The stone to be sliced is first washed clean and dried, the line of the intended section may then be marked in ink as a guide, and the slicer is plentifully lubricated with the oil of brick, a thin oil that is used on account of its limpidity, and not being liable to become thickened by exposure to the atmosphere. Stones of small or moderate size are held in the hand, while the arm is rested upon the edge of the bench to steady it. The stone is then lightly pressed against the edge of the slicer, which is driven with only moderate velocity, or the friction would be liable to heat the stone, and cause it to crack. Care should be taken in the commencement of the cut to present a tolerably smooth surface to the slicer, as if a sharp corner were first advanced, it would be liable to scrape the diamond off the edge; and the diamond may also be torn off the edge if a smooth stone is pressed too forcibly against the slicer with the view of expediting the process. During the slitting the slicer should be kept plentifully supplied with the oil of brick, and the stone should be held steadily, and cautiously managed to keep the cut in a straight line, as from the concave form of the slicer it is rather liable to cut upwards. The principal attention is however required at the first commencement of the cut, and if this be correctly performed, the groove will serve in a great measure as a guide for the completion of the cut.
When the stone to be sliced is too large and heavy to be conveniently held in the hand, it is mounted on the crane, as shown in fig. 1145. The crane consists of an upright rod, mounted between centers, just in front of the perpendicular bar f, and upon this rod slides vertically a horizontal arm j, about 20 inches long, provided with a binding screw, by which it may be fixed to the rod at any height. The stone to be sliced is fixed to the middle of the arm and opposite the slicer, by means of a clamping piece and two binding screws, as seen in the figure, and the whole is drawn forward by a weight k, attached to a line leading from the extremity of the horizontal arm, over a pulley fixed to the end of the bench. The stone to be sliced is carefully clamped, so that the line of the intended division is exactly horizontal, and the precise height is adjusted by sliding the horizontal arm upon the vertical rod, until the line of division just meets the edge of the slicer. The weight then suffices to keep the stone continually pressing against the edge of the slicer, and the operator has merely to keep the latter in motion, and supply the oil.
For cutting parallel slices, it is only requisite between every cut, to shift the horizontal arm upwards upon the vertical rod. This simple contrivance entirely removes all difficulty in holding the stone, but is very seldom resorted to by practical lapidaries, except for large stones. A modification of this instrument to adapt it to the use of amateurs for cutting small stones will be hereafter adverted to.
To remove the marks made by the slitting mill, the flat surfaces of the stones are ground upon the roughing mill, or lead lap, supplied with coarse emery and water, by means of a brush. If the stone be large and thick, it is held directly in the fingers, but more generally the stones are too thin to be thus held, and it then becomes necessary to cement them to a wooden stick to serve as a handle. Large thin stones would also be liable to be broken in working if left unsupported, such stones are therefore cemented upon a handle made as a flat disk of wood, nearly as large in diameter, as the width of the stone, and having a central stem 4 or 5 inches long, and about half an inch diameter.
The cement is made of rosin, tempered with bees-wax and a little tallow, and hardened with red ochre, or Spanish brown and whiting, the smaller and harder the stones, the harder the cement is made by an increased quantity of the powders. For sapphires and other hard gems, a little shell-lac is sometimes added to the cement to increase its tenacity. To cement the stone upon the stick, the wooden disk is first warmed over a lamp, or candle, the cement is then heated, and evenly applied to the surface and edges of the disk, the layer of cement being made sufficiently thick to allow of the stone being fairly embedded, and it is then worked with the fingers nearly to the form of the stone, which is next warmed just sufficiently to cause the cement to adhere, without making it so hot as to be liable to burn the fingers; the surface of the cement is then melted over the lamp, and the warm stone is immediately pressed upon it. Care should be taken to place the stone quite central with the stick, which should also be exactly at right angles with the flat surface of the stone. The cement around the edges of the disk is then worked with the fingers into the angle around the stone, to support it uniformly near the edges.
In charging the lap with emery, a small brush dipped in water is generally applied to the lap to moisten it, and the dry emery is then sprinkled over its surface, and rubbed in with a flat piece of emery stone, or a piece of sheet iron; but some lapidaries prefer to dip the moistened brush in dry emery, and then apply it to the lap. In whichsoever way the emery be applied, it is desirable that as much emery should be supplied at the commencement of the roughing, as it is judged will suffice for the removal of the marks made by the slicer, and should more emery be required as the work progresses, the coarser particles remaining of the emery first supplied are partially crushed, either with a smooth lump of emery stone, or with a piece of soft sheet iron about 1 inch wide and 8 inches long, and the work is completed with a finer size of emery, so as gradually to reduce the coarseness of the grinding powder as the flat surface is approached.
As mentioned at page 1034, under the head Alabaster, Article 3, many lapidaries employ the same lead mill, both for roughing and smoothing the surface of the stones; some lapidaries however employ two benches for these purposes, so that the work may be taken from the roughing mill to the smoothing mill, without the loss of time incurred in crushing the coarser emery quite fine, but when one bench only is used for the roughing and smoothing, the same lap is made to serve both purposes. For large stones, the roughing is generally commenced with grinding emery, and finished with flour emery; but for small stones, superfine grinding emery is sufficiently coarse for the commencement, and fine flour emery is used for the smoothing.