In this chapter, we’ll explore how metals have transformed human society from the Bronze Age to the Internet Age, and how the Earth provided them for us.
ENTER THE BRONZE AGE
The first metal we smelted for crafting tools and weapons was copper. Copper ores are often easy to spot – containing minerals coloured an attractive blue or green – and the metal is easy to smelt: it can be extracted by roasting lumps of the ore with charcoal in the same sort of kiln that is used for firing pottery. The burning charcoal provides both the high temperature needed, and also the ‘reducing’ chemistry to strip the metal from the oxide, sulphide or carbonate it is bonded with in the rock to leave pure copper.
The problem with pure copper is that it is a pretty soft metal: the edges of tools hammered from it dull easily and must be constantly resharpened. A far superior material is offered by mixing copper with another metal to make an alloy – bronze. When larger atoms are interspersed among those of copper they stop the metal being as pliable: they essentially jam the layers of copper atoms from slipping past each other so easily, making the metal mixture harder and more durable. The earliest bronze to be made was an alloy of copper and arsenic, but this mixture was improved on with the copper-tin bronze that was first produced in Anatolia and Mesopotamia in the late fourth millennium BC and then spread to Egypt, China and the Indus Valley.1 One particular advantage of copper-tin bronze is that it melts at a much lower temperature and does not bubble, and so can easily be poured into a mould for casting.2 This enables craftsmen to form any shape of implement required, and then repair or even recast it when it becomes worn or gets broken.3 Bronze soon became the standard material for making ceremonial objects, cooking utensils, agricultural tools and weapons.4 The Neolithic had given way to the Bronze Age.
The pioneering use of bronze in Mesopotamia is something of a surprise as this region does not have its own sources of tin, so this crucial ingredient of the alloy would have been traded over long distances. The tin used in western Eurasia during the Bronze Age came from mines in the Erzgebirge Mountains along the modern German–Czech border,5 from Cornwall and, to a small extent, from Brittany. The mines in Cornwall in particular came to offer the ancient world a large supply of the tin they needed. Ores of this vital metal were created when granite magma intruded up into sedimentary rock layers. The heat of this bulk of magma drove hydrothermal systems underground, with the circulating hot water dissolving metals from the surrounding volume and then redepositing them in fissures and fractures in the overlying rock as veins of rich ore.6
We know that tin was traded by sea from Northern Europe by the Phoenicians sailing through the Strait of Gibraltar from about 450 BC, and before this along overland trade routes to the Fertile Crescent.7 And as tin was scarce in the ancient world, it would have commanded a high price. Copper ore, on the other hand, was more widely distributed, and our planet has made it available to us through a particularly intriguing process.
FROM SEA FLOOR TO MOUNTAIN TOP
Bronze Age craftsmen in the Mediterranean, Egypt and Mesopotamia drew heavily on copper mined from Cyprus.8 Indeed, the island gave its name to the Latin word for copper – cuprum – and hence its modern element symbol, Cu. We saw in Chapter 4 how the geological set-up of the Mediterranean created the perfect milieu for thriving seafaring societies, and the tectonic processes in this corner of the world also provided the crucial raw materials for building civilisations during the Bronze Age.
Copper, along with other metals like zinc, lead, gold and silver, is deposited in high concentrations at mid-ocean spreading ridges,9 where tectonic plates are being heaved apart and magma wells up to form new oceanic crust. Right along the length of these opening cracks in the shell of the Earth hot magma oozes very close to the surface. Seawater trickles and soaks down through the rocks of the sea floor where it meets this magma and becomes superheated. It then surges back up through the crust, leaching minerals out of the surrounding rock as it goes, before it gets squirted forcibly back out of the seabed in hydrothermal vents. When this mineral-rich hot fluid hits the frigid ocean water, particles of metallic sulphide minerals precipitate in a churning, thick, inky dark plume, giving such hydrothermal vents their far more evocative nickname: black smokers. These black smokers form clusters of tall chimney-like structures, like some Gaudí-inspired industrial landscape, in the pitch-black ocean depths.
Black smokers serve as oases on the barren deep sea floor for some of the most extreme life forms on Earth. These exotic communities living far beyond the reaches of sunlight include giant two-metre-long tube worms that were utterly new to scientists when the first black smoker fields were discovered by submersible in the late 1970s, as well as pale-white shrimp, snails and crabs. These sunless ecosystems are powered by microbes able to grow on inorganic energy sources like the metals and sulphides spewing out of the vents.
The particles spurting into the ocean settle back down to smother the area around the vents in high concentrations of valuable metals – copper, cobalt, gold and others – deep on the sea floor, but they are currently inaccessible to miners. It takes special circumstances to make these metal deposits available to us.
As we have seen, at convergent plate boundaries, where two tectonic plates butt head to head against each other, thick layers of sea-floor sediments become folded into mountain ranges. As a result, fossils of sea creatures are commonly found in the mountain peaks of the Himalayas or Alps, for example. These were once ascribed to mythological events like Noah’s great flood, until we came to understand the awesome earth-moving power of plate tectonics. But the oceanic crust itself, holding ancient black smokers, is composed of dense, basaltic rock and is almost always subducted beneath the lighter continental crust and swallowed into the depths of the Earth. Every now and then, however, rare slivers of oceanic crust escape being dragged down, and are instead squeezed up and pushed over the continental crust.10 This appears to happen more frequently with the smallest plates, just as in the Mediterranean with its plate fragments that got caught between Africa and Eurasia as the two crunched together. And this is exactly what has happened on the island of Cyprus.
The oval-shaped mound of the Troodos mountains in the centre of Cyprus is the best example in the world of an ophiolite – a slice of oceanic crust that became beached on top of the continental.11 This oceanic crust was created in deep water about 90 million years ago, at a spreading rift in the Tethys Sea,12 and became scooped up on top of Cyprus with the closure of the Tethys as Africa pushed up into Eurasia. The Troodos mountains haven’t been significantly deformed and so this ophiolite shows a beautifully preserved cross-section of the layers within the oceanic crust;13 there are even recognisable fossils of tube worms and snails alongside the ancient hydrothermal vents. Troodos is like a gently mounded layer cake, and as the mountains eroded down, these layers have been exposed in concentric rings. The highest peak in the centre is made up of mantle rocks that would normally be found over 10 kilometres beneath the sea floor.14
The Troodos ophiolite offers geologists a perfect opportunity for studying how new ocean crust is formed at a spreading rift (which is of course tricky to watch in action at current constructive plate boundaries like the Mid-Atlantic Ridge). But for Bronze Age civilisations it also made conveniently accessible the metals that had been spewed out by ancient deep-sea black smokers. With a chunk of oceanic crust dumped onto land, miners on Cyprus were able to dig into the relevant level on the mountainside where the metal deposits were located. Indeed, Troodos offered fabulously enriched ores, containing up to 20 per cent copper.15
From the second millennium BC Cyprus became the major supplier of copper to Mesopotamia, Egypt and the Mediterranean world.16 As we have seen, in the Bronze Age charcoal was used to bake the metal out of its ore in copper smelting furnaces, and so Cyprus’s productivity also depended on a large supply of timber. In fact, by studying the 4 million tonnes of slag heaps on the island that were dumped as waste once the copper metal
had been extracted from the mined rock, archaeologists have been able to calculate how much timber was required. It turns out that throughout the 3,000 years of copper production on Cyprus, the entire area of pine forests covering the plains and the mountainsides of the island would need to have been cleared at least sixteen times over17 – an early example of sustainable woodland management.18
Much of the Cypriot copper was traded by the Minoans, the first major civilisation in Europe.19 Based on the island of Crete, but with trading posts across the eastern Mediterranean, the Minoans thrived for over a millennium from about 2700 BC.20 We don’t know what these people actually called themselves – the term Minoan was coined by archaeologists in the early twentieth century after the Greek myth of King Minos (with his labyrinth and minotaur) who was believed to have lived on Crete.21 The Minoans built large multi-storey palace complexes and were also expert at water storage and distribution, enjoying well-developed wells, cisterns and aqueducts long before the Romans – and the world’s first flushing toilet at the royal palace in Knossos.22 But above all, they were master bronze-workers and mariners, spreading their cultural influence through their prowess at sea and trade networks that stretched across the eastern Mediterranean.23 Most of the bronze artefacts and tools that the Minoans produced and exported were made with copper mined on the nearby island of Cyprus. The Minoan civilisation grew rich by trading this metal wealth and shipping it around the known world. But as we have seen in the case of Iran, enjoying the spoils of plate tectonics can have a nasty flip side.
The same subduction boundary that created the rich copper deposits on Cyprus runs past Crete, forming a deep trench lying just 25 kilometres south of its shores. One consequence of subduction is that the plunging plate releases blobs of molten rock that rise back to the surface to feed an arc of volcanoes. This row of volcanoes forms directly above the melting point in the mantle and so appears on the surface at a characteristic distance downstream from the subduction line. The Hellenic arc is located about 115 kilometres north of the Cretan trench, and here the volcanic peak of Thera – known today as Santorini – poked above the lapping waves of the Aegean Sea. This active volcano has been erupting sporadically for thousands of years, but at some time between 1600 and 1500 BC Thera abruptly detonated in one of the most violent eruptions in history.
The eruption almost completely destroyed Thera – the submerged caldera left behind is a mere husk of the original mountain – and the vast plume of pulverised rock hurled up into the sky covered Crete entirely with ash. Ports like Amnissos on the northern coast, facing Thera across 100 kilometres of open sea, were devastated, buried by volcanic pumice rock swept there by a tsunami triggered by the explosion. But much like the eruption of Vesuvius destroying the Roman cities of Pompeii and Herculaneum one and a half millennia later, for archaeologists the catastrophe served to capture a snapshot of Minoan life at the time, preserving their distinctive writing, ceramics, artworks and architecture.fn1
It seems that this catastrophic explosion didn’t coincide exactly with the collapse of the flourishing Minoan civilisation, although the precise dating of either event is difficult.fn2 But what is clear is that within a few generations of the Theran eruption, Minoan society was in terminal decline: its palaces were destroyed25 and the island succumbed to invasion by the Mycenaean Greeks.26 What had made the Minoans so successful was their maritime proficiency and trade and so the sudden loss of much of their fleet and ports to the tsunami following the eruption, as well as the destruction of their major trading port of Akrotiri on Thera itself, would have hit their economic infrastructure hard. It’s likely too that the Minoans suffered acute food shortages and even famine with the loss of their fishing boats and the flooding of their agricultural fields by seawater.27 The natural disaster shifted the balance of power in the region and left Crete vulnerable to Mycenaean conquest. But it was the Phoenicians, inhabiting the strip of land that is now Syria, Lebanon and Israel, who came to dominate the shipping in the Mediterranean (see here).28
The Troodos Mountains on Cyprus from which the Minoans had sourced their copper form a large, accessible and exceptionally well-preserved ophiolite, but they are not unique. As plate collision closed up the Tethys Sea to create the Mediterranean, other slithers of ancient oceanic crust were squeezed out on top. Ophiolite metal deposits can also be found in bands around the rim of the Mediterranean, within the Alps, Carpathians, Atlas and Taurus mountain ranges. And around the world other ocean-closing events have also thrown up oceanic crust. Some of the largest mines today, such as Rio Tinto in Spain, Noranda in Canada and those along the Ural Mountains in Russia, dig into rich black-smoker metal deposits of copper, zinc, lead, silver and iron.29
Copper-tin bronze provided humanity with metal tools, utensils and weapons for around two millennia, before it was superseded by a far superior metal – iron.
FROM WROUGHT IRON TO STEEL
In fact, we have been using iron for tens of thousands of years, not for its metallic properties but as a colourful pigment to adorn and express ourselves. Ochre can vary in colour from brown through yellow to vibrant red, depending on the exact iron oxide mineral and how much water it contains in its structure. We’ve ground the various forms of ochre into a powder to decorate our body and colour our hair, and made it into a paint for rock and cave art from at least 30,000 years ago. And it appears that our human species wasn’t the first to have used these natural colours: ochre has also been found alongside flint artefacts in Neanderthal sites dating back over 200,000 years.30
What was truly transformative in the history of civilisation, however, was when we learned to extract pure metallic iron from these rust-coloured oxide ores. As we have seen, although there were a number of copper sources, tin was very scarce throughout the Bronze Age. Iron, on the other hand, is available in huge deposits and widely distributed around the world. But the reason that iron came to be exploited later than copper and bronze is that it is so much harder to tease this metal from its rocky ore.
The first furnace developed to smelt iron was the bloomer, where iron ore and charcoal were fired together but not at temperatures high enough to cause the iron to melt and flow away from the slag. Instead, the hot but still solid, spongy lumps – or ‘blooms’ – of iron mixed with slag were removed from the furnace and then hammered to separate the metal as pure wrought iron. ‘Wrought’, the archaic past particle of the verb ‘to work’, is an appropriate term: it takes an enormous amount of back-breaking labour with hammer and anvil to refine a bloom into pure iron. Iron smelting and smithing in this way was established in Anatolia by around 1300 BC.
A later development was to build much taller furnaces and pump a stream of air up from the bottom with bellows, to attain higher temperatures for melting the iron. This is the blast furnace. Adding limestone as a ‘flux’ helps the slag to flow, improving the separation of the iron and removing impurities. The molten metal can then be drained out of the base of the furnace as pig iron, or cast iron. Cast iron is high in carbon (around 3 per cent), which makes it strong but brittle. The first blast furnaces were operated as early as the fifth century BC by the Chinese, who in the first century AD were also the first to drive the bellows with waterwheels.31 Blast furnaces and cast iron were adopted by the Arabs in the eleventh century, but didn’t arrive in Europe until the late 1300s.32
Beginning at different times in regions around the world, the Iron Age transformed society. Bronze had remained relatively expensive, and so to a large extent it was the preserve of the ruling elites, or was used to equip the armies they pitched against each other. Iron ores, on the other hand, are plentiful and offer a general purpose metal for a whole range of practical artefacts. Iron implements are also much more durable and better at holding a sharp edge than those made of bronze. This was important not just for weapons and armour, but also for everyday tools. Iron axes made a huge difference for clearing forests to open up new areas of farmland. And iron-tipped ploughs not only increased the
productivity of existing agriculture but enabled humanity to transform into fields land that was previously uncultivable. Both these tools opened up whole new regions for settlement.
In particular, the development from the late third century AD of the heavy mouldboard plough, with an iron cutting blade at the front of the ploughshare, made productive agriculture possible in the dense soils of the European landscape north of the Alps. Rather than just scratching a groove into the soil, the heavy plough slices deep into the sod and then flips it over round the curved mouldboard. The effect is to essentially turn upside down the entire topsoil, helping with weed control and mixing in fertiliser, and the furrows also greatly improve drainage of clay soils prone to waterlogging.33 With this iron-made innovation the dense clay soils of Northern Europe became far more productive than the sandy soils around the Mediterranean. So with the help of iron axe and plough, the rolling North European plains were gradually transformed from post-Ice Age forests and waterlogged meadows into a great swathe of grain fields.34 This in turn drove a fundamental shift in the population distribution and urbanisation of Europe over the subsequent centuries.35
If the material properties of copper are improved by mixing it into an alloy, the same is true of iron. Steel is an alloy of iron with a small amount of carbon, typically 1 per cent or less, and therefore sits midway in carbon content between pure wrought iron and cast iron. And like bronze, steel alloy is far harder than the pure metal. The exact properties of the steel can be tuned by varying the content of carbon: from soft but tough low-carbon steel to hard but brittle high-carbon steel. Over the centuries, metalworkers have developed a variety of techniques to achieve the desired amount of carbon: cooking wrought iron with charcoal so that it absorbs a little more carbon, or mixing proportions of wrought and cast iron. But high-quality steel remained laborious to create and so was reserved for critical applications like the cutting edge of knives and swords, or where its flexibility was required in small components, such as the springs in clocks.
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