A bittersweet delight (1)

A bittersweet delight (1)

“[…] La Cuisine moderne est une espece de Chymie. La science du Cuisinier consiste aujourd’hui à décomposer, à faire digérer & à quintessencier des viandes, à tirer des sucs nourrissans & pourtant legers, à les mêler & les confondre ensemble, de façon que rien ne domine et que tout se fasse sentir; enfin à leur donner cette union que les Peintres donnent aux couleurs, & à les rendre si homogenes, que de leurs differentes saveurs il ne résulte qu’un goût fin & piquant, & si je l’ose dire, une harmonie de tous les goûts réunis ensemble […]”

“Modern Cuisine is a sort of Chemistry. Today, the Cook’s science consists of decomposing, digesting and refining meat, of drawing nutritious and yet light juices, mixing and combining them together so that nothing could predominate and that everything could be tasted; finally, this science also brings these ingredients together in the same way as Painters do with colours, making them so homogeneous that the different flavours will turn into a fine, attractive ensemble, and, if I dare say it, a whole harmony of all flavours blended together”

Les Dons de Comus, ou les Délices de la table, François Marin, 1739, pages XX-XXI (the original ortography has been retained in the quoted passage).

“It could be your next blog post”, said P., a colleague of mine, “you could write about the stages of sugar syrup”. Vaste programme1, I would say during a bout of Francophilia…

Now, I’m definitely ready to take up my colleague’s challenge, but an entire blog post on syrup stages could simply taste too sweet for readers to eat. Besides, I know myself all too well: like a frantic honeybee, I will invariably end up flying from flower to flower collecting all nectar that I fancy: dwelling on a single subject does not really suit me, honestly.

That said, let’s start our sticky journey: get ready to wade through thick syrupy swamps and experience some of the hottest environments on Earth -or, maybe, just in the kitchen. What a timely moment to start exploring the universe of confectionery: the festive season is well behind us and some sweet thoughts will help to see us through the late-winter blues.

Spinning threads of words and sugar

When I first met the English confectionery, two similar Italian words came to my mind: a carefully-made wrapping or packaging (confezione) and the candied almond known as confetto. If one looks at etymologies, confectionery and its Italian soundalikes do share a common origin. Confetto, for example, stems from the Latin confectu, the past participle of conficere, meaning ‘to make, to prepare, to consume’, exactly like to confect, confection, and confectionery. Indeed, confezione as confectionery is attested in the Italian language, though with an archaic flavour, and both languages seem to use the very same word to indicate both a type of sweet and a medicinal preparation, usually coated in sugar. The latter was somewhat predominant in the Middle Ages, when table sugar was virtually only used in medicine, and this sweet powder from faraway lands would be stocked alongside expensive spices at the apothecary’s2.  And, incidentally, early chemistry did take on board time-honoured pharmaceutical lore and ‘laboratory practices’3. (However this does not justify the confusing British use of chemist for shop, or person, where medicinal drugs are sold. This is something that the Royal Society of Chemistry itself has stressed in its report on the 2015 survey on public attitudes to chemistry).

Cracking the candy

Back to confectionery, this is a veritable galaxy in its own terms within the universe of food and cooking. When trying to make sense of the mind-blowing variety of confectioneries, a chemical mindset comes in extremely handy. That’s because, as philosophers remind us3,4, chemistry is a hybrid science with a good dose of taxonomy in it: like it or not, chemists have always had to deal with – and classify-the multifarious nature of the material world. Classification, however, is not important only for its own sake: the act of arranging chemical entities can beget chemical laws, as exemplified by periodicity. So, while we navigate confectionery, how can we come up with a taxonomy or an ordering principle of sorts? It would be great to explore the kitchen with the same scope and vision of Linnaeus, but that’s not what blog posts are meant to be (and mine are already quite lengthy). More simply, let’s break down the components of a confectionery product:

-additional structural ingredients (‘scaffolds’)
-tasty bits (‘inclusions’)

A candy, like many other edible things, is similar to a building. Sugars, and the way the cook handles them by controlling the temperature they reach during cooking, turn into crumbly wattle, solid bricks, or hard stones. Additional ingredients (if any) can help make sure that the building will have the texture that the cook and the eater desire, playing the role of concrete, mortar, daub, steel rods…Finally, add extras to taste, much as one would paint, decorate or plaster a wall.

So, when it comes to classifying candies, it is useful in the first place to address each of these three components in a sequential way, along with other specs of the recipe:

-relative proportion of sugars
-when and how the syrup is mixed to the scaffolds
-when and how inclusions are added

Beside these two triplets, the cooling of the reaction mixture is often even more important than all preceding steps, determining to a large extent the success or the failure of the synthesis, er, the recipe. This is where chemistry comes into its own: a cooling candy-to-be is undergoing crystallisation, a fascinating chemical process and a tricky practical operation at the same time.

Swirl down, sweet snow

How many times, when trudging through the white icing on winter’s cake, have you thought: ‘it’s crunching like sugar beneath my feet’? And, caught in a flurry of swirling snow, have you ever likened it to a sprinkle of powdered sugar?

If so, you’re a poet.

From the opening lines of La primavera hitleriana (‘The Hitler Spring’) by Italian poet Eugenio Montale…

Folta la nuvola bianca delle falene impazzite
turbina intorno agli scialbi fanali e sulle spallette,
stende a terra una coltre su cui scricchia
come su zucchero il piede[…]

The thick white cloud of crazy moths is whirling
around the pale lights and the parapets
spreading a blanket on the earth that snaps
like sugar underfoot[…]5

Poetry aside, thinking about snow is a useful starting point to get to grips with sugar crystallisation. Three general facts will come in handy later:

  1. When it’s too warm, it never snows but it pours.
  2. When it’s cold enough, it all begins with a seed.
  3. Wet, fine, frozen, heavy: many words to say snow.

Now, of course, do bear in mind that snow is an example of change of state of matter, while sugar crystallisation involves a solute (sugar) in a solvent (water) that clusters into precipitating (falling) crystals (solids). Both these phenomena, however, show common features.

A dance in three movements

With the help of my favourite food bible6, let’s take on sweet crystals. Looking back at the bullet list written above:

1 ) Let’s start from room temperature. When a big lump of sugar is added to water, (which is the first step of both recipes I will be talking about below), only some of it dissolves. That’s because room temperature is a slow microscopic waltz, and only when we crank up the cooker does the tempo change and quicken: water will then lead more and more sugar into a reeling dance, and the solid is dragged in this invisible, warming whirlpool.

Eventually, this solution will start boiling, not at the same temperature of sugar-free water, but higher, because of the dissolved sugar. How come? Well, think about this: sugar and water love each other. A lot. They make perfect dancing partners, but theirs is also one of those all-consuming, mutually absorbing passions. It takes brute force to separate water from its sweet companion, more than it would be required to separate water from water. Yet, heat is a ruthlessly efficient kidnapper, and more and more water is eventually lost into the air. So, the more we heat, the stickier the situation becomes, because there will be a larger and larger excess of sugar molecules which will not find an aqueous partner, creating a syrup. They tightly cling onto any remaining water, frantically scrambling around in an ever-accelerating chaotic dance, and so heat needs to become increasingly brutal: a thermometer will show that the boiling temperature will keep on creeping up as one continues to heat the syrup.

What does this mean? The longer one boils the syrup (or the higher the temperature reached), the lower its water content. Less water means a harder final product. Bear this in mind. Rule of thumb number one of the art of confectionery.

Without the company of enough water, there could be a way out for desperate, forsaken sugars: meet and cling onto one another, sinking back into a solid. But the heat keeps them apart in a violent motion, bouncing around and so sugars cannot get hold of one another. All it takes for a crystal to form is a slower rhythm – a drop in temperature – and some sort of trigger . Think of the sugar syrup as a boulder teetering on the brink: it just takes a light push to make it fall.

2 ) So, this concentrated hot syrup is primed to form sugar crystals: when the dance slows down and something helps sugars to bump into one another, a crystal will start growing. That’s why we talk about nucleation and growth when discussing crystallisation. These words sound dry and technical, but they can also tell the story of the most bittersweet flavour of life, stories of random encounters (nucleation) turning into ever more passionate love (growth). We love, and so do molecules, in their own way.  The trigger, the nucleus, can be a minute crystal that has already formed, or even a foreign body, like some dust. Then, growing crystals vie with each other for the limited available sugar much as snow crystals do in clouds. It is at this stage that the cooling syrup requires the cook’s full attention.

3 ) That is because sugar crystals can grow to different sizes and shapes, much like snowflakes. So, although crystallisation is indeed ‘order out of disorder’, order itself can come in different versions. Think about boxes: either you stack them up in a single tall pile reaching to the ceiling, or you stack them in pairs but all over the floor.
So we have two main options: large but few, and small but many. What do they mean in terms of candy making?

Big is not beautiful

The hot syrup is a chaotic environment, every forsaken sugar cries and shouts and tries to draw their fellows’ attention, all of this while swirling wildly around. What a dramatic scene! It’s an unruly mob looking for a charismatic leader. Imagine that all of a sudden a voice resonates piercing through the hubbub: it speaks the loudest, it will be heard. The seed of unrest moves swiftly in the heated syrup, and quickly rallies supporters all around. No time for other groups to form: they are dispersed in the crowd. The mob clusters around its leader, ready to follow down that road, the road to crystallisation.

Big crystals form this way, when few seeds collect lots of sugar molecules, often as a result of the crystallisation starting too early when the syrup is still too hot. They contribute to a coarse, grainy texture in the candy, and feel chunky in the mouth. How to avoid them?

Call in the riot police

It looks like that the most finely-textured candy is similar to a pluralistic society: many little crystals all coexisting like many political activists voicing their opinion and gathering small groups of followers. It is (Zygmunt Bauman would approve of it) a fluid society.
All we need to do is to wait for the tempers to cool down, and then stir up some healthy agitation, slowly but continuously, to encourage the engagement of as many seeds as possible.

If a stirring stick is not enough, well, plan B is to draw the baton. So, another way to make sure that law and order reigns in the syrup is to rely on something that will forcefully prevent clustering.

Cooks have used additives to limit crystallisation, in jargon called ‘doctoring agents’, for a long time. They make sure that police cordons are thrown throughout the syrup preventing the clusters of people from growing. Crowd control in the kitchen.

If trained people can control other people’s unrest, sugars can control the crystallisation of sugars. Take note: this is a very important concept. Difficult to understand? Not really.

An exercise with dumbbells

Our everyday life is full of examples of activities involving packing, arranging, ordering things. Someone is tidier, someone is messier (like me): but if the form of the object to be packed does not help, well, it is a no-go. Sugar crystallisation in candy making, invisible though it is, provides just another example of it. How could you turn packing into a nightmare? Simply by adding a handful of oddly shaped items. Believe it or not, it is just the same for ‘doctoring agents’ in confectionery.

Table sugar is sucrose. It looks like a fixed-weight dumbbell with two equal weights, but of different colours. We call one half of it fructose, the other glucose.

So, because you have got two units, you can name sucrose a dimer. Say you are at the gym, and your have got to tidy dumbbells up, arranging them back on the rack. As long as the dumbbell is in one piece, that is piece of cake. But if the handle breaks and the two weights fall down, well, that is another cup of tea. Imagine that enough handles break, or that there are more weights than handles, and this is a recipe for disaster: you will never be able to fill the dumbbell rack. Game over.

Controlling crystallisation in candies rests on either of those two options: have more weights than handles, or snapping the handles to separate the two ways. The former just involves tweaking the sugar proportions, replacing some sucrose with glucose, for example. But chemistry is dead good at snapping handles, at breaking bonds: we will see later how to do this.

Right, enough theory and taxidermy of confectionery for now: let’s talk about the real stuff. Here are two sweet case-studies that have made me sweat and swear in the kitchen in the last few months: lokum (aka ‘Turkish Delights’) and torrone (hard nougat).

The same and not the same

Different as they might seem, chewy pink cubes versus rock hard white slabs, lokum and torrone come to the same critical crossroads: the mixing of hot syrup with the chosen scaffold halfway through the recipe. And both rely on some sleight of hand to avoid unwanted, untimely crystallisation.

Looking more closely6, here are a few differences:


It is an aerated candy, or, put it differently, a toughened egg white foam (a reinforced meringue): the most important step in its preparation involves streaming hot syrup into whipped egg white, while whisking.
Sounds easy? Maybe, but if you want to nail it, not only do you have to get the timing of the mixing right, you must also have a feeling for the intensity of the whisking motion. All of this while the white-hot egg+syrup goo gets splattered all over the kitchen. (See the picture at the start of this post. Like in the lab, safety first: wearing protective spectacles when working with syrup is a golden rule).  At any rate, what matters from a chemical perspective is that nougat could be called candied protein: after all, this is what egg white is made of.


It belongs to the family of jelly candies: their chewy bite is determined by the added scaffolding. Cooks can choose between a variety of ingredients, many of them now widely available in supermarkets, too: agar (from algae), pectin (from fruit) and starch. Lokum relies on the latter, in the form of the humble cornflour.
Watch out: starch is a massively heavy counterpart of the sucrose/dumbbell that we talked about before. Individual weights can be stacked one on top of each other, or arranged in more convoluted ways. Whichever you choose, remember that sucrose was a di-mer a two-unit dumbbell. Starch is a poly-mer a multi-unit object, and losing sweetness is the price to pay when going from one to many. Sucrose was a two-colour dumbbell, with glucose and fructose as weights; instead, starch just contains glucose units.  Just to crack the jargon, because we are talking about sugars, we can also say that starch is a poly-saccharide, which in plain English would sound like multi-sugar. So, if I called (tongue in cheek) nougat candied protein, let me say that lokum is candied multi-sugar, which is slightly scary in these days of low-carb hype.

To sum it up, have a look at this flow-chart showing the main steps in the preparation of lokum and torrone


Turkish delights

I turned out the first batch of lokum one Sunday morning. This time, I decided to have a go at making lokum after reading a review on the production of these sweets7, something that sounds like what we often do in the lab: you come across a paper, an idea flashes, and off you go.

[…]The history of lokum dates back to more than 300 years, making it one of the oldest sweets in the world. Turkish legend has it that in his endeavor to cope with all his mistresses, a Turkish sultan summoned all his confectionery experts and ordered them to produce a unique dessert to add to the collection of the secret recipes for which he was famous. As a result of extensive research lokum was born. In 1776, during the reign of Sultan Abdul Hamid I, Hadji Bekir, a fully apprenticed confectioner, arrived in Istanbul from a small town in Anatolia. Bekir set up a little shop in the center of the city, and quickly won fame and fortune among the people. Fashionable ladies began to give lokum to their friends in special lace handkerchiefs. […] Lokum had been known in Anatolia since the 15th century, but it had become widespread in the borders of the Ottoman Empire[…] 7

Fascinating. Let’s have a look at how to make lokum using our scheme

Sugars Scaffolding Inclusions
sucrose (table sugar) cornstarch red colour
rose water
(pistachios, optional)


Cook syrup to… How to add the scaffolding?
When to add the inclusions?
126-127°C -Dissolve cornstarch in water.
-Add to hot syrup while stirring.
-Continue cooking until the mixture stops giving off steam.
Last step before cooling down

There’s one ingredient missing, and I left it out from the list on purpose. It is the famous ‘doctoring agent’ we were talking about before. To limit crystallisation in lokum, the sucrose-dumbbell is broken up by some lemon juice, or anything edible that has an acidic pH.
Sawing a dumbbell up into two pieces is not exactly the easiest pastime. Yes, it depends on how thick the handle is, but if your saw is not sharp enough, you will have to toil anyway. What about your own strength, then? Are your arms fit enough for the job?
As one can see, there is an interplay of three parameters to work out how efficiently we can break those blessed miniature dumbbells called sucrose. Let’s wrap it up:

  1. How thick the handle is, in chemical terms, how sturdy the bond between glucose and fructose is. It turns out that it is not so frail as it might look.
  2. How sharp the saw is: reactions happen, or not. Or we can make it happen, think about what we found out in the previous post about catalytic converters. Catalysis plays a role in making lokum, too: not only is the acidic pH of lemon juice a sharp saw, it also helps to corrode the handle, to slacken the bond.
  3. How strong your arm is: cooks have an advantage over lumberjacks: they can crank the heat up, which is what they do when the syrup is being cooked. Heat, the water “kidnapper” is also a powerful source of energy for chemical reactions to happen.

Despite the heat, the lemon juice, the amount of sucrose broken up is ridiculously small. Yet, it is enough to avoid untimely crystallisation: after all, lokum contains also a very effective scaffolding agent, starch.

Following the recipe step by step, I boiled the syrup, then I added the starch mixture, and kept on stirring while heating. A glossy, viscous mass was formed, and it started clinging onto the silicone spatula. Call it goo if you like, but this was beautiful in its own way.


Finally, I added the extra bits, some rose water and beetroot extract. Then, I poured the pinkish mixture into a baking tray and I let it cool down. As a last step, I chopped it up into small pieces with scissors (that seemed to be the smartest idea).

I asked a Turkish colleague to taste my lokum: she said that it was a good attempt, and, tastewise, it was close to the real thing. Unfortunately the texture was off the mark, too soft. A couple of days later she brought lokum back from Turkey, an ideal basis for comparison. See for yourself.


My error? The maximum temperature reached by the syrup during the cooking step. My kitchen journal provides irrefutable evidence of my mistake: the recipe that I followed did not mention the temperature and I just tried to make an educated guess: 118°C  Only later did I find out8 that I should have heated the syrup up to…126-127 °C.
Try again.

(If you’d like to try, too, follow this recipe.)


Torrone (hard nougat) is quite another cup of tea. I decided to make it because I felt homesick, back in December: this month sees the consumption of torrone skyrocket, and not only because of Christmas. In some parts of Italy, for example in the north-east, children get presents and sweet treats on St. Lucy’s Day (13th December).

When I found the recipe in a book 8, I could not resist the temptation: the fact that the text classified it as ‘difficult’, just provided some extra thrill. After all, “if there’s a will, there’s a way”, right? Here are the proportions and the operations, from the same recipe:

Sugars Scaffolding Inclusions
sucrose (680 g)
honey (510 g)
corn syrup (170 g)
whipped egg white pistachios, almonds
orange blossom water


Cook syrup to… How to add the scaffolding?
When to add the inclusions?
150 °C -Stream hot syrup into whipped egg white while whisking.
-Keep on whisking for a few minutes.
-Keep them warm in the oven.
-Add to egg white + syrup mixture as last step.

This time, the ‘doctoring agent’ is corn syrup, also known as glucose syrup: it is a crystal-clear, thick fluid containing free glucose, which is one of the two components of sucrose, in a varying amount (10-43 %). So, instead of sawing up the dumbbell handles, we add in some extra weights that we cannot fit on the dumbbell rack.

A bucketful of sweetness

The name corn syrup is a giveaway: it betrays the fact that this product is manufactured from cornstarch (which was the scaffolding of lokum). If you remember that I described starch as a collection of glucose units, that should not come as a complete surprise! In fact, corn syrup contains poly-glucose units of varying length. But…watch out! Honey contains free glucose, too, roughly one third in weight. This means that, regardless of the exact concentration of free glucose in corn syrup, we can approximate the glucose : sucrose weight ratio to 1:3.

Then, I skimmed through the advice given at the start of the recipe:

“Be careful to have all mise en place ready as per the instructions. Once you begin the process, it should not be interrupted until the nougat is cooling

“Sugar cooking temperature is critical. Use an accurate thermometer and cook the sugars carefully”

Ok, here we go. I deployed all pots and pans that I thought I would need and I set about working. I shelled the pistachio nuts and I added them to the almonds, storing them in a bowl in the oven, set at a low temperature (120­ °C). After that, I separated the egg whites, leaving them in another bowl with a pinch of cream of tartar.
Then it was time to start cooking the sugar syrup, without honey, which I heated in a second saucepan, as suggested by the recipe. At first, the temperature was creeping up slowly, degree by degree, on a gentle heat. Then, the rate of increase in temperature quickly sped up, the number on the display fast approaching the target temperature of 150°C. Time to make the egg foam, and do it fast. While keeping an eye on the thermometer, I whipped the egg whites until the foam looked stiff enough. 148…149°C…time was trickling away as the temperature rose higher and higher.  When it reached 150°C, everything happened in the blink of an eye: I grabbed the saucepan with the hot honey and I poured it into the syrup: the temperature dropped slightly, but it bounced back incredibly fast. I placed the metal bowl with the egg white foam in the kitchen sink, I snatched the large saucepan brimming with syrup, my left hand firmly holding the electrical whisk in mid air. I turned the saucepan, a moment that seemed to last forever, as it looked as if gravity would fail me. Suddenly, a thin trickle dived into the fluffy white.  Down, pour it down, stream it into the foam, whisk it in, wield that whip, draw figures of eight as a sweet sticky mess is splattered all around. I kept on whisking for a few minutes; after that, I added the hot mixture to the nuts that I had set aside. I carelessly tossed the metal bowl into the kitchen sink, where it landed with a clanging noise as I was reaching for the roasting tray that I had lined with rice paper. The nougat-to-be was starting to cool down, and so I had to rush. I shovelled the thick white fluid into the tray with a silicone spatula, the pistachios and the almonds barely emerging from the surface as rocks submerged in an ivory sea. I laid the top sheet of rice paper with utmost and loving care, as though I were wrapping the lying nougat in a shroud.


Some sugar-honey syrup had spilt onto my kitchen sink, and set into a glassy amber slab (you can spot it in the featured image above!). I sort of thought I would find a mosquito trapped in one of them.


Later on, after a few hours’ rest, I pried the nougat out and I cut it roughly into bite-sized chunks. It was not the hardest nougat ever, but it seemed almost spot-on, also considering the limitations of my equipment, and the fact that I could not possibly cook the egg-syrup mixture for as long as 12 hours 9.


Little did I know that the hardest part was yet to come. In fact, storing homemade nougat can be tricky because of the hygroscopic nature of this sweet. Sugars will absorb moisture, and the nougat easily starts to ‘weep’, becoming stickier and stickier in the process. Aware of this issue, I thought I would outsmart the nougat this time: I stored the pieces in an airtight (borosilicate) glass container, which I put in the freezer overnight, just to be on the safe side.  On the following day, I shared the nougat with friends and it was a great success, despite its texture.

Then, at a certain point, I flipped the container over, and I spotted a chip, then a fault line, and eventually a spiderweb pattern of cracks. All the bottom part of the nougat was keeping the bottom of the container together, becoming effectively inedible in the process. I salvaged the top layer, but the rest was lost. What a pity.

Trying to get candies right often looks like the toil of Sisyphus, the mythological giant condemned to keep pushing a boulder uphill only to see it roll back down once more.

Yet, “One must imagine Sisyphus happy”, suggests the French philosopher Albert Camus10.

Absurd as it sounds, that is so true. Never can we be freer and happier than when we take up those apparently pointless challenges bound to end, or fail, like cooking, loving, writing a poem or tasting a candy, those most treasured pleasures of our one and only life.

The most bittersweet delight.


  1. Literally meaning ‘vast programme’, the closest English translation of this famous quote by De Gaulle is ‘a tall order’.
  2. “[…]sugar is expensive, a spice that, in the Middle Ages, is produced only in Sicily and Andalusia, where sugarcane is grown. […] In France, sugar is mentioned for its medical applications as of the early 1200s, but it is seldom used as a cooking ingredient until the 1300s […]”.  O. Redon, F. Sabban, S. Serventi, La gastronomie au Moyen-Age. 150 recettes  de France et d’Italie, Editions Stock, 1993, Paris
  3. Chemistry: The Impure Science, Bernadette Bensaude-Vincent and Jonathan Simon, Imperial College Press, 2012 (2nd edition).
  4.  Rein Vihalemm, Philosophy of chemistry and the image of science, Foundations of Science, 2007, 12, 223-,
  5. English translation found online
  6. On Food & Cooking, Harold McGee, Hodder & Stoughton, 2004
  7. A. Baku and B. Kirmaci, Production of Turkish delights (lokum), Food Research International, 2009, 42, 1-
  8. P.P. Greweling, Chocolates and confections :at home with the Culinary Institute of America , 2010, Wiley
  9. See this Wikipedia page (in Italian). This long cooking is required to obtain the rock-hard texture of certain types of torrone which, when snapped, will break and splinter into tiny shards. I remember, as a child, playing with these sticky pieces that would invariably cling onto the tablecloth.
  10. “Il faut imaginer Sisyphe heureux”, Albert Camus, Le mythe de Sisyphe.

Listen to the blog


My chapped hands are wavering on the keyboard. They have weathered the storm.

The past is the bellowing voice of the wind and its gusts. It is blowing from a year ago, echoing a cascade of shed pearls spilling on the floor like tears, the piercing cry of a young tree felled by the howling gales, the creak of the brittle glass beaker that cracked, but that won’t break.

Listen, tonight, listen to the wind speaking.

It reminds me what I once wrote: I was sitting on an inflatable exercise ball, surrounded by scattered boxes, after a hurried relocation.

A year on, boxes and a ball once more, and an envelope I am holding in my hand. The most precious gift.

The hourglass turns, the sand grinds the glass down. Time is a rough substance, it trickles through our fingers, it scratches our skin, it won’t stop. This blog, too, shall go on. Drop by drop, what most counts is that what drips, but is not lost…

To mark this first year, here’s a collection of recordings (click and download, it won’t play otherwise). The voices of the laboratory where I work, sounds without men, voices of instruments, of appliances, of noises that speak.

Like this west wind, fierce and wild.

Can spring be far behind?

A taste of artefact

Qui delle divertite passioni
per miracolo tace la guerra,
qui tocca anche a noi poveri la nostra parte di ricchezza
ed è l’odore dei limoni

Eugenio Montale, I limoni

[…]Here, by some miracle, the war
of troubled passions calls a truce;
here we poor, too, receive our share of riches,
which is the fragrance of the lemons[…]

Eugenio Montale, The lemons (English translation of this poem found online).

Genova and The lemons

Take a map of the regions of Italy (or simply open an online mapping service…), and look northwest: a boomerang arches and embraces the stretch of sea in front of Corsica. This is Liguria and Genoa (Genova, in Italian), its historical and current capital, lies at the centre of this arc, like a keystone in a vault. Affluent port city in the Middle Ages and financial hub, Genoa coexisted with (but at times also fought against) Venice, the other ‘seafaring Republic’, vying for commercial predominance and for the control of trading routes in the Mediterranean. Yet, Genoa has arguably beaten its arch-rival by a naval mile in the contest of literature and music. For example, many think that the finest cantautore (singer-songwriter) of all Italian 20th-century musica d’autore (untranslatable, but literally ‘author’s music’: think of the Italian counterpart to French chanson) is a native of Genoa,  Fabrizio de André. He has even been hailed as ‘the greatest Italian poet of the last hundred years’: although this is definitely far-fetched, he did bring about a sea change in the landscape of Italian music, by writing songs about themes previously neglected, or tacitly banned, in the love-centred and somewhat soppy lyrics of contemporary singer-songwriters.

Now leave Genoa behind you and choose where to turn, right or left: take a look at the western and eastern ends of Liguria, and you will see two of the most celebrated, most influential Italian writers of the 20th century. Prose and poetry face each other: Sanremo, in the western tip, almost bordering France, is the Riviera resort where the novelist, essayist and short story writer Italo Calvino (1923-1985) spent his childhood and youth years; opposite across the sea lie the Cinque Terre (lit. ‘Five Lands’), an area of outstanding natural beauty and World Heritage Site, which greatly inspired the earlier lyrics by Eugenio Montale, born in Genoa in 1896, arguably the greatest Italian poet of the last century. Liguria is the land where mountains choose to escape their earthly nature and dive into the sea, the great salty blue beyond the shore that features in so many of Montale’s first poems; Liguria is the land where the rugged coastline can become complex and intricate, almost like a fractal, like the combinatorial or structuralist schemes informing some of Italo Calvino’s works. Invisible cities is the title of one of them: short descriptions of these “invisible cities” are arranged according to a thematic structure that follows an iterative order throughout the book. The overarching frame story of Invisible cities is Marco Polo’s fictional conversation with Kublai Khan about these cities, something clearly inspired to the 13th-century travel accounts of the real Marco Polo, The Travels of Marco Polo. A theme of Invisible cities includes “cities & memory”, and if I were to write my own account of the cities of my life, I would definitely place Genoa in this group…My invisible Genoa looks like Zora, one of Calvino’s invisible cities:

“Cities & Memory 4

Beyond six rivers and three mountain ranges rises Zora, a city that no one, having seen it, can forget. But not because, like other memorable cities, it leaves an unusual image in your recollections. Zora has the quality of remaining in your memory point by point, in its succession of streets, of houses along the streets, of doors and windows in the houses, though nothing in them possesses a special beauty of rarity. Zora’s secret lies in the way your gaze runs over patterns following one another as in a musical score where not a note can be altered or  displaced. The man who knows by heart of Zora is made, if he is unable to sleep at night, can imagine he is walking along the streets […] This city which cannot be expunged from the mind is like an armature, a honeycomb in whose cells each of us can place the things he wants to remember.[…] Between each idea and each point of the itinerary an affinity or a contrast can be established[…]”
(Italo Calvino, Invisible Cities, translated by William Weaver, Vintage Editions).

There are cities where we sense a resonance with their vibrations: we have never lived there, we do not even love them at first sight, and yet we strangely feel at home. There is usually no point in wondering why: it is just like a subliminal perception. So is Genoa for me, city and memory, and when I tread the stones of its twisted lanes I know that I am a stranger, a casual visitor to a city that maybe I do not even like; nevertheless, I feel at ease, moving effortlessly on those, in Montale’s words, frail “spiderwebs of the memory”, as if a vague reminiscence could still help me find my way. I navigate the visible city following the map of the invisible Genoa within me. On rainy days, gray skies loom large above the Ligurian Sea and you know that they are there to stay: the infamous ‘Genoa low‘ clashes against the mountains, squeezing rain onto the city from its spongy clouds. This is when I would think about Montale’s poem I limoni as I am wandering aimlessly, victim of a literary Wanderlust:

La pioggia stanca la terra, di poi; s’affolta
il tedio dell’inverno sulle case,
la luce si fa avara – amara l’anima.

The rain exhausts the earth then;
winter’s tedium weighs the houses down,
the light turns miserly-the soul bitter.

Then, as I picture myself following in Montale’s footsteps, like a Parisian flâneur in search of a flashing idea, I, too, yearn for that sudden surprise, the golden light of ripe lemons appearing all of a sudden after you turn into a narrow alleyway, the plump fruits hanging heavily from a lemon tree grown in a pot, standing in a courtyard.

Quando un giorno da un malchiuso portone
tra gli alberi di una corte
ci si mostrano i gialli dei limoni;
e il gelo del cuore si sfa,
e in petto ci scrosciano
le loro canzoni
le trombe d’oro della solarità.

Till one day through a half-shut gate
in a courtyard, there among the trees,
we can see the yellow of the lemons;
and the chill in the heart
melts, and deep in us
the golden horns of sunlight
pelt their songs.

A gate left ajar lets us catch a glimpse of the lemons, we would like to go and touch and smell the zesty fragrance of the rind. Yet, the concierge of the mansion spots us and rushes to locks us out. There we stand alone, in the torrential winter rain.

Chemistry and the invisible lemon.

Oxford is a very visible city, but lemons are rarely to be found. This is in stark contrast with Liguria, a land where citrus fruits have always flourished in the mild climate of the region, thanks to the protective embrace of the mountains. Some online sources report the whopping figure of 20-25 million lemons produced in the sole city of Sanremo in  1662. At the same time, somewhere north of the Alps, lemon trees started to be grown, too, requiring orangeries, glasshouses and conservatories: the renowned Botanic Garden of the University of Oxford still houses a couple of citrus plants in its conservatory, where they have been lovingly kept warm ‘since the 1600s’, as its website proudly states.

20150412_155319 20150412_155325

Anyway, it it time for chemistry and the kitchen. Once upon a night I found myself at a friend’s place, ready to do one of my favourite things: improvisation (aka “messing up”) in the kitchen, something I seldom allow myself to do because of the imprinting of my laboratory training. A jar of ready-to-use chickpeas peeping from a shelf was too strong a temptation for us to resist: let’s make some hummus! By the way, one could try and unify the entire Mediterranean world1 (and beyond) under the name of the blessed chickpea. We set sail from the western Andalusian coasts, where espinacas con garbanzos (spinach and chickpeas) are a staple dish, we lay at anchor for a stopover at any port of the Ligurian Sea, where the whole Riviera from Nice to Pisa feasts on flatbread made from chickpea flour (known as farinata in Liguria and socca in the Nice area), and, following the ancient sailing routes, we finally reach the shore somewhere in the eastern Mediterranean, for example the port of Acre, where ships from Genoa would once dock, load cargo, and trade, and where hummus will now keep our hunger pangs at bay.

The soft chickpeas easily yielded to the blade of the blender. Then I added some sesame paste, or tahini, and went on mashing the delicious mixture, tasting it on the go. Looking scrumptious. At this point, I turned to my friend and asked her: “Could you pass me a lemon or some juice please?”. Panic. The air stood still, those silent seconds where people cross their gazes without saying a word, knowing all too well what comes next. “I’ve got no lemon juice, I’ve got no lemons”, said she. Then she pointed to a small, bright green lime: “Use that instead!”, she suggested. I could have, maybe I should have, taken the lime and squeezed it, but I could not help grasping this opportunity for some unexpected “messing up” in the kitchen. I felt like frolicking and so I turned down the offer of the alternative citrus fruit and started thinking: “OK, let’s say that the little lemon juice added to the hummus is just there to correct the flavour: I don’t expect the texture of the hummus to depend a lot on the change in pH…the texture’s more a matter of balance between thick tahini and chickpea water”. In a sense, now I think that I was following the path traced by Italo Calvino in his quote above: “Between each idea and each point of the itinerary an affinity or a contrast can be established“.

Acidity: that is what lemon juice is (almost) all about. The holy book of all food lovers2 shows that the juice is 5 % in acids (as weight/weight percentage). The main sour character is citric acid, an interesting compound that ought to deserve much more than this brief mention in a blog post about hummus. For the moment, let us just remind that it is an allowed food additive, under the EU code E 330. Malic acid comes in a far second in the acid ranking, well below 0.5% 3 . Biochemistry lovers will remember that both citric and malic acid feature in the Krebs cycle, or citric acid cycle.

Then there is a sweet note to the flavour of lemon juice, which arises from the juice sugars, having a total weight content of  3.16 %, slightly lower than that of the acids.

However, as we all know, lemon is much more than its juice: the rind of the golden fruits has a pleasant, smell, arising from some fragrant molecules: limonene and citral in the first place. If we take a look at the molecular structure, we understand the significant difference between the reactivity of these two compounds.

Limonene is a sturdy molecule that belongs to the family of terpenes. You can mistreat it to a certain extent, and it will still be fine. It is a hydrocarbon, there is not much one can do to it unless one targets the double bonds. What I have drawn here is the stereoisomer found in lemon (question for those chemists who loved organic nomenclature as undergrads: is it R or S ?).


Citral, instead, seems poised to shortcut and curl into a comfortable 6-membered ring, with a little help from some acid, and all it takes for citral to become an aromatic molecule is some oxidiser. And beware: citral is actually a mixture of two isomers, neral and geranial, having a different smell.


Too bad citral falls apart in harsh conditions: a lemon-scented bleach or household detergent is the hallmark of cleanliness; if only citral were not so delicate! The quest for a substitute that could be stable in acidic and/or oxidising conditions has almost acquired a legendary status in the perfumery literature. It is cited for example in The Secret of Scent by Luca Turin, a must-read for perfume lovers, and maybe the focus of a future blog post.

Back to the kitchen, how could we introduce some acidity and some lemon-like flavours in the hummus-to-be? Spicing it up with the right ingredients, that was the way to go, but what to add? Fortunately, the kitchen larder was open and lots of spices were beckoning me over: I would quickly find what I needed.

First I sprinkled the hummus with a dark-red powder having a distinctive, tart note: there was sumac, hailing from the same part of the world as hummus itself, and the combination of the two is a match made in heaven. Sumac is a very peculiar spice in that it imparts acidity thanks to its high content in…yes, here we go, malic acid. Moreover, sumac came in with an additional bonus: it also contains some limonene.

Then I opened a small bag full of a brownish powder, and I savoured the fresh smell of ground coriander, its citral notes chiming with the taste I was trying to compose. I sprinkled the hummus with some coriander, I mixed, then I added a bit of olive oil and served. In the end, the lemon-free hummus was given the thumbs up by my friend.

But can one really assemble a lemon from, er… ‘first principles’, bottom-up? Do chemistry and cooking, which are both essentially combinatorial in nature, really have something in common?4

The curse of the alchemist

Regardless of the answer to those questions, what matters here is the idea of artefact contrasted to natural. Oh, here’s another Pandora box popping open…I promise that I will keep it short and simple (look elsewhere5 for the small print), while trying to be careful as I cross this minefield.

In a nutshell, (al)chemists have often been accused of concoct counterfeits, bogus surrogates of the real stuff, their ‘crime’ straddling plain quackery and outright fraud. From the arguments of Scholastic philosophers to today’s alleged supremacy of the natural (whatever it means) over the artificial or the chemical, the perceived image of (al)chemists seems to follows, at least ostensibly, a unique thread throughout the centuries. Dante Alighieri, in his Divine Comedy, places alchemists deep down in hell,  in the Eighth Circle along with fraudsters, and more specifically in the same bolgia (a hole in the ground) as falsifiers. For the 10th-century philosopher Avicenna, alchemy is a form of deception, while the great Thomas Aquinas dismissed alchemy simply as an anomaly outside nature (‘praeternatural’). Today, the label ‘natural’ is often misused as a tacit antonym of ‘chemical’, leading to all sorts of paradoxes, chemistry as a scientific discipline being caught in the crossfire.

Yet, is the rift between the natural and the artificial still so deep? After all, the fear of some chemicals could easily coexist with an appreciation of others5, perceived as a boon to the society’s well-being. Indeed, the 2015 survey Public attitudes to chemistry in the UK by the Royal Society of Chemistry shows that people are quite pragmatic about chemicals, to say the least. Figures show that 60% of the public agree that “everything is made of chemicals” and 70% are in agreement with the statement “everything including water and oxygen can be toxic at a certain dose”. Paracelsus is alive and well, and we chemists should really take the time to sit down and reflect on our own preconceptions about the chemophobic public.

Calvino’s tarots

Calvino prematurely died 30 years ago. His legacy to Italian literature and culture is still under debate: some critics find that Calvino’s style and poetics have provided the following generations of writers with an invaluable framework; others, however, believe that his influence has ended up looming large on Italian literature, more as a magnificent hurdle to overcome than as a reference point.

At that time of his death, the writer was working on a series of lectures on literature to be delivered at Harvard. Fortunately, his notes were edited and collected in a book available in English translation, Six memos for the next millennium. Six seminars, each on a ‘value’ (and its opposite) that Calvino considered worthy of attention with an eye to 21st-century literature. Six memos for the next millennium and Invisible cities are similar in that they single out beacons, vital reference points helping us to navigate, respectively, the vast universe of literature or the everyday chaos that surrounds us. But like any map, we are free to choose if to trust and follow them or not.
At this stage, however, I would like to remember another book, The Castle of Crossed Destinies, in which Calvino crafts short stories by interpreting tarot cards randomly placed on the table. In this novel, mute characters who meet by chance in a castle or at an inn try to tell their own stories by picking, laying and arranging tarot cards; an onlooker, standing in for the writer,  watches and tries to work out what the other characters try to narrate. From this point of view, The Castle of Crossed Destinies also draws on those great works of 14th century literature, Decameron by Boccaccio and The Canterbury Tales by Chaucher, which feature a group of characters taking turns at telling stories -and turns are part and parcel of all card games.
Anyway, in one of these tales, Calvino sees the trump card of The Magician (aka The Juggler, or Le Bateleur in the French original) as a proxy for the (somewhat stereotypical) alchemist, who tells how he sold his soul to the devil. Sounds familiar?

[…]our companion was, in fact, one of those scholars who scrutinize alembics and crucibles[…]trying to wrest from Nature her secrets, and especially that of the transformation of metals[…] from his earliest youth […]he had no other passion […] save the manipulation of the elements, and for years he had waited to see the yellow king of the mineral world precipitated in the depths of his cauldron.[…] This event must have been indicated in the following card, which was the enigmatic First Arcanum, sometimes known as  The Juggler, in which some see a charlatan or magician performing his tricks […].

(The tale of the alchemist, pp. 16-17)6

The second tale talking about alchemists is a much more intriguing piece, in my view: the alchemist is compared and contrasted to the figure of the knight-errant, and both of them come in with an alter ego, Doctor Faust (who else?) for the alchemist, and Perceval for the knight-errant. Put it in chemical terms, I see two resonance structures of the same molecule: the alchemist who “must (instead) free himself of all egoism […] to achieve transformations of matter“, who “tries to make his soul become as unchangeable and pure as gold” and Doctor Faust, “who inverts the alchemist’s rules, makes the soul an object of exchange” (Two tales of seeking and losing, pp. 90-91).

So, what kind of (al)chemist do you think you are?

Only now that I look back do I realise that this post has gone a long way, from Liguria through hummus to Calvino and eventually the Faustian ambitions of his alchemist. It is as if I were trying, as Marco Polo did in his travel accounts to Kublai Khan in Invisible Cities, to give an overview of the entire known world by piecing together his memories. Had he met the Faust of The Castle of Crossed Destinies, he would have agreed with him:
There is not an all, given all at once: there is a finite number of elements whose combinations are multiplied to billions of billions, and only a few of these find a form and a meaning and make their presence felt amid a meaningless, shapeless dust cloud; like the seventy-eight cards of the tarot deck in whose juxtapositions sequences of stories appear and are then immediately undone” (Two tales of seeking and losing, p. 97).

Faust, this time I have got an ace up my sleeve, too.
Such stuff, that all is made on.



1. Neighbours are often too similar to love each other. So, there was a time when Pisa and Genoa, too, were at war. Memories of ferocious naval battles and bloodshed are still alive in the two cities. The same goes with the ongoing Israeli-Palestinian conflict. A humble suggestion: the chickpea could be the seed of durable peace… When I first wrote this footnote I was kind of joking, now I have found out that someone in Isreal has already done so: “Hummus joint gives Jewish-Arab tables 50% off“. As the newspaper writes with a witty humor, that’s chickpeace.

2. On Food & Cooking, Harold McGee, Hodder & Stoughton, 2004

3. Liu, Y., et al., History, Global Distribution, and Nutritional Importance of Citrus Fruits, in Comprehensive Reviews in Food Science and Food Safety, 2012, 11, 530–545. doi: 10.1111/j.1541-4337.2012.00201.x

4. According to a Lebanese cook who took part in a food programme on the French radio, On va déguster, replacing lemon with sumac in hummus is tantamount to blasphemy. If you speak French, you can listen to the programme, focussed on Middle Eastern cuisine from Aleppo to Tel Aviv through Lebanon.

5.Chemistry: The Impure Science, Bernadette Bensaude-Vincent and Jonathan Simon, Imperial College Press, 2012 (2nd edition).

6. All quotes from The Castle of Crossed Destinies, translated from the Italian by William Weaver, Secker & Warburg (London), 1976

Open doors at Chemistry (2): Cupcakes on the Table

Periodic flavours of the elements

This post is a ‘slideshow’ of pictures of the exhibition on the Periodic Table organised by the Department of Chemistry of the University of Oxford on 19th September 2015, following the public lecture of the day before. One of its highlights was an enormous knitted periodic table: a team of fourteen members of the staff of the Department first painstakingly knitted the individual square ‘patches’ housing the elements, which were then assembled, mounted on fabric and completed with plastic letters printed on a 3D printer.

Aerial view of the knitted Periodic Table…
…and a caption putting it into the wider context

Gordon Woods, enthusiastic collector of Periodic tables and related items, provided a magnificent Ukrainian version. Securing it to the wall required the same titanic effort that, long ago, Mendeleev put into trying to make sense of the periodicity of the elements!

A different arrangement for the same elements

The complete set of the elements, a generous loan from Max Whitby and his colleagues at http://www.periodictable.co.uk, was another leading piece of the exhibition, garnering again much attention from visitors of all age. Two metal cylinders, one made of magnesium, the other made of titanium, were tremendously useful in demonstrating the dramatic difference in weight between lighter and heavier metals.

The hands-on table

A series of posters addressed the historical development of ideas about the periodic classification of the elements, along with the philosophical significance of the periodic system of the elements and its influence on culture and literature.

A first poster showed the timeline of the long journey from simple substances to atomic numbers. Note the accumulation of red-dot years just after the Karlsruhe Congress.

Periodic classification of the elements from Geoffroy to Moseley

On the philosophy poster, Aristotle and Epicurus duel to establish the structure of matter and the nature of the elements; at the same time, chemistry tries to reassert its own peculiarity, dispelling the undeserved reputation of being a lesser science than physics or biology, without great ideas. Well, what about the periodic law and the chemical bonding, then? In addition, the continuous development of the ideas on the periodic classification of the elements over two centuries provides a stark contrast to the model of scientific progress proposed by philosopher of science Thomas Kuhn (quieter periods of consensus – the so-called normal science – come abruptly to an end when breakthroughs challenge the previous paradigm, marking the onset of a revolutionary phase).

Next, we explore the interface between the periodic system and literature. Starting from a brief overview of three concepts, periodicity, system and element, which feature for example in postmodern narrative, we address an interview with late Oliver Sacks, who, while terminally ill, compared the years to his life to the atomic number of the elements that he had in front of him on his desk (see the set above). Growing old is a form of nucleogenesis of heavier elements, and, alas, he stopped at 82 last month. The highlight of the poster, however, is Primo Levi’s collection of short stories Il sistema periodico (lit. The Periodic System, but translated in English as The Periodic Table). Excerpts from five stories feature on the poster, and this passage from Silver is an apt description of Primo Levi’s intentions when he set about writing and assembling his book:

I was in search of events, mine and those of others, which I wanted to put on display in a book, to see if I could convey to the layman the strong and bitter flavor of our trade, which is only a particular instance, a more strenuous version of the business of living. […] It did not seem fair to me that the world should know everything about how the doctor, prostitute, sailor, assassin, countess, ancient Roman, conspirator, and Polynesian lives and nothing about how we transformers of matter live: […] I would deliberately neglect the grand chemistry, the triumphant chemistry of colossal plants and dizzying outputs, because this is collective work and therefore anonymous. I was more interested in the stories of the solitary chemistry, unarmed and on foot, at the measure of man, which with few exceptions has been mine“.

For Levi and Sacks, elements are material symbols, stepping stones emerging from the turbulent stream of our lives, which offer us a grid to make sense of the “business of living”, allowing us to keep afloat when facing sorrowful memories, or aiding us to come to terms with our own mortality.

Primo Levi’s portrait shown on the poster is a caricature kindly provided by Italian caricaturist, humor artist and illustrator Marilena Nardi,

20150919_103632Older and newer periodic tables were displayed side-by-side: on the left, a photographic reproduction of a 1920s Periodic Table in use at the University of Turin at the time when Primo Levi was reading chemistry at that university (courtesy of the Archivio Scientifico Tecnologico dell’Università di Torino and the Centro Studi Primo Levi); on the right, a more recent, highly visual version. Which is your favourite?

20150919_104542Elemental cupcakes, arranged in the familiar sequence of periods and groups, were offered to visitors to raise funds for the Sobell House Hospice, in memory of colleague and friend Dr Kristína Csatayová.

Lastly, a few words on my personal experience as science communicator. The audience was primarily composed of Oxford alumni (after all, the exhibition was organised within the framework of the Oxford Alumni Weekend) from all walks of life, whose background was, as a consequence, extremely varied, ranging from no prior knowledge of chemistry, or a lay interest in the discipline, to a decade-long career as chemistry teacher in secondary schools. I quickly realised that I would need to brace for a volley of questions covering the entire gamut of all things periodic. At first, I found it challenging to pitch my answers at the right level, but then I sort of naturally adjusted, striking a balance between depth and breadth. In particular, it was the gist which had to be conveyed in the clearest way, and it was very helpful for me to think in terms of ‘take-home messages’, as in scientific talks. I remember this elderly lady with curious eyes and a keen interest in the poster on the historical evolution of the periodic classification of the elements, who asked me to help her to understand how the periodic system eventually morphed into the familiar arrangement. I was aware that I could never have covered the entire ground from Geoffroy to Moseley, and so I decided to compare and contrast the figures of Lavoisier and Dalton. Luckily, she had a smattering of history of chemistry, which made my task massively simpler, and, in her words, very successful.
On the other hand, I could also mention the Socratic attitude of another visitor: I let myself be dragged into a philosophical discussion on periodicity (and I am proud that I could stand my ground), which eventually led us to agree that individual elements are material simulacra: at the end of the day, what matters is the position in the periodic system, the properties associated with it, and the network of relationships creating groups and periods.

If you are an art lover, do not forget to visit the exhibition Art of the Elements at Compton Verney art gallery! A radio review was broadcast by BBC Radio 4 and is available online.

And I would like to conclude, it goes without saying, by acknowledging the entire team of volunteers who worked behind the curtains and on stage to contribute to this very successful exhibition on the Periodic Table.

Open doors at Chemistry (1): Unbolting the Periodic Stable

The nobility of Man[…] lay in making himself the conqueror of matter, and [that] I had enrolled in chemistry because I wanted to remain faithful to this nobility. That conquering matter is to understand it, and understanding matter is necessary to understanding the universe and ourselves: and that therefore Mendeleev’s Periodic Table, which just during those weeks we were laboriously learning to unravel, was poetry, loftier and more solemn than all the poetry we had swallowed down in the liceo*; and come to think of it, it even rhymed!

* liceo = Italian secondary school for students aged 14-19
Primo Levi, Iron, in The Periodic Table, translated from the Italian by Raymond Rosenthal

The façade of the Inorganic Chemistry Laboratory, South Parks Road, Oxford

Friday 18th September 2015 marked the beginning of a series of events unfolding at the interface between chemistry and art and culminating in the exhibition Periodic Tales: Art of the Elements, which will take place at the Compton Verney Art Gallery (Warwickshire, UK) from 3rd October to 11th December. This art exhibition was launched during a special public lecture at the Inorganic Chemistry Laboratory (Department of Chemistry) of the University of Oxford, one of the first events of the 2015 Alumni Weekend of the University. On the following day, the programme continued with tours of the Department of Chemistry and a drop-in exhibition on the Periodic Table.

What follows is an insider’s account of the lecture and the exhibition, as seen through the lens of a member of the Alchemists, the outreach volunteers of the Department of Chemistry who guided the tours and contributed to organizing and staging the Saturday exhibition. If you want to see with you own eyes and relive the moment, have a look at the video/audio recordings of the Friday public lecutre, available online.

Vicious patterns and shiny reflections: the multifaceted personalities of the elements

No matter how materialistic our philosophical stance may be, we all agree that the human nature is unfathomable, a mystery still to be unravelled. Yet, one could possibly simplify this daunting challenge by trying to identify recurring patterns, repeating units that contribute to shaping who we are. The artwork Fuse (cast iron, Antony Gormley), reproduced on the poster of the public lecture Periodic Tales, can be seen at one time as a poignant reminder of this relentless quest and as the result of the recombination of elementary building blocks in the form of polyhedra. After all, weren’t the elements of ancient philosophy shaped, according to Plato, as regular solids? On the other hand, a more chemical eye could identify in Gormley’s polyhedral man the typical profile of electron microscopy images of metal nanoparticles, with their (more or less well-defined) facets and different crystal orientations.

The poster of the public lecture “Periodic Tales”

When I arrived at the lecture theatre, the audience was flocking in, and I decided to perch onto one of the seats at the back, ready to follow Periodic Tales from this vantage point. No sooner had I taken my seat than I realised that chemical elements were already clashing in front of me, as my very 20th-century notepad-and-pen combination sat next to a sleek laptop computer. Silicon and rare earths versus oxygen, hydrogen and carbon: an apt prologue to the tales of the elements.

Three speakers took turns on the floor before the question time: Hugh Aldersey-Williams, writer and author of Periodic Tales1; Georgiana Hedesan, Wellcome Trust postdoctoral research fellow in Medical Humanities, and Peter Battle, professor of Chemistry at Oxford. The lectures were introduced by Philip Mountford, head of Inorganic Chemistry at Oxford, and Stephen Tuck, director of The Oxford Reserach Centre in the Humanities (TORCH). Periodic Tales is one of the last events of the 2014-15 series of TORCH lectures, book discussions, seminars and debates exploring the frontiers between humanities and science.

Hugh Aldersey-Williams briefly surveyed the landscape of elements and their relevance to literature, art and popular culture: as the author remarks on his website, “the elements come to us“, sometimes openly declaring their presence, while concealing themselves at times. Given my passion for the written word, I found the literary quotes particularly intriguing, in particular the following from Shakespeare’s Merchant of Venice, which, in the author’s own words, was a veritable “epiphany” for the entire project of Periodic Tales. Portia, the leading female character of the play, offers her suitors three caskets, made of gold, silver and lead, only one of which contains Portia’s portrait. He who finds it will win her hand. Aldersey-Williams explained that he regarded the caskets as material allegories of human vices and virtues, inextricably correlated with the character of the different metals (following T.S. Eliot I would say that the caskets are objective correlatives). In Shakespeare’s own verses2:

The first, of gold, who this inscription bears,
‘Who chooseth me shall gain what many men desire;’
The second, silver, which this promise carries,
‘Who chooseth me shall get as much as he deserves;’
This third, dull lead, with warning all as blunt,
‘Who chooseth me must give and hazard all he hath.’
How shall I know if I do choose the right?

A suitor of Portia’s wondering which casket to choose

Thus, gold is the epitome of avarice, silver an invitation to trade (recalling the use of this metal as currency), while lead represents the fatalist choice, because, as the speaker remarked, lead has always been linked with fate, hazard, death and the acceptance thereof: for example, augurs could foretell the future by looking at molten lead that solidifies into bizarre shapes when cast in a bucket full of water. Lead was the material of choice for dice in the Roman world, because heavy dice fall more decisively: to have an idea, think of Caesar reportedly saying “Alea iacta est“, “The die is cast”, when crossing the Rubicon. That lead and death walk hand-in-hand can be explained by mentioning the toxicity of this metal, and this is such a pervasive topic, a powerful combination, that it keeps surfacing again and again in literature and culture: the short story dedicated to the dull metal in Primo Levi’s The Periodic System is a tale of Rodmund, a craftsman/miner of Germanic origins that roams Europe in search of (lead-containing) ores, to extract the metal which will make him rich while condemning him to fatal illness and death, the curse cast on his lineage by the daily handling of the heavy metal. Finally, lead is death by metonymy, as in bullets: Aldersey-Williams discussed an artwork by Cornelia Parker (which will be included in Art of the Elements at Compton Verney), Bullet Drawing (Crosshairs), in which the artist uses lead to trace the trajectory from cause and effect, the metal being the ‘material framework’ unifying them. Lead from a bullet was molten and cast into a net, which was then mounted on paper and distorted as if hit by the bullet.

The author went on addressing several other elements, but I will let the curious reader set off on his/her own personal exploration of the periodic table, perhaps using Aldersey-William’s book as a primer. What matters at this point is the principle inspiring Periodic Tales: elements have a certain meaning for the chemist, but this is intertwined with countless threads from culture, history and myth creating a richly patterned fabric. Looking at every single case in the Periodic Table is like opening the lid of a set of Chinese boxes: once an element is unpacked, its multiple references to our (material and immaterial) culture will pop out, prompting us to keep on exploring.  Aldersey-William’s lecture also prompts a comment on the term element and its multiple meanings. The writer pointed out that ten elements (in modern terms) were known in antiquity (being gold, silver, carbon, sulphur, lead, tin, mercury, antimony, iron, copper and zinc), while Aristotelian elements, as discussed in On Generation and Corruption, just amounted to four (air, earth, fire and water, aether being a fifth, and most obscure, element), each one bearing a certain property (hot, cold, moist, dry) to the highest degree. If one were to stick to Mendeleev’s rigorous notion of element, an abstract entity with an experimentally accessible property (the atomic weight), it would be more appropriate to say that ten “simple substances” of elementary composition were known in the classical world. But I do not want to sound pedantic or dwell too long on this issue in this post.

If there had been an alchemist among Portia’s suitor, he would surely have chosen the gold casket: after all, transmutation of lead into the most precious metal was never accomplished, and several experimental procedures (think of the discovery and use of aqua regia, for example) eventually ended up destroying gold, rather than producing it. This is what I learned as I was listening to the second talk by Georgiana Hedesan, historian of alchemy, who focussed on alchemy as the art of making gold. However, as Hedesan warned, transmutation of base metals into gold was just one of the many goals of alchemy. Gold has attracted humans since the dawn of times for several reasons: its shine (the Proto-Indo-European root for gold means ‘glow’3), its sun-like colour, its rarity; it is a soft, easily shaped metal but at the same time it is also resistant to chemicals and it does not tarnish; it can be turned into stunningly beautiful jewellery, but this requires specialised craftsmanship, much as transmutation calls for the “dark arts” of alchemy. Here is an interesting point raised by the historian: the alchemists did not want to exploit transmutation to become rich, but they were seeking knowledge which would raise them above their contemporaries. For those interested in the history of chemistry, this sounds familiar: alchemists (and later on chemists) were regularly accused of impious, boundless hubris, trying to recreate fake reproductions (‘artifacts’) of the natural world, and in doing so defying the order created by (one or another) god4. This resonates with the opposition natural/artificial so often heard when nasty ‘chemicals’ are pitted against supposedly benign, more wholesome alternatives; in terms of reflections of this theme in literature, just think of the arrogant scientist in Mary Shelley’s Frankenstein, or the modern Prometheus. Interestingly, alchemists aimed to produce gold also because it was thought to be a panacea for good health, a proxy for eternal afterlife; even today, an relic of the alchemical era survives as (Danziger) Goldwasser, a strong liqueur containing gold flakes and dating back to the end of the 16th century, approximately the period on which Hedesan’s own scholarly interests research focus. More recently, elemental gold in the form of nanoparticles has garnered intense interest in research projects across the board seeking to develop effective drug-delivery systems or treatments for cancer. But today’s chemical research is nowhere as universal in its aims as the body of the alchemical knowledge of the 17th century, which, as Hedesan showed, could produce works such as Atalanta Fugiens (1617), which combines poetry, image and music to describe alchemical practices. Its author, Michael Maier, gives us an insight into the vastly broad scope of later alchemy in another work, De Circulo Quadrato Physico, sive de Auro (1616). It was in Maier’s times that the printing press allowed the scrutiny of alchemical practices by uninitiated but “skeptical chemists”, while also offering the inspiration for satirical comedies such as Ben Jonson’s The Alchemist 4,5.

Gold and alchemy

Next came Peter Battle: his lifelong experience on the stages of international conferences was instrumental in delivering an engaging lecture touching on chemistry, art and aesthetics. After pointing out that chemists are mostly interested in compounds rather than elements, Battle said that he regards the latter as ‘building blocks’ and agents which introduce certain desired properties into the structures he and his group design and assemble (his research focusses on magnetic materials). This echoed a remark by the French chemist Michel Pouchard, as quoted in Chemistry: The Impure Science4: “The chemist is primarily the architect of matter as well as its mason; his scale is that of the nanometer, his bricks the hundred or so elements in Mendeleev’s periodic system, and his cement is their valency electrons“. Thus, not only are elements abstract concepts, as suggested by Mendeleev while he was working on the periodic system; they are also very practical raw materials for the wet-lab chemist, ‘bricks’ stacked to assemble structures such as this shown by Battle:

Symmetry in chemistry

The speaker emphasised that if one looks at chemistry zooming out from the level of the single elements to those of compounds, a different type of periodicity is evident: simpler elements repeat following an ordered pattern to create an overall regular structure. One of these simple elements is the unit cell of crystals, which has its eye-catching architectural counterpart in the Atomium in Brussels, a Modernist stainless steel utopia embodying faith in technological progress as instrument of peace.  For a chemist like Battle, the Atomium is a body-centered cubic unit cell, which incidentally is that of iron, the element which Primo Levi, in one the tersest short stories of The Periodic Table, conjures up to talk about freedom, dignity and life-changing choices in a dark Europe.  Battle also stressed that chemical architectures (and the word is not randomly chosen!), from molecules to crystals, often feature symmetry elements: the structure displayed here, for example, has a horizontal symmetry plane reflecting the top half into the bottom half. This is often mirrored by landscapes, such as in the superb view of the still surface of Lake Buttermere (Lake District, UK) reflecting the mountains which rise up towards the skies from the lake shore. The solid state of matter is also characterised by simple components that are repeated periodically, giving rise to a regular shaping, and this is reminiscent of architectural features such as the recurring, imposing arches of Sweetheart Abbey. In this respect, the chemist is definitely an “architect of matter”, and not surprisingly one of the most famous chemists of all times, August Kekulé, first enrolled at the University of Giessen as an architecture undergraduate, falling for chemistry after listening to Liebig’s lectures. Mentioning Kekulé, I cannot help thinking about organic compounds and the symmetries observed in molecular orbital theory, for example in the so-called Hoffmann-Woodward rules. Roald Hoffmann himself has often reflected on the role of aesthetics in chemistry and it is an impossible task to summarise his stance in a few lines here: let us mention at this stage that Hoffmann addresses the shape of molecules and crystals at the start of his 1988 landmark paper Molecular beauty.
What is art’s own image of chemistry? Battle started by showing a painting included in the exhibition Art of the Elements, specifically The Alchemist (there we go again!), by Sir William Fettes Douglas, on loan from the Victoria & Albert Museum. Have a look at the picture and you will understand why the speaker stressed the fact that, annoyingly (at least from the chemists’ point of view), there is an intimate association between chemistry and someone looking at a round-bottomed flask. Here Battle echoes a detailed analysis by Schummer and Spector on The Visual Image of Chemistry, in which the authors maintain that “Whenever today’s chemists want to be portrayed in such a way that anybody can recognize their professional identity, they usually hold up in their hand a flask filled with some liquid that they visually inspect. This posture has become the stereotypical visual icon of chemistry in self-portraits, professional photographs, and clip-art cartoons6 . Unfortunately, it is not ideal for chemistry to be represented by this image, because, as Schummer and Spector remark, it originates from an early medical practice, uroscopy, which would eventually be associated with charlatans and their fraudulent practices. In a sense, this unflattering representation does not come as a surprise, because, at least from the point of view of philosophy of science, chemistry suffered neglect for centuries. Despite his later “conversion”, most likely following Lavoisier’s breakthroughs, a giant of philosophy like Kant was instrumental in asserting the notion of chemistry as “improper science”7. This philosophical stance may have had a fallout in the visual arts, too: Battle showed An Allegory of Chemistry, by the Austrian painter Hans Makart, which belongs to the collection of the Belvedere Palace at Vienna. An Allegory of Chemistry depicts a scantily dressed woman (euphemism!) doing…you guess…yes, right, scrutinising a laboratory vessel, and without wearing safety glasses, as Battle wittily remarked! Compare this to a related painting, Allegory of the Sciences, and you see what Kant meant by improper science: first of all, chemistry has its own painting (an undesirable privilege, in this case), and the second painting depicts a group of serious, somewhat childish-looking scholars all perusing the vastness of the cosmos with telescopes and pondering over massive tomes.

Finally, are there limits to the two-way exchange between chemistry and art? Possibly, as Battle suggested, when remarking that a chemist’s scientific background might prevent a full appreciation of the symbolic message of an artist’s artwork when the latter clashes with chemical facts. So, Gormley’s Fuse uses red iron (oxide) as a proxy for the iron-containing blood in our bodies, but, in Battle’s eyes, Fuse is primarily nothing but rust with a collection of different surface orientations (facets). This is because the chemist knows that iron in blood haem and iron in rust are fundamentally different incarnations of the same element. I find this dichotomy truly fascinating as it brings up the clash between the artist’s freedom in imbuing raw materials with any desired symbolic meaning and the outlook on the world acquired through chemical lenses, which forces the observer to take into account the molecular identity of matter beside its macroscopic qualities (i.e. colour).

Beautiful matter?

Later on, the panel of speakers took questions from the audience. The first question addressed allotropes and how they have (or not) changed our conception of the elements. This prompts us to remind Mendeleev’s philosophical stance, underpinned by the distinction between simple substances (graphite and diamond) and elements (carbon). The second question touched on the equivalence symmetry-beauty, a truly broad topic. Georgiana Hedesan pointed out that symmetry was at the heart of the alchemical thought, from the figures used (squares, circles) to the fact that, supposedly, the four elements came in identical amounts. Peter Battle remarked that, although we tend to see symmetry as neat, too much symmetry might hurt (and – I add – sometimes unexpected properties of materials indeed arise from the suppression of long-range regularities and symmetry, the so-called “defects”). All of this makes me think of Italo Calvino’s reflection on literature in his essay Exactitude in his Six Memos for the Next Millennium, in which, discussing 20th-century literature, he pitted the party of the crystal against that of the flame: “Crystal and flame: two forms of perfect beauty that we cannot tear our eyes away from, two modes of growth in time, of expenditure of the matter surrounding them, two moral symbols, two absolutes, two categories for classifying facts and ideas, styles and feelings…

Again on the issue of symmetry and beauty, we should remember that symmetry in chemistry is a deeply mathematical concept, based as it is on group theory. Hence, we can recall what the philosopher of chemistry Joachim Schummer wrote in his sweeping paper on the aesthetics of molecules: “apart from early Pythagorean views on beauty in nature, it is difficult to find any source in the whole history of western theory of art that considers mathematical symmetry the essence of beauty. Instead, we have severe criticism of that idea as well as aesthetic theories based either on the alternative concepts of proportion and harmony or on the interplay of symmetry and asymmetry in a broad sense8. Schummer’s remark is at odds with what is generally perceived as a natural relationship between between beauty and harmony of proportions, as in Palladian villas or Leonardo da Vinci’s man. As a final comment on symmetry, progress in the periodic classification of the elements took off when scientists stopped trying to force all elements in neatly symmetrical groups and periods (see for example Gmelin’s V-shaped periodic system, almost perfectly symmetrical), allowing for the existence of separate subgroups.

Finally, the audience raised the issue of the aesthetic value of scientific photography, which is becoming increasingly popular as demonstrated by the growing number of contests organised on this theme. I have always wondered what contributes to making a scientific photograph a masterpiece; in other words, which are the, so to say, ‘aesthetic criteria’ of scientific photography? Is it the technical challenge of the experiment producing a certain image? Or, rather, a purely visual appeal of the final picture? Perhaps a combination of the two? Take for example, the EPSRC Science Photo Competition 2015; applicants must submit “a jargon-free extended caption (maximum 150 words) putting the research into context. The caption must be comprehensible to the lay reader and will be taken into consideration by the judges when making their decision“. This central importance of the caption seems to mirror the fact that much contemporary art, in particular in its more conceptual forms, heavily depends on the title to ensure a complete appreciation of a certain artwork.

Ask the chemist, ask the writer

At the end of the question time, the audience streamed to the ‘Abbot’s Kitchen’, one of the oldest sections of the Inorganic Chemistry Laboratory, dating back to 1878.

The visitors could admire a complete collection of the elements of the periodic table; polonium was definitely in the limelight, because of its dark role in the assassination of Alexander Litvinenko.

A full set of the elements of the Periodic Table

Yet, the engagement with the public has always surprises in store: as I was supervising the precious box full of elements, someone came and asked me about the current state-of-the-art in the research on lithium batteries, inspired by the chunk of silvery metal high in the top-left corner of the table.

Lithium galore…and Swiss cheese?

In my view, improvised conversations like these are public engagement at its best because scientists do not have time to get bogged down in technical jargon and the layman can ask all sorts of questions for the sake of sheer curiosity. So, the “fourth wall” between the chemical stage and its lay audience is finally broken.

During the drinks reception I also had the pleasure of chatting with Hugh Aldersey-Williams for a short while: this was a great opportunity to find out more about what it means to be a writer. In my naivety, I was surprised to discover that a writer’s literary agent does contribute in steering the author’s pen towards this or that topic. A timely reminder of the never-ending struggle between the artist’s or the scientist’s creativity and the forces harnessing and reining it in, such as patrons’ own conception of art, and their political or strategic orientations: patronage and support of the artists have always come at a price.

The Elizabethan age and the Queen’s patronage of Shakespeare may well be history, but new characters play on the stage of the digital age, from literary agents to funding agencies and research councils.

And still we stand in front of locked caskets: which one hides our modern Portia, that coveted research grant ?

  1. Periodic Tales, Hugh Aldersey-Williams, Penguin Books, 2012
  2. Act II , scene 7
  3. The root being, as reported on Wikipedia, the slightly scary combination *h₂é-h₂us-o-, from which Proto-Italic *auzom which turned into Latin aurum.  ‎
  4. Chemistry: The Impure Science, Bernadette Bensaude-Vincent and Jonathan Simon, Imperial College Press, 2012 (2nd edition).
  5. Chemistryworld, August 2015: A shared secret?
  6. J. Schummer and T.I. Spector, The Visual Image of Chemistry, HYLE, 2007, 13, 3-41
  7. J. Van Brakel, Kant’s Legacy for the Philosophy of Chemistry, in D. Baird et al. (eds.), Philosophy of Chemistry, Springer, 2006
  8. J. Schummer, Aesthetics of Chemical Products, HYLE, 2003, 73-104

False Wagen

The exhaust contains principally three primary pollutants, unburned or partially burned hydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides (NOx), mostly NO, in addition to other compounds such as water, hydrogen, nitrogen, oxygen, etc. Sulphur oxides, though polluting, are normally not removed by the post-combustion treatments, since the only effective way is to reduce them to elemental sulphur, which would accumulate in the system. Accordingly, it is preferred to minimise sulphur emissions by diminishing the sulphur content in the fuel.

J. Kašpar et al, Automotive catalytic converters: current status and some perspectives, Catal. Today, 2003, 77, 419, doi:10.1016/S0920-5861(02)00384-X


For a chemist who likes cars the tailpipe is the best place to be. One day we will fill our clothes irons with water from our fuel-cell powered cars (and how soon it happens it partly depends on white coats like me…what a weighty responsibility!); for the present, the chemistry of catalytic converters is already interesting enough. So, let us take inspiration from the recent news and have a (very) quick look at it. Luckily, days in the laboratory are dotted with time-outs between experiments, which gives me a handful of snippets of time that I can devote to writing.

Recently, there has been extensive media coverage of a test-rigging scandal involving a major German car manufacturer, which has admitted to equipping its Diesel cars in the US and Europe with an illegal software to cheat during emission tests. As a car enthusiast since the ripe age of 3 (ask my parents), I have often read the road tests published in car magazines, regularly reporting a higher fuel consumption than that advertised by manufacturers in their sleek brochures. However, this is not particularly surprising, because consumption (and emission) tests are defined by standard procedures based on stationary measurements performed on a test rig. Yet, this time the car manufacturer seems to have stepped up its game: after all, when competition is cut-throat, such as in the automotive market, cutting corners is the only way to up the ante and survive, right? I have grasped a rough idea of how the system actually worked by listening to a science programme on the Italian public radio, and so I warn you that what follows is not an exhaustive (sorry for the silly pun) explanation: official enquiries have been launched to uncover the details. In a nutshell, the software running on the electronic control unit featured, along with several ‘legal’ operating modes (those options with catchy names like “sports”, “eco”, “city”) a low-nitrogen oxides (NOx) routine which activated when only two wheels were revolving (exactly as on the test rig). Under these conditions, the emissions of NOx dramatically decrease, falling below the pass threshold of the test. However, low emissions mean higher fuel consumption, and so this operating mode would never be practical while driving on the road, because low consumption is the main asset of a car, at least from the point of view of parsimonious buyers. At any rate, the whole affair will at the very least sap the consumer’s confidence in this car make, and in this case it is going to be a headlong crash from a high reputation. Cars, like friends, come in two different categories: those you go out with just to have fun, and those who you can always trust and rely on regardless of what life throws at you: German cars definitely (used to?) belong to the latter.

As a chemist, it is somewhat flattering to think that nowadays all motor vehicles are chemical reactors on four wheels, equipped as they are with catalytic converters. First of all, mind your words: catalysis is a learned combination from Ancient Greek, meaning ‘dissolution’, among other things1, and when it comes to cars we will be talking about heterogeneous catalysis, because the characters of this play belong to different phases of matter. For hetero-, have a look at a previous post on heterolysis, a word which, yes, recycles once more that Greek root that chemists like so much, λύω,  ‘I release, I set free, I unharness’. Maybe chemists unconsciously think of their discipline as a means to ‘unbind’ a Promethean power which lay locked away. A dangerous idea. More practically, when referring to things λύω means ‘I break up something into its component parts; I resolve’, which would be analysis in Lavoisier’s terms, one of the two poles of chemistry along with synthesis. Anyway, as Gerhard Ertl reminded in his 2007 Nobel Lecture, Jöns Jacob Berzelius was the first to use of the word to describe a chemical phenomenon in 1835; another ‘first’ for this pole star of chemistry rising from the north to shine over the discipline in the first decades of the 19th century, as we saw elsewhere.

For the casual (and brave!) non-scientist who might have landed on this blog by chance, catalysis is the chemists’ equivalent of the fast-forward button in old video players: a videotape has its own normal speed at which we can play it, but what if we cannot afford to wait forever and we really want to watch that particular scene of our favourite film? In this case, catalysis fast-forwards the chemical tape taking us there where we want to go, and fast. In other words, there are reactions that just will not happen unless a fast-forward agent, a catalyst, is introduced in the system, and the catalyst’s secret is its ability to engage in a special relationship with the molecules taking part in the reaction. To visualise this “physical touch”, remember that the video recorder controls the playing speed with a spindle that plugs into a socket on the videocassette, but this connection is by no means permanent, because the cassette can be ejected when we wish to. So, a catalyst must somehow bind the reagents, but unlike diamonds (while sadly similar to most romantic relationships), this bond does not last forever.

As for the type of catalysis operative in catalytic converters, heterogeneous catalysis as we said above, there are three main ingredients:

  • a solid support coated with the catalyst (think of gilding a piece of wood or baser metal)
  • gases emitted by the engine during combustion
  • energy, usually in the form of thermal energy (say: heat)

Imagine an unusual ball game: there is a wall covered with the hooky fabric of touch fastener (“Velcro”), and two players with two different sets of tennis balls, one with the same hooky fabric, the other with a (more tennis-like) hairy fabric. The players start throwing their tennis balls, aiming to bind two balls of different sets. The normal tennis balls will stick to the wall, while the “hooky” counterparts will bounce off unless they hit a hairy ball in the right way and bind to it. When a ball-ball couple (a ‘dimer’) is formed, it is either heavy enough to fall off or light enough to keep hanging onto the wall, which means that we need to go and pull it away. As you can see, the players spend some sort of energy at least once (throwing the ball), and maybe a second time (separating the ball-ball couple from the wall).

Is this just a far-fetched metaphor? Maybe, but Prof. Ertl introduces automotive catalysts in his Nobel Lecture by talking about the following reaction

2CO + O2 → 2CO2

CO and O2 bang against the catalyst (Pt or Pd), CO sticks, O2 hits hard enough for it to fall apart into two O atoms, one of which can bind to CO if it finds some of it in the surroundings, and CO2 falls off as soon as it is formed. A harmful combustion product, CO, is thus transformed into CO2, which can be regarded as harmless. In this respect, a greenhouse gas can be seen as the lesser evil, but this turns up global warming another tiny notch. Bottom line: use the bike instead if you want to go green.

A burning issue

Let’s start from the basics, which reminds me of my undergraduate course in environmental and atmospheric chemistry. It feels such a long time ago. It was the autumn term of 2004 and I was looking forward to exploring this branch of chemistry, and learning how Crutzen, Sherwood Rowland and Molina had won their Nobel Prize in 1995 by unravelling the chemical underpinnings of ozone depletion in the stratosphere … unfortunately the course turned out to be the most boring ever: the lecturer delivering the classes did his best to confuse the audience, and so we hoped that the textbook would come to our rescue. To no avail: the book itself, seemingly typed on an old typewriter, was a dry collection of reaction cycles to be learned by rote. I remember long hours spent scribbling the reaction pathways of atmospheric chemistry on a small blackboard. Funnily, the OH radical rampaging all around, reacting with this and that molecule, is one of the few concepts that has stuck into my memory: for me it was, and is, a poignant metaphor of the ultimate embodiment of life’s wear and tear. After all, “free radicals” purportedly play a key role in ageing.

Memories aside, if one wants to have a rough idea of automotive catalysts it is good to point out a few concepts to start with. Car engines burn fuel to extract energy from it; combustion is a combination of three actors (the good old “triangle”), fuel, oxidant and “heat”, or any suitable source of energy (the spark in petrol engines, for example). Combustion is just yet another chemical reaction, and like all of them, it is wise to weigh out the reactants and make sure we can control their relative proportions. That’s stoichiometry, which reads as the “measure of the elements” 2. The relative proportion of reactants which satisfies the stoichiometry of the reaction can be named stoichiometric ratio. For a simple combustion like:

CH4 + O2 → CO2 + 2H2O

the stoichiometric ratio between methane and oxygen is one molecule to one molecule. In other words, if there is a molecule of oxygen available for every molecule of methane, all fuel is expected to become CO2, while no O2 is left at the end of the reaction. (This in a real world where there are no practical issues with the actual combustion). Put too much O2, and you will end up with some of it in the combustion exhausts; on the other hand, if you are economical with O2, some CH4 will survive the combustion unscathed, or burn to incomplete stage, CO, that requires half as much oxygen:

2CH4 + O2 → 2CO + 2H2O

Similarly, one can define stoichiometric ratios for internal combustion engines. The exact values of these ratios will of course depend on the type of fuel being burned: petrol (gasoline across the Atlantic, and poetic, ephemeral essence in Francophone lands) or Diesel fuel, but the difference between the two types of fuel is not massive. A good idea is to define the air/fuel ratio with respect to the stoichiometric optimal, and call it λ: if λ > 1, there is more air than required, which is called lean mixture in car jargon, if λ < 1 there is more fuel than determined by stoichiometry, leading to a rich mixture. The parameter is not carved in stone, it keeps changing as we drive around. Roughly speaking, if we are cruising in our petrol-powered car on a motorway, λ should be slightly higher than 1, while as we want to go flat out and we put our foot down during hard acceleration λ will be less than 1 to achieve maximum power.

If we now have a look at the combustion products, for two typical engines (Table 1 in a specialised review3, here reported in a modified and shortened version):

Exhaust components and conditions Diesel engine Petrol engine
NOx 350–1000 ppm 100–4000 ppm
Unburned hydrocarbons, HC 50–330 ppm C 500–5000 ppm C
CO 300–1200 ppm 0.1–6%
O2 10–15% 0.2–2%
H2O 1.4–7% 10–12%
CO2 7% 10–13.5%
SOx 10–100 ppm 15–60 ppm
PM 65 mg/m3
λ ≈1.8 ≈1

The exhaust gases contain several nasty fellows, for example CO and NOx; the catalytic converter needs to remove them, which is quite challenging considering that exhausts look like a hotchpotch. The following plot (reproduced here by myself with low-tech tools but originally appeared as a figure in a specialised review3) shows how typical pollutants vary with respect to air/fuel ratios:


The classical example of an automotive catalytic converter is the so-called ‘three-way’ catalyst for petrol engines, which is a fascinating, ingenious device. Its three tasks are:

  1. convert NOx into N2 (mostly done by rhodium)
  2. convert CO into CO2 (mostly done by platinum and palladium)
  3. convert unburned fuel hydrocarbons into CO2 (mostly done by platinum and palladium)

Task 1, the conversion to N2 is in chemical terms a reduction, which can be accomplished with the proper catalyst and a reactant that could act as the reducing agent. The simplest example is H2,which steals the oxygen becoming H2O in turn. As for the actual mixture of exhaust gas leaving the combustion chamber of the engine, one has to make do with the potential reducing agents already present in this exhaust mixture. Possible candidates are CO and hydrocarbons that have escaped complete combustion:

2NO + 2CO → 2CO2 + N2

NO + HC → CO2 + N2 + H2O (not balanced)

Going back to our image of balls hooking onto a Velcro wall, NO sticks onto the catalyst, and two NO molecules nearby will engage in what they think to be a fleeting liaison. Yet, this encounter will change them forever, as they shed their useless, acidic partner oxygen to make perfectly homogeneous pair, and they fly away together, looking forward an eternal, ethereal life as N2. Oxygen is left onto the catalyst, effectively taking up space for other NO molecules to stick, split and click. Individual oxygen atoms yearn for new partners as well, and they are ready for a getaway with any oxygen-loving molecule which happens to fly past. CO is a familiar match, small and thin, similar as it is to NO in many respects; partially burned hydrocarbons are instead burly blokes with charcoal on their faces, who look forward to a life-changing metamorphosis into CO2 by abducting as many stranded oxygens as possible. In a sense, a catalyst surface is the dance floor where couples meet, flirt and swap. Is this image, seemingly taken straight from Zygmunt Bauman’s essay Liquid Love, out of place on a scientific blog? Not really…lysis (λύσις) also means ‘divorce’ in Ancient Greek.

Promiscuity aside, reactions in task 1 help to remove CO and HC, which the pollutants that are to be tackled by tasks 2 and 3, showing a synergy in catalytic conversion. However, let us also note that tasks 2 and 3 involve oxidations: antipodal reactions with respect to the NOx reduction in task 1.

CO + O2 → CO2

HC + O2 → CO2 + H2O (not balanced)

Oxidation reactions like these ideally revel in the presence of extra oxygen. So, now we understand that a catalytic converter treads a narrow line in terms of contrasting demands, and the optimisation of its three tasks at the same time is a tall order achieved by continuous fine-tuning. It is the electronic control unit that calls the shots, finding a delicate trade-off is found. Another simple plot (again reproduced here by myself with chalk and blackboard but originally appeared as a figure in a specialised review3) will show how the efficiency of pollutant removal can plummet quickly as λ moves away from 1.


Managing oxygen is tricky also at a microscopic level, and for this reason catalytic converters also include oxides, for example CeO2 or ZrO2, which have several beneficial effects apart from physically “supporting” the catalyst; in particular, oxides can store extra oxygen when there is plenty, stockpiling a back-up supply of oxygen to keep running tasks 2 and 3 when, for example, a sudden surge in HC floods the catalytic converter as the driver goes at full throttle.

Diesel engines pose different challenges. The air/fuel ratio is higher than petrol engines, which makes it relatively straightforward to burn off CO and hydrocarbons to CO2 (functions 2 and 3 of the three-way catalyst) with a so-called Diesel Oxidation Catalyst (DOC), usually platinum on aluminium oxide (alumina). However, the DOC cannot remove the particulate matter4 produced by Diesel engines. On the other hand, more oxygen means a paucity of CO and unburned hydrocarbons in the exhaust and ready to be used to reduce NOx. This makes a traditional three-way catalyst ineffective for NOx abatement. To make matters worse, lots of oxygen means a very oxidising enviroment and, consequently, more NOx – which, as I narrated in more poetical terms in my previous post on the nitrogen cycle, forms whenever sparks fly in air.

Particulate matter and NOx have to be tackled simultaneously. There is the rub. The following excerpt from a 2011 specialised review sounds like a “writing on the wall”, and its take-home message (valid at the time of publication, but most likely still true today) is: when it comes to reducing particulate and NOx emissions of Diesel engines, striking a balance between the two is very challenging: “The performances of commercial catalytic post-treatment systems are not optimized to fulfill the forthcoming U.S.standard legislation and those that will be implemented in Europe near 2014, particularly the low limit of NOx emissions from diesel engines. The lean-burn engine is actually the most attractive solution combining low consumption and low CO2 emission, recognized for its greenhouse gas behavior. There are also apparent advantages in Europe that might explain a continuous expansion of the diesel car market related to the implementation of a favorable tax system. However, the suitability of this technology, from an environmental point of view implies the minimization of atmospheric pollutants, particularly nitric oxide emissions, which actually represent a serious drawback with no practical solution commercially available. Hence, while CO and unburned hydrocarbons can be easily removed, the simultaneous abatement of NOx and particulates from diesel exhaust gas represents an outstanding issue. The current three-way technology used near stoichiometric conditions is unable to meet upcoming regulations in Europe, United States, and Japan. The existing technical solutions […] involving an exhaust gas recirculation to get an optimal NOx/particulates compromise by controlling the recirculated gas rate or modifying the distribution channel will likely be unable to fulfill the next Euro 6 standard regulation. […] the implementation of an optimal strategy is not an easy task because a reduction of NOx induces an increase in particulate emission […] and reversibly subsequent reduction of particulate matter will induce an increase in NOx emission […]5.

What a catch-22…

NOXious gases

But I feel I am beating about the bush as I promised to take inspiration from the test-rigging affair to talk about catalysis and nitrogen oxides. Here we step into my own turf, and you can read more in a previous post on the nitrogen cycle.

There are various possible strategies to reduce the amount of NOx emitted by a Diesel engine:

  • Special NOx removal catalysts able to tolerate the excess oxygen in the exhausts
  • NOx “traps”, which exploit acid/base chemistry, using oxides or carbonates such as BaCO3 as alkaline counterparts to immobilise acidic NOx as nitrate, which is released and reduced as the engine briefly operates with a rich mixture.
  • Selective Catalytic Reduction, my favourite

Selective catalytic reduction is a reincarnation of a class of reactions called comproportionations, which involve two reactants containing the same element but in two different oxidation states. Most people will remember comproportionations from general chemistry courses as outlandish chimeras standing out from the crowd of ‘normal’ reactions. As for the nitrogen cycle, think of the violent comproportionation that accounts for the explosive decomposition of ammonium nitrate. This salt is intrinsically unstable right because, in its two moieties, it combines the extreme oxidation states of nitrogen, nitrate (+5) and ammonium (−3), which liberates a powerful entropic ‘kick’ by releasing a large amount of gases in a veritable free-for-all which ends in both nitrogen atoms being triply married to each other in a multiple wedlock.

2NH4NO3→ 4H2O+ O2+ 2N2

Selective catalytic reductions are the milder (and more useful) version of this explosion. Developed for stationary sources of NOx (say, fossil-fuel power plants), they require a suitable catalyst and a nitrogen-containing molecule willing to take the lift from a lower oxidation state to the stability of ground floor. Why not ammonia, I hear you say, and so let us take ammonia on board our Diesel cars…but how? Ammonia could for instance attack and corrode the walls of a reservoir, and spill out while we are driving around in our eco-friendly (but leaky, as my favourite burette) cars.

Yet, storing ammonia is just one of the many challenges posed by automotive SCR. Running the reaction at top conversion is another tall order. When looking at the catalyst, the image of the Velcro wall and hairy tennis balls can be conjured up once more. In fact, ammonia sticks very well onto the surface of these catalysts as long as the temperature is not too high (below 473 K, as reported in a topical review); when the exhaust heats up, for example when a heavy lorry goes at full throttle, ammonia can become loose again and do business. It is exactly under these hotter conditions that SCR can occur. Implementing SCR is by no means an easy task, as it requires continuous monitoring of engine and exhaust parameters to avoid, for instance, injecting an excessive amount of ammonia that would end up being released at the tailpipe. Stoichiometry shows up again, and a NH3/NOx ratio less than 1 is often preferred for operations (after all, a short foray into rich-burn conditions could provide extra reducing agents to remove NOx leftovers). The catalyst, once more a thin layer ‘gilded’ on top of a suitable support, is vanadia (V2O5), which is a crowded nightclub where molecules flirt and hook onto each other in all possible ways 5. There is some true SCR going on, like

4NH3 + 4NO +O2 → 6H2O + 4N2

8NH3+ 6NO2 → 7N2 + 12H2O

But the latter could turn into an acid-base neutralisation leading to an old acquaintance of ours, ammonium nitrate:

2NH3+ 2NO2 →N2 + NH4NO3 + H2O

And, last but not least, there is enough oxygen for ammonia to be oxidized on its own:

4NH3 + 3O2 → 2N2 + 6H2O

Vanadium is the DJ playing the tunes for this wild night out, cycling his records between an oxidised form (V=O) that is catalytically active and a reduced one (V-OH) that consumes oxygen to bounce back to action. A complete reaction equation would then be5

NO + NH3 + V(+5)=O →  N2 + H2O + V(+4)-OH

Instead of storing a tankful of ammonia, someone had a brilliant idea that would make Friedrich Wöhler revel in his grave: take urea, instead, and hydrolyse it on board when its needed. Controlling the amount of urea, hence ammonia, injected in the exhaust would also offer another degree of freedom to adjust SCR to engine performance in real time. A concentrated urea solution is vaporised prior to the actual SCR catalyst, and Wöhler’s most beloved molecule first breaks up into ammonia and isocyanic acid5

NH2CONH2 → HNCO (gas) + NH3

In the best-case scenario, the latter acid gives off a second ammonia molecule and CO2 in a following step:

HNCO (gas) + H2O → NH3 + CO2

Things could in principle also go awry: isocyanic acid could start one of those funny and silly versions of the ‘conga line’ in which (tipsy) people line up one after the other in a moving ‘train’. This polymerization could for example lead to melamine (yes, the one that ended up in babies’ powdered milk some time ago in China), which is not great news because it could clog the catalyst (think of no space anymore for single club-goers to move on the dance floor when the snake is around). Luckily, this will not happen if a catalyst for urea hydrolysis and isocyanic acid decomposition is chosen, for example TiO2.

To sum up, the sequence of treatment stages tackling Diesel exhausts is:

  • Diesel Oxidation Catalyst
  • Urea injection and hydrolysis
  • Selective Catalytic Reduction
  • Diesel Particulate Filter

Fed up with noxious car exhausts? Too bad, you deserve some stinky sewage then, but I promise, it will be just a quick dive.

The shortcut to heaven

Miracles happen, or at least sometimes those who seem to be misfits do become famous, eventually end up in the spotlight. This is probably the case for some bacteria, which have had their fair share of fame, rising from complete obscurity to gain a certain notoriety5 in 1999, while taking researchers by surprise. Their metabolism features a reaction called anammox (anaerobic ammonium oxidation), which plays an important role in removing nitrogen species from the oceans:

NH4+ + NO2 → N2 + 2H2O.

This time, no explosive metaphor is exaggerate: these bacteria succeed in handling an anion and a cation which, as a salt, few people would like to work with for fear of explosions. Like desperate lovers imprisoned in nearby cells, nitrite and ammonium will pull all the stops to break free, flee and fly away as N2, tearing down all walls in the process. When swallowed by these bacteria, their separation ends quietly without dramatic escapes. The story of the discovery of these bacteria is fascinating and deserves a post of its own. I happen to know it because I read some of the literature when I was working for my PhD research project at Leiden University. It was my supervisor the one to point out that other Dutch researcher, part of an international team, had identified the elusive bacteria responsible for anammox reaction6. This reaction is exploited in a patented bacterial process (having the same name as the reaction) for the treatment of ammonium-rich sewage or wastewater.

It was to my great surprise that, while I was doing research as a PhD student, we observed that the same recombination between nitrite and ammonia occurs under certain conditions on some surfaces of Pt catalysts when an electrochemical potential is applied7. This time, the catalytic Velcro wall must be of a very special shape for the molecules to stick: the most external Pt atoms must be arranged as if they were located on the corners of a square, and this for the entire surface. No matter how unrelated electrochemistry is to microbiology, the overall reaction

NH3 + NO2 → N2 + H2O + OH

takes place, and, for the most curious among you, I will also mention that the ‘stuck’ (e.g. adsorbed) hairy tennis balls seem to be in this case NO and NHx. The nitrogen cycle perhaps has another surprise in store: a research group based in Québec has proposed that the very same intermediates are involved in the oxidation of NH3 to N2 on the atomically square-shaped Pt surface, and once more performed by help of an electrochemical potential8, but conceptually equivalent to the oxidation of NH3 to N2 by O2 as seen previously in the discussion of the SCR process. Is this really an universal process common to a special Pt surface, ‘anammox’ bacteria and catalysts for NOx removal from Diesel exhausts? Only time will tell, but at the moment it really seems so.

In conclusion, whatever happens in my future career as a scientist, I can already look back and feel somewhat proud, because my own investigation of the nitrogen cycle has followed Roald Hoffmann’s guiding philosophy for chemistry research: “look at hundreds of small(er) problems in chemistry, and keep in mind the connections that must be there” and you “will see the chemical universe” 9. A series of experiments part of a PhD project in electrocatalysis started unravelling a skein of yarn, pulling a thin Arianna’s thread that would lead to the world of SCR for Diesel exhausts and some puzzling bacteria. Eventually, the yarn has been rolled into a ball, and a single line connects all the dots in an elegant common pattern.

A star-studded sky waiting for curious eyes to recognise and trace more new constellations: such is the chemical universe.

Just look above you, and follow the exhausts.

  1. The verb καταλύω encompasses a range of meanings which look quite nasty, such as (in the order reported by this online dictionary) “I destroy”, “I dissolve” (of a political system), “I bring to an end” (of life as well). The name κατάλῠσις follows suit, meaning “dissolution” (of a political system), “disbanding” (a crew, a group of men). If I miss any meaning, forgive me: I used to know someone who had studied Ancient Greek but we are not in touch anymore.
  2. Jeremias Richter (1762-1807), starting from a Kantian perspective and following the “dynamist” approach to chemistry, tried to quantify properties such as acidity and basicity.  Stoichiometry was developed with a view to establishing a “neutralisation” law for the formation of salts (see Chemistry, the Impure Science, Bernadette Bensaude-Vincent and Jonathan Simon, Imperial College Press, 2012).
  3. J. Kašpar et al., Automotive catalytic converters: current status and some perspectives, Catal. Today, 2003, 77, 419, doi:10.1016/S0920-5861(02)00384-X
  4. Defined in J. Kašpar et al., Automotive catalytic converters: current status and some perspectives, as “the most complex of diesel emissions. Diesel particulates, as defined by most emission standards, are sampled from diluted and cooled exhaust gases. This definition includes both solids, as well as liquid material which condenses during the dilution process. The basic fractions of DPM are elemental carbon, heavy HCs derived from the fuel and lubricating oil, and hydrated sulphuric acid derived from the fuel sulphur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates contains (sic) small nuclei with diameters below 0.04 μm, which agglomerate forming particles as large as 1 μm. The non-gaseous diesel emissions are grouped into three categories: soluble organic fraction (SOF), sulphate and soot
  5. P. Granger and V. I. Parvulescu, Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies, Chem. Rev., 2011, 111, 3155–, doi: 10.1021/cr100168g
  6. M. Strous et al., Missing lithotroph identified as new planctomycete, Nature, 1999, 400, 446-, doi:10.1038/22749
  7. M. Duca et al., Selective catalytic reduction at quasi-perfect Pt(100) domains: a universal low-temperature pathway from nitrite to N2, J. Am. Chem. Soc., 2011133, 10928-, doi: 10.1021/ja203234v
  8. D.A. Finkelstein et al., Mechanistic Similarity in Catalytic N2 Production from NH3 and NO2 at Pt(100) Thin Films: Toward a Universal Catalytic Pathway for Simple N-Containing Species, and Its Application to in Situ Removal of NH3 Poisons, J. Phys. Chem. C, 2015, 118, 9860-, doi: 10.1021/acs.jpcc.5b00949
  9. As reported in Q & A Roald Hoffmann: Chemical connector, Nature, 2011, 480, 176, doi:10.1038/480179a

Karlsruhe United

The scales fell from my eyes and my doubts disappeared and they were replaced by a feeling of quiet certainty.

Lothar Meyer, co-discoverer of the periodic law, recalling the Karlsruhe Congress
(as quoted in: Periodic Table: Its Story and Its Significance, Eric R. Scerri, Oxford University Press, 2006)

Have you ever experienced the same eureka moment while getting back home from a meeting? Personally, I have often had sudden brainwaves or flashing ideas while listening to someone’s presentation, but, honestly, never have I felt so close to universal enlightenment as Lothar Meyer did in Karlsruhe in 1860. I am not as brilliant, and, besides that, the scope of my research is not as universal as the quest for a comprehensive periodic law for all the elements.

This month marks the 155th anniversary of a milestone in the history of chemistry: the International Meeting which took place at Karlsruhe from 3rd to 5th September (yes, I know, my post is a few days late…I have been awfully busy as of late). Not only is this congress extraordinary because it is the first international scientific conference ever1: this is also an example of an apparently inconclusive meeting that instead sparked an electrifying sequence of (philosophical and scientific) breakthroughs in the research of a periodic system of the elements. Like a stone thrown in a calm lake, the ripples of the Karlsruhe congress reached the shore only after a while. Scientific ideas migrate slowly: it takes time for them to come home to roost.

No formula one

Say acetic acid, and you know what it is, well, at least what it smells and tastes like. What about its formula? Nowadays it is hard even to imagine that there was a time in chemistry when a textbook could list as many as nineteen different formulae:

By A. Kekulé (edit NobbiP (Lehrbuch der Organischen Chemie, 1861) [Public domain], via Wikimedia Commons, from https://commons.wikimedia.org/wiki/File:Kekule_acetic_acid_formulae.png
To make matters worse, there were even more fundamental problems affecting mid-19th century chemistry:

  • Atomic and/or equivalent weight: Different laboratories used one or the other quantity, which led to an astounding variability in the chemical formulae of chemical substances. In other words, oxygen could weigh 8 or 16 (with respect to hydrogen). The former is the equivalent weight of oxygen in water, the latter its atomic weight. The problem partly stemmed from the fact that equivalent weights have a more immediate significance when doing experiments: they immediately show the mutual proportions of the reactants of a chemical reaction. Atomic numbers, on the other hand, depend on the determination of an empirical formula. In addition, the available values of atomic weights were often significantly inaccurate.
  • Nomenclature: ‘Atom’, ‘molecule’, ‘radical’, ‘equivalent’: there was no common guideline for the usage, and the meaning, of these terms.
  • Theories of the structure of matter. This is where the fundamental problems lay. The remarkable expansion of organic chemistry outpaced the reflection on the structure of matter. Alternative theories of matter coexisted but they were not universally applicable, being mostly suitable for a single class of compounds. For example, Berzeliuselectrochemical theory, which interpreted all chemical bonding in terms of the attraction of fragments (‘atoms’) of opposing charges, stemmed from Berzelius’ titanic work on oxides and salts to draw up an accurate table of atomic weights. It was the prestige of Berzelius’ 1818 textbook Essay on the Theory of Definite Proportions and the Chemical Influence of Electricity that thwarted the acceptance of the so-called Avogadro’s Hypothesis (1811): “equal volumes of different gases at the same temperature and pressure contain the same amount of molecules”. In fact, only apparently is this a purely stoichiometric statement: it implies the existence of diatomic gases like N2, H2, or, in other words, the possibility for an element to combine with itself. This was pure heresy for most of the chemists of the first half of the 19th century: Avogadro’s hypothesis simply did not fit in the theoretical framework of Berzelius’ theory: fragments of equal charge should simply repel.
    From the point of view of organic chemistry, the years immediately preceding the Karlsruhe Congress saw the birth of a structural theory of organic compounds based on the tetravalency of the carbon atom. It has been shown2 that the scientific milieu of chemistry was ripe for the development of this concept in the late 1850s, and that scientific research at Paris exerted a major influence on the three chemists that worked on the structural theory of organic compounds. August Kekulé, a veritable chemistry polymath with an impressively extensive background, had been a research assistant at Paris in 1851-1852; Archibald Scott Couper was working with Wurtz (yes, the same of the Wurtz reaction, an Alsatian chemist who advocated atomicity, the combination of atomic theory and valency2) in the same city when he published a paper on tetravalence in the following year, while Alexander Butlerov, the Russian chemist who, in Markovnikov’s own words “gave full value to this [Kekulé’s and Couper’s] hypothesis and developed it into the whole structural system3 also worked for five months with Wurtz at Paris in 1858.

Going back to the Karlsruhe Congress, one can say that it coincided with the transition from the generation of Liebig and Dumas to that of Meyer (he was 30), Mendeleev (26) and Kekulé (31). Although it would be incorrect to depict the congress as an all-out rebellion of the youngsters against the old guard, the junior delegates would definitely turn out to be more receptive to the new ideas being discussed at the congress. Reportedly4, Meyer approached the congress with significant skepticism, predicting that the “idiotic church-council in Karlsruhe” would end up in the “the election of an infallible [molecular] formula-pope”. On the other hand, it is impossible to understate the role played by Kekulé (then full professor at Gent, Belgium), who, despite his relatively young age (sorry for the ‘relatively’, dear thirty-something reader like me!) , had been one of the main catalysts for the organisation of the Karlsruhe Congress5. Kekulé and Weltzien (a chemist based in Karlsruhe who would be the ‘business manager’ of the conference) went to Paris in March 1860 to meet the third organiser, Wurtz, and draw up plans for the conference together. Looking at the dates, one realises that the conference was actually called on quite a short notice, and this is all the more impressive if one considers that information and people travelled with 19th century means of transportation, the handwritten letter and the steam train.

The invitation letter was sent out in three versions, German, French and English6

Carlsruhe [sic], den 10. Juli 1860


Die Chemie ist auf einem Standpunkte angelangt, wo es den Unterzeichneten zweckmäßig erscheint, durch Zusammentritt einer möglichst großen Anzahl von Chemikern, welche in der Wissenschaft thätig und diese zu lehren berufen sind, eine Vereinigung über einzelne wichtige Punkte anzubahnen […]7

Chemistry has reached a state of development when to the undersigned, it seems necessary that a meeting of a great number of chemists, active in this science, who are called upon to do research and teach, be held so that a unification of a few important points shall be approached[…]8

Good old Lothar Meyer, summarises it once again for us in a note he wrote thirty years after the Karlsruhe Congress:

We now easily recognize that the argument was mainly about three things: electrochemical dualism (i.e. Berzelius’ theory), Avogadro’s Hypothesis, and the relative atomic weights of the elements. However, at the time, this was not so obvious; the most common arguments were about the formulas used to represent how chemical compounds were formed. . . . As a result, there was much confusion, every substance, even the simplest, had a series of formulas, e.g., water: H2O or HO or H2O2, mine gas (methane): CH4, C2H4 . . . . Even a simple compound such as vinegar could have enough proposed formulas to fill an entire printed page9

Meyer’s remark is important in reminding us the role of perspective when looking at the past. From the vantage point of the scientific consensus of the 21st century, it is all too easy to sneer at our chemical forefathers and point out the fallacies of some of the older theories, and their blindly stubborn rejection of Avogadro’s Hypothesis; yet, we should try and imagine how philosophically challenging it was for a chemist of that era to reconcile Berzelius’ and Avogadro’s theories. At the same time, let us not forget that even well-earned prestige, such as in Berzelius’ case, should never convince a researcher to abandon his/her skepticism, one of the four inspiring principles of scientific research, as defined by Robert Merton10:

  • Communism
  • Universalism
  • Disinterestedness
  • Organised skepticism

Bin it or read it

It is thanks to the Stanislao Cannizzaro’s mercurial speech that the Congress reaches its climax just before its end. As the conference is drawing to a close, time is running out and the delegates try to reach an agreement on chemical notation. In particular, they discuss if chemists are to revert back to the principles of Berzelius’ venerable system11, which in Cannizzaro’s view had a major shortcoming: it could not accommodate Avogadro’s hypothesis and the most recent development in chemical theory. It is at this point that, as Wurtz’s minutes of the Congress reports12, “Mr. Cannizzaro takes the floor in order to oppose the second proposition. It scarcely appears fitting or logical to him to move science back to the time of Berzelius”. Cannizzaro’s speech goes on for quite a while, or at least we are inclined to think so because of the extensive notes in Wurtz’s minutes. Moreover, we can only imagine what Cannizzaro’s stage persona looked like (Mendeleev remarks that Cannizzaro’s speeches were “heated” and “animated”8): was he waving hands extensively as Italians are often supposed to be doing all the time when they are talking? Could one hear a thick Italian accent in his French, or his German? Did he make direct eye contact with the audience? Were the listeners engaged or, rather, bored to death and already thinking about “rushing to catch that train back home”, their ongoing experiments, a doctoral thesis to correct? We will never know.

Cannizzaro’s take-home message was simple: the chemical sciences have significantly moved on since Berzelius’ times, and recent, compelling evidence supporting Avogadro’s hypothesis has been published by several groups using different experimental techniques. Therefore, the value of some of the atomic weights in Berzelius’ table must be doubled and these doubled atomic weights should be clearly rendered in notation with crossed-through symbols. Also, the simultaneous use of old and new symbolism must be discouraged to avoid all misunderstanding. Significantly, he concluded by saying: “And if we are unable to reach a complete agreement upon which to accept the basis for the new system, let us at least avoid issuing a contrary opinion that would serve no purpose, you can be sure. In effect, we can only obstruct Gerhardt’s [the newer] system from gaining advocates every day. It is already accepted by the majority of young chemists today who take the most active part in advances in science“. Reflect on this statement. The deliberations of a congress cannot stop the spreading of a novel outlook on chemical facts which is more in line with available experimental evidence. After all, as another French delegate (Boussingault) wittily remarked, “It is not chemistry that grows old, but chemists“; we should praise Boussingault for his objectivity in spite of his own age: in 1860 he was just 59.

Yet, Cannizzaro’s speech alone would probably have been much less convincing if the second Italian delegate at Karlsruhe, Angelo Pavesi, professor at the University of Pavia, had not stepped in. As recalled by Meyer1,8, Pavesi supported his friend’s speech by handing out copies of what I would call Cannizzaro’s educational pamphlet, entitled Sunto di un corso di filosofia chimica (“Sketch of a course of chemical philosophy13) and first published in Italy in 1858. Once more, how did Pavesi actually pull it off? Had he agreed with Cannizzaro to join forces and stage this combined coup de théâtre at the end of the conference? How did Pavesi stop delegates who, probably, were hurrying to Karlsruhe train station? Again, we will never know. Lots of copies must have ended up in the bin, or slipped into briefcases and forgotten there forever. However, Lothar Meyer (and certainly Dmitri Mendeleev) read the Sketch as he was travelling back to Wrocław 8 (then known as Breslau), and he was deeply impressed, hence the quote that opens this post.

Sketch of a course of chemical philosophy, it’s all in the name, those two words that immediately stir up my attention, chemistry and philosophy. But there is more: look at these lines from the first page of this pamphlet: “In order to lead my students to the conviction which I have reached myself, I wish to place them on the same path as that by which I have arrived at it –the path, that is, of the historical examination of chemical theories14. Condensed in a few words there is an entire pedagogical approach to the teaching of chemistry, and remember that Cannizzaro was lecturing on the then frontiers of chemistry, not some commonly accepted “textbook” knowledge. Here is -you will forgive the misuse of this term- a peripatetic approach: retrace your own scientific journey together with your students and let them come to the same conclusions.

No paradigm for social constructivism

A final remark. I can already hear some voices in the background saying that scientific conferences like the one at Karlsruhe are compelling evidence that “science is just social construction”: after all, these chemists actually assembled at Karlsruhe in order to decide what ‘atoms’ and ‘molecules’ are, didn’t they? Well, unfortunately I do not have time to elaborate on this very interesting topic any further, a topic that should be addressed in a post of its own. That said, let me just add a quote from the fourth Reith lecture 2010 by Martin Rees15, already cited in a previous post:

The physicist Steven Weinberg has given an apt metaphor for scientific breakthroughs. He says: “A party of mountain climbers may argue over the best path to the peak, and these arguments may be conditioned by the history and social structure of the expedition, but in the end either they find a good path to the summit or they do not, and when they get there they know it“.

Deciding on the correct use of scientific terms is one thing (by the way, nomenclature has played a paramount role in chemistry since Lavoisier’s times), discovering a law which is valid for all known (and yet to be discovered) elements is quite another cup of tea. Call it tungsten or wolfram as you wish, its place in the periodic system will remain always the same. After all, the Karlsruhe congress failed to achieve consensus, and, despite this, the shock waves generated by Cannizzaro’s paper (and speech) would shake the foundations of chemistry to rebuild this discipline on firmer ground: the periodic law.

A handshake is not a chemical bond.


  1. See also The Karlsruhe Congress: a Centennial Retrospect, Aaron J. Ihde, Journal of Chemical Education, 1961, 38, 83
  2. The interested reader could refer to A History of Chemistry, Bernadette Bensaude-Vincent and Isabelle Stengers, Harvard University Press, 1996
  3. As quoted in Alexander Mikhaĭlovich Butlerov, Henry M. Leicester, Journal of Chemical Education, 1940, 17, 203, doi:10.1021/ed017p203
  4. As reported in When Science Went International, Sarah Everts, Chemical & Engineering News
  5. “L’idée de provoquer une réunion internationale des chimistes appartient à M. Kekulé”, in Compte rendu des séances du congrès international des chimistes réuni à Carlsruhe, les 3, 4 et 5 septembre 1860, Charles-Adolphe Wurtz.
  6. Not surprisingly, these three languages are still today’s working languages of the European Commission, as in http://ec.europa.eu/stages/information/faq_en.htm.
  7. As reported in Der internationale Chemiker-Kongreß Karlsruhe 3.-5. September 1860 vor und hinter den Kulissen, Alfred Stock, Verlag Chemie, 1933
  8. Translation included in The Congress at Karlsruhe, Clara de Milt, Journal of Chemical Education, 195128, 421–425, doi:10.1021/ed028p421
  9. Retrieved on Thriving for Unity in Chemistry: The First International Gathering of Chemists, Michael W. Mönnich, Chemistry International, 2010, 32 . Meyer wrote this quote in the foreword to the 1891 translation of Cannizzaro’s textbook, Abriss eines Lehrganges der theoretischen Chemie. Translation by A. Miolati and edited by L. Meyer. pp. 52–58, Engelmann, Leipzig (1891).
  10. Robert K. Merton, The Normative Structure of Science, 1973. Recently, originality has been added as fifth principle before skepticism to obtain a nice acronym, CUDOS…
  11. Berzelius’ chemical notation had a major impact on chemistry: the symbols of the elements still follow Berzelius’ approach (the first letter of the Latin name of the element, or a capital letter and a lower case letter to avoid confusion), and the use of subscript numbers to indicate the proportions of elements in a certain compound (Berzelius himself used superscripts). The Swedish chemist was certainly influenced by Linnaeus’ nomenclature. Berzelius’ own symbolism incorporated conventions, now no longer used, to shorten the formulae, such as dots above the symbol of an element to indicate oxygen and a horizontal bar to indicate two atoms of a certain element. Berzelius developed his system, which eventually replaced Dalton’s notation, while he was writing a chemistry textbook in 1808. Similarly, both Lothar Meyer and Dmitri Mendeleev would include their own periodic systems in their chemistry textbooks as learning tools. For more information see A History of Chemistry, Bernadette Bensaude-Vincent and Isabelle Stengers, Harvard University Press, 1996
  12. I have not been able to retrieve the original French language version. An English translation is available online: https://web.lemoyne.edu/giunta/karlsruhe.html
  13. Available online.
  14. The original reads: “Per condurre i miei allievi al medesimo convincimento che io ho, gli ho voluto porre sulla medesima strada per la quale io ci son giunto, cioè per l’esame storico delle teorie chimiche
  15. Transcripts available online.