The Saga of the Sunstones

In the Dark Ages, the Vikings set out in their longships to slaughter, rape, pillage, and conduct sophisticated measurements in optical physics. That, at least, has been the version of horrible history presented recently by some experimental physicists, who have demonstrated that the complex optical properties of the mineral calcite or Iceland spar can be used to deduce the position of the sun – often a crucial indicator of compass directions – on overcast days or after sunset. The idea has prompted visions of Norse raiders and explorers peering into their “sunstones” to find their way on the open sea.

The trouble is that nearly all historians and archaeologists who study ancient navigation methods reject the idea. Some say that at best the fancy new experiments and calculations prove nothing. Historian Alun Salt, who works for UNESCO’s Astronomy and World Heritage Initiative, calls the recent papers “ahistorical” and doubts that the work will have any effect “on any wider research on navigation or Viking history”. Others argue that the sunstone theory was examined and ruled out years ago anyway. “What really surprises me and other Scandinavian scholars about the recent sunstone research is that it is billed as news”, says Martin Rundkvist, a specialist in the archaeology of early medieval Sweden.

This debate doesn’t just bear on the unresolved question of how the Vikings managed to cross the Atlantic and reach Newfoundland without even a compass to guide them. It also goes to the heart of what experimental science can and can’t contribute to an understanding of the past. Is history best left to historians and archaeologists, or can “outsiders” from the natural sciences have a voice too?

What a saga

The sunstone hypothesis certainly isn’t new. It stems largely from a passage in a thirteenth-century manuscript called St Olaf’s Saga, in which the Icelandic hero Sigurd tells King Olaf II Haraldsson of Norway where the sun is on a cloudy day. Olaf checks Sigurd’s claim using a mysterious sólarsteinn or sunstone:
Olaf grabbed a Sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun.

An even more suggestive reference appears in another thirteenth-century record of a Viking saga, called Hrafns Saga, which gives a few more clues about how the stone was used:
the weather was sick and stormy… The King looked about and saw no blue sky… then the King took the Sunstone and held it up, and then he saw where the Sun beamed from the stone.

In 1967 Danish archaeologist Thorkild Ramskou suggested that this sunstone might have been a mineral such as the aluminosilicate cordierite, which is dichroic: as light passes through, rays of different polarization are transmitted by different amounts, depending on the orientation of its crystal planes (and thus its macroscopic facets) relative to the plane of polarization. This makes cordierite capable of transmitting or blocking polarized rays selectively – which is how normal polarizing filters work. (Ramskou also suggested that the mineral calcite, a form of calcium carbonate, would work as a sunstone, based on the fact that calcite is birefringent: rays with different polarizations are refracted to different degrees depending on the orientation with respect to the crystal planes. But that’s not enough, because calcite is completely transparent: changing its orientation makes no difference to how much polarized light passes through. You need dichroism for this idea to work, not birefringence.)

Because sunlight becomes naturally polarized as it is scattered in the atmosphere, if cordierite is held up to sunlight and rotated it turns darker, becoming most opaque when the crystal planes are at right angles to the direction of the sun’s rays. Even if the sun itself is obscured by mist or clouds and its diffuse light arrives from all directions, the most intense of the polarized rays still come straight from the hidden sun. So if a piece of dichroic mineral is held up to the sky and rotated, the pattern of darkening and lightening can be used to deduce, from the orientation of the crystal’s facets (which reveal the orientation of the planes of atoms), the direction of the sun in the horizontal plane, called its azimuth. If you know the time of day, then this angle can be used to calculate where north lies.

Ramskou pointed out that polarizing materials were once used in a so-called Twilight Compass by Scandinavian air pilots who flew over the north pole. Their ordinary compasses would have been useless then, but the Twilight Compass allowed them to get their bearings from the sun. So maybe the Vikings did the same out on the open sea? Might they have chanced upon this handy property of calcite, found in abundance on Iceland? Perhaps all Viking ships set sail with a sunstone to hand, so that even on overcast or foggy days when the sun wasn’t visible they could still locate it and find their bearings.

The idea has been discussed for years among historians of Viking navigation. But only recently has it been put to the test. In 1994, astronomer Curt Roslund and ophthalmologist Claes Beekman of Gothenburg University showed that the pattern of darkening produced by a dichroic mineral in diffuse sunlight is too weak to give a reliable indication of the sun’s location. They added that such a fancy way to find the hidden sun seems to be unnecessary for navigation anyway, because it’s possible to locate the sun quite accurately with the naked eye when it is behind clouds from the bright edges of the cloud tops and the rays that emanate from behind the cloud. The sunstone idea, they said, “has no scientific basis”.

That was merely the opening sally of a seesawing debate. In 2005, Gabór Horváth at the Loránd Eötvös University in Budapest, a specialist in animal vision, and his colleagues tested subjects using photographs of partly cloudy skies in which the sun was obscured, and found that they couldn’t after all make a reasonably accurate deduction of where the sun was. Two years later Horváth and collaborators measured the amount and patterns of polarization of sunlight in cloudy and foggy skies and concluded that both are after all adequate for the “polarizer” sunstones to work in cloudy skies, but not necessarily in foggy skies. All this seemed enough to rehabilitate the plausibility of the sunstone hypothesis. But would it work in practice?

Double vision

Optical physicists Guy Ropars and Albert Le Floch at the University of Rennes had been working for decades on light polarization effects in lasers. In the 1990s they came across the sunstone idea and the objections of Roslund and Beekman. While Horváth’s studies seemed to show that it wasn’t after all as simple as they had supposed to find the sun behind clouds, Ropars and Le Floch agreed with their concern that the simple darkening of a dichroic crystal due to polarization effects is too weak to do that job either. The two physicists also pointed out that Ramskou’s suggestion of using birefringent calcite this way won’t work. But, they said, calcite has another property that presents a quite different way of using it as a sunstone.

When a calcite crystal is oriented so that a polarized ray strikes at right angles to the main facet of the rhombohedral crystals, but at exactly 45 degrees to the optical axis of the crystal – at the so-called isotropy point – it turns out that the light in the rays at this position are completely depolarized. As a result, it’s possible to find the azimuth of a hidden sun by exploiting the sensitivity of the naked eye to polarized light. When polarized white light falls on our eye’s fovea, we can see a pattern in which two yellowish blobs fan out from a central focus within a bluish background. This pattern, called Haidinger’s brushes, is most easily seen by looking at a white sheet of paper illuminated with white polarized light, and rotating the filter. We can see it too on a patch of blue sky overhead when the sun is near (or below) the horizon by rotating our head. By placing a calcite crystal in the line of the polarized rays oriented to its isotropy point relative to the sun’s azimuth, the polarization is removed and Haidinger’s brushes vanish. Comparing the two views by moving the crystal rapidly in and out of the line of sight, the researchers found that the sun’s azimuth can be estimated to within five degrees.


Haidinger’s brushes: an exaggerated view.

But it’s a rather cumbersome method, relies on there being at least a high patch of unobstructed sky, and would be very tricky on board a pitching ship. There is, however, a better alternative.

Because calcite is birefringent, when a narrow and partially polarized light ray passes through it, the ray is split in two, an effect strikingly evident with laser beams. One ray behaves as it would if just travelling through glass, but the other is deviated by an amount that depends on the thickness of the crystal and the angle of incidence. This is the origin of the characteristic double images seen through birefringent materials. And whereas Roslund and Beekman had argued that changes in brightness for a dichroic substance rotated in dim, partially polarized light are likely to be too faint to distinguish, the contrast between the split-beam intensities as calcite is rotated are much stronger and easier to spot. “The sensitivity of the system is then increased by a factor of about 100”, Ropars explains. At the isotropy point, the two rays will have exactly the same brightness, regardless of how polarized the light is. This means that, if we can accurately judge this position of equal brightness, the orientation of the crystal at that point can again be used to figure out the azimuth from which the most intense rays are coming.



Double images and split laser beams in calcite, due to birefringence.

The human eye happens to be extremely well attuned to comparing brightness contrasts of fairly low-level lighting. So the researchers’ tests using partially polarized light shone through a calcite crystal showed that, under ideal conditions, the direction of the light rays could be estimated to within 1 degree even for low overall light intensities, equivalent to a sun below the horizon at twilight. The method, they say, will work even up to the point where the first stars appear in the sky.

Showing all this is the lab is one thing, but can it be turned into a navigational instrument? Ropars, Albert Le Floch and their coworkers have already made one. They call it the Viking Sunstone Compass.

It’s a rather beautiful wooden cylinder with a hole in the top, through which light falls from the zenith of the sky onto a calcite crystal attached to a rotating pivot turned by a little handle on the lid. There’s a gap in the side through which the observer looks at the two bright spots projected from the crystal. “You simply rotate the crystal to equalize the intensities of the beams”, says Ropars. A pointer on the lid then indicates the orientation of the crystal and the azimuth of the sun, from which north can be deduced by taking into account the time of day. Ropars says that, even though of course the Vikings lacked good chronometers, they seem to have known about sundials. What’s more, studies have shown that people’s internal body clocks (their circadian rhythm) can enable us to estimate the time of day to within about a quarter of an hour.


The Viking Sunstone Compass made by researchers at the University of Rennes. Note the double bright spots in the cavity.

But never mind Vikings – the Rennes team could probably make a mint by marketing these elegant devices as a luxury item for sailors. Ropars says that a US company is now hoping to commercialize the device based on their prototype.

All at sea

When the findings were reported, they spawned a flurry of excited news headlines, many claiming that the mysteries of Viking navigation had finally been solved. It’s not surprising, for the image of brawny Vikings making use of such a brainy method is irresistible. But what, in the end, did the experiments really tell us about history?

There’s nothing in principle that might have prevented the ancient Greeks from developing steam power or microscopes. We are sure that they didn’t because there is absolutely no evidence for it. So an experiment demonstrating that, say, ancient Greek glass-making methods allow one to make the little glass-bead microscope lenses used by Antoni van Leeuwenhoek in the seventeenth century is historically meaningless. What, then, can we conclude about Viking sunstones?

Because the Viking voyages between the ninth and eleventh centuries were so extensive – they sailed to the Caspian Sea, across the Mediterranean to Constantinople, and over the Atlantic to North America – there is a pile of archaeological and historical research on how on earth they did it. The prevailing view is that, in the Dark and Middle Ages, as much sailing as possible was done in sight of land, so that landmarks could guide the way. But of course you can’t cross the Atlantic that way. So if no land was in sight, sailors used environmental signposts: the stars (the Vikings knew how to find north from the Pole Star), the sun and moon, winds and ocean currents. They also relied on the oral reports of previous voyagers to know how long it should take to get to particular places.

What if none of these clues was available? What did they do if becalmed in the open sea on a cloudy day? Well, then they admitted that they were lost – as they put it, hafvilla, “wayward at sea”. The written records indicate that under such circumstances they would convene to discuss the problem, relying on the instincts of the most experienced sailors to set a course.

However, some archaeologists and historians, like Ramskou, have argued that they could also have used navigational instruments. The problem is that there is precious little evidence for it. The Scandinavian coast is dotted with Viking ship finds, some of them wrecks and others buried to hold the dead in graves. But not one has provided any artifacts that could be navigational tools. Nevertheless, the archaeological record is not entirely barren. In 1948 a Viking-age wooden half-disk carved with sun-like serrations was unearthed under the ruins of a monastery at Uunartoq in Greenland. It was interpreted by the archaeologist Carl Sølver as a navigational sundial, an idea endorsed by Ramskou in the 1960s. More recently another apparent wooden sundial was found at the Viking site on the island of Wolin, off the coast of Poland in the Baltic. A rectangular metal object inscribed in Latin, found at Canterbury and tentatively dated to the eleventh century, has also been interpreted as a sundial, while a tenth-century object from Menzlin in Germany might be a nautical weather-vane.



A Viking ship grave at Oseberg in Norway, and the Uunartoq Viking sundial.

So the “instrumental school” of Viking navigation has a few tenuous sources. But no sunstones. That hasn’t previously deterred the theory’s champions. One of them was Leif Karlsen, an amateur historian whose 2003 book Secrets of the Viking Navigators announced his convictions in its subtitle: “How the Vikings used their amazing sunstones and other techniques to cross the open ocean”. One problem with such a bold claim is that the sunstone hypothesis had already been carefully examined in 1975 by the archaeologist Uwe Schnall, who argued that not only is there no evidence for it but there is no clear need either. “Since then, to my knowledge, no research has contradicted this conclusion”, says Willem Mörzer Bruyns, a retired curator of navigation at the Netherlands Maritime Museum in Amsterdam.

In making his case, however, Karlsen presented a new exhibit. In 2002, just as his book was being completed, archaeologists discovered a calcite crystal in the remains of a shipwreck offshore from the Channel Island of Alderney. It has been made misty by centuries of immersion in seawater and abrasion by sand, but it still has the familiar rhombohedral shape. Finally, tangible proof that sailors carried sunstones! Well, not quite. Not only is it totally unknown why the crystal was on board, but the ship is from Elizabethan England, not the Viking age.


The Alderney “sunstone”.

All the same, Ropars and colleagues claim that it supports their theory that these crystals were used for navigation. They point out, for example, that it was found close to a pair of navigational dividers. But, says Bruyns, “navigational instruments were kept in the captain’s and officers’ quarters, where their non-navigational valuables were also stored.” All the same Bruyns is sympathetic to the idea that, rather than being a primary navigational device, the crystal might have been used to correct for compass errors caused by local magnetic variations (such as proximity to iron cannons), which was done at that time by looking at the sun’s position on the horizon when it rose or set. Ropars points out that birds use the same recalibration of their magnetic sensors using polarization of sunlight at sunrise and sunset. “We’re now looking for possible mentions of sunstones in the historical Navy reports of the 15th and 16th centuries”, he says. But however intriguing that idea is, it has no bearing on a possible use of sunstones for navigation in the pre-compass era. “The Alderney finding is from a completely different period and culture to the Vikings”, Ropars acknowledges.

Finding the right questions

One way to view the latest work on sunstones is that it could at least have ruled out the hypothesis in principle. But don’t historians need a good reason to regard a hypothesis as plausible in the first place, before they get concerned about whether it is possible in practice? Otherwise there is surely no end to the options one would need to exclude. And there is the difficult issue of the documentary record. Lots of what went on a millennium and more ago was not written down, and much of what was is now lost. All the same, there is a rich literature, at least from the Middle Ages, of the techniques and skills of trades and professions, while early pioneers of optics like Roger Bacon and Robert Grosseteste in the thirteenth century offer a pretty extensive summary of what was then known on the subject. It’s not easy to see how they would have neglected sunstones, if these were widely used in navigation. Ropars says that the Icelandic sagas aren’t any longer the only textual source for sunstones, for the Icelandic medieval historian Arni Einarsson pointed out in 2010 that sunstones are also mentioned in the inventory lists of some Icelandic monasteries in the fourteenth and fifteenth centuries, where they were apparently used as time-keeping tools for prayer sessions. But monks weren’t sailors.

The basic problem, says Salt, is that scientists dabbling in archaeology often try to answer questions that, from the point of view of history and anthropology, no one is asking. This has been a bugbear of the discipline of archaeoastronomy, for example, in which astronomers and others attempt to provide astronomical explanations of historical records of celestial events, such as darkening of the skies or the appearance of new stars and other portents. Explanations for the Star of Bethelem have been particularly popular, but here too Salt thinks that it is hard to find any examples of a historically interesting question being given a compelling answer. [See, e.g. J. British Astron. Assoc. 114, 336; 2004]. One of the most celebrated examples, also revolving around optical physics, was the suggestion by artist David Hockney and physicist Charles Falco that painters in the Renaissance such as Jan van Eyck used a camera obscura to achieve their incredible realism. The theory is now generally discounted by art historians.

“‘Could the Vikings have used sunstones’ is a different question to ‘did the Vikings use sunstones”, which is what most historians are interested in,” says Salt. “A paper that tackles a historical problem by pretty much ignoring the historical period your artefact comes from seems to me to be eccentric.” Ropars agrees that “experimental science can exclude historical hypotheses, but isn’t sufficient to validate them.” But he is optimistic about the value of collaborations between scientists and historians or archaeologists, when the historical facts are sufficiently clear for the scientists to develop a plausible model of what might have occurred.

Could it be, though, that we’re looking at the sunstone research from the wrong direction? One of its most attractive outcomes is not an answer to a historical question, but a rich mix of mineralogy, optics and human vision that has inspired the invention of a charming device which, using only methods and materials accessible to the ancient world, enables navigation under adverse conditions. It would be rather lovely if the modern “Viking Sunstone Compass” were to be used to cross the Atlantic in a reconstructed Viking ship, as was first done in 1893. It would prove nothing historically, but it would show how speculations about what might have been can stimulate human ingenuity. And maybe that’s enough.


The reconstructed Viking ship the Sea Stallion sets sail.

Further reading
J. B. Friedman & K. M. Figg (eds), Trade, Travel and Exploration in the Middle Ages: An Encyclopedia, from p. 441. Routledge, London, 2000.

A. Englert & A. Trakadas (eds), Wulfstan’s Voyage, from p.206. Viking Ship Museum, Roskilde, 2009.

G. Horváth et al., Phil Trans. R. Soc. B 366, 772 (2011).

G. Ropars, G. Gorre, A. Le Floch, J. Enoch & V. Lakshminarayanan, Proc. R. Soc. A 468, 671 (2011).

A. Le Floch, G. Ropars, J. Lucas, S. Wright, T. Davenport, M. Corfield & M. Harrisson, Proc. R. Soc. A 469, 20120651 (2013).

G. Ropars, V. Lakshminarayanan & A. Le Floch, Contemp. Phys. 55, 302 (2014).

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Note: A version of this article appears in New Scientist this week. A pdf of this article is available on my website here.

The graphene explosion

I haven’t found any reports of the opening of Cornelia Parker’s new solo show at the Whitworth in Manchester. Did the fireworks go off? Did the detonator work? Here, anyway, is what I wrote for Nature Materials before the event.

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If all has gone according to the plan as this piece went to press, Manchester will have been showered with meteorites. An exhibition at the University of Manchester’s Whitworth art gallery by the artist Cornelia Parker is due to be opened on 13th February with a firework display in which pieces of meteoritic iron will be shot into the sky.

The pyrotechnics won’t be started simply by lighting the blue touchpaper. The conflagration will be triggered by a humidity sensor, switched by the breath of physicist Kostya Novoselov, whose work on graphene at Manchester University with Andre Geim won them both the 2010 physics Nobel prize. The sensor is itself made from graphene, obtained from flakes of graphite taken from drawings by William Blake, J. M. W. Turner, John Constable and Pablo Picasso as well as from a pencil-written letter by Ernest Rutherford, whose pioneering work on atomic structure was conducted at Manchester.

That graphene (oxide) can serve as an ultra-sensitive humidity sensor was reported by Bi et al. [1], and has since been refined to give a very rapid response [2]. Adsorption of water onto the graphene oxide film alters its capacitance, providing a sensing mechanism when the film acts as an insulating layer between two electrodes. These sensors are now being developed by Nokia. The devices used for Parker’s show were provided by Novoselov’s group after the two of them were introduced by the Whitworth’s director Maria Balshaw. Novoselov extracted the graphite samples from artworks owned by the galley, using tweezers under careful supervision.

“I love the idea of working on a nano level”, Parker has said. “The idea of graphene, something so small, being a catalyst.” She is not simply talking figuratively: doped graphene has indeed been explored as an electrocatalyst for fuel cells [3,4].

Parker has a strong interest in interacting with science and scientists. In 1997 she produced a series of works for Nature examining unexpected objects in a quasi-scientific context [5]. Much of her work focuses on connotations of materiality, associations arising from what things are made of and the incongruity of materials repurposed or set out of place. Her installation Thirty Pieces of Silver (1988-9) used an assortment of silver objects such as instruments and cutlery flattened by a steamroller. She has worked with the red crepe paper left over from the manufacture of Remembrance Day poppies, with lead bullets and gold teeth extruded into wire, and with her own blood. Perhaps even her most famous work, Cold Dark Matter: An Exploded View (1991) – the reconvened fragments of an exploded shed – was stimulated as much by the allure of the “matter” as by the cosmological allusion.

“I like the garden shed aspect of scientists”, she has said, “the way they like playing about with materials.” Unusually for an artist, she seems more excited by the messy, ad hoc aspects of practical science – the kind of experimentation for which Rutherford was so renowned – than by grand, abstract ideas. The fact that Novoselov and Geim made some of their graphene samples using Scotch tape to strip away layers from graphite no doubt added to its appeal. Parker also recognizes that materials tell stories. There’s a good chance that both Blake and Rutherford would have used graphite from the plumbago mines of Borrowdale in Cumbria, about 80 miles north of Manchester and the source of the Keswick pencil industry. So even Parker’s graphene might be locally sourced.

1. Bi, H. et al., Sci. Rep. 3, 2714 (2013).
2. Borini, S. et al., ACS Nano 7, 11166-11173 (2013).
3. Geng, D. et al., Energy Environ. Sci. 4, 760-764 (2011).
4. Fei, H. et al., ACS Nano 8, 10837-10843 (2014).
5. Anon., Nature 389, 335, 548, 668 (1997).

Alchemy on the page

Here’s an extended version of my article in Chemistry World on the "Books of Secrets" exhibition currently at the Chemical Heritage Foundation in Philadelphia.

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You thought your chemistry textbook can be hard to follow sometimes? Consider what a student of chemistry might be faced with in the early seventeenth century:
“Antimony is the true bath of gold. Philosophers call it the examiner and the stilanx. Poets say that in this bath Vulcan washed Phoebus, and purified him from all dirt and imperfection. It is produced from the purest Mercury and Sulphur, under the genus of vitriol, in metallic form and brightness. Some philosophers call it the White Lead of the Wise Men, or simply the Lead…”

This is a small part of the description in The Aurora of the Philosophers, a book attributed to the sixteenth-century Swiss alchemist and physician Paracelsus (1493-1541), for making the “arcanum of Antimony”, apparently a component of the “Red Tincture” or philosopher’s stone, which could transmute base metals into gold. It is, Paracelsus averred, a “very red oil, like the colour of a ruby… with a most fragrant smell and a very sweet taste” (which you could discover at some peril). The book contains very detailed instructions for how to make this stuff – provided that you know what “aquafortis”, “crocus of Mars” and “calcined tutia” are, and that you take care to control the heat of the furnace, in case (the author warns) your glass vessels and perhaps even the furnace itself should shatter.

All this fits the image of the alchemist depicted by Pieter Bruegel the Elder in a print of around 1558, which shows a laboratory in turmoil, littered with paraphernalia and smoky from the fire, where a savant works urgently to make gold while his household descends into disarray all around him. Bruegel’s engraving set the tone for pictures of alchemists at work over the next two centuries or so, in which they were often shown as figures of fun, engaged on a fool’s quest and totally out of touch with the real world.


Pieter Bruegel the Elder, The Alchemist (c.1558)

But that caricature doesn’t quite stand up to scrutiny. For one thing, despite all its arcane language that only fellow adepts would understand, Paracelsus’s experimental procedure is in fact quite carefully recorded: it’s not so different, once you grasp the chemical names and techniques, from something you’d find in textbooks of chemistry four centuries later. The aim – transmutation of metals – might seem misguided from this distance, but there’s nothing so crazy about the methods.

Second, the frenzied experimentation in Brueghel’s picture, in which the deluded alchemist commits his last penny to the crucible, is being directed by a scholar who sits at the back reading a book. (The text is, however, satirical: the scholar points to the words “Alge mist”, a pun on “alchemist” meaning all is failed, and we see the alchemist’s future in the window as he leads his family to the poorhouse.)

Books are ubiquitous in paintings of alchemists, which became a genre in their own right in the seventeenth century. Very often the alchemist is shown consulting a text, and even when he is doing the bellowing and experimenting himself, a book stands open in front of him. Sometimes it’s the act of reading, rather than experimenting, that supplies the satire: in a painting by the Dutch artist Mattheus van Helmont (no relation, apparently, to the famous chemist Jan Baptista van Helmont), the papers tumble from the desk to litter the floor in ridiculous excess. “The use of books and texts in alchemical practice may not be discussed frequently, but it becomes obvious when looking at the actual manuscripts used by alchemists and at the multitude of paintings that depict them”, says Amanda Shields, curator of fine art at the Chemical Heritage Foundation (CHF) in Philadelphia.


After David Teniers the Younger, Alchemist with Book and Crucible (c.1630s)


Mattheus van Helmont, The Alchemist (17th century)

The complex relationship of alchemists to their books is explored in a current exhibition at the CHF called "Books of Secrets: Writing and Reading Alchemy". It was motivated by the Foundation’s recent acquisition of a collection of 12 alchemical manuscripts, mostly from the fifteenth century. They were bought from a dealer after having been auctioned by the Bibliotheca Philosophica Hermetica, a private collection of esoteric books based in Amsterdam and funded by the Dutch businessman Joost Ritman. Among the new acquisitions was one of just six existing complete copies of the highly influential Pretiosa margarita novella (Precious New Pearl) supposedly by the fourteenth-century Italian alchemist Petrus Bonus. The CHF already possessed one of the most substantial collections of paintings of alchemists in the world, mostly from the seventeenth to the nineteenth centuries, and while being keenly aware of the difference between the dates of the books and the paintings, Shields and the CHF’s curator of rare books James Voelkel saw an opportunity to use these two resources to explore what books meant for the alchemists and early chemists: who wrote them, who they were intended for, who actually bought them, and how they were read.

Telling secrets

Of course, there weren’t really any students of chemistry in the early seventeenth century. That discipline didn’t exist for at least another hundred years, and its emergence from alchemy was convoluted and disputed. Arguably the first real textbook of chemistry was Cours de chymie, published by the Frenchman Nicaise Lefebvre in 1660, who would have been identified by the transitional terms chymist or iatrochemist, the latter indicating the use of chemistry in medicine. Alchemy was still very much in the air throughout the seventeenth century: both Robert Boyle and Isaac Newton devoted a great deal of effort to discovering the philosopher’s stone, and neither of them doubted that the transmutation of metals was possible. But it wasn’t by any means all about making gold. In the sixteenth century just about any chemical manipulation, whether to make medicines, pigments and dyes, or simple household substances such as soap, would have been regarded as a kind of alchemy.

This is why the whole notion of an “alchemical literature” is ambiguous. Some writers, such as the late sixteenth-century physician Michael Maier, who directed alchemical experiments in the court of the Holy Roman Emperor Rudolf II in Prague, wrote about the subject in mystical and highly allegorical terms that would have been opaque to a craftsperson. Others, such as the Saxon Georg Bauer (known as Agricola), wrote highly practical manuals such as Agricola’s treatise on mining and metallurgy, De re metallica (1556). Paracelsus’s works, which became popular in the late sixteenth century (he died in 1541), were a mixture of abstruse “chemical philosophy” and straightforward recipes for making drugs and medicines. And aside from such intellectual writers both inside and outside the universities, during the Renaissance there arose a sometimes lucrative tradition of “how to” manuals known as Kunstbüchlein, which were hotch-potch collections of recipes from all manner of sources, including classical encyclopaedists such as Pliny and ill-reputed medieval books of magic. These often styled themselves as “books of secrets”, which of course made them sound very alluring – but often they were miscellanies more likely to give you a mundane recipe for curing toothache than the secret of how to turn lead into gold.

In other words, “secrets” weren’t necessarily about forbidden knowledge at all. According to historian of science William Eamon of New Mexico State University in Las Cruces, “the term was used to describe both trade secrets, in the sense of being concealed, and also “tricks of the trades,” in other words techniques.” Eamon adds that the word “secrets” also “carried a lot of weight owing to the medieval tradition of esoteric knowledge”, which remained prominent in the alchemical tradition of the Renaissance. This glamour meant that the term could be useful for selling books. But how could you allude to secrets while writing them down for all the world to read? Some writers argued that there was virtually a moral imperative to do so. In his introduction to the hugely popular Kunstbüchlein titled simply Secreti (1555), Alessio Piemontese (a pseudonym, probably for the Italian writer Girolamo Ruscelli) told an elaborate and perhaps concocted story of how, by withholding secrets from a physician, he had once been responsible for the death of the physician’s patient.

This tradition of compilations of “secrets” was an old one. The historian of experimental science Lynn Thorndike has suggested that “the most popular book in the Middle Ages” might have been a volume called the Secretum secretorum or “Secret of secrets” (how much more enticing a title could you get?), which has obscure origins probably in the Islamic literature from around the tenth century. It was often attributed to Aristotle, but it’s pretty certain that he never wrote it – as with so many medieval books, the association with a famous name is just a selling point. The book does, however, reflect the Islamic writers’ enthusiasm for Aristotle, and as well as alchemy it includes sections on medicine, astrology, numerology, magic and much else. It was a kind of pocketbook of all that the scholar might want to know – in the words of one historian, a “middle-brow classic for the layman.”

But even if some of these “secrets” seemed hardly worth keeping, alchemy was different – for it really could seem dangerous. If it was possible to make gold, what would that do to the currency and the economy? It was largely this kind of worry, rather than any perception that alchemy was wrong-headed, that gave it a bad reputation. In 1317 Pope John XXII made alchemy illegal and imposed harsh sentences on anyone found guilty of trying to make gold. There was, however, also concern – some of it justified – that alchemists were swindlers who were duping people with fake gold. The image of the alchemist as a trickster who blinded gullible clients with incomprehensible jargon was crystallized in Ben Jonson’s 1610 play The Alchemist, in which his wily charlatan Subtle is a figure of fun. What’s more, alchemy was often associated with religious non-conformism. Paracelsus was unorthodox enough to upset all parties during the Reformation, but he was often linked to the Protestant cause and was sometimes called the “Luther of medicine.” When the French iatrochemists, who adopted Paracelsian ideas, battled with the medical traditionalists in the royal court at the end of the sixteenth century, the dispute was as much about religion – Catholics versus French Protestants (Huguenots) – as it was about medicine.

In view of all this, the genuine alchemist had to tread carefully until at least the seventeenth century. He was vulnerable to suspicion, ridicule and condemnation. That’s one reason why alchemical texts were often written with “intentional obscurity”, according to Voelkel. If you wrote cryptically, you could always argue your way out of accusations that you’d said something heretical or illegal. But the alchemical writers also felt that their knowledge held real power and so should be made unintelligible to lay people. A third motivation will be familiar to anyone who has ever read postmodernist academics: if you wrote too plainly, people might think that what you were saying is trivial, whereas if it was hard to understand then it seems profound and mysterious. Even if the recipes were straightforward, you wouldn’t get far without knowing the “code names” (Decknamen) for chemical substances: that “stinking spirit” is sulphur, and the “grey wolf” or “sordid whore” is stibnite (antimony sulphide), say.

Probably all of these motives for concealment and obfuscation were important to some degree, says Eamon – but he suspects that the major factor in the recondite character of many alchemical books was “to enhance the status and mystery of the work.” Also, he adds, “one shouldn’t underestimate the sheer inertia of tradition: secrecy was a very ancient tradition and always connected with that idea of initiation. Its hold over alchemy was strong even after there was little need for it.” Even Robert Boyle, whose The Sceptical Chymist has often been misinterpreted as a dismissal of all of alchemy rather than just its mystical and cryptic excesses, “employed elaborate coding devices to conceal his recipes”, Eamon says – especially those involved in gold-making. Despite insisting that adepts should be less obscure and cagey, Boyle wasn’t averse to it himself. “He may simply have protecting his reputation”, says Eamon - he didn’t want to be associated with an art many regarded as foolish. Isaac Newton, whose notebooks attest to extensive alchemical experimentation, was similarly guarded about that work.

The alchemist’s library

Given the diversity of sources, what would an alchemist have had in his library? The answer would depend somewhat on the kind of alchemy (or chymistry) they did, says Eamon. “The more practically inclined alchemists would probably have owned few books,” he says, “and they would probably have been heavy on books on metallurgy such as Agricola’s De re metallica and works such as the Kunstbüchlein.” Alchemists who were more interested in gold-making and the more esoteric mysteries of the art “would have been drawn to works such as those of [the pseudonymous] Basil Valentine, one of the more celebrated chemists of the period, such as The Triumphal Chariot of Antimony.” The medieval texts attributed to the Arabic writer Jabir ibn Hayyan (Latinized to Geber) would also have been popular among this sort of alchemist, Eamon adds.

Alchemists who wrote about distillation, such as the Frenchman John of Rupescissa and authors who wrote under the name of the Spanish philosopher Ramon Llull, were popular in the sixteenth century, especially for alchemists mainly interested in medicine. “Works by Paracelsus and his followers would also be represented in the chymist’s library”, says Eamon. “For many alchemists, books of secrets would also have been quite useful, of which the most popular was Alessio Piemontese’s Secreti.”

The English writer John Evelyn claimed of Robert Boyle that he learnt “more from men, real experiments, & in his laboratory… than from books”. But in fact Boyle had a very large library that included many alchemical works. “Unfortunately the library was dispersed after Boyle’s death and no library catalogue exists,” says Eamon, “but historians have been able to identify several of his books from his notes.” These included, for example, Agricola’s De re metallica and works by Johann Glauber, Paracelsus and Daniel Sennert. Newton’s library is much better catalogued, and included well-used copies of Paracelsus’s On the Transmutation of Metals and an English translation of Novum lumen chymicum by the Moravian Paracelsian alchemist Michael Sendivogius.

A dialogue in the lab

The CHF exhibition shows that such alchemical books weren’t at all treated like sacred texts. While they were still hand-copied these books could cost a fortune, but that didn’t mean they were kept in pristine form. They are well thumbed and evidently much used, sometimes showing signs of a benchtop life just as the later paintings imply. One book, a collection of recipes from Italian and English sources dated around 1470-75, has pages begrimed with what looks like soot. When the conservator used by the CHF, Rebecca Smyrl at the Conservation Center for Art and Historic Artifacts in Philadelphia, offered to remove the offending substance, Voelkel implored her not to, for he figured that this might be the debris from an actual experiment.


Cooked in the furnace: are these soot stains in a fifteenth-century alchemical text the debris from use in the lab?

What’s more, the readers scribbled all over the pages. Since paper itself was expensive, you might as well use the original text as your notebook, and margins were left deliberately generous to accommodate the annotations. In a copy of Christophorus Parisiensis’ Opera from 1557 there is not a square centimeter wasted, and the notes are recorded in a neat hand almost to tiny to read without magnification. Readers didn’t just mine the book for information: they engaged in a dialogue with the author, making corrections or arguing about interpretations. “There was a real conversation going on”, says Erin McLeary, director of the CHF museum. These markings attest that the books were anything but status symbols to be filed away ostentatiously on the shelf. “Reading was a huge part of alchemical practice”, says Voelkel.


The pages of a sixteenth-century alchemical book with marginal notes from a reader.

The CHF’s newly acquired manuscripts are particularly revealing because they date from the moment when print culture was emerging. The printing press lowered the financial and practical barriers to book ownership. “It made alchemical books widely available and relatively affordable”, says Eamon. “You can already see the decline of the notion of books as luxury items in the early sixteenth century.” Printing enabled the Kunstbüchlein artisan’s manuals to become bestsellers in the early sixteenth century: “they were cheaply printed, widely translated, and produced in large numbers”, says Eamon. Alessio Piemontese’s Secreti went through over 100 editions, and its likely author Ruscelli seems to have been something of a hack (the polite term was poligrafo) churning out whatever his publisher demanded. Print culture drove the trend of writing books in vernacular languages rather than Latin (which many potential buyers couldn’t read), and this opening up of new audiences was exploited as much by religious dissenters – Martin Luther was one of the first to spot the possibilities – as by publishers of scientific tracts, such as the Aldine Press of the Venetian humanist Aldus Pius Manutius.

The transition is fascinating to see in the CHF’s books. The early typefaces were designed to look like handwritten text, and some of the abbreviations used by scribes, such as the ampersand (&) were carried over to print – in this case with the origin as a stylized Latin et still evident. Some early printed books left a space at the start of chapters for the ornate initial capital letters to be added by hand. Quite often, the owners decided to save on the expense, so that the chapters begin with a blank.

As time passed and alchemy turned into chymistry and then chemistry, the image of the alchemist recorded by the painters became more tolerant and less satirical. In the hands of one of the most prolific and influential artists of this genre, the Antwerp-born David Teniers the Younger (1610-1690), the alchemist is less Breugel’s foolish agent of chaos and more a sober laboratory worker. If his floor is still strewn with vessels of brass, glass and clay, that’s simply because it allows Teniers to show off his skill at painting textures. In The Village Chemist (1760) by Justus Juncker, the physician sits calmly taking notes in his well-lit study-workshop; François-Marius Granet’s The Alchemist (early 19th century) shows a sober, monk-like figure in a spacious, sparsely furnished chamber; and Charles Meer Webb’s The Search for the Alchemical Formula (1858) makes the alchemist a romanticized, Gothic savant.

But what are they all doing? Reading (and writing). The text was always there.


François-Marius Granet, The Alchemist (early 19th century)


Charles Meer Webb, The Search for the Alchemical Formula (1858)

Further reading
W. Eamon, Science and the Secrets of Nature (Princeton University Press, 1996).
L. M. Principe & L. DeWitt, Transmutations: Alchemy in Art (Chemical Heritage Foundation, 2002).
L. M. Principe, The Aspiring Adept (Princeton University Press, 2000).