Mother-of-pearl is the inner coat of several kinds of oyster shells, some of which secrete this layer of sufficient thickness to render the shell an object of manufacture. The beautiful tints of the layer depend upon its structure, the surface being covered with a multitude of minute grooves, which decompose and reflect the light. The structure of the pearl shell admits of its being split into lamine, and it can then be used for the handles of knives, for inlaying, or in the manufacture of buttons; but as splitting is liable to injure or spoil the shell, this method of dividing it is seldom resorted to. In manufacture the different parts are selected of a thickness as nearly as possible to suit the required purpose; excess of thickness is got rid of by means of saws, filing, or by grinding upon the common grindstone. In preparing the rough shell, if square or angular pieces are needed, they are cut with saws, as the circular saw or the ordinary back saw; in the one case, the shell is fed up as the saw divides it, and in the other the shell is held in a vice, and the saw operated by hand. If circular pieces of the shell are wanted, such as those for buttons, they are cut with an annular or crown saw, which is fixed upon a mandrel.
It is necessary in sawing that water is plentifully supplied to the instrument, or the heat generated by dividing the shell will heat the saw, and its temper will be destroyed. The pieces of shell are next ground flat upon a grindstone, the edge of which is turned with a number of grooves or ridges, as being less liable to become clogged than the entire service, and hence grind more quickly. It is necessary to supply water, or soap and water, to the stone, as it is then less liable to become clogged. The flat side of the stone, similarly prepared with ridges, may be used instead of the face, if it is desired to have the pieces of shell ground flat, and when of the requisite thinness they are ready for operation in the lathe, or for inlaying.
After the pieces of pearl shell are cut, ground, or turned to the proper form, they are finished with pumice and water; this may be done with pieces of the stone properly shaped, and rubbed over the work as it is held fast in some form of clamp, or held upon the work as it is revolved in the lathe. This process may be followed by an application of ground pumice, which has been carefully sifted to extract all except the minutely powdered portion, and applied with a piece of cork or a cloth moistened with water. The polishing is accomplished with rotten-stone, moistened with dilute sulphuric acid, which may be applied upon a piece of cork or a bit of soft wood. In some turned works fine emery paper may be used, and followed with rotten-stone moistened with the acid or oil. The pearl handles used for razors or knives are first roughed out, then drilled where the rivets are to be inserted, and lightly riveted together in pairs. They are ground to the proper size and thickness, and finished by the means mentioned, the last finishing touch, to produce a fine polish, often being done by the friction of the hand of the workman.
Sometimes it is advantageous to apply the polishing material to the surface of a wheel, and this wheel may be covered with cloth and moistened with water, which will cause enough of the powder to adhere. Separate wheels may be used for the pumice and rotten-stone. Sometimes dry powdered chalk or Spanish whiting is used in place of the rotten-stone.
One process of working pearl is by the aid of corrosive acids and the etching point. The shell is first divided as may be necessary, and the designs or patterns are drawn upon it with an opaque varnish; strong nitric acid is then brushed over the plates repeatedly, until the parts undefended by the varnish are sufficiently corroded or eaten away by the acid. The varnish now being washed off, the device, which the acid had not touched, is found to be nicely executed. If the design is to be after the manner of common etching on copper, the process upon the shell is precisely the same as painting: glass. 131 mixed up together, and fused in an air furnace in a crucible, the fire at first applied very gradually, and the whole repeatedly stirred with an iron rod. The mixture by this calcination, and by being kept for some time in fusion in an intense heat, acquires its fusibility and opacity.