TORTOISE-SHELL.

267

TORTOISE-SHELL ia manufactured in various objects, partly by cutting outthe shapes, and partly by agglutinating portions of the shell by heat. AVhen the shellhas become soft by dipping it in hot water, the edges are in the cleanest possible state,without grease, pressed together with hot flat tongs, and then plunged into cold water,to fix them in their position. The teeth of the larger combs are parted in their heatedstate, or cut out with a thin frame saw, while the shell, equal in size to two combs,with their teeth interlaced, as in fig. 166, is bent like an arch in the direction of the

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length of the teeth, as in fig. 167. The shell is then flattened, the points are separatedwith a narrow chisel or pricker, aud the two combs are finished, while flat, with coarsesingle-cut files and triangular scrapers. They are finally warmed, and bent on the kneeover a wooden mould, by means of a strap passed round the foot, just as a shoemakerfixes his last. Smaller combs of horn and tortoise-shell are parted, while flat, by aningenious machine, with two chisel-formed cutters placed obliquely, so that each cutproduces one tooth. See Rogers’ comb-cutting machine, Trans. Soc. Ms, vol. xlix.,part 2, since improved by Mr. Kelly. In making the frames for eye-glasses, spec-tacles, &c., the apertures for the glasses were formerly cut out to the circular form, witha tool something like a carpenter’s centre-bit, or with a crown saw in the lathe. Thediscs so cut out were used for inlaying in the tops of boxes, &c. This required a pieceof shell as large as the front of the spectacle; but a piece one third of the size will nowsuffice, as the eyes are strained ox pulled. _ A long narrow piece is cut out, and two slitsare made in it with a saw. The shell is then warmed, the apertures are pulled open,and fastened upon a taper triblet of the appropriate shape; as illustrated by figs. 168,169, and 170. The groove for the edge of the glass is cut with a small circular cutter,or sharp-edged saw, about three eighths or half an inch in diameter; and the glass issprung in when the frame is expanded by heat.

In making tortoise-shell boxes, the round plate of shell is first placed centrally overthe edge of the ring, as in fig. 171 : it is slightly squeezed with the small round edge-block g, and the whole press is then lowered into the boiling water : after immersionfor about half an hour, it is transferred to the bench, and g is pressed entirely down, soas to bend the shell into the shape of a saucer, as at fig. 172, without cutting or injuringthe material; and the press is then cooled in a water-frough. The same processes arerepeated with the die d, which has a rebate turned away to the thickness of the shell,and completes the angle of the box to the section fig. 173, ready for finishing in thelathe. It is always safer to perform each of these processes at two successive boilingsand coolings. Two thin pieces are cemented together by pressure with the die e, and auevice may be given by the engraved die /.—See Holtzapffel’s Turning and MechanicalManipulation, vol. i., p. 129.