MARBLE, Polishiny of. The art of cutting and polishing marble was, of course, known to the ancients, whose mode of proceeding appears to have been nearly the same with that employed at present ; except, perhaps, that they were unac quainted with those superior mechanical means which now greatly facilitate the labour, and diminish the expense of the articles thus produced.
An essential part of the art of polishing marble is the choice of substances by which the prominent parts are to be removed. The first substance should be the sharpest sand, so as to cut as fast as possible, and this is to be used till the surface becomes perk•ctly flat. After this the surface is rubbed tt ith a finer sand, and frequently with a third. The next substance after the finest sand is emery, of different degrees in fineness. This is followed by the red powder called tripoli, which owes its cutting quality to the oxide of iron it contains. Common ironstone, powdered and levigated, answers the purpose very well. This last substance gives a tolerably fine polish. This, however, is not deemed suffi cient. The last polish is given with putty. After the first process, which merely takes away the inequalities of the surface, the sand employed for preparing it for the emery should be chosen of a uniform quality. It' it abounds with some particles harder than the rest, the surface will be liable to be scratched so deep as not to be removed by the emery. In order to get the sand of uniform quality, it should be levigated and washed. The hard particles, being generally of a different specific gravity to the rest, may by this means be separated. This method will be found much superior to
that of sifting. The substance by which the sand is rubbed upon the marble is generally an iron plate, especially for the first process. A plate of an alloy of lead and tin is better for the succeeding processes, with the fine sand and emery. The rubbers used for the polishing, or last process. are of coarse linen cloths, such as hop-bagging, wedged tight into an iron plane. In all these processes, a constant supply of small quantities of water is absolutely necessary.
The sawing of marble is pertbrmed on the same principle as the first process of polishing. The saw is of soft iron, and is continually supplied with water and the sharpest sand. The sawing, as well as the polishing of small pieces, is per formed by hand. The large articles, such as chimney-pieces and large slabs, are manufactured by means of machinery, working by water or steam.
Several patents have been taken out for sawing and polishing marble. In IS22, Sir James Jelf patented a com bination of machinery for cutting any description of parallel mouldings upon marble slabs, for ornamental purposes; in which tools, supplied with sand and water, are made to traverse to and fro. Tullock obtained a patent, in 1824, for improvements in machinery for sawing and grooving marble ; and in 1829, Mr. Gibbs, also, for an invention for working ornamental devices in marble.