400-Year-Old Purple Explosive Smoke Mystery Solved

In the early days of high explosives, they were risky, hard to control, and could explode with the slightest touch. These explosives, made from a mix of gold, ammonia, and chlorine, stood out for being dangerous and leaving behind a peculiar purple smoke when they went off. This was something totally different from gunpowder or any other explosives of the time.

Now, over four hundred years since the first “fulminating gold” explosives were created, scientists have figured out why these compounds create this unique purple smoke. The details of this discovery are laid out in a research paper available as a preprint on arXiv.

Creation of high explosives

From the ancient practice of alchemy to the advancements of modern chemistry, the pursuit of transforming lead into gold captivates our imagination, and it’s not just a story from the movies. It actually happened. This quest, known as “chrysopoeia,” led to a groundbreaking discovery in the 1500s: the creation of high explosives.

The process of making fulminating gold was first detailed by the alchemist Sebalt Schwärtzer in his 1585 book, Chrysopoeia Schwaertzeriana. As time went on, prominent scientists such as Robert Hooke and Antoine Lavoisier in the seventeeth and eighteenth centuries refined the method.

Modern science has transformed it from a lengthy four-to-five-day procedure into a quick synthesis achievable in minutes by mixing gold compounds with ammonia. While we now understand the chemistry behind these age-old high explosives, one puzzle reamained: Why does the smoke from fulminating gold appear purple?

Reason for the purple smoke

Researchers speculated that the unique purple smoke might result from gold nanoparticles released into the smoke. There’s some indirect evidence supporting this idea.

In the seventeenth century, German-Dutch apothecary Johann Rudolf Glauber noted that smoke deposition from fulminating gold was used to gold-plate objects. This implies that gold nanoparticles might be present in the smoke, although modern science hasn’t definitively proven this yet.

In their recent preprint, Professor Simon Hall and PhD student Jan Maurycy Uszko from the University of Bristol investigated whether gold nanoparticles could explain the distinct purple color of fulminating gold smoke.

Using a transmission electron microscope (TEM), the researchers examined and captured images of clusters of gold nanoparticles collected from the smoke produced by detonating fulminating gold.

Professor Hall expressed his satisfaction, stating, “I was delighted that our team [has] been able to help answer this question and further our understanding of this material.”

The experiment involved creating fulminating gold, detonating five-milligram samples on aluminum foil through heating, and capturing the resulting smoke with copper meshes. Analysis under the TEM revealed the presence of spherical gold nanoparticles in the smoke, confirming the theory that gold contributes to the mysterious purple smoke.

More implications of the study

Beyond confirming an old theory about the colors from an ancient explosive, the researchers believe this study could have broader implications.

They write, “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.”

Having unraveled one historical, scientific mystery, Professor Hall and his team plan to apply the same methodology to explore the nature of smoke produced by other metal fulminates, including platinum, silver, lead, and mercury, to determine if these compounds might offer additional insights.