The idea of a self-luminescent stone without any form of excitation seems like something impossible. However, in 1603 that was what Italian cobbler cum alchemist, Vincenzo Casciarolo had encountered. He had found greyish bright and unusually heavy stones from the countryside around Bologna, Italy. After heating the stones in the air (this process is known as calcination), they had become fluorescent in the dark. Not only was the fluorescence astonishing, but it was also persistent (for years!). This was one of the first materials to be documented to show such persistent emissions.
San This stone is none other than the famous Bologna stone, named after the city in which it was found The chemistry of this extraordinary discovery was the subject of research over the past 400 years. Early efforts to recreate the natural masterpiece had ended in vain. One of the first attempts was taken by Wilhelm Homberg, a German. Initially, it was thought that his method had succeeded and he had created bright stones.
The method was even included in the famous Cours de chymie which is thought to be the first chemistry textbook published. So what is the chemistry behind the elusive stone?
Interestingly, when a friend of Homberg had tried using the same method in trying to replicate the fluorescence of the stone, the experiment was a success. However, there was one crucial difference in the method. Homberg had used an iron mortar in grinding the stones while in contrast, his friend had used a bronze (typically 88% copper and 12% tin) mortar.
This stark difference in the results had left Homberg baffled and he was to have never discovered the reason behind it.
Eventually, after centuries, scientists had discovered the stone was simply BaSO4 or otherwise known as barite. While barite is rather common, not all forms of it glow. When scientists took samples of the Bologna Stone and visualised it, it was discovered that there were Cu2+ impurities present in the barite. These copper impurities were crucial in giving the barite its fluorescent properties. When light is shone on the stone, the copper ions absorb the light and emit it extremely slowly making explaining the stones persistent glow. However, that is not enough. The calcination process of the stone has to be incredibly precise. The extent of combustion has to be controlled such that the fuel is incompletely oxidised to carbon monoxide which is a reducing agent. If the fuel is completely oxidised to give carbon dioxide, the barite may be converted to barium carbonate.
While the Bologna stone mystery is widely considered to be solved today, it is still undoubtedly astonishing how such a complex and elaborate structure was formed naturally while numerous attempts to recreate it synthetically have failed. The statement sometimes the only way to really understand the history of experimental science is to repeat it (L Principe) cannot be any truer.