The term marble is applied by the mason to any of the materials that he employs, which admit of being polished; but the mineralogist designates thereby the compact carbonates of lime variously coloured. The principal kinds worked in the ornamental arts, are the white or statuary marble from Italy, a variety of coloured marbles, principally from Devonshire and Derbyshire, and the black bituminous marble from Derbyshire, Wales, and various parts of Ireland.*
* The moulds for Mr. Henning's exquisite models of the baa-reliefs of the frieze of the Parthenon, and other antique marbles, are sunk in intaglio in the blue slate. In reference to the choice of this material Mr. Henning says: "Giving my son, Samuel, a lesson in arithmetic, I found him idling and drawing faces upon his slate. I handed him one of my gravers,: desiring him to engrave one of his little sketches: I had previously done about eighteen pieces of the frieze in ivory in relief one-twentieth the full size." The thought was immediately followed, and the lost portions of the originals were restored with great happiness and skill by engraving them in intaglio in slate, although for his works to be appreciated, the original casta from the slate should bo seen, and not their pirated copies obtained from plaster moulds, which it is to be regretted deprive the artist of his merited reward.
The marbles are turned with a bar of the best cast steel, about two feet long, and five-eighths of an inch square, drawn down at each end to a taper point, about two inches lung, and tempered to a straw-colour; this point is rubbed on two opposite sides on a sandstone, and held to the marble at an angle of twenty or thirty degrees; the tool soon gets dull, and must be again rubbed on the sandstone (a bit of Yorkshire flag,) to sharpen it, water should drop on the marble to prevent the tool from becoming heated and losing its temper. The point will keep getting broader by constant grinding, till it forms a kind of chisel an either of an inch wide, after which it will require drawing out again. For cutting in the mouldings, a more delicate point is used, and these are the only tools employed; a flat tool will not turn marble at all.
The black marble is subjected to several processes of embellishment referred to at foot.†
* The Society of Arts has a good collection of marbles. See also the note on page 172, respecting the Museum of Economic Geology.
† Black marble is etched with various figures, imitative of Etruscan vases, and Egyptian hieroglyphics: this is done after the work is polished. All the parts to be preserved are pencilled over with a varnish, in which colouring matter is ground that the artist may see his progress, mastic varnish, with vermilion or flake white will answer: the uncovered surface of the marble is dissolved with any diluted mineral acid, and the muriatic is perhaps the best, the acid is washed off with water, and the varnish is removed with turpentine, after which the etching appears of a dull grey, upon a bright black-ground.
Another method which does not destroy the polish, and in which a bronzy-white is given, is considered to be the removal of the colouring matter alone. Its effect is far more beautiful, but the method is not divulged by the few artists who practise it.
Some works in black marble are engraved with an ordinary graver, such as would be used for copper or steel plates, and the graver is also employed conjointly with the etching and discolouring processes above referred to.
By scratching the marble with a hard steel point it produces a kind of cream-colour, and by slighter scratching a fainter shade, by these means pretty landscapes are produced; but these delicate touches soon lose their colour, by acquiring a partial polish, if wiped for the removal of the dust, such specimens should be therefore kept under glass.
Black marble is also commonly selected for inlaid works, known by the name of Pietra Dura; the device, whether flowers, a running pattern, or any other is drawn upon the marble, and cut in very carefully with a chisel and mallet; thin pieces of the various-coloured marbles and stones of nearly the same hardness, are fitted accurately into the spaces and cemented with shell-lac; after which the whole is polished off together. When the atones thus inlaid differ materially from the
Many of the limestones, although chemically like the marbles, are less compact, and therefore do not readily admit of being polished; of these may be noticed the Bath stone and other oolites, which are aggregations of egg-shaped particles, like the roes of fish; when first raised they may be cut very readily with an ordinary toothed saw, and turned with great freedom. The Maltese stone, of which many beautiful turned and carved works were recently sold in London, belongs likewise to this group; it is very compact, and nearly as soft as chalk, from which in fact it scarcely differs in any respect except in its delicate brown cream-colour; the natives of the island of Malta display considerable taste in the objects turned and carved in this limestone.