The Mysterious Explosions of Fulminating Gold

"Fulminating gold" explodes to produce strange purple smoke. Now scientists have shown for the first time that the smoke contains gold nanoparticles.

By The Physics arXiv Blog
Nov 29, 2023 

(Credit:-strizh-/Shutterstock


One of the great “scientific” endeavors of the Middle Ages was the study of Chrysopoeia, the process of turning base metals such as lead into gold. Most practitioners were doomed to failure but one German alchemist named Sebalt Schwarzer made a single contribution that has stood the test of time.

In his 1585 tome, Chrysopoeia Schwaertzeriana, he describes the synthesis of the world’s first high explosive, a substance known as fulminating gold. This crystalline substance turns almost instantly into a cloud of purple and red smoke in a reaction that travels at supersonic speeds. Hence the high explosive (a low explosive burns at subsonic speeds). Fulminating gold is relatively easy to make but highly unstable, detonated by hear or even touch.

This instability has made it difficult to study. Its crystal structure is poorly understood and even its chemical formula defied analysis until recently. It turns out that fulminating gold is not a specific chemical but a mixture of polymeric compounds of gold, chlorine and ammonia

There is another puzzle too: the cloud of smoke that fulminating gold produces when it explodes. Nobody is quite sure why it is purple or red. Modern chemists have assumed the smoke is made largely of gold nanoparticles and it has long been used to coat objects with a beautiful purple patina. Chemists know that gold nanoparticles can do the same.

But this is merely circumstantial evidence of gold. What’s needed is clear scientific proof of gold in the smoke.

Gold Standards

Enter Jan Uszko and colleagues at the University of Bristol in the UK who have gathered the first conclusive evidence that the reaction is an explosion of gold. “We show for the first time that the explosion of fulminating gold creates gold nanoparticles, ranging in size from 10 to 300 nm,” they say. They add that their discovery may help to produce better nanoparticles in the future.

The team first made several samples of fulminating gold and placed a carbon-coated mesh over them as they were heated. The resulting explosions deposited smoke particles onto the mesh.

The team then scanned the mesh with a transmission electron microscope and characterized the particles they found.

It turns out that the mesh was covered in nanoparticles between 10 to 300 nm in diameter. The electron microscope images revealed that the crystal planes within the nanoparticles have a spacing of 0.24 nm, which is consistent with gold.

The team then compared the electron diffraction patterns with those theoretically expected and from gold and found close agreement. “This work is proof of the long-supposed nature of the cloud produced on the detonation of fulminating gold,” they conclude.

The purple and red coloring comes from the way electrons on the surface of the nanoparticles interact with light via a process called plasmon resonance. Indeed, other scientists recently showed that gold decoration can sometimes develop purple fringes when nanoparticles form on its surface, as at the Alhambra Palace in Grenada in Spain.

All That Glisters...

The Bristol team say that nanoparticles produced in the explosion have some unique characteristics. Nanoparticles usually form as spheres but become misshapen as they grow beyond a few nanometers in diameter. This happens via a process called Ostwald ripening where small particles dissolve and then regrow onto larger particles. This also reduces the variation in size.

But the nanoparticles from the explosive detonation of fulminating gold have a broad range of diameters that are spherical up to 300 nm in diameter. Uszko and co say that’s almost certainly because the speed of the detonation does not allow Ostwald ripening or other similar processes to occur.

Such large nanospheres are unusual and the explosions suggest a new mode of manufacture. “In this way, larger gold nanoparticles can be created with a sphericity more commonly seen in the early stages of formation when the nanoparticles are small,” say Uszko and co.

That’s interesting work solving an ancient problem. Schwartzer would surely be amazed!

Ref: Explosive Chrysopoeia : arxiv.org/abs/2310.15125chemistry

400-Year-Old Purple Explosive Smoke Mystery Solved

Scientists now know why fulminating gold – the world’s first high explosive – produces purple smoke when it detonates.

News  

Published: November 21, 2023

 

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Alexander Beadle

Credit: Yoni Kozminsi / Unsplash.

When high explosives were first invented, they were dangerous, unpredictable and could explode with even the slightest touch.

Made from a mixture of gold, ammonia and chlorine, these explosives became notable for more than just their deadly potential – after detonating, they left behind a strange purple smoke unlike gunpowder or anything else seen at that time.

More than 400 years after the invention of “fulminating gold” explosives, scientists finally have an answer for why these compounds produce their iconic purple smoke. The research is published as a preprint in arXiv.

From ancient alchemy to modern chemistry

The idea of a 16th century alchemist obsessed with their quest to turn lead into gold is an enduring trope in modern media, and for good reason. This fascination with “chrysopoeia”, the artificial production of gold from more common metals, really happened. It gave way to one of the more revolutionary discoveries made in the 1500s – high explosives.

The synthesis of fulminating gold was first described by alchemist Sebalt Schwärtzer in his 1585 book “Chrysopoeia Schwaertzeriana”. Further study by leading scientists such as Robert Hooke and Antoine Lavoisier in the 17th and 18th centuries improved this process, with modern science turning what was a four-to-five-day process into a synthesis that can be achieved in minutes by mixing gold compounds with ammonia.

But while the chemistry behind making these centuries-old high explosives is well understood, one mystery had endured – why is fulminating gold’s smoke purple?

Researchers speculated that the distinctive smoke might be a result of gold nanoparticles that are spat out into the smoke. Circumstantial evidence would back this theory; the 17th century German-Dutch apothecary Johann Rudolf Glauber documents how smoke deposition from fulminating gold was sometimes used to gold-plate objects. This would suggest that gold nanoparticles are present in the smoke to a significant degree, though modern science has never proven this definitively.

Gold nanoparticle clusters create the iconic purple hue

In their new preprint, Simon Hall, professor of chemistry at the University of Bristol and PhD student Jan Maurycy Uszko, synthesized samples of fulminating gold to test whether gold nanoparticles could explain this unusual smoke hue.

Using a transmission electron microscope (TEM), the researchers were able to identify and take images of clusters of gold nanoparticles that had been collected from the fulminating gold smoke.

“I was delighted that our team have been able to help answer this question and further our understanding of this material,” Hall said.

“Our experiment involved creating fulminating gold, then detonating 5mg samples on aluminum foil by heating it. We captured the smoke using copper meshes and then analyzed the smoke sample under a TEM,” he continued. “Sure enough, we found the smoke contained spherical gold nanoparticles, confirming the theory that the gold was playing a role in the mysterious smoke.”

More than just proving an old theory about the colors produced by an ancient explosive, the researchers believe that this research could also have wider-reaching consequences.

“This work is proof of the long-supposed nature of the cloud produced on the detonation of fulminating gold, but also potentially opens the door to fast solvent- and capping agent-free syntheses of metal nanoparticles,” they write.

Having solved one historic science puzzle, Hall and his team say that they plan to use this methodology to study the nature of smoke produced by other metal fulminates – such as platinum, silver, lead and mercury – to see whether these compounds might hold any further answers.

 

Reference: Uszko JM, Eichhorn SJ, Patil AJ, Hall SR. Explosive chrysopoeia. arXiv. 2023. doi: 10.48550/ARXIV.2310.15125

Nanoscale engineering of gold particles in 18th century Böttger lusters and glazes