STONE WORKING. Under QUARRY AND QUARRYING a descrip tion is given of the mode of detaching and shaping blocks of stone at the quarries. A few lines will suffice to convey an idea of the subse quent operations.
Until within a comparatively recent period, few other tools than the saw and chisel were employed in this work ; but now machinery, moved by steam-power, is extensively adopted. The working of marble for ornamental purposes was the chief cause of this change ; seeing that the highly-wrought surfaces might suitably be produced by labour saving expedients. The sawing by hand of blocks of stone for build ings, or slabs for pavements, is familiar to every one. The workman employs a kind of blunt knife, which is converted into a species of saw by the presence of sand and water ; the action of this blade, by a pro cess something between cutting and grinding, severs the block of stone into two. The shaping by hand of blocks of stone is equally familiar ; chisels, of various shapes, urged by a mallet, do the whole work. But it is of more elaborate contrivances that we here treat.
Some of the marble-sawing machines are of the kind shown in the annexed cut. Several saws are fixed parallel in a frame, ut any distance apart according to the thickness of the slab to be cut. The frame slides in vertical grooves ; and it is so balanced that the saws are kept at a height corresponding to the part of the block which is being cut, and descend as the cut deepens. These so-called saws are blades of soft iron, used with sand and water ; the sand being varied in quality according to the kind of stone to be cut. An apparatus is placed over the frame, whereby a little stream of saud and water is made to flow continuously into each saw-cut. Not only does the use of such a machine save much labour, it saves material also ; for in hand workmen never succeed iu keeping the saw in a true plane ; and, as a necessary result, great waste is occasioned in subsequent grinding. Mr. Stewart has recently invented a machine for cutting large masses of stone ; a series of chisel-cutters follow each other iu the same cut, and are fixed to a frame travelling on a kind of railway. The action of each cutter is that of a forcible blow ; the machine being intended to operate upon the harder kinds of stone. Messrs. Hunter, of Manches ter, have devised a machine for cutting pavement-slabs. Twenty or thirty cutters are fixed to the periphery of a revolving disc ten or twelve feet in diameter. These machines are in use in Dean Forest ; some of them will cut 250 square feet of pavement, li inches thick, in ten hours.
When marble or other stone has been cut into slabs, its further working is effected by machines of various kinds. To reduce it to narrow slips, it is exposed to the action of small circular cutters, ranged parallel on one common axis ; the distances between the cutters are made equal to the intended widths of the strips ; the marble is slowly brought up to the revolving cutters by the action of pulleys and weights.
Circular pieces are cut from slabs by ingenious machines. Large circles are cut by means of four cutters placed at the ends of the arms of a horizontal cross ; the size of the cross determines the diameter of the circle to be cut, and the curvature given to each cutter is made correspondent thereto. When the frame or cross is made to rotate,
the four cutters follow one another in the same path, and speedily cut out a circular piece. Smaller circles are cut by means of a hollow cylindrical tool, something like a punch; but it acts by continual rota tion, and not by blows. By a modification of this apparatus, round pillars and hollow cylinders or tubes of stone may be cut.
To produce mouldings, or similar symmetrical cuttings in stone, various machines are employed. The turning-lathe is used for circular objects. Iron cutters, with sand and water, are not used here ; but the workman acts upon the stone with long, sharp-pointed instruments of steel ; and when the shape has been thus roughly produced, it is finished by gouges and other tools. Strips of stone or marble, such as those used for chimney-pieces, have mouldings formed upon their surfaces by different means. The cutters here are in fact grinding tools. They consist of masses of iron, whose surfaces are circular, and have been wrought into various forms, such as hollows, beadings, ogees, &c. ; the tool rotates rapidly in contact with the atone, which is brought up close to it by a weight and pulley; and thus a moulding is formed on the surface of the strip of stone, a counterpart of that on the iron tooL A workman applies sand and water to the iron tooL The apparatus is shown in the annexed cut.
The smoothing, grinding, and polishing of marble and stone are effected by machines variously arranged. Largo slabs are ground by a plate of cast-iron. The slab, placed horizontally, has a reciprocating motion given to it; the iron plate, resting upon it, has a kind of spiral motion ; and the two motions together enable the iron to act equally on all parts of the stone. Sand and water are let down between the two surfaces, through holes in the iron plate. Smaller surfaces of stone or marble are ground by being held down by hand upon the sur face of a revolving iron table, kept moistened with sand and water. The Earl of Caithness has recently devised a machine for dressing the surface of Caithness rag slabs, a stone well fitted for street-pavements. About 30 iron bars are ranged parallel and vertical, each with jagged teeth at the bottom; a crank movement lifts them all to a certain height, one after another, and lets them fall heavily ; the slab slowly moves beneath them, so as to be subjected all over to an equal amount of jagging or chipping. This produces a surface level but not smooth.
Tho polishing of marble requires tools different from those used in grinding or smoothing. The tools are made of lead or some other heavy substance, and are faced with a peculiar kind of felt, which, when wetted and rubbed to and fro by any convenient machinery polishes the marble. For smoothing and polishing of carved marble, or of small pieces shaped in any irregular way, small pieces of cast iron, gritstone, smooth stone, slate, &e., are used in various ways.
STOP. [0aosN.]