Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/317

LAPIDARY 299

diameter and about 7^ inch in thickness. Such disks with their driving gear are termed slitting-mills. When leaden laps charged with emery mud are substituted the arrange ment is called a roughing-mill, and when leaden or pewter laps charged with rotten-stone are used it is known as a polishing-mill. The mills are sometimes worked by steam power, sometimes by hand. In the ordinary pattern of a lapidary's bench the handle turns in a horizontal plane, as shown in fig. 4, where W is the driving-wheel turned by the handle A, and working the pulley P by means of a strap. The pulley is fixed on a vertical spindle, which carries M the disk for slitting or the leaden lap for rough ing or polishing. The upper end of this spindle is conical, and rotates in a socket drilled in a horizontal arm of iron which projects from a vertical wooden rod D. A block of wood C fits on to the end of an iron support termed the gim-peg or germ-peg. This support is used to steady the operator's arm when grinding the edges of small stones, and the wooden block, which is fixed by a wedge, is em ployed for cutting facets at any desired angle, the stone being cemented to the end of a stick S, which is fixed at the requisite angle in one of the holes or notches made in the sides of the socket C. In slicing stones it is necessary not to bring any sharp edge of the stone against the disk, but to commence upon a moderately flat or smooth surface, otherwise the charge of diamond dust or seasoning, which should last for several hours, will be stripped off during the first revolution.

Fig. 4. – Lapidary's Mill.

Fig. 5. – Part of Lapidary's Mill. A, upper summer (cut through); B, lower summer (cut through); C, spindle; D, pulley; E, lap; F, table (cut through); g, g, wooden blocks, adjusted by means of wedges.

Another form of lapidary's mill consists of a strong framework of oak, 8 or 9 feet long by 6 or 7 feet in height, and with a breadth of about 2 feet. It is formed of four square uprights, mortised into a couple of sole-bars, and braced together by a eight cross-bars at top and bottom, which, like all the other parts of the frame, are mortised and strongly bolted together. Half way up the frame a strong board or table is fixed, and above and below this table stout wooden bars or summers run the length of the frame. In each of these summers are two square holes through which slide short oaken rods having square sections and bored out conically at the ends to re-

ceive the upper and lower extremities of the iron spindles which carry the laps or slicers (fig. 5). The remainder of the working parts are very similar to those already described, except that the driving-wheels are very large and the lower extremities of their axletrees, which are conical, rest upon sockets fixed to the floor, while their upper ends revolve in holes in a beam. The driving-wheels drop over pegs which project from the upper sides of collets, imme diately beneath which the axle has a crank. The crank is connected with an arm composed of three flat iron bars, which are fixed together at suitable lengths by square rings. The other end of the crank bar is provided with a stud by which it is attached to a pivoted wooden arm carrying two upright pegs, which serve as handles for the operator, who imparts a backward and forward motion to the arm.

Fig. 6. – Lapidary's Dial. A, section; B, side elevation; a, a, jaws; b, ball; c, tube; d, dial; e, cement rod; f, index; g, quadrant.

A very important substitute for the gim-peg-socket, already described, is the dial, by means of which facets can be cut with great precision. One of the improved forms of the lapidary's dial consists of two jaws a, a (fig. 6), in each of which a hemispherical cavity is ground, and within this cavity a brass ball b is contained by the jaws when they are clamped together. A brass tube is attached to this ball, and carries a circular dial d at its upper end. Into the lower end of the tube is tightly inserted the cement-rod, which is fixed by a set-screw carrying at its lower end e the stone to be cut. At its upper end, which is squared, and projects above the dial, a small index f is fitted. Upon the side of one of the jaws is a divided quadrant g } with the centre of which the centre of the brass ball coincides. The tube bearing the dial can therefore be inclined at any angle corresponding with the divisions on the exposed face of the quadrant, while, by turning the cement-stick and its index, the stone can be easily set, so that a range of facets may be cut with great accuracy.

Fig. 7. – Cut Gems. A, brilliant; B, rose; C goute de suif; D, en cabochon.

Where practicable, the lapidary avails himself of the natural cleavages in the mineral upon which he is going to operate, and these are constant in direction in any one species, but are more easily available in certain minerals than they are in others. When no satisfactory cleavage planes exist, the mineral may be sawn into slices by a thin wire charged with diamond dust. The diamond is cut and polished upon a lap of cast-iron fed with diamond dust and olive oil. Gems having a hardness of 8 and 9 (Mohs's scale) are cut on copper disks, similarly primed, and are afterwards polished with tripoli and water. Stones of inferior hardness are ground upon a leaden lap with emery and water, and are polished on tin disks with tripoli, or on zinc disks with putty-powder and water. In grinding very