TORTOISE-SHELL is procured from a marine tortoise called the hawk's-bill turtle, or tesludo imbricata. Each animal furnishes thirteen principal plates, five along the centre of the back, and four on each side ; and twenty-five smaller scales or plates, which constitute the margin of the shell. The horny plates which constitute true tortoise shell are separated from the bony foundation which forms the shell or covering of the animal by the application of heat ; the whole shell being commonly placed over a fire until the plates begin to start from the bone, and the separation being completed by the aid of a slender knife. The yellow-coloured shell bears a higher price than that which is mottled.
The processes of manufacturing articles of tortoise-shell are very similar to those de scribed under Home MANUFACTURE, but on account of the high price of the material (often three guineas per lb.), it is economised as much as possible. In making the frames for eye-glasses, narrow strips of shell are used, in which slits are cut with a saw, the slits being subsequently, while the shell is warm, strained or pulled open, until they form circular or oval apertures, by the inser tion of tapering triblets of the required shape.
The same yielding or flexible property is made use of in the manufacture of boxes, a round flat disc of shell being gradually forced by means of moulds into the form of a circular box with upright sides. The union of two or more pieces of shell may be effected by carefully scraping the parts that are to overlap, so as to render them perfectly free from grease, even such as might arise from being touched by the finger, softening them in hot water, pressing them together with hot fla tongs, and then plunging the joint into cob water.
In veneering with tortoise-shell, by which very beautiful work may be produced, it i usual to apply fish-glue, mixed with lamp black, vermilion, green, chrome, white, 0] other colouring matter, at the back of fin shell, both to heighten its effect and to con teal the glue or cement by which it is secured to the wooden foundation.