LAPIDARY, a name given to the artist or artisan whose business it is to cut, grind, and polish gems, small stones, &c., for the purposes of jewellery, and also for mineralogical specimens. The name, derived from lapidarius, pertaining to stones (from lapis, a stone), would seem to include the various modes of working or finishing stones in general; the technical use of the word, however, is limited as above noticed.
Minerals differ greatly in the physical property of hardness; and this is so marked a feature, that a scale has been formed in which certain well-known minerals are arranged as standards of comparison; they increase gradually in hardness from No. 1 in the scale, represented by talc, which is very easily cut, to No. 10, the diamond, which nothing will cut. No. 2 is compact gypsum, No. 3, calc-spar; No. 4, fluor-spar; No. 5, apatite; No. 6, felspar; No. 7, quartz; No. 8, topaz; No. 9, sapphire.
Stones are usually cut or polished by means of harder stones, in the form of powder, applied to the edge, or on the surface of certain disks of metal, wood, &c., revolving horizontally, on vertical axes, and called mills. The annexed figure represents the lapidary's bench, consisting of a stout
plank, supported on a frame. The top is formed into two unequal compartments, and a rim rises above it to prevent the dispersion of the waste emery and water. In the compartment A is a hole and collar, through which passes the vertical spindle of the driving-wheel W, which is about 18 inches in diameter, and is worked by the handle represented in A. In the compartment B is another vertical spindle, carrying the mill M, which is 8 or 9 inches in diameter, and also the pulley P, which causes it to revolve by its connection, by means of a cord, with W. D is a square wooden rod, supporting a horizontal iron arm, in which the top conical end of the spindle works. At C is an iron support, called a gim-peg, or germ-peg, the use of which is to support the arm of the workman in grinding the edges of small stones, and also as a guide for the vertical angle in cutting facets, for which purpose a wooden socket C is slipped over the upper part of the peg, and held by a wedge, while holes or notches in the side of the socket enable the operator to place at the proper angle the stick S, upon which is cemented the stone to be cut.
Diamond powder is preferred as the cutting material, even where sand or emery might be employed. It does its work far more rapidly than any other material; it lasts much longer, it can be used with very thin plates, which not only expedite the process, but waste much less of the material in the cutting than thick slicers. Diamond powder is prepared from bor, as diamond fragments are called, or from imperfect stones; they are crushed in a mortar of hard steel, a process rendered easy by the brittleness consequent on the crystalline structure of the diamond. If not made sufficiently fine by this means, the powder is mixed with oil of brick, placed on a flat piece of iron, and worked with another piece of iron as a muller.
The slicer is of planished iron, 8 or 9 inches in diameter, and th of an inch in thickness. The diamond powder is formed into a paste with oil of brick, and is applied to
the edge of the slicer with a piece of stick or a slitted quill, and the particles of diamond are pressed into the iron with a piece of agate or flint applied to the edge. This seasoning, as it is called, will last several hours. The stone to be cut is applied lightly to the edge of the slicer, which, to prevent heating, is made to revolve with moderate velocity by turning the handle, shown in A; the slicer during the cutting being well supplied with oil of brick.
The flat surfaces produced in the above arrangement, which is called a slitting-mill, are ground upon what is called a roughing-mill, which consists of a lead lap, charged with emery and water. It is polished in the polishing-mill, on a lead or pewter lap, supplied with rotten-stone and water.
When a stone is to be worked into a definite shape, a pattern is cut out in card, and this being laid upon the stone, the outline is marked with ink. The stone is then roughly worked into shape by means of flat nippers of soft iron. The stone is then cemented to a stick, and the exposed portion is ground by holding the stick upon a flat mill, and imparting to the stone certain peculiar motions, which are well understood by working lapidaries, who produce rounded, elliptical, and other faces and edges, with surprising accuracy. The numerous forms of faceting are performed with the assistance of the gim-peg, as already explained.
Diamonds are, for the most part, cut at Amsterdam, where the steam-engine is used to give motion to the mills. (C. T.)