Notes

Materials that Changed History: Metals and Alloys

Uncover the properties and reactivity of metals that matter to the story of humankind

1. From the Stone Age to the age of metals

How The Flintstones thinks we spent the 2 million years during the Stone Age

Metals are humankind’s new toy. It is new compared to flints and stones, which were already in use 2 million years ago to make tools. Then, we stoned. It was only until 10000 years ago that we finally learned how to use metals as a new material.

The adoption of metal was so revolutionary that we call the long period before it the Stone Age and the period that follows the Metal Ages.

2. The dawn of the Metal Ages: unearthing copper

One of the oldest copper objects discovered to date in the Middle East (Garfinkel et al, 2014)

Copper was one of the first metals known to humans. This was not a coincidence, but because of its unique chemistry.

Compared to most metals, copper is less reactive. It does not react with water or acid to form compounds. This means that copper can sometimes exist naturally as a free element in the uncombined state. Humans simply had to unearth it, and we got lucky around 10000 years ago.

Conversely, more reactive metals like iron and aluminium exist as compounds in nature. It was hard to revert them back to pure metal, not with the technology thousands of years ago.

Copper is less reactive than most metals. It does not react with water or hydrochloric acid.

3. Bronze Age: going from strength to strength by mixing metals to form alloy

Bronze spear point excavated in Ban Chiang, Thailand (© ISEAA, 2018)

Soon after humans adopted copper, we got curious about how to change the properties of metals. Around 6000 years ago, we began to mix copper with tin. And voila, we got bronze.

Bronze is an alloy, which is a cheem word for a mixture of a metal with other elements.

Compared to pure copper, bronze is much harder and stronger. This allowed metalworkers to construct more ornaments and tools. In Southeast Asia, early metalworkers were making bronze jewellery and weapons around 4000 years ago.

A pure metal and its alloy have very different physical properties. Beyond their different colours, their atoms have dramatically different lattice arrangement.

In fact, all alloys are stronger than pure metals. This means that they can withstand a larger force before they deform or break.

We can explain this property by comparing the lattice structure of pure metal and alloys. Alloys have a mixture of atoms of different sizes. This disrupts their orderly arrangement, preventing the atoms from sliding when we apply a force.

Alloys are a mixture of a metal with other elements. The atoms of different sizes have a less orderly arrangement, preventing the layers from sliding when force is applied.

4. Iron Age: reducing metal oxides with carbon

Around 3000 years ago, humans got hold of iron. Unlike copper, iron is more reactive. So it is almost always found as the compound iron(III) oxide in ores.

2Fe2O3 + 3C ⟶ 4Fe + 3CO2

It was a breakthrough when metalworkers of the day learned how to extract iron. By luck or trial-and-error, they heated iron ores with charcoal to free up the metal. Charcoal contains carbon, which reduces iron(III) oxide to iron.

However, carbon is a so-so reducing agent. It can reduce iron(III) oxide only because iron is not too reactive. It works even better for oxides of less reactive metals, like copper(II) oxide.

Less reactive metals form oxides that are less stable, which can be easily reduced by carbon.

5. Modern metals: isolating the most reactive metals with electrolysis

The original sample of sodium that Humphry Davy isolated (© Paul Wilkinson)

Carbon bullies the weak, but fears the strong and reactive. It cannot reduce oxides of more reactive metals from Group I, II and III. So humanity had to wait until the discovery of electricity, before we could use electrolysis to extract highly reactive metals.

In the early 1800s the English chemist Humphry Davy isolated potassium, sodium, calcium, and magnesium for the first time. What an overachiever.

Aluminium was a latecomer. It was only isolated in the 1820s by the Danish chemist H.C. Ørsted and the German chemist Friedrich Wöhler.

Metals in Group I, II and III are generally more reactive than transition metals. They react to form stable metal oxides, and can only be reverted back to pure metal by electrolysis.

6. Old metals, new functions

Photo by Kateryna Babaieva on Pexels.com

It is incredible how a lot has changed since copper was first used in the Metal Ages, even though metal is a much newer material compared to stones and ceramics.

We have got smarter, with new techniques to discover new metals and alloys. We have got faster, with big furnaces in even bigger factories to extract metal at a breakneck speed. But we have also got greedier that even a pandemic cannot quite slow the increase in copper demand.