The Mehrgarh Artefacts. Credits: Images D. Bagault, B. Mille © C2RMF

Ancient amulet's secrets illuminated by science

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It sounds like something out of an Indiana Jones movie — a 6,000 year-old amulet has been discovered in a Neolithic site called Mehrgarh in Pakistan. But rather than revealing the secrets of summoning the dead, or even aliens, this trinket has revealed how it was made. French scientists used a technique called photoluminescence imaging to look at tiny microstructures within the amulet. They found it contains two types of copper oxide which have corroded differently over the millennia, and was cast as a single piece by pouring very pure molten copper into a clay mould using a technique called lost-wax casting.

Journal/conference: Nature Communications

Organisation/s: Universite Paris-Saclay, France

Media Release

From: Springer Nature

Hidden structures reveal secrets of an ancient amulet

Clues about how a 6,000 year-old amulet discovered in Mehrgarh, Pakistan, was manufactured are uncovered by an imaging method reported in Nature Communications this week. The method, known as photoluminescence imaging, allows visualisation of microstructures that are invisible under other conventional microscopes.

Analysing samples composed of multiple material components over large areas can be challenging as different instrumental designs are required to study details at varying scales and levels of resolution. Mathieu Thoury and colleagues tackle this problem by developing a full-field photoluminescence approach, which can analyse samples by shining light on them and then deciphering the spectrum re-emitted by the sample. They successfully apply the method to visualise the coexistence of two types of copper oxides with different corrosion histories in an ancient amulet from Mehrgarh in Pakistan, which is thought to be the earliest example of lost wax casting — a method for making duplicate metal objects.

The imaging technique confirms that this amulet was cast as a single piece. Moreover, the finer details uncovered by photoluminescence imaging indicate  that the amulet was made by pouring very pure copper melt into a pre-prepared clay mould using lost-wax casting. The copper absorbed a small amount of oxygen during the processing that would explain the observed mixed microstructures.

This study demonstrates the value of this imaging technique for studying archaeological artifacts, but it may also be a useful analytical tool in the fields of geophysics, engineering and environmental sciences.

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    The Mystery of the amulet, or how the study of a 6000-year-old corroded object has led to the discovery of the secret of the invention of lost-wax casting.

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