All the tools are required to have tolerably smooth surfaces, and to be quite free from ridges or hollows. In use they are liable to be worn into minute hollows, which are called creases, and should one of these be formed in the side of a thin tool, such as fig. 1199 or 1201, it would be almost certain to chip off a fragment of the stone, as soon as the crease was embedded in the cut. To prevent the formation of creases in thin tools, the seal engraver makes frequent use of a fine file to smoothen the sides of the tool, and which at the same time serves to prevent the edge of the tool from becoming thickened as it is reduced in diameter by wear.
The tools are charged with fine diamond powder, prepared as described in page 1052, in the mortars shown in figs. 1142 to 1144, p. 1309. Seal engravers generally keep the diamond powder in the mortar in which it was ground; the powder is mixed into a pasty condition with olive oil, and small quantities are taken out as required. Sometimes the diamond powder, or paste, is kept in small quantities in a little conical cup, and the diamond is supplied to the tool, either by holding the cup to the tool, or a little of the diamond powder is removed with a small spatula, and held to the edge of the tool.
More generally, however, the seal engraver wears on the forefinger of the right hand, a ring made of a strip of tin, to which are soldered two little hollow disks, about half an inch in diameter, one of which contains a very small quantity of the diamond paste, the other, one or two drops of the oil of bricks. The diamond paste is occasionally applied to the extreme edge of the tool while it is revolving slowly, the tool is then moistened with the oil of bricks, and the cutting is proceeded with, until the brick oil is nearly expended, when the tool is again moistened. Should the tool be allowed to become too dry, the diamond would become detached from the tool, and instead of the stone being cut, the tool itself would be abraded, and as the brick oil is very volatile, it requires to be frequently applied. Some artists prefer sperm oil for lubricating the tools, it is less expensive, and has not the unpleasant scent of brick oil, but unless carefully prepared, it is liable to become thick, and impede the action of the tools.
The stones to be engraved are always previously prepared to the general form by the lapidary, and frequently they are set by the jeweller before they are engraved, in either case they are too short to be conveniently held in the fingers, they are therefore mounted on a handle about five inches long, and three-quarters of an inch diameter. If the stone has not been set, it is fixed with lapidary's cement upon a wooden handle, and to prevent the cement from adhering to the fingers, it is sometimes coated with sealing-wax. But if the stone has been previously set, it is inserted in a notch made in a piece of cork, or soft wood, that is frequently inserted in the end of a piece of bamboo of appropriate size. When the stones are hard, and have been previously polished on the surfaces that are to be engraved, the latter are roughened, by rubbing them upon a soft steel plate charged with a little diamond powder and oil; an ordinary plane iron when annealed is often used as the plate. Sometimes the polish is removed from soft stones by rubbing them upon a leaden plate charged with emery, but the steel plate and diamond powder is more generally used, as it serves equally well for hard or soft stones.
The roughened surface of the stone is required partly because the tools penetrate more readily into the rough surface, and are less liable to slip, but principally to enable the outline of the device to be sketched upon the stone with a brass point, which is abraded by the rough surface, and leaves a distinct line. In drawing the design upon the stone, the general outline alone is first carefully sketched, the entire surface enclosed within the outline is then sunk, and the details of the design are afterwards sketched and sunk in succession.
For example, in engraving a shield with quarterings upon a seal. The outline of the shield is first drawn with the brass point, this is then dotted round with a small tool, such as fig.
1199, having a thin edge, and called a sharp or knife tool. The dots, which are about the thirtieth of an inch long, and about half that distance asunder, serve to secure the outline, and prepare a path for a thicker tool with a rounded edge, such as fig. 1201, with which the outline is perfected. The bulk of the material, within the outline, is then removed with a thicker and larger tool; having a rounded edge like fig. 1203, the larger tool operates more rapidly, and is also less liable to leave the surfaces irregular, and therefore as large a tool is employed as can be conveniently applied to the purpose. When the body of the shield has been sufficiently lowered, the surface is smoothed with a smaller and flatter tool, like fig. 1202, which cuts smoother, and also allows of being applied closer into the angles. The fine lines for the quarterings are next sketched, and cut with the sharp tool, and the figures or bearings on the quarterings are afterwards sketched, and sunk in succession. If, as frequently happens, two of the quarterings are similar in design, they are sketched and cut together, in order to avoid the frequent change of tools, and also to ensure greater similarity; the same tools being used for corresponding parts of the design. The bolder portions of the bearings are of course cut first, and the smallest details are left to the last. Should the escutcheon have a shield of pretence, this would be sunk after the quartering lines had been cut, and if supporters and garniture were required, the entire outline would be first sketched, and the whole advanced equally, so as to keep the general effect uniform.
The cutting of the fine parallel lines on the field, called colour lines, presents considerable difficulty, as they are very shallow, and to give them a uniform appearance requires much care, and a light but steady hand. To assist in cutting these lines equidistant, a tool is used, having two knife edges, as shown in fig.