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


The search for the recipe for gold took some peculiar turns in earlier times. In the 16th century, German alchemists Basilius Valentinus and Sebalt Schwertzer reported a mysterious discovery. A substance, a grimy olive-green powder, with variations in color such as yellowish, brown, or black depending on the manufacturing process. Upon combustion, the goldmakers observed a purplish-red smoke.

Experiments with the powder proved perilous, as naturalist John Dalton experienced firsthand. In 1809, a vessel containing the substance exploded in his hand, causing severe injury. Knallgold, the name of this hazardous mixture, was one of the earliest primary explosives, detonating with minimal thermal or mechanical influence.

However, the chemical processes involved in this mixture remained unclear until recently. Knallgold forms when gold salts mix with ammonia or ammonium salts, but the precise chemical reactions remain hidden from chemists for a long time. According to earlier studies, the manufacturing process appears to produce materials with a variety of structures, hinting at the potential for polymer compounds.

A World of Nanospheres

Now, scientists have made progress. A team from the University of Bristol in the UK, under the direction of Simon R. Hall, sheds light on the enigmatic purplish-red smoke in a study that has not yet been subject to peer review and is available on a preprint server. According to a report by “Spektrum,” it consists of golden nanoparticles exhibiting a reddish color. Under the electron microscope, researchers could trace the structures of a sample previously vaporized with the smoke down to the atomic level. They discovered a world of tiny spheres, each a few nanometers in size, with structures resembling metallic gold. The association between metallic gold and the color red has been known for some time. Solutions of fine gold also display a reddish shimmer.

However, the study still does not elucidate the specific chemical structures behind the colorful Knallgold powder. Further research will be necessary for such clarification. Given that the current study’s primary motivation is academic curiosity, interest in these findings is probably going to remain low. To date, no practical application has been found for Knallgold. Its unpredictability in explosive production and its high cost as a raw material have rendered it impractical.

In the 17th century, chemist Johann Rudolf Glauber recognized that the purplish smoke carried fine gold dust. He used this knowledge to gild objects placed near the cloud. However, this method also proved to have no lasting significance.