Everyone enjoys stone, although not everyone is aware of it. It represents safety; protection; home. So says geologist, surveyor and stone consultant Barry Hunt as he introduces a series exploring the main categories of natural stone used in construction.
When caves were first entered for protection from the elements and predators, stone became a friend that may not always be present but is never far away. Tools and objects were made with stone and later, stone was used for building.
Surviving stone buildings are often the main reminder of past Civilizations that might otherwise have been forgotten. Just stand and gaze in awe at the Pyramids, take a stroll through Petra, enjoy a bus ride in Rome, drive down to Salisbury Plain and see Stonehenge.
We all enjoy seeing the sights and we all benefit from the utility of stone in the built environment. But how often do most people really think about or look at the stone?
Because of our familiarity with stone we may well take it for granted and just consider it natural to be surrounded by it. It is such a natural part of the landscape – whether you are in a big city or the back of beyond; in a skyscraper or your own home.
With globalization and technological advances, stone from around the world can be chosen from your local stonemason. Everyone has the chance to appreciate and use this versatile, durable and beautiful material.
Working stone is part magic and alchemy – breathing life into a stone as it is shaped and formed for a purpose – part science.
Because of the mass appeal of stone, the world of building stones has been simplified into just 10 major stone groups. The main five will be described in more detail in the pages of Natural Stone Specialist, the monthly magazine of the stone industry in the UK and Ireland, in the months ahead as I explore their unique properties and explain how they fit into the grand scheme of construction.
We will begin our journey through building stones with limestone, probably the first widely used masonry material – not forgetting that most pre-historic caves are found in limestone massifs (defined sections of the Earth’s crust).
Some of the UK’s best known building stones are limestones – Portland and Bath, Cotswold, which defines the attractive villages and many of the mansions of that region, Ancaster in the Midlands and many, many more.
Limestone often has an ease of extraction and use that belies the style and variation in aesthetic qualities that make it so desirable. Rarely is anything this good won so easily.
Archaeologists have determined that some of the World’s oldest stone structures are found on Gozo and Malta, where the local materials include globeriginid limestone that could be easily cut and shaped with simple tools using the same stonemasonry principles that still apply today, 6,000 years later.
When masons are preparing limestones they are concerned about the ease of working them, the ability of the stone to hold a good edge (arris) and that the stone contains no obvious splitting planes.
After limestone, I will consider sandstone, which might be regarded as the stone most easily understood by the layperson – basically sand that has been turned to stone.
Sandstone is one of the most widely used stone building materials, being both common in many geological sequences and easy to use for construction purposes, although it is highly abrasive and takes its toll on the tools used to process it.
It is not always the most versatile of natural stones, due principally to the constraints created by the original bedding process. Where sandstone is derived from marine deposition, the beds tend to be relatively thin and these laminations can create weaknesses, although it is these very weaknesses that can be exploited when splitting the stone to use as paving and flooring, and sometimes as roofing.
The quarrying of marine-derived sandstones is often a slower process than with other dimension stones because of interbedded shales and other waste or unusable materials.
The finished sandstone product can represent as little as 10% of the stone taken from the quarry face in some locations.
More consistent properties are often achieved with sandstones formed from desert sands, where bed heights can reach tens of metres and occur across large regions. The sorting of the sand grains also makes many desert-derived sandstones more consistent in their appearance and properties.
These desert sands often contained iron oxide, giving rise to a red colour. While these stones are typically strong, they do not always have sufficient abrasion resistance to be used for paving, which is a major use of many sandstones.
The American Society for Testing & Materials (ASTM) simplifies sandstone classification on the basis of density, dividing it into three categories: sandstone, quartzitic sandstone and quartzite.
The classification works reasonably well and few stones that would not be regarded as sandstone by geologists manage to fall into the ASTM classification.
Sandstone probably has the edge over limestone for traditional block stone masonry use, as even the lower quality sandstones do not experience the level of damage in-service that limestones often suffer (although much of the damage experienced in the past by limestones was related to the acidic conditions resulting from sulphur emissions from coal burning, which no longer have the same influence).
Possibly the main difference between limestone and sandstone is the lack of a microporous network in sandstones, which means they tend not to hold on to water as long as limestones and thus suffer less from frost action.
In the UK, limestones and sandstones are the major building stones, along with granite, which we shall come to later.
First, though, a word about marble.
Marble was recognised by early civilisations as a stone that, while being harder to work than the more ubiquitous limestone and sandstone, was better suited to fine detail and carving.
Its dense structure allowed early sculptors to represent life more convincingly and so it became the choice of Kings and Emperors to fulfil their cravings to be immortalised.
Perhaps it is fanciful, but both the Greek and Roman Empires had good local resources of marble and were in warm climes where marble buildings created an atmosphere of coolness and tranquillity. Would the thoughts of Socrates, Aristotle and Pythagoras have been less lofty had they not had the luxury of marble halls in which to soar?
Marble can take a high polish with less effort than other polishable stones (such as granite and porphyry) but it is not just this property that makes it so favoured. The typically large calcite crystals allow light to penetrate without great absorption, bringing a depth to the stone that is unmatched. The crystals sometimes sparkle like sugar where there is reflection from cleavage planes and hence petrologists apply the term ‘saccharoidal’ to this effect.
What constitutes a marble in building terms can be broad, but these days is generally considered to be a stone that complies with a broad geological view that marble is a limestone recrystallized by the heat and pressure of the earth.
Some of the rarest and most expensive marbles are the most highly coloured and figurative, and yet, perversely, those often considered of highest quality have the least veining, spotting and other features.
Two of the best known and most highly regarded marbles today are the uniformly white and noticeably translucent marbles of Carrara in Italy and Pentelicon in Greece – the same marbles appreciated by our ancestors 3,000 years ago.
Most commercial marbles tend to be of the recrystallised limestone variety rather than derived from other stones such as the forsteritic marble that results from the alteration of less common dolomite.
Because marble often appears in the ground in huge, consolidated deposits, it can be sawn from the face of a quarry in large, regular blocks that make processing easy and waste minimal, so that recovery of finished product can be as high as 80% of the extracted stone.
Images of quarries from which marble has been extracted in this way are frequently seen and are often what customers think of as a dimensional stone quarry. This preconceived idea can result in quite a shock when those same people visit a typical British limestone or sandstone quarry that does not look anything like that.
Marble is not ideal for exteriors in the UK, where the still acidic atmosphere can rapidly remove the polished surface. The most notable exception is possibly Marble Arch in London, completed in 1828 using Carrara marble to John Nash’s design for the main entrance to Buckingham Palace. The arch was moved to the current site by Hyde Park when the palace was extended in 1851. The arch has withstood the ravages of one of the most polluted locations in the UK.
Otherwise, the external use of marble has been confined mostly to statues and figurines, although it has been used in many interiors as floors, columns, pillars and wall cladding, and is these days widely used in fairly modest bathrooms and for floors.
And so to granite. This is viewed by many as the antithesis of marble, with its severe appearance and archetypical durability unchanging across millennia.
It has earned such a reputation with good reason, as many of the oldest stone antiquities are of granite, often little altered from the day they were carved.
Granite is probably the most widely available stone on the planet. Perversely, although it is sometimes known for its grey blandness, its abundance has made it probably the most diverse of stones, with thousands of varieties commercially available.
Another reason for granite’s popularity is its ability to be applied to almost any building stone use from paving to cladding and statues to ashlar.
Furthermore, granite can be highly polished and even under extreme weathering conditions can retain its polish for many years.
To the uninitiated, almost any igneous rock (ie one that is formed from the magma of the earth) is called granite and little differentiation is made by the stone industry other than those that are black, which are simply referred to as ‘black granites’ (although to a geologist they are not, in fact, granite but gabbros, basalt, or some other kind of hard rock).
The highest quality granites are usually considered to be those with little variation, although some of the rarer examples featuring large porphyritic and rapakivi textures are much sought after.
Granite is another stone found in large, highly consolidated areas that can be sawn from the quarry face with diamond wire saws, so the recovery rate in most instances is high – as much as 80%.
It is not only the enduring qualities of granite that make it desirable as a building stone but also the uniform nature and potentially massive size of the blocks available. This was first exploited by the Egyptians with the building of their great temples and obelisks, which include some of the World’s largest monuments made from single pieces of stone.
The temple complex of Amun-Ra at Luxor is home to the obelisk of Hatsheput, which, at 30m, is the tallest in Egypt. It reportedly took just seven months to carve from a single block of granite taken from the Aswan quarry. The Aswan granite was also used for the burial chamber of the Great Pyramid of Khufu at Giza and the lower portions of the casing stones of the other two pyramids.
In the UK today, granite can be seen in most towns and cities as cladding on office buildings, in particular banks, which like to create an impression of impregnability. In the past 20 years it has become uniquitous as kitchen worktops in commercial and domestic settings.
One of England’s granites still in production as building stone and paving is DeLank, in Cornwall, while some of the best UK examples of granite buildings are in Aberdeen – notably Marischal College, reputedly the second largest solid granite building in the world.
The oldest structure in London is the pink granite obelisk called Cleopatra’s Needle, built in 1475 BC for the Pharaoh Thothmes III and originally erected in ancient Heliopolis (North Cairo).
The final broad category of stone used in construction that will be explored in this series is slate – possibly the greatest gift of the Earth to human construction.
The ability of slate to be readily split into thin sheets capable of resisting breaking under considerable loading and keeping out water has made it highly desirable, especially, of course, as roofing.
The amazing properties of slate were not fully realized in the UK until after the Romans had left. There is some debate over the first use of slate for roofing but, naturally, Wales, the home of many of the UK’s slates, claims to have thought of it. But the French also claim to have first used slate for roofing, in the Ardennes from at least 466AD.
The earliest known contemporary documentation of the use of slate for roofing was of 800,000 blue slates shipped from Dartmouth and Totnes to Winchester between 1171 and 1187.
With the Industrial Revolution and the building of railways, slate production eventually hit a 500,000tons-per-year peak in the UK during the 1870s, 90% of it coming from 670 quarries in Wales. Today there are half a dozen, with one or two more producing occasionally, although the largest, Penrhyn, recently gained planning permission for an extension that will prolong its working life.
Slate is defined in a geological sense as a fine grained, regionally metamorphosed mudrock with a well developed penetrative cleavage, which is a foliation of
sub-microscopic phyllosilicate minerals that allow the rock to be split into the familiar platy sheets.
The cost of slate production is mostly dictated by the amount of waste, which can be huge – 99% for some high quality products and typically 90-95%.
Therefore a good slate deposit with low wastage can lead to a dramatic reduction in costs, something that both Spanish and Chinese resources have benefited from in recent years, although they are often branded (wrongly) as inferior materials.