Ever wondered why proponents of medieval alchemy thought they could turn lead into gold? Such a transmogrification seems the stuff of fantasy to us with our modern understanding of matter, chemistry and physics, but alchemists had none of this. And, as it turns out, their thinking was quite sophisticated for the Middle Ages. Strip away our knowledge and look at the world as they did, and you will start to see what that means. Alchemists lived in a world of magic, where metals could be burned out of rocks and the application of various caustic, reactive or catalytic ingredients could fully change one substance into another. We know that lead into gold is not possible because both are elements, but they did not, and they hoped to discover the process as they had so many other “magical” transformations. Furthermore, there is a certain logic to the choice of lead which may have made them think is would be relatively straightforward. Lead and gold are very similar: they are both soft, heavy metals which rarely react.  This is borne out by our modern science, too. Lead and gold are very similar at the atomic level, with lead having only three more protons and electrons than gold. Lead is therefore the heavier element, and it was only reasonable for alchemists, observing this, to conclude that lead contained something else. The only difference the alchemists could plausibly see was that lead was poisonous, and their logic amounted to an attempt to remove this toxic aspect of the metal, hopefully leaving the unreactive and safe gold behind. Such a reaction is in fact possible if you have a particle accelerator, but sadly nobody in the Middle Ages had one of those, so no dice. So, what did the alchemists have to instruct them in how to turn lead into gold? They turned to their alchemical manual: the emerald tablet of Hermes Trismegistus. Secret Wisdom OK, so you have a lot of questions. Let’s start with Hermes Trismegistus. From roughly the second century BC Greek texts in Egypt started to reference this figure, and the trove of ancient hidden knowledge that he possessed. His name is a combination of the Greek god Hermes and the Egyptian god Thoth, associated with esoteric knowledge and wisdom. The number of texts associated with this figure grew in the following centuries and these were collected into a large body of work known as the Hermetica which covered everything from medicine to astrology to magic, all with a heavy sprinkling of mysticism. The Emerald Tablet is one such work. The earliest surviving example appears in Arabic in the 9th century, although it is believed that this is based on a much older text in Greek, in line with the rest of the Hermetica. It may be as old as 200 AD. The text in which it is found is called Kitâb sirr al-Halîka, The Book of the Secret of Creation. In it the author (here named Balînûs, the Arabic name for Apollonius of Tyana) describes finding the tablet in a crypt under a statue of Hermes Trismegistus, held by an old man. Much of the book discusses how all the elements of the world come from mercury and sulfur (mercury is, incidentally, a far better candidate for transmutation into gold than lead, differing from gold in only a single proton in its nucleus). But the emerald tablet itself is translated, in full, at the end. And it is revealed to be…very short, and very cryptic. Furthermore, it doesn’t really say anything about lead, gold, alchemy or any of these other cornerstones of medieval “magical” science. It has been translated many times but its talk of the Sun and Moon, of Earth, wind and fire, has never been fully understood. Some medieval practitioners saw it as a coded explanation of alchemical processes to transmute one element into another, This, it is said, is the “secret” of Hermes Trismegistus, and once his meaning can be understood the processes described in the rest of The Book of the Secret of Creation can be replicated. Alchemists from the medieval and Renaissance period offered their own explanations and interpretations for the content of the tablet, as an increasingly large commentary on the tablet and the Book came into being. Luminaries such as Roger Bacon, Albertus Magnus and Isaac Newton offered their thoughts. All thought that what was described in the tablet related to the “philosopher’s stone” and how it acted as the key ingredient in turning base metal into gold. Also known as the “powder” or the “tincture,” this obscure chemical was the key. Use it on the metal, follow the instructions on the tablet, and presto! You have gold. It is this unknown substance which lends medieval alchemy its modern mystique. We don’t know what it is, and so we cannot say with certainty what it can do, goes the argument of modern believers. First mentioned in roughly 300 AD, the pursuit of this substance became the mainstay of medieval alchemy: it was the missing piece of the puzzle. Texts exist which describe it in detail, but we have no idea what it actually is beyond being a white (or sometimes red) powder. We do, surprisingly, know how to make it using a process called the Magnum Opus, the “Great Work”. Or rather, we have multiple processes, all of which are complex and difficult and none of which work.  These recipes involve observations of the ingredients as they interact chemically with each other, described in terms of their changing colors, until eventually we are left with what we need. The main problem is that the starting ingredient, the “prima materia” is not a real thing. This primordial substance was thought to be the basis of all things. Without it, we cannot make the philosopher’s stone, and without that we cannot start to figure out what the Emerald Tablet is telling us. So much pseudoscience, then? Probably. But there may be hidden wisdom in