Imagine you suddenly had to build a new house for everyone in the world. Let’s say 7 billion people spontaneously need a 25m2 living space which we propose to form with a 200mm reinforced stone slab. How much stone would we need? Is there enough? The total volume of used stone would be 35km3, equating to to a hole of roughly 40km square and 20m deep. This is a big hole, but in global terms it’s a pinprick: the Earth’s crust has a volume in excess of 5 billion km3. Between 5 and 70km deep it is mainly granite under the land, and under the sea it is mainly basalt. Stone is being replenished all the time through plate tectonics in unimaginable volumes. It is inexhaustible, a replenishable natural resource created by geothermal energy.
Almost all construction materials are extracted like stone. Timber, one of the few which isn’t, is considered a sustainable material with good reason, but growing it occupies a lot of space with monocultural forests for long periods of time. A tree takes 25 years to grow 1.5m3 of material. If we quarry the stone under the tree we get 500m3 of stone in a month. Although quarrying is highly intrusive and disruptive while it is being carried out, fully exploited quarries can be returned to nature, backfilled with unwanted spoil or reused for other purposes. The quarry that supplied most of the distinctive sandstone for all of Edinburgh’s New Town, built in the 18th and 19th centuries, has now been filled in and forgotten, sitting under Craigleith Sainsbury’s car park.
Technological development in stone largely ceased with the advent of reinforced concrete and steel in the 19th century. Billions of pounds have been spent developing these new technologies, funding academic research that has filled civil engineering departments with teachers who are specialists in these materials. This has created an educational bias that has limited contemporary designers’ understanding of how to build with stone in a more meaningful way.
The advent of coal changed the way we build. Before the Great Fire of London in 1666, buildings reflected the scarcity of energy and materials at the time. Tudor buildings exposed their braces, joints and pins, and were infilled with lightweight material, while Gothic churches consisted of thin shell flying buttresses. It was an architecture born of its environment, composed of limited timber and stone but also of more abundant twigs and mud. After the fire, Charles II declared that all new buildings in London were to be built with bricks – which were fired with coal. This event marked the beginning of the widespread influence of coal on building in Britain. Coal allowed the industrial-scale production of not only brick but iron and steel, and later the development of the concrete industry, burning limestone to make cement.
The fossil fuel era is a short blip in history but it has radically changed the lives of people the world over. An abundance of cheap energy has allowed rapid human development, an unparalleled population explosion and more prosperity than ever. ‘Energy is the power to do anything,’ Barnabas Calder writes in Architecture: From Prehistory to Climate Emergency. ‘Without it nothing can be heated up or moved, nourished or destroyed. The sum total of human activity has always been constrained by the total amount of energy humanity could harness.’ Today, cheap energy trumps labour, craft and know-how. It is cheaper to extract and burn several tonnes of coal to make over-simplified steel and concrete structures than it is to employ people to design and form more complex, materially frugal structures.
The energy required to make cement and steel is used in extraction, processing and transportation. The processing is the fiery business of changing limestone into cement or iron ore into steel. In the case of stone, the energy is only required for extraction and transportation, with no fiery processes at all. Making stone has about half the carbon footprint of concrete and stone is often more than 2½ times stronger; in the case of some dolerites, stone is as strong as steel in compression.
In most places, however, stone has been largely relegated (or should we say elevated) to a high-end decorative material. Stone quarries seek out stones of fashionable shades and extract vast quantities, that then for aesthetic reasons are rejected. Because of this high-status selectivity, stone is expensive. If stone were quarried simply for its strength, many more stones would become useful and the prices would drop. Quarries frequently use reject blocks to form walls and barriers; to them a 1m3 block of discoloured marble is cheaper and more immediately available than chain link fencing. Stone could be cheapened further by not being overdressed. Much of the old architecture of Lyon is composed of large rough stone blocks, used for their strength alone and historically covered up. These are being gradually exposed.
In fact, stone is highly versatile and can be used for most structural architectural purposes. It can be used in place of brick and concrete blockwork and it can be used in place of concrete foundations. In East Africa, stone buildings are the norm: it is more economical to use than concrete blockwork. Nairobi is a rapidly growing city that sits among diverse volcanic deposits. Much of the land to the north of the capital sits over trachyte (lava stone) deposits, which are overlain by tuff (weaker cemented volcanic ash). It is more economical for Kenyan builders to excavate these stones and cut them into blocks, using trachyte in foundations and tuff for walls.
After the Second World War, France’s government turned to stone as reconstruction efforts were hampered by a shortage of materials such as steel, concrete and coal, funding industrialisation in the quarrying industry. The architect Fernand Pouillon pioneered a stone building system on a number of schemes in both southern France and Algeria. They are astonishing, modern buildings ennobled by their structural honesty and use of natural materials. The Totem Tower in Algiers reaches 62m in height (26 storeys high) and sits in the middle of a significant earthquake zone. The building, built by the French colonial government to provide desperately needed housing, was erected at low cost in just 96 days. Pouillon used stone produced by quarryman Paul Marcerou, whose stone cutting machine was developed using grant money from a French government industrialisation fund to create large-scale stone blocks to millimetre-level accuracy.
Strains of this movement continue today in southern France. Gilles Perraudin, who in many ways is a successor to Pouillon, has continued to build housing in solid stone such as a social housing project in Cornebarrieu in 2013. Offering a new aesthetic of stone used structurally in a more raw state, Groupwork’s 15 Clerkenwell Close housing project in London (AR July / August 2018) uses a monumental stone exoskeleton. The practice’s building at 317 Finchley Road, currently in construction, will be one of the largest load-bearing stone structures since the last stone cathedrals. Stone can replace steel and concrete columns: also in London, Eric Parry and engineers Whitbybird used stone columns in their 2002 project in Finsbury Square. Stone can also be used in thin layers to form walls or in panel form – Elastico Farm’s innovative use of large format granite panels for their project Houses of Cards in Italy, built in 2020, shows the potential of building with bigger stone elements. Although using stone in a relatively unsophisticated way, as simple compression elements, they create a precedent for stone to be more than decoration. As recently as 2001, in the Chinese province of Shanxi, the government built a 95-metre stone arched road bridge. The bridge crossed the Danhe River and supported a four-lane motorway. This incredible achievement points to the potential of building large-scale infrastructure in stone.
‘Making stone has about half the carbon footprint of concrete and stone is often more than 2½ times stronger’
Hybrid structures using timber are now beginning to emerge. So far, these are largely combinations of timber and concrete, using concrete in compression and timber in tension, but this material combination could also work with stone. Some of Studiolada’s work is pushing this idea, such as the nursing home extension in Vaucouleurs in France, completed in 2018. Savonnières stone sheets, 40mm thick, are mounted on a timber frame with flexible joints – the project required a huge amount of research as the timber frame moves and expands much more than the stone. This method of combining timber and stone is half the price of widely used stapled stone facade systems on concrete frames.
Stone is a new material in our technological context. It is a new material in search of a language. When materials are adopted into the canon of building materials they tend to ape the materials that came before them: the Iron Bridge, opened in 1781 in Shropshire in the UK and the first major bridge made from iron, used cast iron as if it were wood, reflecting one of the designers Thomas Gregory’s background in carpentry. Early 20th-century architect Maxwell Ayrton frustrated engineer Owen Williams by designing concrete buildings that looked like classical stone. It didn’t take long for those materials to forge their own paths: Owen Williams dumped Ayrton and struck out on his own, producing fabulous concrete structures like the Boots factory in Nottinghamshire, built in the 1930s.
Historically, stone would be broken by drilling, wedging and splitting, or dynamiting, and broken into blocks by hand. Today’s quarries use electrical diamond saws to cut blocks directly from the face. An array of processes is used to cut the stone blocks into useful pieces such as slabs and tiles. When asked whether he used CNC or robotic cutting to produce stone elements, Pierre Bidaud of The Stonemasonry Company replied: ‘What’s the matter with creating employment?’ His answer questions why we favour the machine over the human: we tend to overdesign and avoid complexity while having a high global unemployment rate, due to taxation of the human and non-taxation of the machine. Nevertheless, most contemporary stonemasons do not like to stand in workshops for long hours grinding pieces of stone manually and are in favour of programming machines to do the actual work. CNC means that stone can be shaped in many ways, allowing complex and potentially more economical structures.
Engineers designing with stone are cowed by its brittle unpredictability. It is a naturally occurring material full of cracks and fissures, so it is common practice to use large factors of safety, leading to wasteful and overengineered structures. Because of the rarity of structural stone use, there are no specific codes of practice for the design of reinforced stone structures. Similarly there are no material standards for strength classification. Currently, stone strength is assessed by taking multiple samples and crushing or breaking them at the discretion of the designer. This is slow and liable to give misleading results eliciting excessive factors of safety. Ultrasound and X-ray grading is a potential solution to this and would allow stone to be tested non‑destructively leading to a subsequent reduction in factors of safety.
Modern methods of analysis mean that stone structures can be developed in ways that they couldn’t before. Finite element analysis is a tool that is available and familiar to most engineers and allows designs to be tested in ways that were not available a hundred years ago. It is now possible to create computer simulations of any structure and see the stresses in three dimensions, allowing a greater degree of experimentation and greater confidence to build structures that could only be proven empirically in the past. While Gaudí used chains and sacks of sand to find a virtuous geometry for the Sagrada Família, Philippe Block at ETH Zürich has spent years developing analytical methods to ‘form find’ funicular structures numerically, producing an impressive dry-jointed limestone vault for the Venice Biennale in 2016.
Building stone structures only in compression plays to its advantages, but these forms are not always practically convenient and are expensive to erect and form. Today, adding steel reinforcing bars to stone frees structures from acting in compression only. Blocks of stone can be drilled through with modern diamond drills, high tensile steel wire is commonly available, and cheap jacks facilitate easy post-tensioning. Post-tensioning means that stone blocks can be strung tightly together like square beads on an elastic cord to form a beam. The steel content of a reinforced stone beam is a fraction of that of a steel beam: to produce a steel beam requires iron ore to be excavated in Brazil or Australia, a blast furnace and a rolling mill; an equivalent stone beam can be produced at a quarry near you with a diamond saw, a drill and a strand jack.
Either because we will have to self-impose restrictions on energy consumption due to the climate crisis, or simply due to scarcity, the fossil fuel era is coming to an end. If the fossil fuel era hadn’t happened, what would have been the arc of technological development in building? How would we have advanced the design and construction of buildings without coal, gas or oil? What kind of retro-futuristic neo-Tudor alternative future would we be living in?
After repeated flooding and other extreme weather events, voters of the future will demand action on carbon. Punitive carbon taxes will force up the cost of steel and cement, making them unaffordable. Strong stones will be sought in all countries in the way we currently explore for oil. Wherever these stones are found, giant high-tech quarries will be established, cutting out blocks using solar powered, computer controlled cutters. The quarries will produce hundreds of stone beams and columns an hour. Automatic X-ray fault identification will massively improve the reliability of stone products, reducing safety factors and permitting slender, high strength elements. These standardised products will be consigned to building sites regionally, minimising transportation. Post-tensioned stone will become as ubiquitous as steel and concrete today.
‘Stone is a new material in our technological context. It is a new material in search of a language’
This new low-carbon construction industry will enable us to build the buildings that people need without damaging the planet in the way it once did. It will break the dichotomy that we currently have between giving people the buildings they need to live and damaging our habitat with the processes of construction. It will unleash a new architecture that we can hardly imagine today.
Lead image credit: Glasshouse Images / Alamy