TORTOISE-SHELL, is manufactured in various objects, partly by cutting out the shapes, and partly by agglutinating portions of the shell by heat. When the shell, has 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 heated state, or cut out with a thin frame-saw, while the shell, equal in size to two combs, with their teeth interlaced, is bent like an arch in the direction of the length of the teeth. The shell is then flattened, the points are separated with it narrow chisel or pricker, and the two combs are finished, while flat, with coarse single cut files and triangular scrapers. They are finally warmed, and bent on the knee over a wooden mould, by means of a strap passed round the foot,just as a shoemaker fixes his last. Smaller combs of horn and tortoise-shell are parted, while flat, by an ingenious machine, with two chisel-formed cutters placed oblique ly, so that each cut produces one tooth.
(See Rogers' Comb-cutting Machine, Trans. Soc. Arts vol. xlix., part 2, since improved by Mr. Kelly.) In making the frames for eye-glasses, spectacles, &c., the apertures for the glasses were for merly cut out to the circular form, with a tool something like a carpenter's cen tre-bit, or with a crown-saw in the lathe. The discs so cut out were aced for in laying in the tops of boxes, &c. This required a piece of shell as large as the front of the spectacle ; but a piece one third of the size will now suffice, as the eyes are strained or pulled. A long nar row piece is cut out, and two slits are made in it with a saw. The shell is then warmed, the apertures are pulled open, and fastened upon a taper tablet of the appropriate shape. The groove for the edge of the glass is cut with a small cir cular cutter, or sharp-edged saw, about three-eighths or halt an inch in diame ter; and the glass is sprung in when the frame is expanded by heat.