Today, if you encountered a glowing rock, you are likely not to touch it with your bare hand. Who knows what it might do to your flesh? It might be radioactive. It might be an egg of a dangerous alien species, ready to emerge and gruesomely kill you. Modern science and sci-fi have conditioned our response to be cautious about glowing rocks. But you’d still be curious about it. Very curious. You might contact NASA – maybe it fell from the sky? Or perhaps a scientist at the local university? All this assumes you’ve taken pictures and video with your smartphone, all at arm’s length, to document your find!
Back in 1602, a cobbler found that certain stones, after being roasted in a furnace, could glow in the dark. These stones only came from a specific location: along the slopes of Monte Paderno near the city of Bologna, Italy. These stones were phosphorescent; they absorb light, and then emit light for some period of time before the effect fades away. (Not to be confused with fluorescence which is closely related, but the light emission is much quicker and is not persistent. Also not to be confused with chemiluminescence where chemical energy is converted into light emission.) This became known as the Bologna stone. It only worked with the stones from this particular region.
I learned about the Bologna stone reading Peter Wothers’ book on the origin of element names in today’s Periodic Table. The cobbler, Vincenzio Cascariolo, who discovered the stones’ phosphorescence didn’t know anything about its chemistry. He called it spongia solis, meaning ‘sponge of the sun’, thinking that the stone soaked up the rays of the sun akin to a sponge. The famous scholar and polymath Athanasius Kircher thought that “the stone was a kind of magnet acting on light in the same way that an ordinary magnet acts on pieces of iron.” Turns out that moonlight can also be sponged up by the Bologna stone, and it became known by many names including lapis phosphorus, meaning ‘the stone that carries light’.
It turns out that the Bologna stone has no phosphorus. To learn more about the history and chemistry, I read Lawrence Principe’s article, “Chymical Exotica in the Seventeenth Century, or, How to Make the Bologna Stone” (Ambix 2016, 63, 118-144). Principe has also written a superb book on the history of alchemy that is now on my bookshelf, one of the rare instances where I buy the book after reading a copy from the library. In the Ambix article, Principe details the investigations of Wilhelm Homberg on the Bologna Stone. But Principe isn’t just an armchair historian, and he goes through the process of trying to reproduce the lost art of making these stones phosphorescent. In the process, he discovers that it’s not just the particular type of ore (chemically-speaking) that you begin with, but impurities present in the preparation can enhance or inhibit the phosphorescence.
The story takes many twists and turns. To acquire his vast expertise in these ‘chymical exotica’, Homberg trades in chemical secrets. I’ll tell you a secret preparation that I know if you tell me one that you know. No one outside of Bologna could prepare these stones, so Homberg travels there and learns. Eventually the methods were published, but they were hard to reproduce. One might suspect that perhaps a crucial step or ingredient was purposefully left out in the published procedure to maintain the value of the secret, but this does not seem to be the case. Homberg himself was very confused when after being successful in Italy, he was unable to reproduce the effect in Paris where he now had a prestigious position in the Academie Royale des Sciences. After failing over and over again, and trying to avoid his imploring fans and colleagues to show them the process, he stumbled upon success in a fascinating tale that Principe elaborates. Here’s an excerpt with Homberg as narrator.
What chagrined me more was that I had promised to teach one of my friends the method of making the stones luminous, and he was pressing me strongly to keep my word to him. After many excuses, I ran into this friend one day on the street in his neighbourhood, and he led me to his house and showed me some raw Bologna stones and a furnace which he had had made expressly for this calcination according to the design I had given him… Being thus pressed, I began again the operation which had so often failed, and to speak the truth, I was trembling all the while, for I had not told him that I had always failed at it in Paris. When the operation was finished I found the stones the most brilliant and luminous that I had ever seen. My astonishment was enormous, for I had changed nothing in the operation. These were the same stones as mine, for I had given them to him. After having examined everything well, I found no difference except that in this last operation I used a bronze mortar… in place of the iron mortar which I had used in my laboratory in Paris.
The primary ore of the Bologna stone is barium sulfate (BaSO4) which does not exhibit phosphorescence. Roasting in the furnace drives off the oxygen turning it into barium sulfide (BaS). For that, you need both a reducing agent and the right (high) temperature to facilitate the chemical reaction. But the key to phosphorescence is the impurity of the Bologna stone. It contains trace amounts of copper(I). Wothers explains: “During exposure to light, electrons in the copper ions become energetically excited and trapped in defects in the barium sulfide crystal. Over time, the electrons return to their lower-energy tate, emitting the stored energy as light once again.” The presence of iron significantly inhibits the phosphorescence, which is why Homberg failed in his Paris laboratory. Bronze, on the other hand, is an alloy containing almost 90% copper. Thus, the grinding in the bronze mortar introduces more copper impurities and the brighter glow!
Principe learns much more from his process. It begins with going to Bologna to look for the stones and finding that no one sells them, he has to make use of the seventeenth century accounts to find them in nature. It is a credit to the early writers in their specificity and a wonder that modern development had not destroyed the original site that Principe was able to find the right ores. Many crystalline ores in the vicinity looked similar, but Principe knew that the barite (common name of BaSO4) would feel much heavier in the hand. Getting the furnace with the right conditions is also tricky. The standard chemical explanation that you just need to burn it with charcoal (elemental carbon) as the reducing agent will work. We can even write a balanced chemical equation for it (BaSO4 + 2 C à BaS + 2 CO2). I even used this equation in a stoichiometry G-Chem exam question some years ago.
When Principe designed the furnace according to Homberg’s description, he figures out that there won’t be complete combustion because the vents are too small. He also notices that the flames have a purplish hue just outside the vent (but not inside the furnace). Thus, the reducing agent acting on barite at the furnace temperature is actually carbon monoxide. If there was complete oxidation within the furnace, carbon dioxide can instead react with the ore to form barium carbonate (BaCO3) instead of the desired BaS. In his conclusion, Principe notes that “chemical processes, even the ‘simple’ ones, frequently turn out to be far more complicated in practice than one would imagine, and this is often the case because of subtle or unnoticed differences in materials… [These materials] have their own histories, which include the problems of finding starting materials that are correct and consistent, and developing methods of preparation, many (perhaps most) of which will fail, more often with some practitioners than others… Considering the enormous variation in materials such as purity, particle size, and origin, as well as more obscure factors like scale, climate, and the reactivity of vessels and instruments, it is amazing that anything beyond the most tri vial chemical reaction actually works the same way twice.” There is also a visceral feeling that you can only get when you repeat the experiment, and not as an armchair theoretical chemist. The stones, when correctly prepared, stink. Principe finds that he “could accurately predict how brightly a stone would glow based on how strongly it smelled… when the calcined Bologna Stone ages and ceases to luminesce, the odour of Sulphur likewise vanishes.”
I learn from Wothers that while the Bologna Stone was unique when first discovered, eventually more glow-in-the-dark were discovered. Seventy years later, in a serendipitous discovery Balduin’s Phosphorus – a preparation of calcium nitrate that contained no phosphorus whatsoever – was discovered. It gave rise to the word ‘phosphoresence’. Around the same time, phosphorus was discovered and purified. The light emitted from phosphorus does not come from phosphorescence. Rather it is chemiluminesence. Phosphorus (the ‘white’ version that is molecular P4) oxidizes quickly and flashily and is dangerous to handle. And in the eighteenth century, fluorite (calcium fluoride ore, CaF2) was found to emit light, but it does so only when heated up, having absorbed radioactive rays over a long period of time that have trapped excited electrons in the crystal. Once the emission is over, the fluorite cannot easily and quickly be recharged (unless exposed to a strong radioactive source). Turns out this is a great way to carry out ‘thermoluminescent dating’ of ancient pottery and ceramics.
Today, modern chemists can easily make glow-in-the-dark novelties that employ a variety of physicochemical mechanisms. They’re so common now, perhaps they are no longer novelties. But back in the day when the Bologna Stones were ‘discovered’, it must have been an enigma to experience.
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