TORTOISE-SHELL This beautiful substance, or at least the best kind of the material which goes under the name, is procured from a marine tortoise called the Hawk's-bill turtle, or Testudo imbricata ; the Latin name being derived from the mode in which the scales upon the back are arranged, overlapping one another like the tiles upon the roof of a house, one kind of which were called by the Romans itabricea In most other tortoises the several scales of which their covering is composed adhere to one another by their edges, like inlaid work. 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 size and thickness of the plates depend on the magnitude and age of the animal, a fresh layer being produced every year ; and at the margin of the large plates may be seen distinctly the edges of the layers as they thin off In succession. The horny plates are separated from the bony foundation 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 shell varies much in value, being frequently injured by barnacles, limpets, and other shell-fish adhering to the turtles while alive, and interfering with the growth of the shell where they attach themselves. Occasion ally plates of a uniform yellow colour are met with ; and such are in great request among the Spanish ladies, who will give at least twice as much for a comb of plain tortoiseshell as for a mottled one. The belly-plates of the tortoise are yellow, and are sometimes found suffi ciently clear for use.
Before working, the shell needs to be softened or tempered, which is usually done by dipping it for three or four minutes, or longer if it be very thick and brittle, in boiling water. Some manufacturers flatten and temper the shell, with hot flat Irons, similar to those used by laundresses; the tortoise-shell being, in the course of the operation, frequently dipped in cold water to prevent scorching. Generally, however, the less the shell Is heated and pulled about the better, because from its apparent want of grain or fibre it Is apt to become very brittle. Being also less fusible than horn, tortoise-shell cannot be made soft enough to be moulded without some Injury to the colour; and accordingly the manufacturers, at least in England, never attempt to produce tortoise-shell combs with ornamental open work by means of dies. Such work is produced by pasting a piece of paper over the tortoise-shell, drawing the pattern upon it, cutting it out with drills and fine saws, and, after the paper has been removed by steeping in cold water, finishing the ornaments with the graver. The cutting or
parting teeth of combs by machinery is described under COMB MANU FACTURE; but we may here notice another mode which is occaaionally followed, and which illustrates the convenience arising from the flexi bility of the material when warmed. A piece of tortoise-shell, large enough to make two combs, is bent or bowed in the direction of the length of the teeth, to such a degree of convexity that they may be cut with a straight bow-saw without cutting through either of the edges of the pieces of shell, which are required to form the top or back portions of the combs. The shell is then flattened and the ends or points of the teeth aro separated with a narrow chisel or pricker, after which the combs are finished with files and scrapers, and bent to any required curvature upon wooden moulds, The frame& for tortoise shell eye-glasses are usually formed out of narrow slips of shell 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 insertion of tapering triblets of the required shape. The same yielding or flexible property is made use of in the manufac ture 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 hand, softening them in hot water, pressing them together with hot flat tongs, and then plunging the joint into cold water.
In veneering with tortoise-shell, by which very beautiful work may be produced, it is usual to apply fish-glue, mixed with lamp-black, vermilion, green, chrome, white, or other colouring matter, at the back of the shell, both to heighten its effect and to conceal the glue or cement by which it is secured to the wooden foundation. In making knife-handles and some other ornamental work, metallic foils aro put beneath the tortoise-shell veneer with excellent effect.
The qualities of tortoise-shell as brought to market aro thus dis tinguished : Manilla, fine and large ; Singapore, nearly as good as Manilla; West India, large and heavy, but red; Honduras, darker, but with largo dark red spots; Calcutta, dark, heavy, and badly coloured ; Bombay, the worst quality. Sometimes plates aro obtained as large as 13 inches by 8 inches, and a quarter of an inch thick in the middle. The price of the best tortoise-shell is generally about 60s. per lb. A small import duty, formerly imposed, was repealed in 1S46.