The Bologna Stone was discovered in 1603, at the base of a dead volcano near Bologna. When treated with heat, and exposed to sunlight, it would glow for hours—sometimes days. It took 400 years to figure out why.

In 1603, Vincenzo Cascariolo was digging in the volcanic rock near Bologna, Italy. The man was a shoemaker by trade, but he hoped to get rich by alchemy. When he found a milky-white stone he decided to take some samples back to his workshop. There, he most likely heated a sample of the stone, possibly in a special oven that let him control the exposure of the material to both the source of heat (coals or flames) and the oxygen in the air. The process is called “calcination.” Cascariolo failed to come up with the Philosopher’s Stone, but he can’t be faulted for thinking he did. After all, after the treatment, if exposed to sunlight or flames this particular stone glowed in the dark for hours.

The stone he found is now known as baryte. It’s BaSO4, a mix of barium, sulfur, and oxygen, but not all kinds of baryte glow, even after they’ve been calcinated. What Cascariolo found, which made him the discoverer of the world’s first “persistent luminescent material,” was a special radiating kind of baryte. It was much sought after by alchemists for years.

It took scientists a long time to figure out what it was that made baryte glow the way it did. Over four centuries after its discovery, scientists took a lab to samples of the Bologna Stone. What they found was an impurity in the baryte. Copper ions, denuded of two electrons each, were sprinkled through the baryte. When exposed to light, they would absorb energy, and then slowly emit it over multiple days.

Today, because no one thinks it will turn lead to gold or make them immortal, the Bologna Stone isn’t a hot commodity. It is still used. Here’s someone who made “Bologna stone pies,” by adding, among other things, copper chloride to barium sulfate and baked the result.

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Top Image: Carles Millan