LAPIDARY-WORK, the art of cutting, grinding, and polishing small pieces of orna mental or precious stones for jewelry. (For the engraving of fisaires on precious stones, see CAMEO and GENts.) The working of the less precious ornamental stones has made great strides within the last 20 or 30 years, and nowhere has it reached greater perfec tion than in Scotland. A large trade is now carried on in this kind of work between Birmingham and some towns of Germany, where the Scotch patterns are imitated; and although the foreign productions are of inferior workmanship, their comparative cheap ness commands a ready market.
Stones are cut by rubbing the powder of a harder stone against a softer one. There are 10 types of hardness (q.v.), from talc up to diamond; but in practice it is found most convenient to employ either diamond-powder or emory, which is next to it, for the cutting of all kinds of stones. Diamond-dust is found to cut 10 times faster than emery; to that, except where the machine is driven by water-power, it is found more profitable to employ diamond-powder, notwithstanding its high price. Diamond-powder is prepared from the inferior kind of diamonds (q.v.) called bort (costing about a guinea per carat), by grinding in a steel mortar.
To produce a plain polished surface on any stone, say a jasper, it goes through the three processes of cutting or slitting, grinding, and polishing. The slitting-wheel, which is driven by means of the handle, is .a mere disk of thin sheet-iron, from 6 to 9 in. in diameter, with a turned edge, and is generally placed in a horizontal position. The diamond-dust, mixed with a little sperm-oil, is applied to the edge of the slitting wheel with the finger, and is then pressed into the soft iron with a smooth hard stone. The wheel will then continue to cut for several hours without any renewal of the pow der. When the wheel is tints prepared, a stone held by the hand to the cutting edge is rapidly slit through. During the operation, sperm-oil is kept dropping from a can, to keep the wheel from heating.
The grinding is performed on a uorizontal lead-wheel, charged on its upper surface with emery-powder; the stone to be ground being pressed against it with the hand until it is smooth enough for polishing. In polishing, a tin wheel is substituted for the leaden one, the polishing material being rotten-stone.
If, instead of a plane flat surface, sonic ornamental surface is required, say an agate brooch in the shape of a butterfly, a.model is produced in plaster of Paris, to serve as a guide, and metal size-plates are prepared for the pieces. of stone which are to form the wings, etc. For these, thin slices of agate are cut at the slitting-machine, or chipped off with a hammer and chisel, and are then formed roughly into shape, by means of soft iron nippers. The several pieces are now ground and polished, as already described, and the brooch is finished. When pieces of stone are too small to be held in the hand, they are attached with cement to a wooden handle, and then applied to the wheels.
One of the most elaborate operations of the lapidary is the cutting of cairngorm (q.v.) stones. In faceting the surface, which so much enhances their beauty, the lapi dary uses the ordinary grinding-wheel, with the addition of a wooden peg, stuck round with projecting "Wires. The stone is fixed with cement on the end of a stick, having a hole at the other end on the wire-points, which, being at different heights, enables the stone to be held at any angle to the grinding surface. With this simple guide, the lapidary proceeds to cut the facets, dividing them off by the eye, aided by his sense of feeling; and in this way, in about a fortnight's time, as many as 700 facets are produced of perfect regularity upon a stone, say an inch in diameter. A cairngorm of good color, so cut, may be worth about £30.