building stones

building stones It is probably true to say that all the rocks that are accessible to humans have been used for constructional purposes at one time or another. In general, the stones used for building are those that have sufficient strength and rigidity to support free-standing structures, and are amenable to the technical and organizational resources of the culture wishing to use them. The innumerable varieties of rock are not uniformly distributed over the surface of the Earth, and any one culture will consequently have only a limited range with which to work. Stone, at least in its natural state, is a bulky product of low value, which until modern times was rarely transported great distances. This put tight constraints on the types of building that could be erected, and led to culturally distinctive forms of architecture, ranging from the comfortable oolitic limestone villages of the English Cotswolds to the sandstone and granite temples of the Nile Valley.

Building in stone has an extremely long history. It developed to satisfy several requirements: better protection from the weather and from enemies; longevity of the structure; and prestige for the builders. Building is essentially a craft industry, and knowledge of the reaction of stone to the environment has developed by experience over the generations. Rocks are relatively strong when placed under compression, as in traditional block-on-block construction, and even material as weak as sun-dried mud is capable of supporting multi-storey buildings, such as are common in Arabia and North Africa. When subjected to tensional (stretching) forces, however, all rocks are weak. Because of this fact the distance that can be bridged by a single slab of stone used, for example, as a door lintel, is rarely more than three or four metres, and flat ceilings of stone are almost unheard of. (This limitation is apparent in the temples of ancient Egypt.) The arch was developed to overcome this problem, which it does by smoothly diverting vertical stresses around the open space. In short, the traditional building stands by harnessing gravitational forces to balance the structure.

Of the 2000 or so mineral species currently known to science, only about a dozen are important as structural components of building stones. Quantitatively the most import-ant are quartz, feldspars, micas, the clay minerals, pyroxenes, amphiboles, and the carbonate minerals (calcite and dolomite). Of lesser general importance are two sulphate minerals, gypsum and anhydrite, and iron hydroxides (‘limonite’ and haematite). Individual rock types are usually composed of no more than four essential minerals, and it is the relative proportions and interrelations of these, together with a degree of post-formational alteration that gives the variety of appearance and physical characteristics in the building stones that are available.

Rocks, although durable, are not indestructible. They have each formed under restricted environmental conditions and when subjected to a different set of conditions may undergo mineralogical alteration. Stone in buildings has been removed from a relatively stable environment within the Earth's crust and has been exposed to the atmosphere, to the effects of changes in temperature, to the weather in general, and to man-made pollutants. The result is that the stone suffers from the adverse effects of weathering or decay. This process can be rapid, as measured on a human timescale.

Classification of building stones

The stone trade has its own nomenclature, which in several respects is at variance with geological usage. For commercial purposes, building stones are usually divided into three lithological groups, which are scientifically ambiguous, although based on geological classification.

(1) Igneous and metamorphic rocks, also referred to as ‘granites’ or ‘crystalline’ rocks.(2) Sedimentary rocks: limestones, sandstones, and ‘slates’.(3) ‘Marbles’: rocks of any type, regardless of origin, that are used for decorative rather than for structural purposes.

It should be appreciated that rocks show continuous ranges of compositions and structures. The classification of a building stone within the three groups listed above depends much on the use to which it is to be put.

Igneous and metamorphic rocks. Rocks that result from the crystallization of molten magma within the Earth's crust, or from surface extrusions of lava, are called ‘igneous’, those produced by molecular reorganization in the solid state are ‘metamorphic’. Both types are products of crystallization in the mass.

In general, igneous and metamorphic rocks are composed of silicate minerals, among which quartz, feldspars, and ferromagnesian minerals, such as the micas and pyroxenes, predominate. The individual crystal grains that comprise these rocks have grown by molecular accretion, and the resultant interlocking structure is commonly extremely strong when the crystals are randomly orientated. When crystallization has occurred within an environment of uneven pressure, elongate or flat crystals tend to form, with parallel orientations that result in planes of weakness within the rock. Schists and slates are formed by pressure-induced metamorphism, generally from muddy sediments in which recrystallization of the mud has resulted in a fabric of parallel aligned flakes of clay or mica. Such weakening characteristics can be exploited in the production of thin sheets for roofing (slates) or paving.

Sedimentary rocks. Sedimentary rocks are formed by the deposition of detritus and solutes derived from the chemical and physical breakdown, as a result of weathering, of pre-existing rocks of all kinds. All sedimentary rocks have a layered structure, or bedding, produced by the successive settlement of material. The beds represent the sea floor or other surface upon which successive layers are deposited. Many sedimentary rocks also display joints, at right-angles to the bedding. The presence of closely spaced joints can greatly reduce the value of a stone for building purposes. Both bedding planes and joints are planes of weakness exploited by quarriers, and control the maximum block size that can be obtained from a stratum. They may be very thin (laminations) or several metres in thickness. Mechanical deposition, by water or wind produces clastic rocks composed of broken fragments, generally of silicate minerals and small pieces of rock: sandstones and conglomerates. The physicochemical and biochemical concentration of dissolved salts results in limestones and alabaster.

Sedimentary rocks can be thought of as being composed of three parts: (1) the fabric of relatively coarse sand grains, granules, and shell fragments; (2) an original fine-grained component or matrix of clay minerals; and (3) a mineral cement introduced after deposition. It is the relative proportions and interactions of these three components which determine the usefulness of a rock. The mineral cement, which controls the strength and weather resistance of the stone, is especially important.

Marbles. In scientific parlance, marbles are limestones that have suffered metamorphic recrystallization, but common usage applies the term to any rock that may be used for decorative rather than for structural purposes. An unmetamorphosed limestone with an attractive appearance may thus be called a ‘marble’ by the stonemason.

A great variety of rocks have been used for building. In the past the choice was usually, as indicated above, confined to what was readily available in the locality. Limestone was used for many of the buildings of classical Greece. Tufa was among the materials used in ancient Rome. Andesite was used for Borobudur, in central Java, the world's largest Buddhist monument, built in 778–856. The Maya pyramids of Yukatan were built of limestone. The Jaisalmer Citadel in north–west India, dating from the sixteenth century, is built of sandstone. In the absence of other sources of building stone, glacial erratics have been extensively used in Finland and northern Poland.

The structural and decorative uses of building stones are usually distinct. The stones used for decoration are usually more expensive than structural stones, and in many instances are applied as thin slabs. The Taj Mahal in India, the Cathedral of San Marco in Venice, and (a modern illustration) Canary Wharf in the London Docklands are among many examples of decorative stonework that could be cited. Sedimentary, igneous, and metamorphic rocks have all been used for decorative purposes. (One igneous rock, a syenite from the Oslo region of Norway, was used so extensively in London that it was nicknamed ‘public-housite’ by metropolitan geologists.)

Durability of building stones

Rocks are natural materials, and they can vary significantly within relatively short distances. A illustration of this fact was the choice in 1839 of a Permian dolomite for the construction of the Houses of Parliament in London. In the event it became necessary to obtain most of the stone from another quarry, where it was extracted unselectively from a thickness of more than 30 metres. The result was a disaster, and in 1927 it was decided to restore the building using a sandstone at a cost of more than a million pounds—a huge sum in those days.

The observation of some very simple rules can help to increase significantly the effective life of building stone. For example, sedimentary rocks will resist erosion much better if they are put in place with their bedding planes horizontal—that is, in their natural position.

One of the unforeseen effects of the Industrial Revolution has been the need to use complicated measurements of physical characteristics in engineering calculations. This fact, combined with the desire of architects to build gravity-defying structures, has made it increasingly necessary to understand the materials used in construction. Modern buildings are commonly built of cast reinforced concrete, and the exterior is clad with thin slabs of stone, or some other material, to give protection from the weather. Instead of being supported on four or five sides by adjacent blocks, as in traditional buildings, the cladding slabs are held by a small number of brackets which grip into slots cut into their thickness. Among the industrial nations, such organizations as the British Standards Institute and the American Society for Testing Materials have sought to develop test regimes of general application. For building stones the results have been generally disappointing, and no unified testing system has yet proved entirely satisfactory. At best, it might be possible to relate a combination of factors such as porosity and saturation coefficient to the known past behaviour of a limited range of building stones, but the results should not be extrapolated to other stones without careful consideration.

The results of failure are dramatically illustrated by a high-rise building in Chicago, where the stone used for cladding reacted badly to exposure to the elements and fell off in large quantities: a serious hazard to life as well as a commercial loss of considerable magnitude.

Problems of supply

Suppliers, especially those in industrialized countries, are finding it difficult to meet the demand for stone for building and other purposes as existing quarries become worked out. Possible sites are few, and there is—understandably—strong resistance to the opening of new quarries. Large ‘super-quarries’ in remote locations that have suitable anchorage for large ships have been opened up in such places as the west of Scotland, Newfoundland, and the Yucutan peninsula of Mexico. Other large quarries have also been developed at inland sites, even in the United Kingdom.

R. W. Sanderson

Bibliography

Burton, M. (ed.) (1999) Designing with stone. Ealing Publications Ltd, Maidenhead.
Clifton-Taylor, A. (1987) The pattern of English building (4th edn). Faber and Faber, London.
Hund, R. (ed.) (1990) Dimension stones of the world. Marble Institute of America, Farmington, Michigan.
Smith, M. R. (ed.) (1999) Stone: building stone, rock fill and armourstone in construction. Geological Society Engineering Geology Special Publication No. 16.