Feet of clay? The science of statues

Concept art for the Terry Pratchett statue (c) Paul Kidby

Concept art for the Terry Pratchett statue (c) Paul Kidby

Yesterday we received the exciting news that a statue to commemorate Sir Terry Pratchett and his work has been approved by Salisbury City Council. Hurrah! So, even if we don’t quite manage to get octarine into the periodic table (and thus into every science textbook for ever more), it’s looking very likely that there will still be something permanent to help keep his memory alive.

But this got me thinking about everyday chemistry (who am I kidding, I’m always thinking about everyday chemistry!) and, in particular, bronze – the material from which the statue will be made.

Bronze, I hear you say, what’s that good for apart from, well, statues? And maybe bells? Is it really that interesting?

Well, let’s see. Bronze is an alloy. Alloys are mixtures that contain at least one metal, but they’re stranger than the word ‘mixture’ might perhaps suggest. Imagine combining, say, sand and stones. You still be able to see the sand. You could see the stones. You could, if you could be bothered to do it, separate them out again. And you’d expect the mixture to behave like, well, stony sand.

Alloys aren’t like this. Alloys (other well-known examples include steel, brass and that silver-coloured stuff dentists use for filling teeth) look, on all but the atomic level, like pure metals. They’re bendy and shiny, they make pleasing ringing sounds when you hit them and they’re good electrical conductors. And unlike more simple mixtures, they’re difficult (though not impossible) to separate back into their constituents.

Perhaps the most interesting this about alloys is that their properties are often very different to any of the elements that went into making them. Bronze, in particular, is harder than either tin or copper, and hence The Bronze Age is so historically significant. Copper is one of the few metals that can (just about) be found in its pure form, and so is one of the oldest elements we know, going back at least as far as 9000 BC. But while quite pretty to look at, copper isn’t ideal for making tools, being fairly soft and not great at keeping an edge. Bronze, on the other hand, is much more durable, and was therefore a much better choice for for building materials, armour and, of course, weapons. (War, what is it good for? Er, the development of new materials?)

Hephaestus was the God of fire and metalworking; according to legend he was lame.

Hephaestus was the God of metalworking. According to legend he was lame, could it have been because of exposure to arsenic fumes?

Today we (well, chemists anyway) think of bronze as being an alloy of tin and copper, but the earliest bronzes were made with arsenic, copper ores often being naturally contaminated with this element. Arsenical bronzes can be work-hardened, and the arsenic could, if the quantities were right, also produce a pleasing a silvery sheen on the finished object. Unfortunately, arsenic vaporises at below the melting point of bronze, producing poisonous fumes which attacked eyes, lungs and skin. We know now that it also causes peripheral neuropathy, which might be behind the historical legends of lame smiths, for example Hephaestus, the Greek God of smiths. Interestingly, the Greeks frequently placed small dwarf-like statues of Hephaestus near their hearths, and this is might be where the idea of dwarves as blacksmiths and metalworkers originates.

Tin bronze required a little more know-how (not to mention trade negotiations) than arsenical bronze, since tin very rarely turns up mixed with copper in nature. But it had several advantages. The tin fumes weren’t toxic and, if you knew what you were doing, the alloying process could be more easily controlled. The resulting alloy was also stronger and easier to cast.

teaspoon in mugOf course, as we all know, bronze ultimately gave way to iron. Bronze is actually harder than wrought iron, but iron was considerably easier to find and simpler to process into useful metal. Steel, which came later, ultimately combined superior strength with a relatively lower cost and, in the early 20th century, corrosion resistance. And that’s why the teaspoon sitting in my mug is made of stainless steel and not some other metal.

Bronze has a relatively limited number of uses today, being a heavy and expensive metal, but it is still used to make statues, where heaviness and costliness aren’t necessarily bad things (unless, of course, someone pinches the statue and melts it down – an unfortunately common occurrence with ancient works). It has the advantages of being ductile and extremely corrosion resistant; ideal for something that’s going to sit outside in all weathers. A little black copper oxide will form on its surface over time, and eventually green copper carbonate, but this is superficial and it’s a really long time before any fine details are lost. In addition, bronze’s hardness and ductility means that any pointy bits probably won’t snap off under the weight of the two-millionth pigeon.

So how are bronze statues made? For this I asked Paul Kidby, who designed the concept art for the statue. He told me that he sculpts in Chavant, which is an oil-based clay. It’s lighter than normal clay and, crucially, resists shrinking and cracking. He then sends his finished work away to be cast in bronze at a UK foundry, where they make a mould of his statue and from that, ultimately (skipping over multiple steps), a bronze copy. Bronze has another nifty property, in that it expands slightly just before it sets. This means it fills the finest details of moulds which produces a very precise finish. Conveniently, the metal shrinks again as it cools, making the mould easy to remove.

And just for completeness, Paul also told me that the base of the statue will most likely be polished granite, water jet cut with the design of the Discworld sitting on the back of Great A’Tuin. I can just imagine it – it’s going to be beautiful.

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Gold! Bright and yellow, hard and cold

200px-Gold-49956Let’s talk about element number 79.  It’s one of the oldest known elements, used for quite literally thousands of years.  It’s constantly at the heart of conflicts and politics.  Poets have waxed lyrical about it, authors have written about it, economists and prospectors have hinged their livelihoods on it.  And, of course, chemists have studied it.

As an element it’s unusual.  It’s a metal, but instead of the boring silvery-grey of most metals it glows a warm yellow.  It’s also one of the most unreactive elements, and yet has found use a catalyst – speeding up chemical reactions that otherwise would be too slow to be useful.  It’s rare, making up only about 0.004 parts per million of the Earth’s crust, and yet its annual production is surprisingly high: 2700 tonnes in 2012.  Its density makes it heavy – weighing over nineteen times more than the same volume of water – but it’s also relatively soft, so soft that it’s possible to scratch a pure piece with your fingernail (in theory, and if you have fairly robust fingernails).

Yes, gold.  Chemical symbol Au, from its latin name aurum meaning ‘shining dawn’ or ‘glow of sunrise’ (how lovely is that?)

The history of gold is fascinating.  You could easily write a whole book about it.  In fact, someone has.  I won’t attempt anything so ambitious, but it does have some very interesting chemical stories associated with it.

Because of its unreactivity, gold is one of the relatively few elements that’s found uncombined in nature.  In other words, you can pick up a piece of pure gold from the ground or, more likely, out of a river bed.  Thanks to this property it’s very probably the first metal that humans as a species interacted with.  It’s too soft to be much use as a tool, so its earliest uses were almost certainly ornamental.  Decorations and jewellery had value and could be traded for other things, and ultimately (skipping over a chunk of history and early economics) this led to currency.

And so it was that early alchemists, some two thousand years ago, became obsessed with the idea of a quick buck.  Could other metals be turned into gold?  They searched long and hard for the mythical philosopher’s stone (like in Harry Potter, only not exactly) which could turn base metals into gold or silver.  Of course they never found it, because it doesn’t exist.  It’s not possible to change one element into another during a chemical reaction.  This is because what defines an element is the number of protons in its nucleus, and chemistry is all about the electrons. Chemical processes don’t touch protons, which are hidden away in the nuclei of atoms.

But where there’s a will there’s almost always a way.  Two millennia after alchemists were hunting for a magical stone, the chemist Glenn Seaborg managed to transmute a minute quantity of lead, via bismuth, into gold by bombarding it with high-energy particles.  Apparently, these days particle accelerators ‘routinely’ transmute elements, albeit only a few atoms at a time.

The trouble is, this method costs a fortune – way, way more than the value of any gold produced.  Gold, after all, is ‘only’ worth about a thousand pounds for a troy ounce (31 grams).  Particle accelerators cost billions of pounds to build, and yet more in running costs.  If you really want gold so desperately, these days there may be more mileage in harvesting it from defunct bits of electronic equipment.

Or just ask people to send you their old jewellery through the post in exchange for cash.  Even Tesco have got into that game now.  Through the post!  Honestly, people fear putting a tenner in a birthday card but gold jewellery in a paper bag?  No problem.

But anyway, back to gold’s reactivity, or rather lack of it.  Gold isn’t the most unreactive element (depending on how you’re defining reactivity, that honour probably goes to iridium) but it’s up there.  Or perhaps I should say down there.  It keeps its shiny good looks even when it’s regularly in contact with warm, damp, salty, slightly acidic skin, which is quite handy from the jewellery and money point of view.

But there is one thing gold reacts with: aqua regia.  Aqua regia is a mixture of nitric and hydrochloric acid and ancient alchemists gave it its name – which literally means ‘royal water’ – because it dissolves the ‘royal’ metal, gold.  It’s pretty cool stuff, in a slightly scary way.  Freshly-prepared it’s colourless, but quickly turns into a fuming, reddish solution.  It doesn’t keep – the hydrochloric and nitric acids effectively attack each other in a series of chemical reactions which ultimately result in the production of nitrogen dioxide, accounting for the orange colour and nasty fumes. Screen Shot 2013-06-04 at 00.20.27The fire diamond (remember those?) for aqua regia has a 3 in the blue box, putting it on a nastiness par with pure chlorine, ammonia and, funnily enough, oxalic acid (the stuff in rhubarb).  It also has ‘ox’ in the white box, telling us it’s a powerful oxidising agent, which means it’s effectively an electron thief.

All atoms contain electrons but they can, and frequently do, lose or gain them during the course of chemical reactions.  Acids in general are often quite good at pinching electrons from metals, but aqua regia is particularly good at it, and especially with gold.  Much, much better than either nitric acid or hydrochloric acid on their own because, in fact, the two work together, as a sort of two-man gang of acid muggers.  When metal atoms lose electrons they become ions, and ions dissolve very nicely in water.  Hence, aqua regia’s fantastic property of being able to dissolve gold.

Which leads me to a really great story.  During World War II it was illegal to take gold out of Germany, but two Nobel laureates – Max von Laue, who strongly opposed the National Socialists, and James Franck, who was Jewish – discretely sent their 23-karat, solid gold Nobel prize medals to Niels Bohr’s Institute of Theoretical Physics in Copenhagen for protection.  All well and good, until the Nazis invaded Denmark in 1940.  Now, unfortunately, the evidence of von Laue and Franck’s crime was sitting on a shelf in a lab, just waiting to be found.  This was serious: if the Gestapo found the gold medals they would persecute von Laue and Franck, and probably take the opportunity to make things very unpleasant for Bohr as well, particularly since his institute had protected and supported Jewish scientists for years.

Nobel_PrizeWhat to do?  At the time a Hungarian chemist called George de Hevesy was working at the institute, and it was he that had the bright idea of dissolving the medals in aqua regia.

It would have taken ages, because although aqua regia dissolves gold, it doesn’t do it quickly, and these were chunky objects.  He must have been anxiously looking over his shoulder the whole time.  But he managed it, and eventually ended up with a flask of orange liquid that he stashed on a high shelf.  The Nazis searched the building but didn’t realise what the flask was, so they left it.  Iit stayed there undisturbed for years, in fact until after the war was over.  At which time, de Hevesy precipitated the gold back out and sent the metal back to the Swedish Academy, who recast the prizes  and re-presented them to Franck and von Laue.

So there we have it, you can’t turn lead into gold (at least, not without a particle accelerator) but, if you know what you’re doing, you might just be able to turn a flask of orange liquid into two solid gold Nobel prize medals!


The title of this post comes from a poem by the British poet, Thomas Hood, 1799-1845. Here it is in full:

Gold! Gold! Gold! Gold!
Bright and yellow, hard and cold
Molten, graven, hammered and rolled,
Heavy to get and light to hold,
Hoarded, bartered, bought and sold,
Stolen, borrowed, squandered, doled,
Spurned by young, but hung by old
To the verge of a church yard mold;
Price of many a crime untold.
Gold! Gold! Gold! Gold!
Good or bad a thousand fold!
How widely it agencies vary,
To save – to ruin – to curse – to bless –
As even its minted coins express :
Now stamped with the image of Queen Bess,
And now of a bloody Mary.