Wednesday, July 01, 2015

Perkin's purple: a journey around London

I have just presented one of BBC Radio 4’s Science Stories, a new series looking at episodes in the history of science. This one tells the tale of William Perkin’s purple coal-tar dye and how it changed the course of chemistry. That, of course, is the kind of grand and often contentious claim these programmes inevitably end up making, but I do feel that there is a case to be made for it here.

The initial plan was for me to take a journey across London, visiting the key locations en route: from Shadwell in the East End to the Royal College of Chemistry in the West End and then the site of the Perkins’ factory in Greenford Green on the outskirts of west London. In the end it didn’t quite happen that way, but I got a few pictures of some of the relevant locations as we recorded, and so wanted to include these here with the original draft of the script – it changed considerably, and I’m sure very much for the better, but this at least tells and illustrates the story. For more details, see Simon Garfield's excellent book Mauve, Tony Travis's authoritative The Rainbow Makers, and my own Bright Earth.


“A reservoir of dirt, drunkenness and drabs” – that’s what Dickens called Shadwell, and I’m not sure that he wasn’t being affectionate. There’s not a lot of Dickens’ Shadwell left: whatever the bombs didn’t destroy during the war disappeared soon after in the slum clearances. But I can’t say that what took its place has added much to its appeal: all these ugly flats and traffic bollards.

But here’s the place I want. King David Lane. Just down here in the mid-nineteenth century there was a big old house at 1 King David Fort, but now it’s just a council block.

Visiting the site of William Perkin’s family home in Shadwell – on a very blustery spring day!

This was the home of the Perkin family, who were wealthy by the standards of Shadwell. George Perkin was a successful carpenter who could afford to indulge his son William’s passion for chemistry. William had a little home laboratory on the top floor of the house – just a simple place, with a table and bottles of chemicals, no running water, no gas. But when he was 18 years old and still a student, he discovered something here that for once justifies that awful cliché: it changed the world.

There’s a blue plaque here to back me up. “Sir William Henry Perkin, FRS, discovered the first aniline dyestuff, March 1856, while working in his home laboratory on this site, and went on to found science-based industry.”

The blue plaque marking the spot where Perkin discovered mauveine.

Listen to that again: “went on to found science-based industry”. In other words, what Perkin discovered led to the whole idea that industry might be based on science.

That’s an astonishing claim. What could this young lad have found that was so important?

Let’s start with a gin and tonic.

For the British army in India in the nineteenth century, this drink really was medicinal. The troops were issued with their bitter tonic water at daybreak, but the officers started taking this medicine on the verandah as the sun set, not just with a spoonful of sugar but with a splash of lime and a generous shot of gin.

You see, the bitter taste was due to quinine, the only effective anti-malarial drug then known. This stuff was extracted at great labour and expense from the bark of a Peruvian tree called the cinchona. The bark had been known since the seventeenth century to help treat and prevent malaria. No one really knew what was in it until two French chemists separated and purified quinine in 1820. With quinine to protect them, the Europeans were able to begin the colonization of Africa, the consequences of which are still reverberating today.

You really didn’t want to get malaria. Chills, convulsions, fever, vomiting, delirium, and quite possibly at the end of it all – death. But quinine cost a fortune. Peru was then just about the only place where the tree was found and the bark contained only tiny amounts of it. And the Peruvians kept a monopoly by outlawing the export of cinchona seeds or saplings. In the nineteenth century, the East India Company was spending about £100,000 every year to keep the officers and officials in the colonies healthy.

But what if, instead of extracting this stuff drip by drip from tree bark, you could make it from scratch?

What does that mean? Well, over the previous centuries, chemists had found how to take simple chemical ingredients and get them to combine to make entirely different chemicals: useful substances like soap, soda, bleach. Might they be able to make a complicated natural drug like quinine?

One man in particular had this dream of using chemistry to reproduce and even rival nature. He was a German chemist called August Wilhelm Hofmann, and many people, including Prince Albert, hoped that he’d be the savior of British chemistry. In 1845 Hofmann was appointed director of the Royal College of Chemistry in London, which had been set up at Albert’s request.

August Wilhelm Hofmann

So what do we know about Hofmann? Well, according to the sign that now marks the spot in Oxford Street where the Royal College of Chemistry used to stand [it’s next to Moss Bros, opposite John Lewis’s], he “inspired the young to do great things in chemistry, and relate them to both academic and everyday life.”

The plaque erected by the Royal Society of Chemistry to mark the former site of the Royal College of Chemistry in Oxford Street, London.

There were two aspects of everyday life that Perkin, walking down these streets in the mid-nineteenth century, couldn’t fail to have noticed. In the lanes and docks of Shadwell, Dickens said, everyone seemed to be wearing rough blue sailors’ jackets, oilskin hats and big canvas trousers. But up here in the fashionable West End, it wasn’t so different to the style emporiums of today: ladies wore the latest colours: yellow silks from France and fabrics printed in patterns of rich madder red and indigo. Those last two colours were plant extracts, and they faded after lots of washing and being out in the sun. But the yellow silk, which had graced the Great Exhibition in 1851, was coloured with a new dye that was made artificially – by chemistry.

And the stuff it was made of was a by-product of the other thing that distinguished the splendor of Oxford Street from the gloomy alleys of Shadwell: the street lights. They had brightened up the evenings since the start of the century, burning gas that was extracted from coal.

Left over from that process was a thick, smelly tar called, naturally enough, coal tar. At first it seemed to be just noxious waste, and was often just dumped into streams. But then folks figured out that coal tar might be useful. Charles Macintosh used it to make waterproof raincoats. And if you distilled it, then you could extract a whole range of chemicals, like coal itself primarily composed of carbon. They often had an acrid smell – aromatics, the chemists called them. One was carbolic acid, also known as phenol. You remember that stinky old coal-tar soap? That’s phenol you were smelling, and it was in there to act as a disinfectant, one of its main uses since the 1850s.

But phenol was also the starting ingredient for the yellow silk dye that rich ladies bought from Lyon. Yes, this coal tar had some valuable stuff within it.

No one knew that better than August Hofmann, who had become pretty much the world expert on coal-tar compounds. So when William Perkin enrolled at the Royal College of Chemistry in 1853, pretty soon he found himself working on coal tar.

And when Hofmann set Perkin the challenging task of trying to make synthetic quinine in 1856, the coal-tar compounds seemed like good materials to start from.

We need to do some chemistry now. But don’t worry. I’ve got a Scrabble set to help me. You see, molecules are like poems: you have to get the words in the right order. Each word is a cluster of letters, and we can think of each letter as an atom. Making molecules is like stringing together these letters in an order that has some meaning. Now, some molecules, like polythene or DNA, really are a lot like strings of atoms. But others have other shapes. Benzene, for example, which is at the heart of all the coal-tar aromatic compounds, is a ring of six carbon atoms, each with a hydrogen atom attached. I take all six C’s for carbon – and yes, this isn’t exactly a regular Scrabble set – and put them in a ring.

But the problem was that in Perkin’s day no one knew that molecules have shapes like this, with atoms in particular arrangements. All they knew was the relative amounts of each kind of element, like carbon and hydrogen, a substance contained. Benzene was equal parts of carbon and hydrogen, rather like a G&T is one part gin to three parts tonic water.

So then, what Hofmann and Perkin knew about the element cocktail that is quinine was that it is twenty parts carbon, to twenty four of hydrogen, two of nitrogen and two of oxygen.

What gives quinine its meaning – what lets it cure malaria – is its particular arrangement of these atoms. But Perkin knew nothing about that. His strategy – so crude that in retrospect it was obviously hopeless – was, roughly speaking, to take a compound that had half of these amounts – ten parts carbon and so on – and try and stick them together, as if mixing up these two piles of letters is going to miraculously give them the same meaning as quinine.

It’s not surprising he didn’t succeed. When he did the experiment at home one night, instead of colourless quinine he got a red sludge.

He could have been forgiven for just flushing it down the drain. But he was too good a student for that, which is why Hofmann had made Perkin his personal assistant.

Instead, he thinks, well, what seems to be going on here? Let’s try the same reaction with another two identical piles of letters, rather like the ones before but a bit simpler. And so he goes through the same procedure with a different coal-tar extract, one of Hofmann’s own favourites: a compound called aniline.

Well, this time the result is even worse. Now the gunk is black. Even so, Perkin keeps going. He dries the stuff and swills it around in methylated spirits.

And now at last, something nice. It dissolves to turn the liquid a beautiful purple.

Here Perkin thinks of those fine ladies of Oxford Street in their bright silks. He knows that the textile industry is hungry for new dyes. And so takes a piece of white silk and dips it into the liquid, and when he pulls it out the colour has stuck fast to the fabric.

So what now? Perkin manages to get hold of the name of a dye works in Scotland and he sends them a piece of his purple-dyed silk. When the reply comes a few months later, it must make his heart beat faster:
“If your discovery does not make the goods too expensive it is decidedly one of the most valuable that has come out for a very long time. This colour is one which has been very much wanted in all classes of goods and could not be had fast on silk and only at great expense on cotton yarns… the best lilac we have… is done by only one house in the United Kingdom… and they get any price they wish for it, but… it does not stand the tests that yours does and fades by exposure to air.”

So there it was: Perkin had a potential new dye on his hands.

But remember what the man had said: “If your discovery does not make the goods too expensive”. Well, aniline was expensive. If this dye was going to succeed, Perkin had to find a way of making it cheaply – which meant, on an industrial scale.

He realized that he wasn’t going to be able to do that while he was still a chemistry student. So he told Hofmann that he was quitting. But Hofmann had made the young man his protégé, and as Perkin recalled many years later, “he appeared much annoyed”. What was his best student thinking of, abandoning a promising career in pure research to go into industry? As Perkin recalled,
“Hofmann perhaps anticipated that the undertaking would be a failure, and was very sorry to think that I should be so foolish as to leave my scientific work for such an object, especially as I was then but a lad of eighteen years of age.”

The funny thing is that purple was already fashionable even before Perkin discovered his aniline dye. From the 1830s a purple dye called murexide became popular, though probably its fans had little idea that it was made from Peruvian bird droppings. Another purple dye was made from an extract of lichen. In the year that Perkin made his discovery, the Pre-Raphaelite Arthur Hughes painted his picture April Love, showing a young woman in the kind of long flowing purple dress then in style. The French, who even at that time called the shots in fashion, had a word for these rather pale purples. It was what they called the purple-flowered mallow: mauve.

April Love (1856), by Arthur Hughes.

But he did leave, and when he couldn’t find a backer for the factory he proposed to build, his father George put up his life savings, even though he’d never wanted William to become a chemist in the first place. William’s older brother Thomas chipped in to help too.

Now they had to give aniline purple a catchy trade name. Perkin thought of the famous royal purple of Rome, originally made in the Phoenician city of Tyre from a substance extracted a drop at a time from shellfish. Why not call it Tyrian purple?

But it didn’t catch on. Soon enough the aniline dye he’d intended to call Tyrian purple had become synonymous instead with the colour mauve.

There was nowhere suitable in the East End for the coal-tar dyeworks of Perkin & Sons, and in the end they found a meadow right over in Greenford Green, near Harrow, northwest of London, conveniently close to the Grand Junction Canal. In less than six months, a factory was turning it into purple for the dyers of Great Britain.

Well, I can’t say that the industrial estate in Greenford Green is much of an improvement on the faceless modern development in Shadwell. But I guess it wasn’t any better in Perkin’s day. His dyeworks grew quickly, and it looks pretty grim in old engravings and photos, with its tall chimneys belching smoke and toxic nitrous fumes. He found a way to make aniline cheaply on the site from benzene, sulphuric and nitric acid, so goodness knows what the factory’s chemical vats spewed into the canal. The chemical process was dangerously explosive, and none of the Perkins had any experience with industrial-scale chemistry. It’s a wonder the whole place didn’t go up in smoke.

A photograph of the Perkins’ dyeworks in Greenford Green.

The last traces of the old factory were destroyed in 1976, but there’s a blue plaque here to mark its place… and here it is. “William Henry Perkin established on this site in 1857 the first synthetic dye factory in the world.”

The blue plaque at Greenford Green where the original coal-tar dye factory of Perkin and Sons once stood.

It became so much the rage in London that it even drew comment from Dickens in 1859:
“As I look out of my window, the apotheosis of Perkin’s purple seems at hand – purple hands wave from open carriages – purple hands shake each other at street doors – purple hands threaten each other from opposite sides of the street; purple-striped gowns cram barouches, jam up cabs, throng steamers, fill railway stations; all flying countryward, like so many migrating birds of purple Paradise.”

Perkin’s Greenford Green factory marks the end of the beginning – for aniline dyes and for the entire synthetic chemicals industry.

Perkin & Sons couldn’t get the French patent rights for their mauve, and within a year French and German companies started to make it too. Soon the coal-tar dyes were everywhere – not just purple but green, red, blue, black. The liberation of colour had arrived, and fashion became positively gaudy.

Bright colour – once the preserve of the rich – could be worn in all walks of life. Gone was the colour-coding of social hierarchies that had existed since the Middle Ages. Colour became a matter of individual expression.

What began as a stroke of serendipity in Shadwell was now becoming an exact science. Chemists came to understand that the particular arrangement of atoms in a molecule determines what it does – what, as I said earlier, the molecule means. And what it does might include which colours it absorbs and which it reflects, when light shines onto it.

So on the one hand, it became possible to make new colours to order. By carefully studying aniline dyes, chemists in the late nineteenth century could predict from the architecture of these compounds what colour they were likely to have. This is now the entire business of synthetic chemistry: constructing molecules with particular atomic arrangements and therefore particular properties.

On the other hand, if there was a substance found in nature that had useful properties – like quinine, say – then if you could figure out the shape of its atomic framework you had a chance of working out how to make it synthetically, perhaps more cheaply than harvesting it from plants.

But what became of the natural dyes, such as indigo and madder? They didn’t go out of fashion; instead, synthetic chemistry re-invented them. Getting these substances pure and in large amounts was costly and labour-intensive, and indigo plantations in India were the British Empire’s most lucrative business in all of Asia.

But as chemists came to understand that molecules were made of atoms linked together into particular architectures, they turned themselves into molecular architects who could even aspire to construct the molecules of nature. They figured out how, from simple ingredients like coal-tar substances, they could string together atoms to make the very molecules that gave indigo and madder their colours.

The molecular structures of indigo (top) and alizarin (bottom), which gives madder red its colour.

When two German chemists figured out how to make synthetic madder red in 1868 from the coal-tar compound anthracene, William Perkin quickly figured out how to do it more cheaply and on an industrial scale. By 1873 he’d got rich enough from this and other dyes to sell his company and return to pure research.

The blue plaque in Victory Place, near Elephant and Castle in southeast London, showing where the dyeworks of Simpson, Maule and Nicholson was situated. The company was established here in 1853, and in 1860 it began to manufacture aniline red dye, known also as magenta. Three years later they marketed an aniline violet, discovered by August Hofmann, that offered Perkin’s mauve some stiff competition. In 1873 William Perkin sold his dye company to the firm that Simpson, Maule and Nicholson had become, called Brooke, Simpson and Spiller. I was terribly excited when I discovered this plaque on my usual cycling route into London; I suspect I was the only person who could say that for a good many years.

Portrait of William Henry Perkin, painted in 1906 by Arthur S. Cope.

Perkin’s main competitor for synthetic madder was the German chemicals company BASF. If you’re like me, the name BASF will put you in mind of cassette tapes. But that’s just an example of how the dye companies diversified into other areas, because BASF stands for Badische Anilin und Soda Fabrik: the aniline and soda makers of Baden.

In 1877 one of their academic consultants, the German chemist Adolf Baeyer, worked out how to make indigo from the coal-tar extract toluene. BASF was soon producing it by the hundreds of tons. Within just a few years the price of indigo plummeted and the colonial plantations were put out of business, which the British government declared a national calamity.

Doesn’t this then make the chemist a kind of modern Prometheus? If you can control the shapes of molecules, what can you not create?

These colour manufacturers now pervade our language, our material world, our history. ICI, Hoescht, Agfa, Novartis – all began with dyes. In 1925 some of the major German dye companies merged to form the notorious cartel IG Farben, a force powerful enough to dictate its terms to Hitler. The diversification into pesticides left IG Farben with the patent for the poison gas Zyklon B, which it licensed for use in the concentration camps.

The diversification of the great dye companies into areas like pharmaceuticals had begun by the late nineteenth century. The coal-tar dyes themselves showed the way. In the 1870s the German physician Paul Ehrlich began to use the dyes for staining cells, which made them easier to see and distinguish under the microscope. He found that some dyes actually killed the microorganisms they stuck too.

That sounded useful. In 1909 Ehrlich discovered an arsenic-containing dye that would destroy the microorganism responsible for one of the most feared and deadly afflictions of the day: the disease that dare not speak its name, syphilis. Other coal-tar dyes worked as antibiotics.

Before this time, most drugs were, like quinine, extracts from natural sources, mostly from plants – like the extract of willow bark called salicylic acid that had long used as a painkiller. In 1897 a chemist at the German dye company Bayer turned phenol into a compound related to salicylic acid but which worked even better. The company started selling it under a trade name: aspirin.

To make sense of the science behind all this, chemicals companies couldn’t just any longer rely on hiring the services of academics. They started to employ their own chemists, who could design products like drugs based on a rational understanding of how the molecules needed to be shaped, and what they would do.

This, then, is what science-based industry is all about. It’s what the pharmaceuticals industry looks like today.

All the same, the revolution that Perkin began is in some ways still just getting started. We now know that there’s more to the way a drug works than just a good fit with the biological molecule that it aims to latch onto, like a lock and key. But we still can’t always fully understand or predict how a given drug will behave: you can’t be sure of designing it at the drawing board. Instead, most drug discovery still relies on trial and error, on shuffling molecular fragments into many different shapes and then seeing which ones work best.

What’s more, synthetic chemistry still has plenty of problems to solve: scientists struggle to put together some of the complicated molecules that nature produces. And even if they succeed, the route is often too long and too expensive to be useful in industry. This is why chemical synthesis is still as much an art as a science.

But Perkin is now regarded as one of its finest early stylists: a man who first gave us a glimpse of what might be possible if we can get clever enough at molecular architecture. And for that we have to thank the colour purple.


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