Built

by Roma Agrawal

  To turn a single brick into a usable structure, we need a special glue or mortar that can bind the units together to form a whole. The ancient Egyptians used the mineral gypsum to make a plaster (also known as plaster of Paris, since it was commonly found and mined in the Montmartre district of the city). Unfortunately, however, gypsum isn’t stable in the presence of water, so gypsum-sealed structures will eventually suffer damage and degradation. Fortunately, the Egyptians also used a different mixture that had lime mortars. This hardened and strengthened as it dried (and absorbed carbon dioxide from the atmosphere) and is more resilient than the gypsum recipe. When made correctly, mortars give strength to the structures they form and can last a very long time: parts of the Tower of London were built largely with lime mortar, and are still standing strong more than 900 years later.

  Other materials are often mixed into the mortar to give it different properties. In China, the mortar used to build The Great Wall had a small amount of sticky rice added to it. Rice is mainly composed of starch – this made the mortar bond well with the stone, but also allowed some flexibility, so it wouldn’t crack easily if the wall moved slightly as it heated and cooled with the seasons. The Romans added the blood of animals to their mortars, believing it helped the mortar stay strong when it was hit by frost. The dome of the Taj Mahal is held together with chuna, a mixture of burnt lime, ground shells, marble dust, gum, sugar, fruit juice and egg white.

  Bricks are used in most UK houses today because they are cheap. But they have their disadvantages. You need specialist labour to lay the units one at a time, and it’s a relatively slow process. And because of the standard size of the unit, you have less flexibility in the shapes of the structures you can create. Brick structures are also very weak in tension: the mortar glue between bricks, and the bricks themselves, can crack if pulled apart. Bricks can only be used in structures in which they are being compressed most of the time. They aren’t strong enough to carry the weight of taller structures (steel and concrete can take far more compression than brick, as we’ll see) so are impractical for, say, high-rise buildings or the larger bridges. However, their popularity remains where cost is the driver. Approximately 1.4 trillion bricks are made each year around the world; China alone manufactures about 800 billion, and India about 140 billion. LEGO, by comparison, makes a mere 45 billion or so bricks per year.

  This ancient building block, born from the earth and baptised by fire, is so versatile that it was used in the construction of pyramids, the Great Wall of China, the Colosseum, the medieval Castle of the Teutonic Order in Malbork, the famous dome of the Catedrale di Santa Maria del Fiore in Florence, and even my own house. I love that in our modern, fast-paced world, with all the technology we’ve developed, we continue to rely heavily on a building tool that has been in use for over 10,000 years, created from a material that was 50 million years in the making.

  METAL

  In Delhi in India, there is a pillar of iron that doesn’t rust. This column stands discreetly within the Qutb complex, a historic compound filled with extraordinary examples of Islamic architecture. The cavernous tomb of Iltutmish, in which every inch of the arched walls is decorated with loops and whorls, and the imposing Qutb Minar, a gracefully ridged, tapering tower – and at 72.5m the tallest brick minaret in the world – are simply breath-taking. At first glance, the dark grey column – about as thick as a tree trunk and barely seven metres tall – seems insignificant and out of place: a stray cat in a zoo of exotic animals. But it made a big impression on me.

  The pillar predates the architecture around it. It was made in around AD 400 by one of the kings of the Gupta dynasty, as an offering to Lord Vishnu, the Hindu god worshipped as the Preserver of the Universe. Originally it was topped with a statue of Garuda (Vishnu’s part-human, part-eagle steed, believed to be large enough to block out the sun). People consider it lucky if you can stand with your back to the pillar and wrap your arms around it so your fingers touch, but a fence now protects the monument from tourist limbs. I wasn’t interested in luck, though, I was fascinated by another peculiar property of the pillar: in defiance of its natural propensities, this iron hasn’t rusted in over 1,500 years.

  The iron pillar that never rusts at the Qutb complex, Delhi, India.

  The Iron Age followed the Bronze Age, which came to an end as copper and tin, the raw materials for making the metal, became difficult to obtain. The Iron Age is believed to have started around 1200 BC in India, and in Anatolia (modern-day Turkey). Archaeologists studying the ruins of Kodumanal, a small village in the middle of Tamil Nadu state in southern India, found a trench dating back to around 300 BC on the southern edge of the village. In this was a furnace that still contained some iron slag (a by-product left over from the smelting of metals). Indian iron – mentioned in the writings of Aristotle and in Pliny the Elder’s Historia Naturalis – was famous for its excellent quality. It was exported as far as Egypt for use by the Romans, but its secret recipe was carefully guarded.

  To build the Iron Pillar, the ancient Indians made discs of iron, which they then forged (heated up and hammered together), before striking and filing the outer surface to make it smooth. The iron used to forge the column was extraordinarily pure, except for the higher than usual amounts of phosphorus it contained; a result of the extraction process used by the ironmongers. It is the presence of phosphorus that prevents the pillar from rusting. Rust forms on iron when it is exposed to oxygen and moisture; at first, the metal would have corroded but, in the dry local climate of Delhi, the phosphorus was drawn to the interface between the rust and the metal surface, creating a very thin film. This film prevents air and moisture from reacting with the iron. And so the pillar hasn’t rusted any further. Modern steel is not made with those relatively high levels of phosphorus because the steel would become susceptible to cracking when it is ‘hot-worked’, which is a typical part of the manufacturing process where the metal is deformed at high temperatures. Take a look at structures made from iron or steel that are exposed to the atmosphere and you’ll see they are painted to prevent the formation of rust, which would weaken them. But the steel beams and columns in our air-controlled buildings are left unpainted – unless painting is necessary for fire protection – because the lack of humidity means they won’t rust much.

  While the ancients recognised the wonders of iron, it was mostly used to make household vessels, jewellery and weapons, because the iron they extracted was too soft to build with, and they didn’t know how to strengthen it enough to create an entire building or bridge. There are nonetheless rare examples of structures that use it: in A Record of Buddhistic Kingdoms, the Chinese monk Fa Hsien wrote about suspension bridges held up by iron-link chains in India around the time the pillar in Delhi was made. And the monumental marble gateway to the Acropolis in Athens, the Propylaea (built in around 432 BC), has iron bars to strengthen the ceiling beams. That’s how the ancient engineers used metal: in little snippets to help strengthen their stone and brick structures. Before iron (or its cousin steel) could be used in large-scale structures, scientists and engineers had to learn more about its character.

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  Bricks and mortar crack easily when pulled apart, but metals don’t. They are fundamentally different because of their molecular structure. Like diamonds, metals are made from crystals – but not large shiny ones like we see shimmering on the dresses of glamorous Bollywood actresses. Metal crystals are tiny – so small, in fact, that you can’t see them with the naked eye – and they are opaque.

  These crystals are attracted to each other, and this attraction bonds them together in a matrix or grid. However, when you heat up a metal, the crystals vibrate faster and faster until the bonds weaken. The metal then becomes malleable, and may even melt into a liquid if the temperature is high enough. Because of the flexibility of the bonds, metals are ductile, which means they can stretch and move to a limit without breaking; the process of hot-working mentioned above makes sure this characteristic is retaine
d. A thick plate of steel, say 100mm thick, can be rolled into a very thin sheet of 0.1mm thickness without splitting (like my pastry normally does). The matrix of crystals and the bonds between them can be softened, reshaped and moved around.

  Another property the bonds give metals is elasticity. If a metal is pulled or squashed by a force (within a certain range), it adjusts back to its original shape when the force is removed. It’s similar to when a stretched rubber band is released and returns to its normal size and shape – unless it’s overstretched, in which case it deforms. The same thing can happen to metals.

  In combination, these characteristics – the bonds, ductility, elasticity and malleability – make metals resistant to cracking. This gives them a very special property that makes them ideal for construction: they are good in tension. It was this property of metals that revolutionised the way we build. Before, structures had been designed mainly for compression, but now for the first time, we could create structures that could stand up to significant compression and tension.

  While pure iron is good in tension, it’s too soft to resist the immense loads in larger structures because the bond between its crystals is quite fluid and flexes. So engineers of the past could make decorative pillars, but pure iron was not strong enough for large, complex structures. It needed to be strengthened somehow. The crystals that make up iron are arranged in a lattice, so scientists and engineers began devising ways to stiffen it.

  One way to do this is to add atoms to the lattice. A simple (and tasty) illustration of this involves taking lots of Maltesers and rolling them under your hand on a table, during which you’ll find that they move around very easily. But if you then add a few chocolate-covered raisins to the mix, you won’t be able to roll them as easily as before. Okay, you can eat the experiment now, but the point is that the ‘impurities’ – the raisins – lodge themselves in awkward positions and stop the Maltesers from moving around as smoothly. Similarly, if carbon atoms are added to iron they jam the crystal lattice.

  There is a balance. Too few carbon atoms and the iron is still too soft. Too many, and the lattice becomes so stiff that it loses its fluidity and the material ends up very brittle, cracking easily. As if this wasn’t complicated enough, iron naturally contains some carbon (and other elements like silicon) as an impurity – usually too much – but the amount varies, so the quality of the iron varies. Scientists had great difficulty trying to determine precisely how much carbon to remove to create iron that was neither too soft nor too brittle. Results of their experiments include cast iron (which, being resistant to wear, is good for cooking pots, but is not used much in buildings because it’s brittle, like an Italian biscotti); wrought iron (which is not used much commercially any more, and which has a texture more like the soft, luxurious chocolate-chip cookies I used to eat as a child in America); and steel. While wrought iron was a decent enough building material – the Eiffel Tower is made from it – steel turned out to be the ideal compromise between strength and ductility. Steel is simply iron with about 0.2 per cent carbon content. The process of removing all but 0.2 per cent of the carbon was originally very expensive, so until someone worked out how to manufacture steel cheaply and on a large scale, it didn’t make a splash in the structural world. Engineer Henry Bessemer finally solved this long-standing problem and revolutionised the steel-making process, facilitating the development of railways across the world and allowing us to begin building skywards.

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  Henry Bessemer’s father, Anthony, ran a factory that manufactured typefaces for the printing press that he kept under lock and key. The protection was designed to safeguard his secrets from his competitors, but the young Henry often broke in to try and figure them out. Realising that his disobedient son was adamant about learning a trade, Anthony relented, and began training him in the factory. In 1828, when he was fifteen, Henry left school to work with his father. He loved it: he excelled at metalwork, had a natural talent for drawing and eventually began making his own inventions.

  During the Crimean War (1853–1856), Henry Bessemer turned his attention to the guns the French and British were using against the Russians. The principal drawback of these guns was that they could only fire one shot before they had to be reloaded. An elongated shell that could carry more explosive seemed like a valuable improvement, so Henry tested this in the garden of his home in Highgate, North London (much to the annoyance of his neighbours). The British War Office, however, wasn’t interested in his design, so he showed it to the French emperor, Napoleon Bonaparte, and his officers. Although impressed by the shells, the officers pointed out that the extra firepower would make their brittle cast-iron guns explode. As far as they were concerned, the shells were too big. Bessemer disagreed: the problem was the guns, not the shells – so he took on the challenge of finding a better way to make them.

  He decided to improve the quality of the iron being used to make the guns by developing another way of casting it. He set about formally experimenting in his homemade furnace, but the invention that made his name happened almost by mistake.

  One day, in his workshop, Bessemer was heating pieces of iron in a furnace. Even though he turned up the heat, a few pieces on the top shelf refused to melt. Frustrated, he tried blowing hot air into the top of the furnace, and then prodded the pieces with a bar to see if they had finally melted. To his surprise, they were not brittle like cast iron but instead were ductile and flexible. Noticing that they were the ones closest to the hot air, Bessemer realised that the oxygen in the air must have reacted with the carbon and other impurities in the iron – and removed most of them.

  The Bessemer Process, developed for producing steel on an economic scale, led to radical developments in the construction industry.

  Until now, everyone had tried to purify iron by heating it with coal or other fuels in an open furnace. Bessemer decided to use a closed furnace with a current of warm air running through it – and without using any fuel. This is like blowing hot air into a pan which has a lid covering it, rather than heating up an open pan on a gas hob. You would normally expect burning gas to create more heat than hot air, but this is not what happened.

  Bessemer must have watched cautiously as sparks emerged from the top of the furnace when the chemical reaction began. Then, a raging inferno started up – there were mild explosions and molten metal splashed around, erupting from the furnace. He couldn't even approach the machine to switch it off. Ten terrifying minutes later the explosions petered out. Bessemer discovered that what was left in the furnace was purified iron.

  The furnace inferno was the result of an exothermic reaction: a chemical reaction that releases energy – usually in the form of heat – during the oxidation of impurities. After the silicon impurities had been quietly consumed, the oxygen in the air current reacted with the carbon in the iron, releasing a huge amount of heat. This heat raised the temperature of the iron far beyond what a coal-fired furnace was then capable of, so Bessemer didn’t need to use external sources of heat. The hotter the iron became, the more impurities burned off, which made the iron hotter still, so it burned off even more impurities. This positive loop created pure, molten iron.

  Now having pure iron to work with, Bessemer found it easy to add back precisely the right amount of carbon to create steel. Until his invention, steel’s prohibitive manufacturing costs meant it was used to make cutlery, hand tools and springs, but nothing larger. Bessemer had just swept away that huge barrier.

  He presented his work at the British Association meeting in Cheltenham in 1856. There was huge excitement about his process because his steel was almost six times cheaper than anything else available at the time. Bessemer received tens of thousands of pounds from factories all over the country to replicate his process. But his lack of understanding of chemistry was nearly the end of him.

  When other manufacturers tried to reproduce Henry’s methods, they failed. Furious at the amount they had spent on the licence to use the process, they sued Bessemer,
and he returned all their money. He then spent the next two years trying to figure out why the process worked perfectly in his brick-lined furnace but not in others. Finally, he cracked it: the iron he was using contained only a small amount of phosphorus as an impurity. His peers, however, had been using high-phosphorus iron which, it appeared, didn’t work in a brick kiln. So Bessemer experimented with changing the furnace lining, and realised that replacing brick with lime was the answer.

  However, the perplexing and financially frustrating failure of his original process had bred a mistrust of Bessemer that meant no one believed him. Finally, he decided to open his own factory in Sheffield to mass-produce steel. Although it took a few years before suspicions faded, after that factories started manufacturing steel on a truly industrial scale. By 1870, fifteen companies were producing 200,000 tonnes of steel each year. When Bessemer died in 1898, 12 million tonnes of steel were being produced worldwide.

  High-quality steel transformed the railway networks because it could be made into rails quickly and cheaply, and they lasted ten times longer than iron rails. As a result, trains could be bigger, heavier and faster, clearing up the clogged veins of transport. And because steel was cheaper, it could now be used in bridges and buildings – ultimately opening up the sky.

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  Without Bessemer’s steel, I wouldn’t have been able to design the Northumbria University Footbridge, which literally hangs on steel’s ability to carry tension. The bridge was, in fact, the very first structure I worked on, fresh out of university. I can still vividly remember the first day of my brand new job, taking a packed Tube train to Chancery Lane in London, and being swept up and out of the station by the hurrying throngs of other professionals in suits. Feeling excited, nervous and a little awkwardly formal, I threaded my way along the pavement towards my destination – a five-storey office building clad in white stone.