Metals and materials

Pick up a gold ring and it is heavier than it looks. How much mass a material packs into a given volume is called its density. A cubic centimetre of water has a mass of one gram. A cubic centimetre of aluminium has 2.7 grams, of iron 7.9 and of gold 19.3.

The measurement is old. In the eleventh century the scholar al-Bīrūnī built a flask with a spout in its neck and filled it to the spout. A sample dropped in pushed out its own volume of water, which he caught and weighed. He found gold to be about nineteen times as dense as water, very near the modern figure.

Density helps decide what a metal is used for. Aircraft and drink cans are made of aluminium because it is about a third as dense as steel, so the same shape weighs about a third as much.

Gold can be picked out of a stream bed as shining nuggets. Nobody has ever found a nugget of aluminium, though aluminium is millions of times more plentiful in the Earth’s crust. The reason is that metals differ enormously in how readily they react with other substances, such as oxygen, water and acids.

Chemists rank them in a list called the reactivity series. At the top are metals such as potassium and sodium. Potassium is so reactive that a piece dropped on water bursts into flame. Lower down come aluminium, zinc and iron, then copper and silver, and at the bottom, gold. Gold reacts with almost nothing, so it lies in the ground as the pure metal. A reactive metal such as aluminium combined with oxygen long ago, and it lies in rock locked into compounds, joined chemically to other elements.

The series is more than a list. It predicts what happens when metals meet. Stand an iron nail in a blue solution of copper sulfate, and within minutes the nail is coated with pink-brown copper. Slowly the blue fades. Iron is higher in the series than copper, so it has taken copper’s place in the compound and pushed the copper out as metal. This is called a displacement reaction. Try it the other way, copper in a solution of an iron compound, and nothing happens. A more reactive metal displaces a less reactive one, and never the reverse.

Acids give a second test. Hydrogen, though no metal, has a place in the series, between lead and copper. A metal above it, such as zinc or iron, pushes the hydrogen out of an acid as a fizz of gas. Copper, silver and gold lie below hydrogen and cannot push it out.

Because most metals are reactive, most of them are found combined with other elements, usually oxygen or sulfur. A rock that holds enough of such a compound to be worth mining is called an ore. Haematite, the main ore of iron, is a compound of iron and oxygen, much the same substance as rust. Bauxite is the ore of aluminium.

Getting the metal out of its ore is called extraction, and it always means the same thing: something must take the oxygen or sulfur away from the metal. How hard that is depends on the metal’s place in the reactivity series. A metal low in the series lets go easily. A metal high in the series holds on, and the higher it is, the more energy its extraction takes. This is why humanity’s metals arrived in order: copper first, then iron, and aluminium only in the nineteenth century.

Iron sits in the middle of the series. Carbon, though no metal, has a place there too, a little above iron, and it is cheap. Ironmaking rests on that. Heat iron ore with carbon and the carbon takes the oxygen, leaving the iron.

It is done in a blast furnace, a tower as tall as a ten-storey building. Iron ore and coke, a form of carbon made from coal, are fed in at the top, and hot air is blasted in at the bottom. The coke burns and raises the temperature above 1,500 °C. In that heat the carbon, working mostly through a gas it forms as it burns, pulls the oxygen off the iron and carries it out of the furnace. Molten iron runs to the bottom and is tapped off. A large furnace runs day and night for years and can make ten thousand tonnes a day.

Aluminium is above carbon in the series, so carbon cannot take its oxygen, however hot the furnace. In the middle of the nineteenth century the only ways of freeing it were so costly that aluminium was dearer than gold.

The answer came in 1886. Aluminium is more reactive than carbon, but electricity can do what carbon cannot. The ore is purified to aluminium oxide, which is dissolved in a bath of molten mineral, and a very large electric current is passed through it. The current pulls the compound apart, so that aluminium gathers at one terminal and oxygen at the other. Splitting a compound with electricity is called electrolysis, and sodium, higher still in the series, is made the same way.

It takes a great deal of electricity. Melting down used aluminium takes about a twentieth of the energy, which is why recycling drink cans is worth the trouble.

Pure metals are often too soft to be useful. Pure gold is too soft for a ring that is worn every day, and pure iron bends easily. The softness comes from the same thing that makes metals malleable: the layers of identical atoms slide over one another.

Now mix in atoms of a different size. A metal blended with small amounts of other elements is called an alloy. The foreign atoms sit among the regular layers like pebbles in a deck of cards. They block the layers sliding, and the metal becomes harder and stronger.

Almost every metal object you own is an alloy. The gold in a ring is mixed with copper or silver, and a coin is a blend of several metals. By choosing what to add and how much, a metalworker can tune a metal to be hard, springy or resistant to rust.

Three alloys changed history. The first was bronze, copper mixed with about a tenth of its weight in tin. It is harder than copper and easier to cast, and from about 3300 BC it gave its name to an age.

The second is steel. Add a little carbon to iron, usually less than one part in a hundred, and the soft metal becomes strong enough for bridges, ships and rails. Steel is by far the world’s most-used metal.

The third is younger. Ordinary steel rusts. In 1913 the English metallurgist Harry Brearley made a steel with about an eighth of its weight in chromium and found that it would not stain. Chromium reacts with the air to form a thin, invisible skin of oxide that seals the surface, and if the skin is scratched it grows back. This is stainless steel.

Extraction takes a metal out of its compound. Left to itself, the metal slowly goes back. The gradual eating away of a metal by reaction with its surroundings is called corrosion, and for iron it has a special name, rusting.

Rusting needs two things at once, oxygen and water. In dry desert air an iron tool can last for centuries, and a nail sealed in boiled water, which holds no air, stays bright. Give iron both, as in damp air, and it rusts. Salt speeds the process, which is why cars rust fastest near the sea.

Rust is iron oxide with a little water bound in, much the same compound as iron ore. The trouble with it is that it is flaky. It crumbles away and exposes fresh metal beneath, so the rusting does not stop until the iron is gone.

Since rust needs oxygen and water, the simplest defence is a barrier. A coat of paint on a bridge, oil on a bicycle chain and grease on a hinge all work the same way: each keeps oxygen and water out. The weakness of a barrier is that one scratch lets the rust in.

A thin layer of another metal makes a tougher barrier. In electroplating, the object is hung in a solution of a metal compound, and an electric current deposits an even coat of that metal on it. A tin can is steel plated with tin.

Aluminium needs no help. It is more reactive than iron, yet an aluminium window frame lasts for decades untreated. Within moments of meeting the air its surface forms a thin oxide layer, and this layer does not flake. It clings and seals the metal beneath.

There is a cleverer defence than a barrier, and it uses the reactivity series. Zinc is more reactive than iron. Coat iron with zinc and the zinc keeps air and water off, like any coating. But scratch it, and something better happens. As long as zinc is touching the iron, the zinc corrodes first and the iron beside it is spared. Iron or steel coated with zinc is said to be galvanised, and that is why farm gates, buckets and crash barriers last for decades.

The same idea works without a coating. Blocks of zinc are bolted to the steel hull of a ship below the waterline. In the sea water it is the zinc that corrodes, in the hull’s place. Such a block is called a sacrificial anode, and when it has been eaten away a new one is bolted on.

One idea ties this reading together. When a metal reacts, its atoms give away electrons. In the simple sense, oxidation means joining with oxygen. Chemists use the word more widely, for any loss of electrons. The opposite, a gain of electrons, is called reduction.

The two always come as a pair, since electrons given up by one substance must be taken by another. A reaction of this kind is called a redox reaction, from reduction and oxidation. When iron rusts, the iron is oxidised and the oxygen is reduced. In a blast furnace it runs the other way: the iron in the ore is reduced back to metal, and the carbon is oxidised.

So the reactivity series is at bottom a ranking of how readily each metal gives up its electrons. Potassium does so most readily and gold least, and everything in this reading follows from that order.