Marbles are generally cut up in the same direction in which they are quarried; this is known as sawing with the grain. Sometimes it is necessary to cut them against the grain, which renders them more difficult to work. Some marbles can only be sawn in the direction in which they are cut up. The marble worker is often obliged to rough hew and work without the help of the saw, casings, columns, and other articles with curved outlines; sometimes, but rarely, he re-works with the chisel badly-executed sawings; he then squares each piece with the saw or chisel to the required dimensions, and finally mounts the marble upon its stone core, and sets up the work in its place. The working of mouldings takes much time and trouble. The first operation is to saw the arris, then to work with a notched chisel making several successive groovings, on account of the contour and expansion, in which but very small pieces of the material are taken, for fear of splintering it; finish with small common chisels, which should be sharp and well tempered. Cylindrical pieces, such as round pedestals, columns, urns, and vases, are worked with a chisel, and then if portable, finished on a turning lathe.
When it is impossible to place the pieces in a lathe, they are thickly grooved, bolstered with the puncheon, and the desired contours obtained by means of thick panels they are then worked with a small chisel, which removes the dust, and thus prepares the marble for polishing. Polishing. - (a) Polishing includes five operations. Smoothing the roughness left by the burin is done by rubbing the marble with a piece of moist sandstone for mouldings. Either wooden or iron mullers are used, crushed and wet sandstone, or sand, more or less fine according to the degree of polish required, being thrown under them. The second process is continued rubbing with pieces of pottery without enamel, which have only been baked once, also wet. If a brilliant polish is desired, Gothland stone instead of pottery is used, and potters' clay or fullers' earth is placed beneath the muller. This operation is performed upon granites and porphyry with emery and a lead muller, the upper part of which is encrusted with the mixture until reduced by friction to clay or an impalpable powder. As the polish depends almost entirely on these two operations, care must be taken that they are performed with a regular and steady movement.
When the marble has received the first polish, the flaws, cavities, and soft spots are sought out and filled with mastic of a suitable colour. This mastic is usually composed of a mixture of yellow wax, rosin, and Burgundy pitch, mixed with a little sulphur and plaster passed through a fine sieve, which gives it the consistency of a thick paste. To colour this paste to a tone analogous to the ground tints or natural cement of the material upon which it is placed, lampblack and rouge, with a little of the prevailing colour of the material, are added. For green or red marbles, this mastic is sometimes made of gum lac, mixed with Spanish sealing wax of the colour of the marble; it is applied hot with pincers, and these parts are polished with the rest. Sometimes crushed fragments of the marble worked are introduced into this cement; but for fine marbles, the same colours are employed which are used in painting, and which will produce the same tone as the ground; the gum lac is added to give it body and brilliancy. The third operation of polishing consists in rubbing it again with a hard pumice under which water is constantly poured unmixed with sand.
For the fourth process, called softening the ground, lead filings are mixed with the emery mud produced by the polishing of mirrors or the working of precious stones, and the marble is rubbed with a compact linen cushion, well saturated with this mixture; rouge is also used for this polish. For some outside works, and for hearths and paving tiles, marble workers confine themselves to this polish. When the marbles have holes or grains, a lead muller is substituted for the linen cushion. In order to give a perfect brilliancy to the polish, the gloss is applied. Well wash the prepared surfaces, and leave them until perfectly dry; then take a linen cushion, moist-ened only with water, and a little powder of "calcined tin of the first quality. After rubbing with this for some time, take another cushion of dry rags, rub with it lightly, brush away any foreign substance which might scratch the marble, and a perfect polish will be obtained. A little alum mixed with the water used penetrates the pores of the marble, and more speedily gives it a polish. This polish spots very easily, and is soon tarnished and destroyed by dampness.
It is necessary, when purchasing articles of polished marble, to subject them to the test of water; if there is too much alum, the marble absorbs the water, and a whitish spot is left.
(6) If the piece to be polished is a plane surface, it is first rubbed by means of another piece of marble, or hard stone, with the intervention of two sorts of sand and water; first with the finest river or drift sand, and then with common house or white sand, which latter leaves the surface sufficiently smooth for its subjection to the process of gritting. Three sorts of grit stone are employed; first, Newcastle grit; second, a fine grit brought from the neighbourhood of Leeds; and lastly, a still finer, called snake grit, procured at Ayr, in Scotland. These are rubbed successively on the surface with water alone; by these means the surface is gradually reduced to that closeness of texture, fitting it for the process of glazing, which is performed by means of a wooden block having a thick piece of woollen stuff wound tightly round it; the interstices of the fibres of this are filled with prepared putty powder, or peroxide of tin, and moistened with water; this being laid on the marble and loaded, it is drawn up and down the marble by means of a handle, being occasionally wetted, until the desired gloss is produced. The polishing of mouldings is done with the same materials, but with rubbers varied in shape according to that of the moulding.
The block is not used in this case; in its stead a piece of linen cloth, folded to make a handful; this also contains the putty and water. Sand rubbers employed to polish a slab of large dimensions should never exceed 2/3 of its length, nor 1/3 of its width; but if the piece of marble is small, it may be sanded itself on a larger piece of stone. The grit rubbers are never so large that they may not be easily held in one hand; the largest block is about 14 in. in length and 4 1/2 in breadth.
Marble workers mount and fasten their works upon plaster mixed with a third part of dust, as pure plaster repels the marble, and causes it to swell out and burst. These are joined together by cramps and gudgeons of iron and copper, which should be carefully covered, in order that the oxides may not spot the casings. Marble chimney-pieces should be lined with lias stone or plaster.
Examine each piece, note its beauties, and endeavour to hide its defects before cutting or working it. When fine pieces are found, endeavour to cut them into two or three parts, in order to multiply them, cutting them in such a manner that these happy accidents may be reproduced according to taste.
Veneering upon wood is preferable, in every respect, to that on stone. For this purpose, as marble, particularly the black, would break by heating it in the usual manner, place the slabs of marble in a caldron, tightly closed, in which let them boil.
Then take them from the caldron, and after this preliminary operation, subject the marble to the heat of the fire to receive a mastic of tar. The wood having been prepared in a similar manner, press the marble, coated with the mastic, upon the wood, and a perfect cohesion is effected. The cases of ornamental clocks are hollow, for the movement of the pendulum and other works. This hollowing cannot be effected on stone without detriment to its solidity. When wood is used, a frame is made of it, upon the exterior parts of which marble is to be veneered. The mixture of glue with tar is found an improvement in effecting this veneering.