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[Music] Mercury is the only metallic element in existence that's a liquid at room temperature, and it's absolutely mesmerizing to look at. So I'm always left feeling amazed by the fact that humans actually had access to this stuff thousands of years ago. For instance, vials of mercury have been found in ancient Egyptian tombs that are around 3,500 and 12,000 years old.
Similarly, excavation of the ancient Mesoamerican city of Teotihuacan has revealed some fascinating discoveries. Archaeologists have unearthed a chamber containing thousands of treasures which had remained undisturbed for 18,800 years. Not only that, but they discovered a miniature mountainous landscape that had been constructed, which had pools of liquid mercury in some of the tiny valleys to represent lakes.
But here's my favorite story about mercury in the ancient world. In the 3rd century BC, China consisted of various warring kingdoms. Qin Shi Huang was the king of the Qin state, and after years of conflict, he successfully conquered all of the other states and became the first ever emperor of a unified China. Towards the end of his life, he became obsessed with finding an elixir that would grant him immortality, and he would regularly drink mercury, thinking that it contained magical properties. What he didn't know was that mercury is actually poisonous, and long-term ingestion can be extremely detrimental to your health. In the year 210 BC, at the age of 49, he died. Ironically, his quest for immortality was probably the very thing that cut his life short.
But that's not the end of the story. He was put to rest in perhaps the most impressive mausoleum ever built. Much of what we know about it comes from an ancient historian, Sima Qian, who wrote about the mausoleum about two centuries after its construction. According to him, it took a total of 700,000 men almost four decades to build. He described how it would have been considered a serious breach if the craftsmen who constructed the mechanical devices and knew of its treasures were to divulge those secrets. So after the funeral ceremonies were completed and the treasures were hidden away, the inner passageway was blocked, and the outer gate was lowered immediately, trapping all the workers and craftsmen inside so that no one could escape. Trees were then planted on the tomb mound, and nowadays it's easy to mistake it as just another hill. Surrounding the tomb are thousands of lifelike, intricately sculpted warriors, archers, chariots, and horses, now known as the Terracotta Army. The mausoleum itself was modeled on the layout of Xianyang, the capital of the Qin Dynasty. The tomb was filled with rare artifacts and priceless treasure, and allegedly continuously flowing all around this recreated city were 100 rivers of pure mercury. Now, if it's true, that's a lot of mercury.
The thing is, nobody knows for sure if it's true or not. Archaeologists have excavated the surrounding areas of the tomb and famously discovered the guarding Terracotta Army, as well as mass graves of some of the workers, but nobody has entered the tomb itself. This is partly due to a strong desire to minimize any destruction of the tomb and with the associated difficulties in excavating it, but it's also due to legends of booby traps, such as crossbows and arrows primed to shoot at anyone who attempts to enter the tomb. From its description, it sounds like the only person who could survive would be Indiana Jones himself. As well as the possibility of lethal traps, there's the potentially dangerous levels of mercury. So it's unlikely that anyone's going to be walking around in there anytime soon. According to the World Health Organization, just the inhalation of mercury vapor can produce harmful effects on the nervous, digestive, and immune systems, lungs, and kidneys, and may be fatal. There's a reason we don't use mercury in thermometers anymore. So if the legends are true, it could be bad news for anybody who wants to excavate the tomb. But are they true?
Well, various soil analyses around the site of the mausoleum over the last few decades have shown significantly elevated levels of mercury. In addition, in 2020, a team of researchers used a technique called mobile differential absorption lidar to analyze not the soil, but the atmosphere surrounding the mound, and they discovered much higher levels of mercury vapor directly above the mound compared to surrounding areas. This makes sense when we consider mercury's volatility; if it's in the soil in high concentrations, it'll also evaporate into the air. All of this suggests that there might be some truth in the old legends. Could there actually be rivers of mercury running through the tomb of the first Chinese emperor? Quite possibly. Maybe one day we'll know for sure.
So thousands of years ago, from Egypt to America to China, people were handling mercury. But how? How did all of these ancient civilizations get their hands on this amazing substance? The answer is mercury(II) sulfide, which is found naturally in a beautiful red ore, more commonly known as cinnabar. It's the most prominent naturally occurring mercury ore, and there's evidence that humans have been mining cinnabar since the Neolithic times, most likely to be used for paints and cosmetics. At some point, its toxic nature became known. In ancient Rome, for instance, slaves and criminals were often forced to work in cinnabar mines, and it was effectively considered a death sentence for them. But then how did these ancient civilizations take this rock and extract liquid mercury from it?
In 2022, a team of chemists and historians from the University of Bologna read through surviving documents written by alchemists almost 2,000 years ago, and they attempted to recreate these ancient methods in a modern lab. They published a fascinating paper about their findings, which is completely open access, so I'll put the link in the description if you'd like to read it. There were loads of ancient recipes describing the extraction of mercury from cinnabar, but the researchers categorized all of them into two groups: cold extraction and hot extraction. With cold extraction, they said the earliest known procedure for extracting mercury was recorded by the natural philosopher Theophrastus, who stated in his work On Stones in the 4th century BCE, "Mercury is produced by grinding cinnabar with vinegar in a copper mortar with a copper pestle." Modern chemists immediately recognize copper—that is, the metal from which pestles and mortars are made—as a key reagent. So what the ancient alchemists wouldn't have known is that the copper pestle and mortars were actually reacting with the mercury sulfide in the cinnabar to form copper sulfide and mercury. The researchers recreated this procedure by grinding synthetic cinnabar with copper turnings and acetic acid. After an hour, they started to see droplets of liquid mercury forming. The researchers also said that it is clear that the acid—meaning the vinegar—plays a catalytic role, perhaps by removing the oxide coating on the metal surface. They then used a technique called X-ray diffraction to analyze the crystal structure of their product, and they found that the original mercury sulfide (shown in red) had been transformed into a completely different crystal structure—that of copper sulfide (shown in blue)—giving evidence for the success of the reaction. So just the simple act of grinding cinnabar with copper and vinegar can produce liquid mercury.
How about the hot extraction techniques? Here the researchers said, according to ancient sources, the hot extraction method of processing mercury from cinnabar can be divided into three procedures: 1) simple heating of cinnabar; 2) heating cinnabar in a closed vessel in the presence of iron; and 3) heating cinnabar in the presence of so-called natron oil in a closed vessel. The first of these is just the reaction between mercury sulfide and oxygen to form sulfur dioxide and mercury. In the 1st century BC, Vitruvius—by whom Da Vinci was inspired to draw his Vitruvian Man—described how when unrefined cinnabar was baked in a kiln to dry it out, mercury was always found on the floor of the kiln. So it seems like the simple act of heating cinnabar led to the formation of mercury. The researchers tried to recreate this method by heating synthetic cinnabar within an alumina crucible that was covered with a lid and found that liquid mercury condensed on the lid of the crucible. However, they said that the amount of oxygen estimated to be inside the crucible is insufficient to complete the reaction. To overcome this hurdle and make the extraction technique more efficient, we can assume that ancient alchemists tried to add other ingredients before roasting cinnabar in closed vessels. This brings us on to the second hot extraction method: heating cinnabar in the presence of iron. In this case, the mercury sulfide would react solely with the iron and not with any oxygen, giving us iron sulfide and mercury. The researchers recreated this method by placing the cinnabar on a small iron plate within an alumina crucible that was then covered with a lid.
Finally, we have the third and most mysterious hot extraction technique: heating cinnabar with natron oil in a closed vessel, which was described by the alchemist Pseudo-Democritus. By the way, this Pseudo-Democritus is not the same person as the philosopher Democritus, who is one of the fathers of atomic theory. If you want to learn more about him, check out my series on the history of the atom. Anyway, the reason why this method is the most mysterious of all is because it's not clear as to what natron oil actually is. According to Wikipedia, natron deposits are sometimes found in saline lake beds which arose in arid environments. So the solid form of this mineral is something that ancient civilizations had access to, but how they made it into an oil isn't clear from the alchemical documents. From translating old texts, the researchers' best guesses are that the oil form likely constituted a solution of sodium carbonate dissolved in water, oil, or vinegar. Their next step was to investigate the effect of sodium carbonate, and they found that the addition of solid sodium carbonate without any liquid vastly improved the first hot extraction method, which, remember, had led to an incomplete reaction. With the addition of sodium carbonate, the cinnabar fully converted into mercury. So sodium carbonate seems to be a very effective reagent for the production of mercury. Now, to test it in a liquid form, they made up solutions of sodium carbonate dissolved in water, vinegar, and vegetable oil and tried heating mercury sulfide in each one. For the first two solutions, although mercury was produced, no significant improvement was observed in comparison to the reactions that occurred under dry conditions. For the third solution, they couldn't manage to produce mercury at all.
To summarize, many of these ancient techniques for the extraction of mercury were proven to work remarkably well. Ancient alchemists could have made the liquid metal by grinding cinnabar and vinegar with a copper pestle and mortar, or by heating the cinnabar alone, or with iron, or with some form of a sodium carbonate solution. So given enough power, influence, finances, and resources, perhaps it's not so crazy after all that an ancient Chinese emperor had the means to spend eternity surrounded by rivers of flowing mercury. Let me know in the comments if you've heard any other amazing stories of mercury being used throughout history, and if you enjoyed this video, I'd really appreciate you subscribing. It really helps out the channel. Thanks for watching, and I'll see you in the next one.