Marble is a compact carbonate of lime which varies in color, some specimens being pure white, others perfectly black, while others are green, red, veined, mottled, etc. The famous Mexican onyx, so-called, is also a carbonate of lime, and notwithstanding its hardness and beauty is liable to injury from the same causes that affect ordinary marble.
Marble is easily dissolved, with escape of carbonic acid gas, by the mineral acids, sulphuric, nitric, hydrochloric, etc., and it is also acted upon, though more slowly by vinegar, the acids of fruit, etc. It is also soluble in water containing an excess of carbonic acid, and therefore dissolves rapidly in the ordinary "soda" water that is so generally sold as a beverage, for this fluid, in its pure state, consists solely of water holding a large amount of carbonic acid in solution. Consequently bottles and glasses of this liquid should not be placed where there is any danger of spilling it on mantel pieces, table tops, etc., as it will infallibly destroy the exquisite polish upon which the beauty of such articles of furniture depends.
Finely carved articles of marble, when exposed to the rain of our northern climates, are apt to suffer corrosion, and the delicate tracery of the sculptor is soon lost. Therefore, while marble answered very well in the comparatively dry climates of Greece and Egypt, it is unsuited for statues, etc., exposed to the open air, in England and America, the rainfall in these countries being very great, and the moisture heavily charged with carbonic and sulphurous acids.
In cleaning marble ornaments, etc., great care must be exercised to use nothing corrosive like acids, chlorides, or metallic salts, such as are usually recommended for removing stains of inks and dyes from wood and textile fabrics. When marble has been stained by ink or vegetable coloring matter, the only way to remove it is to apply warm water abundantly and for a long time. If the marble is very compact, and the stain consequently quite superficial, the article may be scraped and repolished, but of course this is applicable only to objects which have plane surfaces, or those with simple curves. Elaborately carved or sculptured objects could not be so treated.
Greasy stains may be removed by covering them with a paste of chalk and potash or soda. The alkali will convert the grease into soap, which will be gradually absorbed by the chalk and thus removed. In such cases, however, the stains, especially if old, may require a long time and several repetitions of the process. Alkalies (potash, soda and ammonia) may be applied to marble without injuring it, and any staini-which they can remove may be taken out by their means.
Marble is easily worked either on the bench or in the lath..
In the latter case, however, great care must be taken to avoid anything like a heavy cut, since marble is so rigid and brittle that if the cut be heavy the article is apt to be broken. The only tool that can be used is a steel point, tempered to a straw color. The tool requires frequent grinding, and when it gets broad it must be forged over again, as a flat tool will not turn marble at all.
For working and finishing marble on the bench the following is the process: After the marble is sawn into slab, the first operation is to grind it down with a flat coarse sandstone and water, or with an iron plate, fed with fine sand and water, until all the marks of the saw are perfectly removed; secondly, a fine sandstone is used with water until the marks made by the first stone are removed; thirdly, a finer sandstone is applied to work out the marks of the former; fourthly, pumice stone with water, and fifthly, snake stone is used, and this last finishes what is called the grounding.