Domestic Encyclopædia (1802)/Marble

MARBLE, in Natural History, a genus of stones that admit of a bright and beautiful polish: they are composed of small separate concretions, moderately hard; not emitting fire, when stricken against steel; effervescing with, and soluble in acids; and calcining in a moderate fire.

The finest modern marbles are those of Italy, Blankenburg, France and Flanders. In some of the Western Islands of Scotland, very fine specimens of this fossil have lately been discovered. When chemically examined, marble consists of calcareous earth united with fixed air; and is, like lime-stone, or chalk, convertible into a strong quick-lime.—Black marble derives its colour from a partial admixture of iron.

Staining of Marble.—The pieces to be coloured should be of the hardest kind; previously well polished; and be divested of every spot or blemish. Such only are calculated for supporting the heat which is always necessary, in order to open their pores, and render them susceptible of the colours. On the other hand, too low or too high a degree of heat are equally injurious: hence a due temperature ought to be preserved; and this, without making the marble red, will cause the liquor to boil on its surface.

The principal colours used for staining marble are, red, yellow, and blue: the two first of these tints may be imparted to it, by reducing dragon's blood, or gamboge to a powder, and grinding them separately with spirit of wine in a glass mortar. But, in experiments on a small scale, a little of either of those powders should be mixed with spirit of wine in a silver spoon, and dissolved over a charcoal fire. Thus, a strong tincture will be extracted; with which, by the aid of a pencil, the finest traces may be drawn on marble, while cold: on heating the latter in an oven, the marks will penetrate deeply, and remain perfectly distinct.

A fine blue colour may be communicated to marble, by a watery solution of the drug, known among dyers by the name of Canary Turnsol, and tracing the marks designed with a pencil. These will strike deeply into the stone, and the colour may be increased, by drawing the moistened pencil repeatedly over the same lines. The staining liquor must always be laid on, cold; nor should the marble afterwards be heated; yet such blue is apt to spread itself irregularly, unless its outlines be circumscribed by wax, or other adhesive matter. This colour possesses the advantage of being applicable to marble that has already been stained with other drugs: it affords, besides, a very beautiful shade, and is not liable to be easily effaced.

In 1778, a patent was granted to Mr. Richter, for his invention of an art or method of inlaying scagliola, or plaster in marble or metals, so as to imitate flowers, fruits, trees, birds, beasts, landscapes, and every kind of ornament. This patent is now expired; but, as it is practicable only by statuaries and artists, the inquisitive reader will consult the 10th volume of the Repertory of Arts and Manufactures.

For the easiest method of cleaning marble, or alabaster, see p. 25 of our first volume.