|Posted by Mark Cantrell on February 1, 2017 at 3:50 PM|
New technique uncovers 6,000-year-old mysteries of humanity’s transition to metal
Talk about casting light on history, writes Mark Cantrell: when scientists wanted to learn more about how a 6,000 year old copper amulet was made, they turned to a photoluminescent imaging technique used to check semiconductors for defects
AN ancient amulet cast in copper was persuaded to give up its secrets when it was subjected to a new photoluminescent imaging technique based on a method used to monitor semiconductors for defects.
The 6,000-year-old amulet was discovered about 30 years ago in the Mehrgarh area of Baluchistan, in Pakistan, and is the oldest known lost-wax cast object. It dates from a period known as the Copper Age, an era that marked the transition from the Neolithic – stone age – to the Bronze Age.
Lost-wax casting is a method used to duplicate metal sculptures cast from an original model. The technique, also known as ‘investment casting’ is still used to this day – and for some high-tech end uses at that, such as aerospace, aeronautics and biomedicine – using high-performance alloys of steel and titanium.
Researchers from the IPANEMA laboratory and synchrotron SOLEIL at the University Paris-Saclay studied the amulet using the new approach. The project and its findings were presented in a paper published in the journal Nature Communications late last year.
Image: D Bagault © C2RMF
Corrosion of the metal had previously hindered a fuller understanding of the craftwork that went into its creation. But the research on the amulet managed to shed a little light on the invention of this form of high-precision foundry, enabling a greater understanding of the metallurgical sequences used by the pre-Bronze-Age metalworkers.
“Scientists had reached the limits of what they could learn from the amulet with traditional imaging techniques since its discovery three decades ago,” said Mathieu Thoury, one of the authors of the paper. “We designed a full-field photoluminescence approach to look at the object’s structure and composition in greater detail.”
As the paper explains, photoluminescent spectroscopy is a key method for monitoring defects in semiconductors from nanophotonics to solar cells. The thing is, semiconducting materials such as the silicon wafers used in microchip manufacture, are highly pure materials.
However, when it comes to the more complex crystalline structures of ‘messier’ materials, such as those encountered in environmental, medical, ancient materials sciences, and engineering, its “great sensitivity to small variations in local environment” becomes a handicap. The new technique described in the paper is said to overcome that problem.
In collaboration with scientists from C2RMF, Pre-Tech, ArScAn and TRACES laboratories, the IPANEMA team was able to reveal a hidden microstructure by shining light in the UV/visible range, generated by a contrast of luminescence induced by variations of defects within the crystalline structure of the corroded amulet.
This “ghost microstructure”, which was invisible to other advanced imaging techniques, revealed the full metallurgical sequence used in the creation of the amulet and its subsequent degradation during burial.
The technique confirmed that the ‘spoked wheel’ shape of six small rods on a ring of 20-mm diameter had been cast in a single piece. There were no soldered parts. Furthermore, the researchers were surprised to discover that the ancient metalworkers of Mehrgarh had used very pure copper.
Apparently, this has given insight in innovations in metallurgy in these bygone millennia, as the use of high-purity copper was subsequently rejected as the metalworkers of the Pakistan region discovered that adding lead increased the metal’s fluidity.
Located on the SOLEIL synchrotron site at the University Paris-Saclay, IPANEMA is a laboratory dedicated to research and development of advanced methodologies for analysing materials in the fields of archaeology, paleao-environments, palaeontology, and research into cultural heritage. It’s a hi-tech approach to examining the past.
“Although it has never been used in archaeology, the photoluminescence imaging technique developed here holds great promise for this field and other disciplines, including environmental sciences and geophysics,” said Loic Bertrand, another of the paper’s authors.
It’s tempting to suggest: beat that Indiana Jones.
The full paper, “High spatial dynamics-photoluminescence imaging reveals the metallurgy of the earliest lost-wax cast object”, published November 2016, can be read online at Nature Communications.
Image: D Bagault & B Mille © C2RMF