The hardest, least absorbent, and most compact and uniform stones are of ancient geological formation, and with time and increase of superimposed pressure have become dense and very hard. The softer stones are of later formation, and are usually lighter in weight and more porous. A good stone should ring clearly when struck with steel, and a fresh fracture should on examination be bright, clean and sharp in texture and free from loose grains. A dull earthy appearance indicates an inferior stone.
The most usual method adopted for preserving stonework is to paint the exposed surfaces with ordinary oil colour. The painting must be redone every four or five years. Boiled linseed oil is sometimes used on stonework, one or more coats being well brushed in after cleaning it. Its use deepens the colour of the stone, and unless very carefully done the work is apt to appear patchy. A large number of processes consist of coating the stone work with a solution of soluble silica. In Kuhlmann's process a solution of silicate of potash or soda is brushed into the stone and, aided by the carbonic acid in the air, acts upon some of the con stituents of the stone and forms a hard surface which is not liable to decay. In Ransome's process, a solution of silicate of soda is applied until the surface of the stone has become saturated. This is allowed to dry and a solution of chloride of calcium is then applied in a similar manner. The two solutions act together, and by decomposition produce an insoluble silicate of lime which fills the pores of the stone and binds its particles together thereby checking decay. Baryta water will, when applied to limestone that has decayed owing to the action of sulphurous fumes, pene trate into and solidify the crumbling portions, with the result that the stone is reconstituted and becomes hard and quite solid. Prof. A. H. Church employed this method in arresting the decay of the frescoes in the Houses of Parliament and the stonework of the chapter house at Westminster was also treated by him in the same manner. Fluate is the name given to a siliceous preservative specially recommended for use upon the limestones from the Bath district. It may also be applied to other limestones, and to bricks, tiles, terra-cotta, etc. It does not materially change the appearance of the stone but enters the pores and prevents decay.
The natural bed of a stone is that surface on which it was originally deposited. But volcanic and other disturbances may have occurred since that time and completely altered its "lie"; and therefore it frequently happens that a horizontal line does not coincide with the natural bed of stone as it rests in the quarry. Care must be taken before using the stone to find the proper bed and to set all stones with their laminae quite level. Stone fresh from the quarry is found to contain a quantity of moisture called "quarry sap," on account of which all stones (even granite) are comparatively soft when first quarried. This water gradually evaporates, and after some months' exposure stones that were quite soft and weak when quarried acquire hardness and strength. For these reasons it is desirable from an economical point of view to "work" the stone to its desired shape and mould and carve it when soft and easily workable.
Limestones consist chiefly of calcium carbonate with small proportions of other substances. They are often classified under four heads : Compact limestones consist of carbonate of lime, either pure or in combination with clay and sand. Granular or oolitic limestones consist of grains of carbonate of lime cemented together by the same substance or mixed with sand and clay. The grains are egg-shaped (hence the name "oolite") and vary in size from tiny particles to grains as large as peas. Shelly limestones consist almost entirely of small shells, cemented together by car bonate of lime. Magnesian limestones are composed of carbon ates of lime and magnesia in varying proportions, and usually also contain small quantities of silica, iron and alumina. Stones hav ing less than 15% of magnesia are not classed under this head.
Dolomites are limestones containing equal proportions of carbon ate of lime and carbonate of magnesia. Many of the finest build ing-stones are limestones. In England typical examples are the Bath stones, Portland stone and Kentish ragstone, and in America those from the States of New York, Indiana (Bedford quarry, light brown stone), Illinois (Grafton and Chester quarries) and Kentucky (Bowling Green stone, light grey, similar to Portland). Notable French limestones are obtained from the quarries at Peuren (cream), Château-Gaillard (white), Abrots, Normandoux (white), and Villars (light brown). The hardest and closest grained of these are capable of taking a fine polish. Limestones should be used with care as they are uncertain in their behaviour and usually more difficult to work than sandstones, and as a gen eral rule they do not stand the action of fire well. On being treated with a dilute acid, limestones will effervesce and by this test they can easily be identified. Limestones weigh between 13o lb. and 166 lb. per cu.ft. They vary in colour but most of them are cream or yellowish brown. Marble is a limestone which has been changed by the action of heat and pressure into a crystalline form. Many beautiful varieties are found which are suitable for interior decoration, such as for columns, wall lining, paving, etc., and in dry sunny climates they may be employed with great effect in external situations. They will take a high polish and the fine grained varieties are well adapted for intricate carving. The prin cipal European supplies of marble are from Italy, Belgium and France, but the marbles from Ireland and those from Devonshire and Derbyshire possess a remarkable range of colour and variety of markings. America has few notable coloured marbles; most of the stones quarried are white or black. The States of Vermont (West Rutland and Sutherland Falls quarries), Tennessee and Georgia produce large quantities of marble. Marezzo and scag liola are plaster imitations of marbles (see PLASTERWORK).