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400-Year-Old Alchemical Mystery Solved: Why Does Gold Explode Purple?

It's a rare win for the turning-lead-into-gold crowd.

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Dr. Katie Spalding

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Dr. Katie Spalding

Freelance Writer

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

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Purple smoke drifting on a black background, looking moody and dramatic

It's taken 400 years to solve the mystery of why alchemists were creating purple explosions.

Image credit: Vagengeim/Shutterstock.com 

It’s easy to laugh at the alchemists of yesteryear, with their spotty understanding of science and dogged determination to discover nuclear transmutation five centuries before the discovery of the nucleus. But can we really blame them? After all, it’s only now, after 400 years of advancements in chemistry and physics, that we’ve finally solved the mystery of how they were creating purple explosions all those years ago.

Fulminating gold – the name comes from the older meaning of “fulminate”, viz, “explode” – was the first high explosive ever discovered. The earliest reference to its creation comes from 1585, in a book by the German alchemist Sebald Schwaerzer, and it’s been popular ever since with just about everybody – from academic chemists to popular YouTubers.

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Why? Well, it’s easy to make, fun to use, and the icing on the cake: it gives off an unusual purple smoke when it detonates. But despite its chemical makeup being thoroughly understood for centuries now, the reason for that violet smog has thus far stumped science.

It’s not that people haven’t had their suspicions. “[It] is often stated [that] the source of the unusual red or purple coloration of the smoke… is due to the presence of gold nanoparticles,” notes a new paper (currently in preprint form, meaning it has not been peer-reviewed) from researchers at the University of Bristol.

It might sound strange that the presence of gold should color something purple, but there’s actually some pretty strong circumstantial evidence to support the idea. “[Fulminating gold] has been used to coat objects in a purple/crimson patina,” the authors explain, “much in the same way that solutions of gold nanoparticles can be used to coat substrates with purple/red layers.” 

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But so far, nobody has been able to prove the hypothesis one way or the other – until now. 

“Our experiment involved creating fulminating gold, then detonating 5mg samples on aluminium foil by heating it,” said Simon Hall, Professor of Chemistry at the University of Bristol, who authored the new paper alongside his PhD student Jan Maurycy Uszko, in a statement

“We captured the smoke using copper meshes and then analyzed the smoke sample under a transmission electron microscope,” he explained. “Sure enough, we found the smoke contained spherical gold nanoparticles, confirming the theory that the gold was playing a role in the mysterious smoke.”

Transmission electron microscopy image of a cluster of gold nanoparticles captured from detonated fulminating gold.
Transmission electron microscopy image of a cluster of gold nanoparticles captured from detonated fulminating gold.
Jan Maurycy Uszko et al, Arxiv 2023


Having located a rare win for the alchemy crowd, the team now plans to use the same methods to investigate the smoke produced by other metal fulminates such as platinum, silver, lead, and mercury (if that last one sounds familiar, you might be remembering the time Walter White used it to blow the freaking roof off of Tuco’s drug den.)

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Just like fulminating gold, the precise nature of these clouds remains a mystery – though perhaps not for much longer. And the results aren’t just of use to those interested in the history of science or peculiar chemical reactions: even with just the purple problem solved, the team is already talking about potential applications in the fast and quick synthesis of super-regular metal nanoparticles – useful in fields as diverse as medicine, bioengineering, or anything involving nanotechnology.

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

The preprint can be viewed on ArXiv.


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