Stone. Under the head BUILDING STONE (see also ROADS) the more important kinds of stone used for architectural purposes are noticed, and some of these, such as limestone, marble, sandstone, flagstone, flint, slate, granite, basalt (greenstone), serpentine, and porphyry, are again referred to under their respective names. See under MILL, OVEN, and WHETSTONES for millstones, firestone (leckstone), and hones. Purely ornamental stones other than gems are noticed under ALABASTER, FLUOR SPAR, JADE, JASPER, LABRADORITE, LAPIS LAZULI, MALACHITE, and other heads. The beautiful material called Mexican onyx marble (stalagmitic aragonite) has only been known for a few years. Algerian onyx marble, also a handsome stone, wants the bright-coloured veins of the Mexican. The most recent addition to these ornamental stones is the jasperised wood of Arizona, many pieces of which are of striking beauty. In recent years some beautiful porphyries and granites from Norway and other countries have been cut and polished for decorative purposes at some of the Aberdeen granite-works. Besides its well-known granites Scotland possesses quite a number of handsome siliceous stones suitable for architectural decoration, hitherto not utilised.
Stone-dressing.—Ashlar stones (see MASONRY), whether of limestone or sandstone, after being chiselled on the face, generally get the tool-marks removed by smoothing them with a bit of soft sandstone and water. In England a stone so finished is technically said to be rubbed; in Scotland, polished. It has recently become the custom in Scotland, where sandstone is the only freestone, to dispense with the 'polishing' and leave irregular chisel-marks visible. In former years there were in use various effective ways of dressing stones for the fronts of buildings. One of these was tooling or doving—i.e. covering the face with small flutings by means of a broad chisel; another was broaching or incising the face with narrow parallel grooves by the use of a pointed tool. Many buildings of a by no means unpretentious kind recently erected in Scotland and elsewhere have their ashlar stones dressed only on their beds and joints, their faces being merely 'pinched' from the edges, leaving the exposed part of the stones rough and hackly. When the face of granite is not polished it is generally dressed with a nidding hammer, which gives it a chiselled appearance.
Preservation of Stone.—The preservation of stone can be effected to a great extent by coating the surface with boiled linseed-oil, or with oil-paint; but these methods are not much in favour, as they destroy the crystalline appearance which constitutes the beauty of most natural stones. There is, however, no other efficient way known of preserving a sandstone with a tendency to decay. More hope may be entertained that certain chemical solutions will prevent the wasting of oolitic and magnesian limestones so much used as building stones in London and southern England generally. The coating of these with an alkaline silicate and the subsequent application of calcium chloride, as proposed by Ransome, has not had the beneficial result which was expected from this treatment. Quite recently the owners of the Bath stone (oolitic) quarries have recommended a preserving solution under the name of 'Fluate.' The exact nature of this material does not appear to have been disclosed, but in October 1890 The Builder published analyses of the stone before and after being fluated. It may be inferred from these analyses that its durability is increased by the application of this fluid to its surface, but there can be no certainty till the treatment is tested by time. The preservation from decay of a porous substance like freestone (whether sandstone or limestone) in a climate like that of northern Europe is a problem of the greatest difficulty. One precaution ought never to be neglected, and that is to see that a 'damp-proof course' is put through the walls of a building just above the ground, to prevent the ascent of moisture from the soil. See BUILDING.
Artificial Stone.—Burnt clay in the form of bricks or terra-cotta blocks of larger size, though not usually classed as artificial stone, is by far the best substitute for real stone. Portland Cement, (q.v.) mixed either with sand alone or with sand and broken stones, forms one kind of artificial stone (see CONCRETE). In the north of Italy paving tiles with beautiful patterns are made by inlaying Portland cement with small pieces of marble, serpentine, and other ornamental stones. This kind of work is obviously suited for external wall decoration of a very effective kind, provided that even the best Portland cement has the durability which some of its advocates claim for it. Seagliola (q.v.) is a polished plaster for internal decoration, somewhat similarly ornamented. Von Fuchs of Munich, Kuhlmann of Lille, and Ransome of Ipswich have successively done material service in enabling an artificial stone to be made of the silicate of soda or potash (soluble glass) and sand (see GLASS, Vol. V. p. 245). Ransome's artificial stone is a hard substance formed by mixing sand with a solution of this alkaline silicate, then pressing it into moulds, and when partially dry soaking it in a bath of chloride of calcium, which to some extent penetrates the 'stone,' forming the insoluble silicate of lime (calcium silicate). Chimney-pieces, vases, and architectural ornaments of various kinds have been made of this material. Ransome's 'patent concrete stone' is made in the same way, with broken pieces of stone added.