Architects, specifiers, designers, clients; choosing and using stone is an experience and one for which we recommend you ask for professional advice. Stone is a naturally occurring material and as such, has to be chosen like no other building material. Stone Federation members are fully trained, experienced and competent professionals who should be consulted as soon as you decide to use natural stone on your project. They can advise you and prevent any costly mistakes being made before it is too late.
Domestic use of stone
Using stone in your home or property can enhance your living space but you must be aware of the questions to ask when making your choice of stone.
Stone, correctly selected, can enhance the value of your property and be durable, and virtually maintenance free. However, incorrectly selected it can cause you problems. By purchasing stone you are making an investment so protect your money and get the right advice. Stone Federation suggests you always consult the experts first.
General types of natural stone
Classification
There are many different classification schemes for stone, which have prompted the industry to simplify descriptions. This has led to many problems when, for instance, a stone laid as a granite is actually found to be a different stone type altogether and does not perform as expected. The British Standard BSEN12440 (Denomination of natural stone) addresses the classification problem and insists upon the correct identification of stone type and origin. For the purposes of this guidance however, the simplified descriptions remain and it must be stressed that all stones should be considered individually for their merits in whatever use they are being considered for.
The term granite has been applied to almost any igneous stone that can retain a polish. True granites provide many of these stones but other types of igneous stone that may fall into this classification include syenites, gabbros, dolerites, and diorites. The metamorphic stones gneiss, schist and granulite are frequently also included in this ‘granite’ classification.
The formation of these igneous rocks by the slow cooling of molten minerals such as quartz, feldspar and hornblende has resulted in a wide variety of colours and grain patterns.
In the United Kingdom the quarrying of granite is concentrated in a small number of locations. Those of particular importance may be found in Devon and Cornwall, Cumbria, at Peterhead and Aberdeen and on the east coast of Scotland.
The main sources for imported granite are Scandinavia, South Africa, Sardinia, Portugal, Spain, India, North America, China and Brazil.
Applications
The interlocking crystal structure imparts both the high strength and low porosity necessary for a wide range of applications other than just flooring; these characteristics are also important in allowing successful use of slabs that are thinner than other types of stone.
Appearance
The immense colour choice includes fine and medium grained silver grey from Devon and Cornwall and fine grained pink from Scotland, whilst other countries provide dense, very fine grained materials with reflective crystals such as larvikite (e.g. blue and emerald pearl) from Norway; reds from Scandinavia and India; large grained brown from Finland; yellows from Brazil; and the delicately mottled greys, fawns and pinks from Sardinia.
The surface finish affects the appearances of granite and those available include sawn, flame textured, dolly pointed, fine axed, rough punched, honed, bush hammered and water jet. It is in the gloss polished form, however, that these granites reveal fully their colours and grain patterns.
Performance
Very hardwearing and physically strong, granite is largely unaffected by erosion, pollution or atmospheric attack. Facades are generally self cleaning, although eye level areas benefit from occasional cleaning as for glazing. A polished finish is almost indestructible.
SANDSTONEThe term sandstone is used to describe almost any stone of sedimentary origin with a granular texture. Some other types of stone that may fall under this classification include gritstones, siltstones, greywackes, conglomerates and marls. Aside from particle size variations, the dominant factor affecting sandstone performance is the grain cement that may be siliceous, calcareous, clay bearing or iron-rich.
Geologically it is a sedimentary type rock made up of various mineral particles mainly quartz, mica and felspar. All these minerals are bonded together with natural cements such as silicas, calcium carbonates, iron oxides and clays. The individual make up of each sandstone gives a wide range of colours. A pure silica sandstone is white with the colours arising from other minerals. Iron oxides can cause the stone to vary from buff or brown through to the deepest red. Grey colour stone is due to the presence of clay and green could indicate the presence of glauconite which contained potassium.
Sandstone is a natural material traditionally regarded as the building stone of the North. However, sandstone can be seen throughout the United Kingdom in many prestigious buildings from structures which form part of our heritage to modern office blocks.
Sandstone is traditionally used as a building material in many areas where it occurs locally particularly in Derbyshire, Yorkshire and the North East of England, Wales and Scotland. Many of the “stone cities” of the North derive their particular architectural character from the use of sandstone.
Most of the major sandstone quarries working today are located in the North of England, Derbyshire and Scotland. For flooring purposes, sandstones are also imported.
Appearance
Sandstone has many colours ranging from white, grey and buff to various shades of pink and red.
The colour and texture of stone is self evident, however the colour of dry stone can change quite dramatically on wetting. Natural weathering can play a major part in the change of colour. Such colour change can sometimes be viewed on abandoned faces of the quarry although it is much more important to view existing buildings constructed from the same stone.
Sandstones generally have an even texture, which may vary from course to fine, but the stone from some quarries also show attractive natural markings.
Applications
Many types of sandstone are suitable for paving or flooring. Some can be split (“riven”) otherwise the normal finish is sawn. Tooled, sandblasted and flame textured finishes are also available.
There is a vast variety of surface finishes available from the traditional rubble walling to wrought stonework with its finishes to different forms ie. boasting, pitched, broached, hammer dressed and rusticated, to the very intricate looking vermiculated. But most importantly, credit must go to the craftsmen who hue the sandstone to all the various forms from large stones with a high degree of moulding to the most intricate of carving work. Sandstones are suitable for blcok construction in most building and engineering work and can now be cut to relatively thin sections for cladding to steel framed or concrete structured buildings.
All in all sandstone is one of the few historic natural minerals still in use today. It provides excellent durability and is virtually maintenance free. It is aesthetically appealing to the eye whilst giving great value for money to the client.
Performance
Most sandstones have good load bearing and weathering properties. Correct detailling with regard to weathering is of prime importance, especially on buildings designed with sloping surfaces and without overhangs. Great care is required in the design of non-ferrous cramps and corbels for fixing to structures.
SLATESlate is found extensively, throughout the British Isles and has, for centuries, been a major source of building stone. Historically, and, in common with other stones, it was first used in and around the immediate areas of availability.
The term slate is often used to describe any rock that can be easily split into thin sheets, principally for roofing purposes. True slate is defined by the presence of a ‘slaty’ cleavage; this allows the slate to be split at almost any point through the stone parallel to the cleavage plane. Most true slates are metamorphosed sediments, often formerly mudstones; however, some British “slates” are derived from volcanic ash sequences and are not true slates in the strict geological sense.
Appearance
The major UK sources of slate, with their distinctive colours, are the Lake District blue/grey, light green, olive green and silver grey, North Wales blue, grey, blue-black and red, and Cornwall - grey.
Slate is easily split (“riven”) into thin sections, giving a natural finish. Other finishes include sawn, sanded, fine rubbed, flame textured, bush hammered and water jet.
Applications
From the very early, basic applications, British Slate is now used for very wide and diverse purposes. Modern quarrying and machine processes allow prime blocks to be sawn or split into large slabs and a variety of thicknesses. Current products include: roofing, cladding, cills, copings, flooring & paving, treads, plaques & memorials, worktops & fireplaces.
Slate is typically a very durable construction material and rarely exhibits visible degradation, even when placed under extreme atmospheric conditions for many years. For flooring purposes U.K. slate is unlikely to delaminate or exhibit efflorescence.
High flexural strength and typically very low porosity makes slate eminently suitable for flooring purposes and it requires little maintenance.
Caution should be exercised when choosing material marketed as slate as some may not be suitable for flooring. If any doubt exists about the material being considered, specialist advice should be sought.
Very thin, split, natural riven sections are used for roofing and the same methods produce thicker material for flooring and cladding. Slateis readily available in sawn, fine rubbed and other finishes wuch as sand-blasted and flame textured.
Performance
British Slate has a proven record of durability and being supplied fit for its intended purposes. This is readily seen on countless buildings in the UK and many areas of the world. Such examples of installation are witness to the ability of slate to withstand the rigours of extreme atmospheric conditions. Slate is extremely stable and will not degrade, warp or twist. Quality Assurance, management systems are widely used throughout the industry.
LIMESTONE
Stone is a material that man has used in building for several thousand years and it still lends itself to modern usage and design, and limestone in its various geological forms is probably the type of natural stone most widely used for this purpose. It occurs naturally in many parts of the UK with the limestones of the Jurassic period being those of most importance in this field. They run broadly in a belt from Dorset and Somerset in the south west, through the Cotswolds and Oxfordshire, on into Northamptonshire and Lincolnshire, to just north of the Humber.Limestone is a sedimentary rock. Many of the commonly used varieties were formed by the accretion of the hard remains of former organisms such as corals and shells. These materials principally comprise calcium carbonate (calcite), as does the cement. Calcite is a relatively soft mineral and careful selection will be needed when you choose this for use as flooring. Variations in the types and quantities of shell and other remains and the nature of the cement provide a huge range in the types of limestone available.
The limestones of these areas were used for building in Roman times and, in later centuries and to-date, have been the traditional building material of their localities, Many buildings throughout these counties bear witness to this. In addition, some limestones are extensively used outside their areas of origins as is evidenced by the many fine buildings to be seen throughout the British Isles, particularly in major cities and towns. Cathedrals, churches and numerous public and private buildings provide outstanding examples of the durability and beauty of these limestones and they show through the skill of the mason, how this stone is so eminently suitable for producing masonry of excellence in all aspects of plain and detailed work that may be required. Among the better known limestones of the Jurassic age are those of Purbeck, Clipsham, Ancaster, Ketton, Bath, Doulting and Weldon, whilst Portland stone is a characteristic feature of London architecture. From the earlier Carboniferous age come the limestones of Derbyshire and Cumbria such as Hopton Wood, Sheldon, Orton Scar and Salterwath.
Imported limestones, most notably French, German, Portuguese, Spanish, Turkish, Egyptian and Israeli are also available in the UK.
Appearance
The colours of limestones range from almost white to the warm honey tone of Bath stones with grey/blue and darker browns a feature in some limestones. Textures range from fine even grained stones to the smooth fossil bearing types and to the coarse open textured Portland roach. Certain limestones such as Purbeck and Hopton Wood take a polish and can be used as "marbles".
Applications
Limestones are used extensively in both new building and restoration where their ease of working facilitates the production of cladding, ashlar and other forms of walling. They are also suitable for flooring.
Many limestones are particularly suited to carved and moulded work. Components such as cills and jambs, soffits and copings, heads and mullions are readily produced to add interest and enrichment to building facades.
Performance
Limestones generally have good load bearing properties and weathering characteristics although correct detailing is important as with all building materials. Limestones must not be used above sandstones on exterior elevations.
The properties of individual types of stone can vary considerably and advice should always be sought on the selection of suitable stones, both for general and particular applications.
MARBLETrue marbles in the geological sense are metamorphosed limestone and are principally composed of recrystallised calcite formed into an interlocking granular structure. Some hard or partially metamorphosed limestones are incorrectly referred to as marble, even though they exhibit many of the characteristics of marble.
In its ‘purest’ state marble is typically white, however the presence of other minerals can often provide colour(s). In some cases the colour appears as irregular shaped patches (brecciation) or as substantial veining. Specialist advice should be obtained as to the suitability of such marbles in particular locations.
Greater care must also be taken during installation as other construction defects easily taint the pureness of marble, particularly from underlying materials, and may result in staining at a later date.
It is common to use marble with a highly polished surface finish and, as a consequence, slip resistance should be carefully considered. Marble is imported.
QUARTZITES
Quartzites are typically the metamorphosed product of an original sedimentary rock e.g. sandstone, composed almost entirely of quartz.
In general quartzites are hard wearing, have low porosity, a high compressive strength and good durability making them suitable for use as flooring. The metamorphic varieties in particular may be brittle and so some care is required in assessing a material prior to use. While composed largely of quartz, the presence of some impurities can lead to the development of different colours and the typical range is from white to yellowish brown, through to green, brown, gold, grey and blue. In Eire, the metamorphic quartzites have a complex geological history which has tended to result in them having a relatively small natural block size. All operating quartzite quarries in the British Isles occur in Eire. Elsewhere quartzites are commercially available from Norway, Sweden, Italy, South Africa, USA and Brazil.
TRAVERTINE
Travertine is the name normally given to a type of precipitated calcite associated with the cooling of waters around hot springs or in caves. Being formed very slowly it often incorporates dead matter such as tree debris that later rots away to leave a voided structure.
The performance of travertine greatly depends on the size and density of the voids, which, in most instances, are routinely resin surface filled before the finished stone is supplied. The nature of the fill material must be compatible with the travertine and not subject to discoloration or decay.
Traditionally travertine was naturally bedded and generally referred to as vein cut. Recently travertine cut face bedded and referred to as classico has appeared on the market. Whilst the vein cut material has a long and proven history of use, the cross cut or classico travertine has not yet proved to be as suitable because of surface collapse and filler failure. Before this material is considered, its limitations in performance must be taken into account.
Generally travertine is suitable only for internal flooring purposes, and then only after the most careful consideration. Strength may be lower than most other stones and the presence of hidden voids below the surface may lead to ongoing repair when the filling material collapses.
Unfilled travertine has been used for flooring purposes but only after strict quality control of the size and distribution of the voids. All travertine is imported.
GREEN MARBLE OR VERDE
Green or verde marble is considered separately to true marble because its origin is often the result of the metamorphism of rocks other than limestones. The green colour is typically derived from the presence of serpentine minerals. These minerals are relatively soft and the structure often weak, the stone commonly requiring reinforcement from matting glued to the underside.
Verdes are rarely used for whole floors and are preferred for providing coloured inserts to other flooring materials. Care must always be taken to ensure their compatibility with other stones.
A special consideration for any verde is the presence or otherwise of asbestiform minerals that may be naturally present e.g. chrysotile (white asbestos) and the amphiboles tremolite and actinolite. It may be prudent to establish that, where present, fibres of these minerals in the respirable range are not generated during any cutting, grinding and polishing operations, particularly if carried out dry.
The Asbestos (Prohibitions) Regulations have banned the import, supply and use of blue, brown and white asbestos (crocidolite, amosite and chrysotile respectively) essentially when part of an asbestos admixture product. Natural stone that contains chrysotile for example, and is used for flooring, would not be classed as an asbestos admixture product.
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