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Physical Chemistry

Podcast: Historians decode centuries-old chemical secrets

Researchers tell Stereo Chemistry about their quests to recreate chemical recipes lost to time

by Kerri Jansen , Gina Vitale
January 19, 2021


Photo collage depicting a transparent green screen on top of a book, a hill covered with gray stones, and a small wooden boat on fire.
Credit: Courtesy of Marjolijn Bol/Lawrence Principe/John Haldon
Historical objects like green reading glass, Bologna stones, and Greek fire are a source of fascination for some modern researchers.
Credit: C&EN

Researchers want to invent the technologies of the future, but there are plenty of chemical questions lurking in the past. In this episode of Stereo Chemistry, C&EN assistant editor Gina Vitale joins host Kerri Jansen to explore the centuries-old secrets and nagging mysteries that keep science historians up at night—and how these researchers go about solving them.

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The following is the script for the podcast. We have edited the interviews within for length and clarity.

John Haldon: Once we’d thought about building one of these, the three people I was working with all said, “You’re going to kill yourself if you build this, because guaranteed it’ll explode.”

Kerri: That’s John Haldon. He’s a Princeton University historian who specializes in the Byzantine Empire—that’s the medieval eastern Roman Empire. John is among a select group of scholars who have attempted to unmask a mysterious historical chemical weapon that the Byzantines used in naval battles. It had such devastating effects that sources describe enemy fleets fleeing in terror to avoid it. Over the years, this weapon came to be known as Greek fire.

John Haldon: If you read any of the sort of standard history textbooks, they always mention it. But it always struck me as quite an odd thing. Incendiary projectiles have been known about forever, but what was described in the medieval sources was clearly something very different. It was coming out of a siphon or a squirt, and it was covering ships and it was very hot, and it burned on water, and it, you know, did all sorts of other stuff that these traditional incendiary weapons clearly didn’t do at all.

Kerri: For centuries, scholars have debated what Greek fire was made of and how it worked. But this weapon is a military secret from a fallen empire, so its details have been lost to time. John thinks he’s found the key to this legendary substance. And he has the scorched boats to prove it.

In this episode of Stereo Chemistry, we’re going to explore how modern researchers illuminate the chemical secrets of the past. We’ll hear from experts about what motivates them to study chemistry lost in time, what inventions and substances from the past they’ve studied, and what chemical questions remain unanswered. And we’ll hear from John about his experience recreating the Byzantines’ potent weapon and why he believes the secret of Greek fire has been right in front of us all along.

I’m your host, Kerri Jansen.

To help me tell this story, I’m going to bring in someone whom regular listeners will recognize, Gina Vitale, an assistant editor at C&EN.

Gina Vitale: Hi Kerri!

Kerri: Hello. Thanks for joining us. You’ll recognize Gina’s voice from our recent episode on Rosalind Franklin. Gina and I have both collected some stories from this area of research, which we hope you’ll enjoy as much as we have.

Gina: Yeah, I was surprised at how many people are not just looking at these old scientific texts, but actually going through the steps and trying to do those experiments in the way early scientists did. Through that process, they feel like they’re learning about how people of the past thought about their world in a way they can’t just by reading.

Kerri: Right. C&EN has explored some of those efforts to revive ancient chemical recipes before. And I was curious about what drives these scientists to do this work and what chemical puzzles from the past remain unsolved. As we learned, recreating those old experiments is far from a straightforward task.

Gina: Yeah. Often because the information they’re working from is incomplete or unclear.

Marjolijn Bol: For almost everything we do, we only have snippets of information that for one reason or another came down to us. And so you have to indeed detective your way through it and try to make sense of it.

Gina: That’s Marjolijn Bol, who studies the historical relationship between materials and art at Utrecht University in the Netherlands. But that often involves taking a closer look at centuries-old recipes for things that we have never seen—objects or substances that are lost to time. In Marjolijn’s case, that could be anything from varnishes used on paintings to imitation gemstones.

Marjolijn Bol: It’s a little bit strange, because I’m an art historian, so I should actually be fascinated by things that are still there, right? By art objects in museums. And I like those as well. But I actually became really fascinated by things that aren’t there anymore, but of which we have written records such as these recipes.

Gina: Marjolijn told me about this recipe for a green reading glass, a sort of transparent screen, which people in the 15th century thought would protect your eyes from the strain of reading if you held it in front of the text.

Kerri: So it was like the 15th-century version of today’s blue light–blocking glasses for computer screens?

Gina: Sure.

Marjolijn Bol: And I was fascinated by it, because it’s again a practice where we don’t have these screens anymore, and I was wondering, what is it? Like, what were they doing with it?

So you have to take the skin of an animal—so parchment, basically—and this you have to put in a mixture of copper green. So again, this pigment verdigris . . .

Gina: The recipe is pretty long and complicated. Basically, it calls for soaking a piece of parchment in a copper solution, pulling it very thin and taut into a frame, and rubbing varnish over it. And boom, you’ve basically got a green-tinted, see-through sheet of parchment, almost like a thin piece of glass. But it turns out hunting down the equivalent of 15th-century parchment isn’t as simple as grabbing something off the shelf.

Marjolijn Bol: So I went to a parchment specialist and I told him about the recipe. And I asked him, could you give me some parchment? And we decided because the recipes explain it needed to be very fine and clean parchment, so calf [skin] would be a really good one to try.

Gina: So Marjolijn took home some calfskin parchment, which is very smooth, and tried the experiment.

Marjolijn Bol: And it didn’t work. And because, well it looked beautiful, but it wasn’t see-through at all. And that’s because what he gave me was parchment that was prepared for writing on. And of course when you want to write on something, you don’t want it to be see-through because you want to write on both sides, because it’s more economical.

Photo of a square, transparent green screen on top of a page of text.
Credit: Courtesy of Marjolijn Bol
After a failed first attempt, Marjolijn Bol successfully recreated green reading glass.

So I went back, and I explained what I got and that it didn’t really work. And he’s like, “Oh, if only you told me that! Now I know what you need.” And so he gave me this skin that hadn’t been prepared for writing on. And it was already completely see-through.

Gina: Now having discovered the likely starting material, Marjolijn was able to recreate the transparent green screen described in the recipe. Like many other historians, she feels that studying old recipes is worthwhile just for its own sake. But a more practical goal of her work is to identify methods, maybe hundreds of years old, that could help us with problems today or at least serve as a starting point.

Marjolijn Bol: We are sometimes trying to reinvent the wheel, and the wheel might have already been invented, but we forgot about it. And so I think there’s a lot of interesting practices in the past that we can learn from today and then understand also how we can maybe deal with certain problems today.

Gina: This idea of trying to learn from how ancient experimenters thought about their craft, and how it related to the culture of the time, is something I heard from several researchers I spoke to.

Kerri: Yeah. And what Marjolijn was saying about reinventing the wheel really resonates as well. Something I heard from a lot of folks I talked to is that people in the past had more knowledge than we sometimes give them credit for. Here’s chemist and historian Larry Principe.

Lawrence Principe: It didn’t make sense to me that chemistry suddenly sprang out of nowhere at some point in the 18th century.

Kerri: Larry’s a professor at Johns Hopkins University, and he studies the history of alchemy as a window into the origins of modern chemistry.

Lawrence Principe: So, much of what I’ve been doing is trying to figure out what kind of practices and ideas the alchemists had and how those relate to later chemistry to argue for a continuity of ideas and practices, really, from the Middle Ages, or even before from the Islamic Middle Ages, up to the present.

Kerri: Larry tells this story about a mineral, found in Italy in the 17th century, that fascinated the alchemists of the day.

Lawrence Principe: So it’s a very heavy, gray stone. In fact, most of the time, if you just saw this on the ground, you wouldn’t pick it up because it just looks like an ordinary rock. Not very interesting, until you pick it up. And you realize that it’s incredibly heavy. It’s much heavier than you expect.

It was discovered by a shoemaker, actually, who went and found these minerals out on a hill near his house in Bologna. Put them into a fire because he thought, oh, they’re so heavy. They might contain precious metal. Maybe I can smelt gold or silver out of them. He didn’t get any. But what he did discover is that after the stones cooled, if you exposed them to light and brought them into a dark room, they glowed like a burning coal.

Photo of a hill covered with gray rocks.
Credit: Courtesy of Lawrence Principe
Lawrence Principe traveled to Italy to collect Bologna stone specimens from their native environment.

Kerri: This was history’s first artificially prepared, persistently luminescent substance, which we now know is barium sulfide. Back in the 17th century, these remarkable glowing stones came to be known as Bologna stones. Word traveled, and many became interested in making their own. Some experimented with stones from Bologna to try to improve the recipe—tinkering with heating methods, for example, or grinding the stones into powder before heating. But success was elusive.

Lawrence Principe: Even though the process was published many, many times, people could not get it to work. The only people that seemed to be able to get it to work were those who actually traveled to Bologna and stood side-by-side with somebody who knew how to do it. One of the people who did this was a guy I just wrote a book about by the name of Wilhelm Homberg, and he tells this wonderful, wonderful story that I think any chemist today would understand. He goes to Bologna, he learns how to make them, he develops a new method that works better than anybody’s method ever worked before. He leaves Bologna, he gets to Paris, he says to people, “Look what I discovered!” And all of a sudden the process doesn’t work anymore. So he tries to figure out what on earth went wrong. He discovers that when he was in Italy, he was using a bronze mortar to grind the powder that he used. Whereas in Paris, he didn’t have a bronze mortar, he used an iron one that was available there. And what he basically discovered at this point was something that was not rediscovered until the early 20th century. That is that iron is a luminescence poison and that your persistently luminescent materials need a dopant.

Kerri: It turns out that traces of iron that rubbed off when Homberg ground the stones in the iron mortar quenched the luminescence of the stones. That’s why his experiment in Paris didn’t work. Homberg also observed that while iron contamination prevented luminescence, copper impurities were actually necessary for the Bologna stones to glow. Today, chemists would call that copper a dopant. Too much copper could decrease or prevent the luminescence. So although the fundamental cause of luminescence wasn’t described until much later, Homberg’s 17th-century experiments did reveal that the necessary amount of copper dopant falls within a narrow range, which we now know is between one part in 20,000 and one part in 5,000.


Lawrence Principe: So in 1683, he makes this discovery, which is about basically things in parts per 10,000 concentration. So it shows how clever he was with a very careful kind of analysis.

Kerri: And Homberg’s findings are borne out by Larry’s own research, which involved a visit to Bologna to collect specimens.

Lawrence Principe: What I discovered was that it’s only a very, very small area where this stone actually has the correct amount of copper as an impurity already. But if you’re outside that very, very small region, they don’t work anymore, unless you use a bronze or copper mortar to do the preparation, which is what Homberg discovered. Quite remarkable, really.

Kerri: But there is something Larry’s modern experiments couldn’t answer.

Lawrence Principe: This is the mystery that I haven’t solved yet. So I collected all the reports of people who went to Italy and saw successful Bologna stones glowing in the dark. And almost all of them are described as glowing orange like a glowing coal, like a piece of charcoal. Now this makes perfect sense. A barium sulfide persistently luminesces—emits—orange light, no problem. The ones that I made emitted an orange light too. So, great. But there are several examples where someone says they saw a stone that glowed blue. Now there’s the mystery. What was it that glowed blue? So I have not yet succeeded in making a stone that glows blue, but I’m working on it.

You know, many of us are interested in mysteries from the past. But in this case, this is one where you can actually access the mystery yourself if you discover the right method of preparation. So that just makes it twice as exciting, I think.

Gina: So Larry was quite literally retracing the steps of ancient experimenters to solve his mystery.

Kerri: Exactly.

Gina: Now Kerri.

Kerri: Yes, Gina.

Gina: We’ve heard some cool stories of chemical mysteries from the world of art history and glowing rocks. But I think you owe us the rest of the story about Greek fire.

Kerri: That’s right, I do. But first, we’re going to take a short break. When we come back, we’ll revisit John Haldon’s quest to rekindle a legendary lost flame.

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Gina: So Kerri, last we heard, John the Byzantine history expert was contemplating building something that everyone warned him was likely to explode.

Kerri: Right. John told me about his project to recreate the legendary incendiary weapon known as Greek fire. Remember, the literature described this weapon as liquid fire that clung to enemy ships and burned fiercely even in water, which is an unusual combination of properties!

John Haldon: So when I was a postdoc back in the, dare I say it, 1970s, I got interested in this just as a sort of sideline. I just thought that’s a bit weird, you know, the Byzantines had this thing. And nobody seems to know how it works. And it’s supposed to be a great mystery. So I read the literature. There are all sorts of crazy hypotheses. One is that it’s a mixture of saltpeter and other things, it might be an early form of gunpowder. But none of them really had a satisfactory answer.

Kerri: The thing is that although details of it have been lost, there are a lot of clues about this substance found in literature, which John laid out for me. One of the earliest mentions of this weapon refers to “cauldrons of fire,” for example, and other sources say the Byzantines installed furnaces and vessels of oils on their warships. And there are reports from battles where it was used describing smoke and a loud roaring noise when ships unleashed the weapon. And, as John mentioned at the beginning of the episode, sources describe the substance being squirted from ships using some sort of bronze device.

John Haldon: So if we add all these different things together, we can start to build up a picture of how it works. How can I describe this? Imagine you can heat this liquid, project it through a tube, and ignite it as it comes out of the tube, and you’ve basically got Greek fire in the way that the sources describe.

Kerri: In the ’70s, John worked with a chemist he knew, Maurice Byrne, to come up with a model for how Greek fire worked. They suggested the substance was some form of crude oil—a flammable substance that fit descriptions in the literature of an oily liquid.

John Haldon: We couldn’t build a full-scale one at that point. But we did a few experiments basically taking some crude oil and putting in a hypodermic syringe and heating it up and then projecting it at some paper ships in a basin. And the principle worked.

Kerri: Satisfied with what they had accomplished, John set the project aside. Until a couple decades later—he was at the University of Birmingham at the time—when a colleague connected him with a London-based documentary crew interested in exploring Greek fire.

John Haldon: The stupid title Machines Time Forgot. I hate the way TV does this, you know? Anyway, they’ve got to have a dramatic title. So Machines Time Forgot. So they rung me up and actually, they were quite good. They said, “Look, we have to make it popular. Otherwise, we won’t get an audience, we won’t get a contract. But we want to try and stick with the science as much as we can. And would you be prepared to put a team together and we’ll make a documentary about it.” And so I basically did.

Kerri: The documentary team wanted John to make a full-scale model of how Greek fire was used—with wooden ships and everything. Doing so presented a couple of issues. In their earlier work, John and Maurice had developed a theory that the siphon device projected this flaming oil from a pressurized container. The team realized the risk was high that a period-accurate bronze container might explode—even if they added a very non-Byzantine pressure gauge. So instead they built a manual pump for the liquid.

The other issue was they weren’t totally sure about the exact composition of the oily liquid. Their theory was it was crude oil, but that raised some questions.

John Haldon: So the questions that we had were, if you’re collecting crude oil, how did you keep it volatile? How is it stored? Was it distilled? Was there indeed a secret? But the key thing with crude oil is, how do you store it, because if you collect crude oil, even in liquid form, within 24 hours, it’s turned to pitch or tar, because all the light fractions will evaporate away, and you’re left with sticky, black gunk. And clearly the Byzantines had a liquid that they were using. So we had those questions. And that’s when I was talking to a chemist friend, who’s also a geologist, and she said, “Well, you know, there are different sorts of crude oil.” I had no idea!


Kerri: With his geologist friend’s help, John pinpointed a region within the Byzantine Empire where there is a particular kind of crude oil with unique properties. The oil fields are centered on an area between the Black Sea and the Caspian Sea.

John Haldon: Those oil fields produced a particular type of oil. It’s very low in nonhydrocarbons. And it’s very high in kerosene fractions, which means that it evaporates very slowly, which is obviously an advantage. So, you can collect it. It retains a high inflammability or burning index—in other words, its gases ignite very easily. And the other thing about this, it’s easily collected from surface seeps—you don’t need to have an oil well. It collects on the surface. So if you add all this together, the Byzantines have control or regular access to these areas, they can collect a particular type of crude oil, which purely by chance happens to have all the right characteristics for production of a sort of projectable instrument of war.

So I spent a bit of time rereading the texts. And I suddenly thought, oh, of course, the texts are telling us what this is, it’s really obvious. Once I looked at it all again, it was like duh, you know, of course that’s what it’s actually telling you. What was weird was that nobody had seen that before. Because, you know, didn’t take any stroke of genius. As soon as you think about it, it’s sort of obvious, really.

Kerri: John also added some pine resin to his formulation to give it the stickiness described in medieval accounts of its use.

John Haldon: So then we built the full-scale model. We did indeed heat a large vat of crude oil on a wooden ship, we built a furnace and put the cauldron on it, and we heated it to the appropriate temperature, we mixed pine resin in, which made it nice and sticky. And then we projected it.

Kerri: So Gina, I’m going to show you a clip from that documentary.

Gina: I’m so ready.

Photo showing a recreation of Greek fire destroying a wooden fishing boat.
Credit: Courtesy of John Haldon
A wooden fishing boat was no match for John Haldon's recreation of Greek fire.

Documentary audio: With only a gentle wind, it’s the best conditions they could ask for. They’re ready.

Kerri: All right, skipping forward a little bit here. So this is happening in a big water tank on the coast of Malta, basically a film set—nice controlled environment. The documentary team bought a couple of wood fishing boats to try their Greek fire out on. And here they go.

Gina: Holy cow! Look at that thing go!

Kerri: So you can see they’ve got the spigot device there on the larger ship. And you can hear them really straining to work that manual pump.

John Haldon (in video clip): Very impressive. Very impressive.

Kerri: This is John.

John Haldon (in video clip): Brilliant jet of flame. The boat’s in a dangerous position, blistered, the sails have gone. Absolutely superb example of how it would work. Really impressed. Much better than I expected. Super!

Gina: That is a lot of fire going on. Yeah, that boat is toast.

Kerri: And here’s John again, in our interview.

John Haldon: That’s the second boat, we sank the first one pretty quickly. Because it’s sticky, it burned and burned and burned. It didn’t go out. It’s an incredibly scary and effective weapon.

Kerri: So, John says he’s satisfied that the mystery of Greek fire has been solved. And, in fact, that it wasn’t really a big secret after all—just clever construction and a lucky geological coincidence. But it was that nagging question of how they pulled it off, and the chance to try it for himself, that led him to put the final pieces together.

John Haldon: I think it’s just curiosity, basically. And then specialists who are interested in either chemistry or military weaponry are the ones who’ve looked at this. And they’re intrigued by you know, what could it have been? Is it a precursor for gunpowder? You know, that was the origins of the whole thing. Did we actually have gunpowder and then lose it, and it had to be reintroduced again via China in the 13th century? And the answer is no, it’s nothing to do with gunpowder at all. And I’m pretty sure that all it is is crude oil with probably pine resin which is readily available all over the eastern Mediterranean. You just pour it in and stir it around over a cauldron of crude oil, which sounds mad, but it worked.

Kerri: Once again, that documentary series was called Machines Time Forgot and the episode John appears in is named “Fireship” if you want to go track that down for yourself. And if you’d like to learn more about recreating ancient chemical experiments, we’ll include some links in the script for this episode on C&EN’s website. The stories in this episode are truly just scratching the surface.

Gina: You know, I was thinking we should try out the Greek fire thing for ourselves. Get a boat, get some matches . . .

Kerri: I think this is where I point folks to our two-part episode on lab safety! Gina, maybe give that a listen, too?

Gina: All right, I’ll put it in my queue.

Kerri: This episode was written and produced by me and Gina Vitale. Story editing by Michael Torrice and Amanda Yarnell. The music in this episode was “Artful Thief” by Kadir Demir and “Plain Loafer” by Kevin MacLeod.

Next month on Stereo Chemistry, we sit down with two Nobel-winning biotech pioneers: Frances Arnold and Jennifer Doudna. In the same room together! Well, virtually, anyway. That’s out in February. Subscribe or follow Stereo Chemistry now to make sure you catch this incredible episode.

Gina:Stereo Chemistry is the official podcast of Chemical & Engineering News, which is published by the American Chemical Society. Thanks for listening.


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