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Nature Communications : Lost-wax casting : a 6000 years old technique invented at Mehrgarh (Pakistan)

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High spatial dynamics-photoluminescence imaging reveals the metallurgy of the earliest lost wax-cast object. M. Thoury, B. Mille, T. Séverin-Fabiani, L. Robbiola, M. Réfrégiers, J.-F. Jarrige, L. Bertrand. Nature Communications, 15 novembre 2016.

A pluridisciplinary research team associating IPANEMA, C2RMF, PréTech, TRACES, the SOLEIL synchrotron, ArScAn and the National museum of Asian arts–Guimet has elucidated the production process of the oldest object manufactured by lost-wax casting, through the development of a method used for the first time in Archaeology. This work has just been published in the journal Nature Communications.

Lost-wax casting is a precision foundry technique, which uses for a start a model shaped in a low melting point material, such as beeswax. This model is covered with clay, then heated to remove the wax, and baked. The mould containing the object’s shape is filled up with molten metal. In a last step, the mould is broken to release the metal object. The oldest objects that were manufactured with this technique originate from the Mehrgarh site (Indus Region, now in Pakistan). This site, discovered and excavated since 1974 by Mission Archéologique de l’Indus headed by Jean-François Jarrige (1940–2014) in collaboration with the Department of Archaeology and Museums of Pakistan, is known to be an intense place of innovation since the Neolithic Period : early animal domestication, textiles, dental care, glazed beads and ceramics, as well as metallurgy.

In 1985, a small wheel-shaped amulet was discovered at MR2, a sector of the Mehrgarh site that has only been inhabited in the Chalcolithic times between 4500 and 3600 BC. A first study, carried out in the 2000s, concluded on the use of the lost wax technique. However, complete corrosion of the object prevented any understanding of its production process, and the study was therefore stopped. In the frame of a PhD thesis of the new Paris–Saclay University, a method based on UV/visible photoluminescence imaging has been developed at the IPANEMA laboratory and at the DISCO beamline of the SOLEIL synchrotron, and applied to collect new information. In parallel, C2RMF and PréTech had just launched a programme on the origin and development of lost-wax casting from the Indus to the Mediterranean. The amulet, as an emblematic artefact, was selected to test the new approach.

The team then observed an unexpected phenomenon : the complete fossilisation of the original state of the metal material from the scale of a centimetre to a micrometre. The discovery of a eutectic morphology, not visible through the other approaches tested, allowed understanding the whole production process of the object. After its high-temperature solidification, the material was made of a eutectic consisting in cuprous oxide rods (cuprite) in a copper matrix. Over the years in the soil, this metal matrix also oxidised to cuprite. The contrast observed through photoluminescence results from a difference in the crystal defects between the two cuprites : a variation between the content of oxygen atoms in the cuprite of the eutectic and in that resulting from corrosion. This observation is the key to reconstruct the operating sequence used to produce the object at an unprecedented level for such a highly corroded object – from the choice of the ore used (a particularly pure copper), to the oxygen content absorbed by the molten metal, and even the melting and the solidification temperatures (close to 1066°C).

The production of the object discovered at Mehrgarh demonstrates the outstanding technical control of the craftsmen of that period. The Chalcolithic metallurgists experimented a revolutionary production process – lost-wax casting, which led to the development of a new artistic expression, metal sculpture, from the 4th millennium on. Until today, this technique of high-precision metal foundry is still the most adapted method for fine manufacturing of art objects, but also for the aeronautic and health sectors, etc. The imaging method that was developed to study the object is of great interest for the investigation of a very large range of systems : biological tissues, semiconductor materials and… archaeological objects. The publication is also an opportunity to pay tribute to the memory of Jean-François Jarrige, archaeologist, who discovered the Mehrgarh site and died in 2014.

What is Photoluminescence Spectral Imaging ?

Following excitation of a material by a light beam (here in the visible or the UV range), photoluminescence images indicate the distribution of photons resulting from the deexcitation of electrons in the material. In this work, emission indicates the presence of crystalline defects in the cuprite. The experiments presented in the publication result from the use of the DISCO beamline at the SOLEIL synchrotron with specific developments carried out during the Ph.D. work of T. Séverin-Fabiani (IPANEMA / Synchrotron SOLEIL).

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Voir en ligne : Article on Nature Communications website