The slicer is made of a disk of sheet iron, usually about eight or nine inches diameter, and two hundredths of an inch in thickness. It is of course necessary that the edge of the slicer should run exactly in one plane; it is therefore planished or hammered in the manner explained in vol. i., pp. 414 to 422. But if so thin a plate were made perfectly flat, like a circular saw, it would be very feeble sideways, and would be readily distorted by the resistance of the work, and therefore, to give greater rigidity, the slicer is hammered into a slightly arched or dished form, the concavity being about one-sixth of an inch in the entire diameter. This trifling concavity materially increases the stiffness of the slicer, and does not interfere with its use for cutting straight sections; as when the slicer is properly hammered and turned true, the extreme edge runs exactly in one plane for the commencement of the cut; and when the slicer has penetrated a small depth, the trifling curvature of the plate gives way, and it is flattened by the groove it has itself cut, and in which it is compelled to run. The slicer is further stiffened by being firmly clamped, like a circular saw, between two flanges on its spindle, which is made of such a length that the edge of the slicer may be about 3 inches above the level of the bench, in order to allow room for the hand, and also for large stones.
The preparation of the diamond powder for charging the slicer has been already described on page 1052, under the head Diamond, Article 1; but it may be added that the usual criterion for the fineness of the diamond powder used by lapidaries, is that the particles should be so small that no sparkling is perceptible when the diamond powder is exposed to the light. Slight differences are made in the forms of mortars for crushing diamonds, but that represented in fig. 1142 is the more generally preferred. The mortar a, has a deep cylindrical hole terminating at the bottom in a spherical cavity of hardened steel, embracing from about one-third to one-sixth of a circle, into which the pestle b, is accurately fitted by grinding. The long cylindrical fitting serves as a guide for keeping the pestle upright, and also prevents any of the valuable particles from flying about when the pestle is struck with the hammer; the cover c, is also added for the latter purpose. In some mortars for crushing diamonds, the bottom of the cavity is made flat, and the pestle is then made square at the end, as shown in fig. ] 143, in which a represents the base of the mortar, b, a short cylindrical tube fitted into a shallow cavity in the base, and c, the pestle, which is fitted within the tube. But this form of mortar is seldom employed by working lapidaries.
Sometimes, when the diamond has not been crushed sufficiently fine in the mortar, the lapidaries grind the diamond powder; for this purpose they commonly mix it with a little olive oil or the oil of brick, and spread it upon a flat piece of iron, generally an old laundry iron, and any small piece of iron is used as a muller. The mortar represented in fig. 1144 is, however, greatly preferable for grinding the diamond powder. The base a of the mortar has a spherical cavity, of hardened steel and about two inches radius, to which is fitted the pestle b, which is also made of hardened steel and fixed in a wooden handle. The diamond powder is placed in the center of the cavity, and a few drops of oil are added; the diamond is then ground as fine as required by rubbing the pestle within the mortar with moderate pressure.
In applying the diamond powder to a new slicer, or as it is called seasoning the slicer, it is mounted in the machine, and the edge is turned quite true and smooth, with a graver supported upon the rest h, fig. 1141; or in some cases it is afterwards smoothed with a fine file, as it is of importance that the edge of the slicer should be quite true, and free from even minute notches, or otherwise the irregularities would be liable to catch the stone, and throw it out of the hand, or if the stone were firmly held, the slicer would become distorted.
A small quantity of diamond powder, mixed with the oil of brick, is then taken out of the cup with a small piece of stick, or a better practice is to employ a piece of an ordinary quill about 1 inch long, prepared by splitting the barrel of a quill lengthways into three or four pieces, and rounding the ends. The quill is dipped in the cup, and a little of the diamond powder, or rather paste, is taken on the concave side of the quill, which is then held vertically against the edge of the slicer, so that the curvature of the slicer may nearly agree with that of the quill; the latter is then held steady while the slicer is moved slowly round, in order to distribute the diamond uniformly on the extreme edge of the slicer. To fix the particles therein, a smooth piece of any hard stone, such as agate or flint, from half an inch to an inch wide, is immediately applied with gentle pressure against the edge of the slicer.
In order that both hands may be at liberty for charging the slicer, the wheel is sometimes turned by an assistant, and the lapidary supplies the diamond powder with one hand while he holds the charging stone to the edge of the slicer with the other. As soon as the diamond begins to cut the stone, the latter is shifted to another position, as, if the slicer were permitted to cut a groove in the charging stone, the diamond powder would become fixed in the sides of the slicer, which must of course be avoided. As soon as the small quantity of diamond resting on the edge of the slicer has been pressed into it, the margin of the slicer is carefully wiped on both sides with the forefinger, in order to remove any small portions of the diamond that may have become accidentally lodged on the sides, and these particles are pushed to the edge of the slicer, and pressed in with the charging stone. When the whole of the diamond powder has been pressed in to the extreme edge, a second quantity is applied in the same manner, and which is generally sufficient for charging or seasoning a new slicer. After the edge of the slicer has been once fairly charged with the diamond powder, a single application is generally sufficient for restoring the cutting edge, and under the hands of the practical lapidary, a single seasoning will endure several hours' work.