Gold, forgery and redox reactions

Since antiquity, alchemists tried (and failed) to create gold out of base metals. Instead, their work laid the foundations for modern chemistry.

 Illustration by  Elizabeth Newton

Illustration by Elizabeth Newton

No other metal has made as significant an imprint on our collective consciousness as gold. The oldest known metal, it has been mined, hoarded, ruminated on and counterfeited for millennia. Obssessed with finding another method of attaining the noble metal, generations of alchemists devoted their lives to the impossible pursuit of chrysopoeia, the transformation of base metals into gold.

So-called “alchemist’s gold” was formed from a myriad of recipes and varied widely in its resemblance to true gold. But far from being a fruitless endeavour, the alchemists’ attempts to pass off less precious metals as gold paved the way for today’s assaying and laboratory practices, enhanced our understanding of chemistry and had an influence on our relationship with products that look and feel like the real thing.

The art of alchemy likely had three independent birthplaces: China, India and the ancient Egyptian hub of Alexandria. In early Alexandrian alchemy, sacred Egyptian science merged with the Greek belief that matter was composed of the four elements of nature: earth, water, fire and air. This Hellenistic phase of alchemy lasted until the dramatic expansion of Islam in the seventh century AD.

The etymology of the word alchemy is unclear, but one possible origin is Al-Khemia, meaning “the Black Land” in Arabic. What is clear is that for the next 500 years after the Arabian lords’ military conquests, the chief practitioners of alchemy were Islamic. European alchemy, which did not reach its peak until the 16th and 17th centuries, owed its existence to the translation of Arabic alchemical texts into Latin.

One of the most important texts was De Mineralbus, by the Persian philosopher and alchemist Avicenna (AD980–1037). Avicenna described all metals as being composed of varying amounts of mercury and sulphur, which in turn had “earthy” and “watery” components from the four natural elements. Mercury and sulphur would come to be described by medieval alchemists as the “principles” of metals.

There is no honest man! not one, that can resist the attraction of gold!
— Aristophanes, Plutus (388 BC), 230

William R. Newman, Professor of History and Philosophy of Science at Indiana University, described how alchemists believed that the principles varied in appearance and properties based on their “fixed” and “unfixed” properties. Unfixed properties, such as the volatility of mercury and combustibility of sulphur, were treated as characteristics which could be eliminated, while fixed properties were intrinsic to the principles.

If metals could be purified into their respective components, it stands to reason that they could also be transformed into different metals — even into the most precious of all metals: gold. Medieval alchemists believed that alchemia transmutatoria, the transformation of metals, though difficult, was possible.

But to successfully create gold, alchemists first needed to identify it. The earliest alchemists considered colour to be the most decisive criterion for identifying gold, and this belief persisted for centuries.

Bold as brass

Metallurgy predates alchemy by several millennia, so it’s only natural that the earliest attempts to produce gold used ancient metallurgical techniques. The copper-zinc alloy brass, which was developed as far back as the fifth millennia BC in China, was commonly used to imitate gold, while bronze (a redder copper-tin alloy) was widely used for humdrum utensils. Because brass was such a popular faux gold, the terminologies for golden-coloured metals became hopelessly muddled in multilingual Europe. 

The transmutation of copper into gold is described in an ancient Indian treatise by Nagarjuna, alleged to be one of the founders of Indian tantric alchemy. Nagarjuna was working with calamine (a mineral ore of zinc, usually zinc carbonate), which he roasted with copper.

 
  An alchemist at a furnace with a large ‘receiver’ (a vessel for use in distillation), with diagrams of alchemical apparatus. Woodcut. Johannes Rudolph Glauber, 1658.   Wellcome Images/ Wikimedia Commons .

An alchemist at a furnace with a large ‘receiver’ (a vessel for use in distillation), with diagrams of alchemical apparatus. Woodcut. Johannes Rudolph Glauber, 1658. Wellcome Images/ Wikimedia Commons.

 

A chemical transformation does indeed occur at temperatures over 1300°C. In fact, many alchemical transformations can be interpreted in terms of redox (oxidation and reduction) reactions. Chemical species such as atoms, molecules or ions (charged atoms or molecules) are reduced by grabbing electrons from other species, or oxidized by losing their electrons.

In the case of Nagarjuna’s metalworking, heating reduces the zinc ions in calamine to metallic zinc. The zinc in turn diffuses into the copper, but rather than gold, the product is an alloy known as calamine brass. Nevertheless, if the right proportions of calamine and copper are used, the resulting brass can convincingly resemble gold.

In Europe, metallurgists and craftsmen employed similar techniques to produce golden metals. However, whereas metallurgists recorded clear and simple processes, alchemical recipes tended to be vague and contain many potentially superfluous steps, according to Chemistry Professor Vladimir Karpenko of Charles University, Prague. A mysterious recipe for “Spanish gold” in the 12th-century treatise De Diversis Artibus was one such example, calling for “red copper, basilisk powder, human blood, and vinegar”. These were almost certainly code-words – basilisks tending to be hard to find outside of mythology and Harry Potter.

A more intelligible recipe for golden metal came from the German physician and chemist Andreas Libavius (1550–1616). Libavius was a vocal critic of alchemists, mainly due to their use of what he believed were deliberate obfuscations. Nevertheless, he believed in the transmutation of metals and published many of his experiments to produce golden-coloured metals.  Libavius recommended that the best way of producing imitation gold from brass required firing four parts of copper and one part of calamine in a large, long crucible in alternating layers, with a liberal sprinkling of glass gall (the saline whitish scum sometimes produced in the fusion of molten glass).

 
  As well as several copper allloys, pyrite, otherwise known as “Fool’s Gold” was used in several alchemical recipes trying to transmute base metals into gold.   PollyDot/Pixabay  CC0

As well as several copper allloys, pyrite, otherwise known as “Fool’s Gold” was used in several alchemical recipes trying to transmute base metals into gold. PollyDot/Pixabay CC0

 

Brass was not the only alloy that could produce a passable imitation of gold. The copper amalgam Aurum sophisticatum (also known by the German term Truggold, or false gold) was prepared by cooking sulphuric acid with mercury and molten copper. Other recipes employed pyrite, otherwise known as “Fool’s Gold”, as the main ingredient.

Not all techniques would have achieved realistic outcomes. The 16th-century Swiss physician and alchemist Paracelsus also dabbled in chrysopoeia with a variety of metallic compounds. Like the equally hot-headed Libavius, Paracelsus was highly critical of his contemporaries, who he believed did not give due consideration to the importance of chemistry. Despite his focus on the scientific process, his alchemical recipes were often unnecessarily complicated:

Take of calcined sulphur…two parts, of…mercury ten parts. Take also some water of sal ammoniac [ammonium chloride] and imbibe frequently…Place a little on heated iron….take one part of this elixir to a hundred parts of Venus [copper]….adding continually a little crocus of Mars [red ferric oxide]….Then melt in iron…Take one part to hundred parts of purified Mars [iron]. It becomes gold better than that of Nature.

Paracelsus

Understanding these recipes requires some chemical conjecture. It’s presumed that first, a mixture of mercuric sulphide and compounds of mercury with ammonia would have formed. These may have then reacted with copper, ammonium chloride and ferric oxide to produce copper and iron salts. The salts would later decompose under the high melting point of iron. The copper could have imparted a red or reddish colour, but considering that it was to be mixed with a hundred-fold amount of iron, it is highly unlikely to have resembled gold.

Beneath the surface

Surface treatment of metals or alloys with inorganic compounds to produce a golden appearance was practised since antiquity. In ancient China, lead was treated with potassium nitrate and sulphur to “transmute” it into gold. The most famous Chinese alchemist Ko Hung (~AD281–361) produced a comprehensive treatise that also included stranger methods, such as heating tin with alum (a hydrated double sulphate of aluminium and potassium), ammonium chloride and horse dung, which was meant to produce “a cluster of gold beans”. Amazingly, a modern chemist reproduced this recipe and found that it did in fact produce golden-coloured crystals of tin sulphide.

Real gold is not afraid of the melting pot.
— Chinese proverb

Andreas Libavius developed a method for imparting a golden appearance to rusted, impure mineral compounds. His recipe included solution of alum in water, Hungarian vitriol (copper sulphate contaminated by iron sulphate), auripigment (arsenic sulphide), rock salt and Kupferrost (greenish-coloured rusted copper, usually copper carbonate). Upon boiling, copper dissolves into its salt form and reacts with arsenic sulphide to produce yellow inorganic compounds. The solution is then streaked onto metallic iron like a slightly more toxic furniture glaze. The metallic iron will lose electrons, while the copper ions in the solution gain electrons. This reduction reaction forms metallic copper, which deposits onto the iron. The combination of yellow inorganic compounds and reddish deposited copper could well have produced an imitation of gold.

Some more dubious methods used organic compounds, such as plant extracts or other yellow substances (Paracelsus recommended egg yolks), to colour the surface of a metal yellow. These would almost certainly have produced, if anything, only the most short-lived effects.

Testing times

Examining the alchemists’ recipes begs the question: did they believe that they were actually producing gold? Marco Beretta, Professor of Psychology at the University of Bologna, suggests in his history of chemistry, Enlightenment of Matter, that if alchemists genuinely did not notice the impossibility of alchemia transmutatoria, they were either obsessively blind, pathological forgers or simply made no experiments, observations or tests.

However, historians tend to agree that alchemists were highly concerned with the purification, exact measurement, and testing of the materials they synthesised. Newman writes in Instrumentation and Experimentation in the History of Chemistry that there was a strong link between alchemical practice and modern assaying, the compositional analysis of metallic materials.

  Engraving of Jabir ibn Hayyan Geber, an Arabian alchemist, from the Pleasures and Lives of Illustrious Men, Andre Thevet, 1584.   Wellcome Images/Wikimedia Commons .

Engraving of Jabir ibn Hayyan Geber, an Arabian alchemist, from the Pleasures and Lives of Illustrious Men, Andre Thevet, 1584. Wellcome Images/Wikimedia Commons.

An extensive series of assaying tests for metals was described in the seminal work Summa Perfectionis Magisterii, attributed to Pseudo-Geber, a Latin alchemist whose identity is disputed. These included cupellation and cementation (controlled operations performed in high temperature to separate noble from base metals), exposure to various chemicals and inspection for changes in colour, weight or volume. Alchemists had come a long way from being satisfied that golden-coloured metals were gold.

Alchemy is now described as an obsolete science, but the chemical and metallurgical processes employed by alchemists are far from obsolete today. Chemists still use the fire assay process, which involves fusion with molten lead, cupellation, separation and weighing, to find out the purity and amount of metal in a substance. While still regarded as the most accurate technique for classifying gold, it has largely been replaced by modern spectroscopy techniques such as X-ray fluorescence which have the advantage of not destroying the item you are testing. 

In my conversation with Professor Newman, he elaborated further that “the assaying tests [of Pseudo-Geber] served two purposes, one obvious, the other less so.” Newman explained that, while the obvious purpose was to authenticate the alchemist’s gold or silver, “the less obvious purpose, but perhaps the most important one, was to provide a means of determining the composition of individual metals based on their relative quantities of fixed and unfixed sulphur and mercury”.

Alexander von Suchten, a mid-16th century Prussian nobleman who developed a recipe for “antimonial gold”, also strived to verify the authenticity of his creations. Very little is known about Suchten, but from his laboratory notebooks, we can see that he was certainly a meticulous chemist. His recipe entailed reducing the natural ore of antimony, stibnite, to produce “regulus of antimony” (metallic antimony). The regulus was then alloyed with gold or silver and the resultant alloy used to “acuate” quicksilver (liquid mercury). Alchemists believed the acuated mercury had the power to penetrate and separate the antimony into its principles (mercury and sulphur), isolating the volatile or unfixed gold within the regulus.

 
 

Woodcut prints from Alexander von Suchten's 1680 treatise Theeosophy, Alchemy and Medicine. “The position of man in medical theosophy” (left) and “Philosophical representation of the comparison of properties” (right). Alexander von Suchten/Wikimedia Commons (public domain)

Suchten’s antimonial gold received the approval of a professional goldsmith, but the alchemist remained unsatisfied, despite subjecting his handiwork to numerous assaying techniques. Finally, he amalgamated it with his acuated mercury, heated the mixture for a month, and then distilled the product. After driving off the mercury, Suchten found that he was left with exactly the amount of gold that had been originally added to the regulus of antimony. This null result showed the original gold had not “acuated” the antimony. Despite passing all other tests, Suchten’s gold failed by quantitative analysis, leading a friend to conclude that alchemia transmutatoria was “a lunatic, melancholy fantasy”.

Nevertheless, some alchemists were convinced they had been successful. Early in his career, the 17th-century American alchemist George Starkey believed he had managed to produce alchemical silver using Suchten’s recipes, but Newman notes that examination of Starkey’s notebooks reveals that the alchemist later had second thoughts about the process.

Subverting nature

Alchemy not only had a transformative effect on science, but had influential effects on law and economics. Alchemy had its share of highly vocal opponents, from philosophers who practiced science to religious figures and policy makers. Economists today may worry that cryptocurrencies could affect traditional economies, but back in the day, many feared that alchemical gold would cause an economic bust-up. The Arab historian ibn-Khaldun even claimed that chrysopoeia could foil God’s plan for maintaining economic stability.

Christianity shared equally strong anti-alchemical sentiments. The 13th century Archbishop of Bourges, Giles of Rome, was highly concerned that a hidden property in alchemist’s gold, which was sometimes used in medicines, “might greatly harm the human complexion”.

These early fears parallel modern consumer concerns about chemical additives and preservatives in foods and cosmetics. As chemical and biological processes become increasingly sophisticated, consumers find comfort or reassurance in products which are “pure” and “natural” – terms that can be more nebulous than anything found in an alchemist’s recipe book.

Widespread accusations of alchemical fraud led to a papal decree issued by Pope John XXII in 1317 condemning the counterfeiting of coins. The kings of France and England also banned transmutational alchemy, but King Henry IV later awarded licences for the practice. Newman explains in his study of the Summa Perfectionis, “Latin alchemy could not be wiped out by proclamatory or official means. The vision of human power in the realm of technology…was too seductive to be repressed for long”.

Our relationship with the chemical manipulation of nature has become even more complex than it was in the medieval laboratories of the alchemists. The historical figures who attempted to create – or just imitate – gold would have found today’s concepts of genetic engineering, cloning and CRISPR fantastical and subversive. Perhaps not that much has changed after all.

Edited by Nathan Mifsud and Diana Crow