As these possess, a smooth surface, the substance which should fasten them to the marble cannot incorporate itself with them intimately enough to join both and render them inseparable. It is therefore necessary to interpose between the metal and the marble a third body, which should force them to perfectly adhere; this is effected by the use of sand-paper.
Take a plate of zinc of about 1/10 inch thick; make a frame of this of the form of whatever article may be wished; upon this form glue sand-paper, leaving the rough side outermost, and upon this rough side apply the marble, having first prepared it by heating in a water bath, and placing between the marble and the sand-paper a coating of mastic of tar. By this means, so perfect an adhesion between the marble and the zinc is effected, that the marble could be more easily broken than removed. The application of marble upon zinc can also be effected by grooving the metal in every direction with strokes of the file, but the sand-paper produces the best results. Zinc is preferred to other metals, because it possesses resistance and cheapness, and causes no other expense in the manufacture than that of cutting up to form the model. Tin does not possess the same resistance or cheapness; sheet iron is dearer; cast iron is too heavy; copper is expensive; by the application of marble upon zinc, articles can be manufactured at the same price as those veneered upom wood. In fastening marble to the metallic plating, the tar which is used in the application of marble to stone will not be sufficient.
The parts must first be heated in a water bath, or over a furnace prepared for this purpose, and then, by a sieve, sprinkled with one of the following mordants: - Crushed glass, grains of emery of all sizes, copper filings, castings of any metal, finely-rasped lead, or any kind of powdered stone, such as sandstone, marble, granite, or pumice, and rubber, can also be used. When the sheets of metal and of marble have received sufficient mordant, join with a coating of tar, which fastens them strongly together. Any web of linen or cotton can be placed between the marble and the metal; this web being covered with grainy substances, stuck on by glue.
The marble is first sawn to thickness and form required for the dressing case or box to which it is to be applied. The wood, usually white wood, oak or fir, is cut a little smaller than the marble which is to cover it. This wood is lined with a shaving of beech wood, to prevent warping. This lining is only placed on the side which is to receive the marble; each piece of marble is then applied to the corresponding piece of wood, and stuck on by glue or other mastic. When the marble has been applied, the opposite side of the wood is thinly lined with rosewood or mahogany, so that this lining forms the inside of the box, which is thus prepared for receiving the necessary divisions. The four parts are then dovetailed together, and the top and bottom parts are fastened flatwise on the four sides with glue or mastic. The box being finished, the outside is pumiced and polished, and any applications of gilding can be made.
Prepare a varnish by pulverising Spanish sealing wax, and dissolving it in spirits of wine. Trace on the white marble, with a crayon, the design which is to be formed in relief, and cover this delicately with a brush dipped in the varnish; in about 2 hours the varnish will be dry. Prepare a dissolvent of equal parts of spirits of wine, hydrochloric acid, and distilled vinegar; pour this solution upon the marble, and it will dissolve those parts which are not covered by the varnish. When the acid has ceased to ferment, and, consequently, will no longer dissolve the marble, pour on some fresh, which continue until the ground is sufficiently grooved. ~ When there are delicate lines in the design which should not be grooved so deeply, they should at first be covered with varnish, to prevent the action of the acids upon them; then, when the reliefs have been made, the marble should be well washed, and the varnish removed from these delicate lines with the point of a pin; then pour on new acid, which will groove it as deeply as desired, care being taken to remove it at the proper time.
When the acid has acted upon the marble, it corrodes beneath the varnish, and enlarges the lines in proportion to its depth; therefore draw the lines in relief a little larger than it is desired to leave them. When the work is completed, remove the varnish with spirits of wine, and, as the grounds will be very-difficult to polish, they may be dotted with ordinary colours diluted with the varnish of gum lac. The marble being thus grooved, the cavities may be filled in inlaid work with gold, silver, tin, sealing wax, sulphur, or pearl shell reduced to powder. These designs can be made either in moulding or in relief,, without changing or injuring the marble; every sort of writing, however delicate it may be, can also be thus; traced; and the execution is very rapid, whether in groovings inlaid with gold or silver, or in relief, which can also be gilded or silvered.