400-Year-Old Purple Explosive Smoke Mystery Solved

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 16^th 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 17^th and 18^th 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 17^th 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

This article is a rework of a press release issued by the University of Bristol. Material has been edited for length and content.

This article is based on research findings that are yet to be peer-reviewed. Results are therefore regarded as preliminary and should be interpreted as such. Find out about the role of the peer review process in research here. For further information, please contact the cited source.