Many mysteries surround the chemical makeup of gold. In a recent news article, we explored why the precious metal sits so close to the Earth’s surface despite scientists believing it should settle near the planet’s core due to its siderophile composition, meaning it’s attracted to iron. Another one of the precious metal’s strange anomalies is how it turns a surprising shade of purple when it fulminates or explodes.
Scientists have been dazzled by fulminating gold for centuries. Gold was actually the first ever fulminating element discovered when German alchemist Sebald Schwaerzer isolated the compound, dissolved the sample, and discovered the high explosive, describing its purple emanating smoke in his book “Chrysopoeia Schwaertzeriana.” Since then, many have tried to unveil the reasoning behind gold’s transformation from a yellow metal to purple smoke once detonated, including highly renowned chemists to popular YouTubers conducting questionable at-home experiments.
Fulminating gold has proven to be a highly challenging topic to study due to its explosive nature. The highly unstable compound can be detonated simply by heat or touch, creating low explosive burns at subsonic speeds. Once detonated, the crystalline substance instantly transforms into a cloud of purple smoke in a reaction that moves at supersonic speeds.
Until recently, the violet smog has stumped alchemists.
“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,” according to The Physics arXiv Blog.
Some have developed theories, though none have been proven until recently. Researchers at the University of Bristol developed a peer-reviewed theory citing nanoparticles as the source of the emanating purple shade.
“[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,” the paper explained.
The researchers explained that “[Fulminating gold] has been used to coat objects in a purple/crimson patina much in the same way that solutions of gold nanoparticles can be used to coat substrates with purple/red layers… We show for the first time that the explosion of fulminating gold creates gold nanoparticles, ranging in size from 10 to 300 nm.”
While this theory may seem sound, no one was able to prove its basis until recently.
Simon Hall, a Professor of Chemistry at the University of Bristol, alongside Jan Maurycy Uszko, a doctoral degree student, conducted an experiment on fulminating gold to capture smoke samples and analyze their components.
“Our experiment involved creating fulminating gold, then detonating 5mg samples on aluminium foil by heating it,” Hall explained. “We captured the smoke using copper meshes and then analysed the smoke sample under a transmission electron microscope. Sure enough, we found the smoke contained spherical gold nanoparticles, confirming the theory that the gold was playing a role in the mysterious smoke.”
The team confirmed that violet smoke coloring stems from how surface electrons on the nanoparticles interact with light through a process called plasmon resonance. This process can also explain why some gold decorations develop purple fringes after nanoparticles form on the surface. A perfect example of this would be the Alhambra Palace in Granada, Andalusia, Spain.
The team in Bristol also noted that nanoparticles produced during gold fulminations have unique characteristics, potentially offering future applications for nanotechnology fields. Typically, nanoparticles form into spheres, though they become misshapen as they grow beyond a few nanometers, dissolving and re-growing following the Ostwald ripening process.
The nanoparticles examined in the fulminating gold experiment were all spherical, though, despite their sizes ranging up to 300 nanometers. Hall’s team explained that this anomaly is likely due to the supersonic speeds of the detonation not allowing for the ripening process to occur.
“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,” Hall’s team explained. Essentially, fulminating gold displays the potential for a new manufacturing process.
This revelation offers potential applications for the fast synthesis of super-regular metal nanoparticles, which could be useful for numerous fields, including bioengineering, medicine, and other industries involving nanotechnology. The recent discovery not only answered an ancient 400-year-old mystery but has paved the way for future scientific breakthroughs, with gold as the pivotal key.
“I was delighted that our team have been able to help answer this question,” Hall stated, “and further our understanding of this material.”