Bible Encyclopedias
Stone

Building-stone

In selecting a stone for building purposes many important points have to be considered. The stone must be strong enough to bear the load placed upon it, it must be durable and weather well in the atmosphere of the district, and its colour and appearance need to be studied. It must further be ascertained whether a sufficient supply is available, and the price also must be taken into account; some difficulty is often experienced in obtaining a suitable stone at a moderate cost, and considerations of expense frequently have more to do with the choice of a stone than the architect would wish. Where there is risk of fire, as is often the case in business and factory premises, it is necessary to select a stone able to stand the effect of a great heat without damage. Great experience of the strength of stones and of their behaviour in different situations is desirable; but even when this knowledge is given and the greatest care is combined with it, some point may be overlooked. For example, the stone facing of the Houses of Parliament at Westminster was chosen on the recommendation of a committee composed of men of eminent scientific and technical skill; yet it has not weathered well because it is not constituted to resist the destroying effects of the London atmosphere.

The prime factor in the choice of a building stone should be the climate to which the material has to be exposed. Stone that in the pure country air has proved extremely durable may quickly decay in an impure city atmosphere, or when subjected to the strong salt winds from the sea. Extremes of temperature, too, are, generally speaking, prejudicial to the life of stone, the alternations of heat and cold setting up movements in the substances of the stone, which, though slight, will in many cases hasten its disintegration. There are few materials which more quickly decay and fail than stone placed under unsuitable conditions. An analysis, made by E. G. Clayton, of a sample of incrustation found on the Portland stone masonry of St Paul's Cathedral, London, gave the following result: Weight per cent. Water (lost at Too°). 206 Water (lost at 150 0) 22.48 Carbon (soot) I. 10 Calcium sulphate 59'38 Calcium phosphate 2.22 Calcium silicate 163 Magnesium silicate 0.67 Iron silicate 2.40 Sand and uncombined silica 806 10000 The deposit when reduced to a fine grey powder and placed under the microscope did not appear to contain any organic matter. Mr Clayton says that this test points to the fact that the principal constituent of limestones, namely calcium carbonate, has been changed into calcium sulphate by the action of sulphurous and sulphuric acids ever present in the smoky London air. Impurities of this nature lodge on the face of the stone and are diluted and driven into the pores by subsequent rain. Having by their chemical action destroyed a portion of the substance of the material, they cause a slight crust to form on the surface which is in turn washed off. Carbonates of lime and magnesia, the chief constituents of ordinary marbles and limestones, are very susceptible to the solvent action of these acids. Pure water has little or no chemical action upon most building stones, but a danger arises to a porous stone even when situated in pure air. Water will soak into some stones in considerable quantities, and in frosty weather this fact constitutes a serioUs menace to the rock; for water when passing from the liquid to the solid state exerts if checked an enormous pressure, and the face, and sometimes the bulk, of the stone is frequently damaged in this way. One of the best precautions that can be taken by an architect is a personal visit to the quarry, to examine the stone in its natural situation. This, of course, will give little clue to its behaviour in an impure atmosphere, and therefore, if the particular stone has been previously used in the same district, the buildings in which it has been employed should also be inspected. A hard and lasting stone will show the marks of the tooling, and the arrises of the blocks will be sharp and good, even after many years' exposure.

The colour has a considerable bearing upon the selection of a stone, but this, although a very important matter, must give way. before the question of durability. In large towns and manufacturing districts this is most emphatically the case, for within a few years of erection the exterior of a building in such districts is disguised under a coating of soot and grime.

Should the stone contain iron, especially in the form of "pyrites," there is a great likelihood of its being stained more or less badly by iron "rust." If the metal is distributed evenly in small particles throughout the mass the rusting may do no more harm than merely deepen the tone of the stone, but if present in large pieces the stain may be so serious as to spoil the appearance entirely.

When the durability of stone has not been tried over some considerable period in a building actually erected, the most careful and chemical tests should be made. If the stone passes the following tests satisfactorily it may safely be assumed to be of good quality and likely to prove durable: (I) Resistance to crushing; (2) acid test; (3) absorption test; (4) microscopical examination.

The resistance to crushing varies to an enormous extent with the different kinds of stone, from a little over 60 tons per square foot, which is the limit for a weak limestone, up to a load of over 1300 tons necessary to crush the hardest granites. In general practice the load placed upon stone should not exceed one-tenth of the crushing weight as found by testing typical specimens. A sixinch cube is a convenient size often adopted for the blocks to which the crushing test is to be applied.

The effect produced by soaking pieces of stone for some days in a i % solution of sulphuric and hydrochloric acids will decide roughly whether it will be durable in a city atmosphere. The vessel containing the test should be agitated twice a day; the action of the acid is to dissolve any portions of the stone that would be decomposed by the action of smoke and acid fumes.

A block of the stone under consideration should be dried thoroughly in a warm kiln or oven and carefully weighed before it has time to absorb moisture from the air. It must then be placed to soak in clean water for twenty-four hours and after removal again weighed. The difference between the weights registered will give the weight of water absorbed, and this should not be more than ro % of the weight of the dry block. There are, however, exceptions to this test, some very porous stones being capable of taking up a large quantity of water and at the same time proving durable in use. But such material is liable to allow damp to penetrate through it to the interior of the building in which it is employed.

The microscope is the best means of determining the structure of a stone, and of recognizing the presence of matter likely to affect its usefulness adversely. Should iron pyrites be discovered in any quantity the stone should be rejected, as this impurity easily decomposes on exposure, and badly stains and sometimes splits the stone.

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.

A simple test for determining whether a stone contains much earthy matter is this: Some small chippings from the stone are placed in a vessel with sufficient water to cover the pieces, and are left undisturbed for about three-quarters of an hour. The water is then gently agitated. With stone of a highly crystalline nature, having its particles well cemented together, the water will remain clear, but stone containing earth and clay will cause the water to become thick and cloudy in appearance.

The action of the air of certain districts has been shown to be prejudicial to the durability of many stones. A striking instance of this peculiarity is afforded by Cleopatra's Needle fives. on the Thames Embankment. This is an Egyptian monument of carved granite which undoubtedly stood for some thousands of years with little deterioration on the spot from which it was removed. But since its erection in London it has been found necessary to coat it periodically with a preservative solution in order to check the rapid decay set up by the impurities of the London atmosphere. Similarly the Egyptian obelisk in Central Park, New York, U.S.A., has for the same reason been coated with a preparation of paraffin containing creosote dissolved in turpentine. The surface of the stone was heated by means of lamps and charcoal stoves, and the compound applied hot.

The most usual method adopted for preserving stonework is to paint the exposed surfaces with ordinary oil colour. This fills the pores of the stone and forms a coat which, though weatherproof, completely hides the natural beauty of the stone. 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 stonework 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 constituents 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, penetrate into and solidify the crumbling portions, with the result that the stone is reconstituted and becomes hard and quite solid. Professor 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, &c. It does not materially change the appearance of the stone but enters the pores and prevents decay. Stonework that is much decayed may be restored by Tabard's Metallic Stone, which is a natural stone of trachyte origin reduced to powder. The stone is restored to its original condition by mixing the powder with an acid which softens and reunites the molecules without decomposition. The invention is of French origin and has been used for much important work on the continent of Europe and in England.

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, however, before using the stone in a building, to find the proper bed and to set all stones with their laminae quite level. Exceptions to this rule occur in the projecting stones of cornices and string courses, especially those with undercut members which would be likely to drop off were the natural bed level; in these cases the stones should be placed on edge with the laminae vertical, except of course at the angles of the building where the stone must be specially selected and laid on its natural bed. Limestones and sandstones which are granular in structure and are found with wide planes of cleavage, giving deep beds which can be quarried in large blocks having no tendency to split in any particular direction, are known as freestone.

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. Seasoning. 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. By adopting this method a considerable saving in carriage will be effected, and the durability of the stone is enhanced, for the quarry sap on drying out leaves a hard outer crust or protective skin which would be removed if the working of the stone were left until it had become seasoned. It is an interesting fact that Sir Christopher Wren directed that the stones used in the erection of St Paul's Cathedral should be seasoned for three years on the sea beach.

Building-stones are divided into several groups; limestones and. sandstones are classified as aqueous or stratified rock, granite being the principal igneous or unstratified stone. 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 carbonate of lime. Magnesian limestones are composed of carbonates of lime and magnesia in varying proportions, and usually also contain small quantities of silica, iron and alumina. Stones having less than 15% of magnesia are not classed under this head. Dolomites are limestones containing equal proportions of carbonate of lime and carbonate of magnesia. Many of the finest building-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 general 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 130 lb and 166 lb per cubic foot. 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, &c., 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 principal supplies of marble are drawn 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 scagliola are imitations of marbles, and their manufacture and use are described in Plasterwork.

Sandstones are composed of grains of sand held together by a cementing substance to form a compact rock. The cementing medium may be silica, alumina, carbonate of lime or an oxide of iron. Those stones that have a siliceous cement are the most durable. Sandstones vary more in colour than limestones, the colour being largely due to the presence of iron. Cream, brown, grey, pink, red, light and dark blue, and drab are common colours. Typical British sandstones are Corsehill (red) from Dumfriesshire, the Yorkshire sandstones (brown), Pennant stone and Forest of Dean (blue and grey) from Gloucestershire. In America sandstones are quarried in many states, principally Connecticut (brown stone), New York (Potsdam red stone), Ohio (Amherst Berea and other quarries, light brown or grey stone) and Massachusetts (Longmeadow brown stone). The texture of sandstones varies from a fine, almost microscopical, grain to one composed of large particles of sand, It will generally be found that the heaviest, densest, least porous and most lasting stones are those with a fine grain.

Granites are igneous rocks formed by volcanic action and are of all geological ages. Granite is composed of quartz, felspar and mica intimately compacted in varying proportions to form a hard granular stone. Quartz is the principal constituent and imparts to the rock the qualities of durability and strength. Stones containing a large proportion of quartz are hard and difficult to work. Felspar of an earthy nature is opaque in appearance and is liable to decay; it should be clear and almost transparent. The characteristic colour of the granite is generally due to this substance, but the stone is often affected by the nature of the mica it contains, whether it be light or dark in tint. Granite is the hardest, strongest and most durable of building-stones, and is difficult and costly to work. When polished, many varieties present a beautiful and lasting surface. By reason of its strength and toughness this stone is often used for foundations, bases, columns, kerbs and paving and in all positions where great strength is required. The granites from the Peterhead and Aberdeen districts of Scotland and from Cornwall and Devonshire in England are much used. In the United States good granites are quarried in Connecticut, Massachusetts and Minnesota. Canada, especially the eastern provinces, supplies many excellent varieties of granites. Much granite is also exported from Norway and Sweden. Syenitic granite contains hornblende in addition to quartz, felspar and mica. True syenite consists of quartz, felspar and hornblende, the latter taking the place of mica. It obtains its name from a stone found at Syene in Egypt, but it has since been discovered that this stone is not a "syenite" as it actually contains more mica than hornblende. These rocks are very hard and are used more for paving and road-metalling than for building purposes.

Slates

The slate used for roofing and other purposes in building is a fine-grained and compact rock composed of sandy clay which has been more or less metamorphosed by the action of heat and tremendous pressure. Such rocks were originally deposited in the form of sediment by the sea or river, afterwards becoming compacted-by the continual heaping up of superincumbent material. Owing no doubt to some sliding motion having at some time taken place, slaty rocks are capable of being split into thin sheets which are trimmed to the various marketable sizes. A good slate is hard, tough and non-absorbent, will give out a metallic ring if struck, and when trimmed it will not splinter nor will the edges become ragged. Slates range in colour from purple to grey and green. The best-known British slates are those of the Welsh and Westmorland quarries. In America good slate is found in the states of New York, Pennsylvania and Maine. (See also RooFs.) There are several kinds of artificial stone on the market, consisting of fine cement concrete placed to set in wooden or iron moulds. Artificial Although from an artistic point of view its use is not desirable, it is prepared with such care that its cheap ness, strength and uniform character have led to its wide employment. One of the best-known varieties is Victoria stone which is composed of finely crushed Mount Sorrel (Leicestershire) granite and Portland cement, carefully mixed by machinery in the proportions of three to one, and filled into moulds of the required shape. When the blocks are set hard the moulds are loosened and the blocks placed in a solution of silicate of soda for about two weeks for the purpose of indurating and hardening them. Many manufacturers turn out a material that is practically non-porous and is able effectually to resist the corroding influence of sea air or the impure atmosphere of large towns.

See Rivington's Notes on Building Construction, vol. iii.; F. E. Kidder, Building Construction and Superintendence, vol. i.; P. Merril, Stones for Building and Decoration (American); H. Blagrove, Marble Decoration; W. R. Johnson, Report on Building Stone for Extension of United States Capitol; Report of Committee upon the Decay of Stone at the Palace at Westminster. (J. BT.)