This was my submission to Nature Chemistry’s “In Your Element” essay competition, which involved writing an informal essay about one of six elements. I chose copper and wrote about the prehistoric copper ax that was found with the Alpine Iceman. My essay wasn’t selected for publication in Nature Chemistry, but hey maybe you’ll enjoy it. I’ve also added a short description of how the reproduction ax pictured above was made.
There are billions of miles of copper wire strung across the globe, buried beneath cities, spanning even the most remote landscapes, winding through our homes, our computers, and our cell phones. Although some of this global tangle of wires has lately been replaced by fiber-optic cable and wireless signals, we still depend on copper and its ‘wandering’ S electron to light our cities, power our appliances, and transmit the electrical signals we use to communicate. While the importance of copper today is sometimes overshadowed by the ubiquity and strength of modern steel, for thousands of years copper was the only practical metal known to humans.
Our past relationship with copper was dramatically brought to light, quite literally, on a bright autumn day twenty years ago when two tourists hiking in the Tyrolean Alps stumbled upon a desiccated corpse protruding from a receding glacier. The hikers and the Austrian authorities they called to the scene assumed that they had found the remains of a recent avalanche victim, but after the body was crudely excavated from the glacial ice and inspected at a nearby morgue, it became clear that it belonged to a man who had died over 5,000 years ago during the Neolithic period. The man was nicknamed Ötzi after the Ötztal region where he was found.
Ötzi the ‘Iceman’ became an immediate media sensation, and with good reason. His remains were naturally mummified and provided researchers with an unprecedented opportunity to directly study the life and death of a prehistoric European man. Ötzi was found wearing a full set of animal skin clothing and was surrounded by a variety of supplies and accoutrements, including a copper-bladed ax. Ötzi’s ax is one of the oldest and most intact copper tools ever discovered, and was undoubtedly one of his most treasured possessions.
Humans first discovered how to isolate elemental copper from mineral ores around 7,000 years ago during the Late Neolithic era, a transitional period when nomadic groups began planting crops and settling into permanent towns and villages. The circumstances of this discovery are unknown, but one possibility is that brightly-colored copper ores were used to make pottery glazes. When pottery that was glazed in this way was fired at high temperatures in a charcoal-burning kiln, carbon in the form of charcoal would have acted as a reducing agent, and small amounts of elemental copper could have been produced unintentionally. However it happened, the invention of copper smelting represented a technological revolution that allowed humans to produce a wide array of durable tools from malleable metal.
Archaeologists have found the earliest evidence of copper production in Southeastern Europe and the Near East, but knowledge of copper smelting had spread to the Tyrolean Alps by the time Ötzi was born. Analysis of Ötzi’s ax blade revealed that it was made of greater than 99% pure copper, but it was not immediately obvious how it had been made. Was the blade hammered into shape from cold or heated copper, or was it cast from molten copper? To answer this question, researchers at the University of Milan examined the crystallographic texture of the ax using neutron powder diffraction, a non-invasive technique. The random orientation of the copper crystallites in the ax indicated that it had been cast in a mold without further working.
In an effort to wrest every last clue from Ötzi’s remains everything has been examined and meticulously analyzed, including his hair. Ion beam analysis revealed elevated levels of arsenic, a common impurity found in copper ore, in Ötzi’s hair, as well as copper particles on the surface of his hair. Ötzi could have inhaled the small amount of arsenic found in his hair from vapor released during copper smelting, and routine polishing of copper objects could account for the copper particles.
These findings suggest that Ötzi himself may have been involved in copper smelting. As an early smelter, he would have been a pioneer in the field of metallurgy. In the centuries and millennia after Ötzi’s death, metallurgy became increasingly important as humans discovered new metals, and metal-workers learned to produce stronger alloys. In its early days, metallurgy relied on the methods of mysticism and magic, but as the practical need for strong, reproducible metals became greater, metallurgy slowly became more scientific. The development of a scientific understanding of how to purify and manipulate metals was crucial to the birth of the modern discipline of chemistry. In this regard, prehistoric copper smelters like Ötzi were some of the first chemists.
A Fine Piece of Ax (or The Making and Remaking of a Prehistoric Ax)
Ötzi’s copper ax was made using ancient, rudimentary metal-working technology. So when my brother, Greg set out to reproduce the ax as part of his graduate studies in art conservation, he used techniques similar to what would have been used in Ötzi’s time. For a prehistoric smelter, the main obstacle to isolating copper was making a fire that was hot enough to extract the metal from a mineral ore. Even at its hottest, a run-of-the-mill campfire only reaches about 900 degrees Celsius, a good 200 degrees below the melting point of copper. In order to reach higher temperatures, bellows must be used to fan the flames. Archaeological evidence suggests that the earliest bellows were simple blowpipes and bag bellows. Greg used bag bellows to heat charcoal embers in a simple, baked-earth hearth. The set-up looked like this:
When the black bag is compressed, the attached blowpipe directs a blast of air into the hot coals, gradually increasing the temperature of the fire. While it’s impossible to know for sure, a similar set-up may have been used to produce Ötzi’s ax blade (minus, of course, the modern convenience of duct tape). To cast the ax blade, Greg made a sand mold by pressing a carved wood blank in the shape of the ax blade into damp sand, and poured the molten bronze into the mold. After the bronze had solidified, he removed the ax blade from the mold and quenched it in cool water:
We know that Ötzi’s ax blade was also cast from molten copper, and while we don’t know what kind of mold was used, it seems likely that his ax blade was also cast in a simple sand mold. In Ötzi’s day it took a great deal of time and effort to collect and smelt copper ore, re-melt the purified copper, make a mold, and cast the molten copper into the desired shape. So Ötzi’s ax was a rare and valuable tool that he probably used for a variety of purposes from cutting wood, to hunting, and maybe even during raids on neighboring groups. The rarity and value of the ax have led some researchers to conclude that Ötzi was a high-status individual such as a warrior or tribal leader.
Special thanks to Gregory Bailey for providing the images and details of how he made the reproduction ax for this post.
References and Further Reading
Ötzi is back in the news. His frozen remains were recently thawed and researchers performed an autopsy, the results of which may shed new light on the circumstances of his death. National Geographic has a feature this month about the recent thawing and autopsy. See a gallery of photographs from the autopsy here. Ötzi’s complete genome was also recently sequenced, and although the results have not been officially released, they are sure to help us understand more about who Ötzi was and how he lived. Preliminary results have already proven that Ötzi had brown hair and eyes, and he may have been lactose intolerant.
ABRAHAM, M. (2004). Ion beam analysis in art and archaeology: attacking the power precisions paradigm Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 219-220, 1-6 DOI: 10.1016/j.nimb.2004.01.018
Artioli, G., Dugnani, M. Per. Mineral. 73, 5-16 (2004)