1902 Encyclopedia > Lapidary

Lapidary




LAPIDARY (lapidarius, from lapis, a stone), one who cuts, grinds, polishes, and engraves small pieces of stone, especially gems. The prehistoric stone implements found in cave deposits, peat-mosses, river-gravels, &c, may be regarded as the earliest examples of the chipping and grinding of stone. Small cylinders of serpentine and other soft stones, on which figures and inscriptions were engraved, were fabricated in very early times by the Assyrians. Similar cylinders were afterwards made in rock-crystal, chalcedony, haematite, &c, and these harder substances were engraved by means of drills charged with the powder of still harder minerals. The use of such drills is said to date as far back as the year 730 B.C. These cylinders were perforated by round holes, and were strung as necklaces. The stone scarabei and other amulets of the Egyptians were carved or chiselled, and, according to King (Handbook of Engraved Gems, 1866), these people do not appear to have followed the Ninevites and Babylonians in the use of the drill. This tool was, however, largely employed by the Etruscan lapidaries, who also used a diamond point in finishing their work. Signets were used by the Greeks as early as 600 B.C. and in the time of Alexander gems of all kinds were cut and engraved, with the exception of the diamond. The art of cutting diamonds was probably known to the Hindus and the Chinese in very early times, but it was unknown in Europe until nearly the close of the 15th century, the diamond cut and polished for Charles the Bold, duke of Burgundy, in 1475 being the first recorded example. The diamond point was extensively used in engraving the gems of the 16th, 17th, and early part of the 18th centuries, when skilful imitations of the antiqu^gems were fabricated, and the employment of this tool is already spoken of by Pliny (H. N., xxxviii. 15) and Solinus, c. 52. The drills used by the ancients were worked either by hand or with a bow. Holes are now drilled in stone by means of an iron or copper tube, fed with diamond dust and
oil (fig. 1).

Fig. 1

Fig. 2

Fig. 3.

The small tools used for engraving stones are set in a horizontal position, and are worked by vertical driving gear (fig. 2). They are of various forms, some of which are shown in fig. 3, and are made of soft iron and charged with diamond dust and oil. Any substance finely pul-verized, and of greater hardness than the material operated upon, may be used for cutting and grinding stone, but diamond dust is preferred as it can be used sparingly by the employment of very thin slitting disks, into the edges of which it is imbedded by the application of an agate or glass roller, the dust being previously worked into a paste with oil. Oil of brick or soft soap is used freely as a lubricant during the process of cutting. The diamond powder is procured by finely pulverizing imperfect stones, usually the coarse variety termed bort or carbonado, in a steel mortar; or it is ground between flat iron slabs with oil of brick.

The iron slicing disk in common use is 8 or 9 inches in diameter and about 1/200 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

FIG. 4.—Lapidary's Mill.

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 whisb. 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 fiat or smooth surface, otherwise the charge of diamond dust or seasoning, which should last for several hours, will be stripped off during the first revolu-tion.

FIG. 5.--Part of the Lapidary's Mill. A, upper summer (cut through); B, upper 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 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.





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, 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. 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.

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 vhich he is going to operate, and these are constant in direction in any one species, but are more easily available in certain minerals than are in others. When no satisfac-tory cleavage planes exist, the mineral may be sawn into slices by a thin wire charged with dia-mond 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 fragile or soft stones disks of hardwood are employed. Precious stones are cut in forms known as brilliants and roses, and the several parts are designated as shown in fig. 7 (side-elevations). Turquoise, opal, cats-eye, car-buncle, asteria, and a few other stones are cut en cabochon.

Prior to engraving on a stone, the polish is removed with emery from the surface to be engraved, and the device marked on it with a brass point; the outline is then sharply incised, and the work continued by means of small drills, the diamond point, &c.

Within the last few years a great advance has been made in our knowledge of the minute structure and mineral constitution of rocks by cutting and grinding small slices of them so thin that they readily transmit light, and can then be examined under the microscope, and the optical properties of their constituent minerals conveniently studied.

Fig. 8

Sections suitable for this purpose may be prepared by grinding thin flakes or splinters of a rock or mineral upon a cast-iron plate smeared with emery powder and water. The emery employed for the first grinding should not be very fine, that of medium grain being best suited for the purpose. The fragment is pressed by the fingers against the slab, and ground uniformly over all parts of the plate with a circular motion. When a flat surface is obtained,
the fragment should be carefully washed from all traces of the emery mud, aud a finer face should be imparted to it by a second grinding with the finest flour-emery and water, smeared upon a slab of plate-glass or a smoothly-planed brass slab. When thoroughly cleaned, the smooth face of the chip is warmed and cemented to a small piece of plate-
glass with Canada balsam (fig 8). The older and drier the balsam the better it answers this purpose. A little should be placed on the piece of glass and warmed until it liquefies (it must not boil). The smooth surface of the stone is then laid upon the balsam and pressed tightly against the glass ; when the balsam has hardened, the grinding process is renewed, tbe.piece of glass serving as a handle, and the flour-emery sho >dd generally be employed as soon as the fragment is thin enough to transmit light. When finished, the glass and section are cleaned, the glass is warmed, and the section is pushed off with a blunt needle or wire into a saucer of turpentine, which should be gently warmed, and all traces of dirt should bs removed with a camel's hair brush. The section is then lifted from its bath by
means of a needle and allowed to subside gently upon a drop of fluid Canada balsam placed on a clean glass slip which has been previously warmed. A thin covering-glass is then slightly warmed and placed over the preparation, care being taken not to include any air-bubbles. The process of grinding sections by hand is necessarily a rather slow one, and, although in the finishing it cannot well be superseded by other methods, still the rough grinding may be much more expeditiously done by means of various machines which have been devised for this purpose. Some of these are worked by hand, others by a treadle. Among the latter, the apparatus devised by Mr J. B. Jordan, and manufactured by Messrs Cotton & Johnstone of Grafton Street, Soho, and that made by Fuess of Berlin are those in most general use. These machines are provided with slitting disks for cutting thin slices with diamond dust. This saves much grinding, but presents some difficulties to the novice. The grinding laps with which the machines are supplied are generally cast in lead or pewter, while occasionally prepared corundum disks are employed, and disks of hard wood are now and then used for imparting a final polish. (F. K.)



Footnotes

Described and figured in The Study of Rocks (Longman's Text-Books of Science).








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