and tightened by nuts on the outside. The saw frame works between vertical guide posts to keep it upright, and it is reciprocated horizontally by a connecting rod fixed to a crank driven by the engine. The connecting rod is attached to the frame by a loop, which can be placed at various heights so as always to keep the stroke of the connecting rod nearly horizontal notwithstanding the gradual descent of the saw in the cut.

These saw frames are sometimes made as large as 16 feet long, and 10 feet high, for cutting huge blocks of marble; and to prevent the great weight of these frames from pressing on the cut, they are suspended at each end by chains or slings which vibrate with the saw, and are connected with a counterpoise weight, that is adjusted to allow of the necessary pressure for the cutting, which is effected with sand and water supplied in the same manner as for the stone saw used by hand, but the introduction of the guide principle, renders the chasing of the stone for the entry of the saw unnecessary. In some cases smaller saws of similar construction are used for cutting thick slabs into narrow slips, and sometimes several cuts are made at once by an equal number of saw blades, arranged in a rectangular frame, that is suspended horizontally by vibrating slings, and works between vertical guide posts.

In the horizontal sawing machine for marble patented by Mr. James Tulloch in 1824, the entire arrangements are combined in a very effective manner, for cutting a block of marble into a number of parallel slabs, of any thickness, at the one operation. The iron framework of the machine, shown in fig. 1091, consists of 4 vertical posts strongly connected together at the top and bottom, to form a stationary frame from 10 to 14 feet long, 4 to 5 feet wideband 8 to 12 feet high, within which the block of marble to be sawn is placed. The two upright posts at each end of the stationary frame have, on their insides opposite to each other, perpendicular grooves, within each pair of which slides up and down a square vertical frame; to the lower end of each of these slides is affixed a spindle carrying two guide pulleys, or riggers, upon which the horizontal saw frame rests, and is reciprocated backwards and forwards. The saw frame is thus traversed within the fixed framing, and supported upon the four guide pulleys of the vertical slides, which latter are themselves suspended by chains coiled upon two small drums placed overhead. On the same spindle with the drums is a large wheel, to which a counterpoise weight is suspended by a chain. The weight of the counterpoise is so adjusted as to allow the saw frame to descend when left to itself, and which thus supplies the necessary pressure for causing the penetration of the saws.

Fig. 1091

The Production Of Plane Surfaces By Abrasion Part  30043

The saw frame is made rectangular, and from 2 to 3 feet longer than the distance between the vertical slides, in order to permit of the horizontal traverse of the saws, which is from 18 to 20 inches. To allow of the blades being fixed in the frame with the power of separate adjustment, every blade is secured by rivets in a clamp or buckle at each end; the one extremity of the buckle embraces the saw, the other is made as a hook, the buckle at one end of the saw is hooked upon a horizontal bar fixed across the end of the saw frame, and the opposite end of the frame has a groove extending its entire width, through which a separate hook, provided with a vertical tightening wedge, is inserted for every saw, which thus admits of being replaced without deranging the position of the neighbouring blades.