X-ray vision deciphers archaeological mystery

27-10-2015

Synchrotron light has pierced the mysteries of a small 17th Century metallic box thanks to X-ray imaging techniques developed at the ESRF. Scientists were able to virtually reconstitute, in 3D and with astounding resolution, the inaccessible contents of the very fragile and badly damaged box.

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The box was discovered on the archaeological site of the Saint-Laurent church, now the archaeological museum of Grenoble (MAG).  Restoration of the very badly damaged and fragile box had been limited to stopping the oxidation process, without providing any insight into the nature of its contents.

Thanks to the non-destructive techniques and high resolution imaging of the ESRF, in particular synchrotron X-ray phase contrast micro-tomography, the research team, made up of members of the MAG and ESRF, were able to identify the contents, and the finer details of three medals.

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Relics exhibited in the nave of the Saint Laurent church, now the Archaeological Museum of Grenoble (MAG). Credit: Frédéric Pattou

The success of this research and the astounding results obtained open new possibilities in the field of archaeological research.

Located in the heart of one of the oldest districts of Grenoble, the Saint Laurent church has been the object of archaeological research for the last twenty years under the direction of archaeologist and expert in history and civilisations of Antiquity and the Middle Ages, Renée Colardelle. The excavation has revealed buildings (mausoleums, churches), more than 1500 tombs and 2000 objects which provide a testimony to the evolution of mentalities and beliefs since the IVe century.

 

This small metal box, measuring just 4 cm, was buried alongside a corpse in one of the 195 graves from a phase of burials dating from the XVIIe century. The box, which was in a bad state of degradation and very fragile, was restored by the CREAM (Centre de Restoration et d’Etude Archaeologique Municipal) in Vienne, France, to stop the oxidation process eating away the metal. Through the broken lid of the box, archaeologists could suppose the box contained three circular elements resembling coins, however the inscriptions on the objects were totally illegible.  As Renée Colardelle explains, “This minimal restoration, which was also applied to archaeological relics to preserve their authenticity in a state as extracted from the earth, today allows us to perform non-destructive investigations with astounding results.”

The Casemate, Grenoble’s scientific and technical cultural centre, steered the MAG towards Paul Tafforeau, scientist and paleoanthropologist at the ESRF, to try to decipher the mystery of the small box without opening it. The box was scanned using phase contrast synchrotron X-ray micro-tomography on the ESRF’s BM5 beamline. This technique, which can be likened to a highly powerful medical scanner, is capable of producing high resolution 3D images of the inside of a sample in a non-destructive manner. “It was only supposed to be a small feasibility study to produce an image for an exhibition. However, the results were so astounding that it turned into a full scale research project”, says Paul Tafforeau who carried out the experiments and produced the 3D images of the box.

X-ray vision!

Tomographic sections of the box revealed the contents not to be metallic coins but small clay medals, plus two pearls. The two pearls and the block of three medals were virtually extracted from the box by 3D segmentation. The three medals were in very poor condition and stuck together. However, after manual segmentation of the contact zones and the cracks in the medals, it was possible to separate them. The scientists then applied rendering and 3D virtual lighting techniques to produce images that revealed a multitude of details impossible to see with traditional lighting.

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Left: central panel of a tryptch by Pietro di Cristoforo Vannucci, known as Le Pérugin (~1448-1523), collection National Gallery of Art, Washington D.C. ©DR. Right: 3D reconstitution using phase contrast synchrotron micro tomography at the ESRF. Credit: P. Tafforeau/ESRF

The medal in the middle of the stack proved the most legible. On one side, Christ can be seen on the cross with two figures standing at the foot of the cross. On the other side, Christ’s resurrection is symbolised by a figure bearing a crown of thorns, with one leg out of the tomb and holding a standard of the resurrection in his right hand.

The two other medals, which are identical, were damaged in different places. By combining the images obtained from the two identical medals, it was possible to reconstitute the illustrations and even the inscriptions on the medals. On one side, the scene of Christ’s baptism by Saint John the Baptist can be distinguished. The inscription is from John 1.14. : VERBUM CARO, FACTUM EST (“And the word became flesh”). The reverse side of the medal shows the three Magi bringing gold, frankincense and myrrh to the infant Jesus seated on Mary’s lap. The presence of Joseph behind Mary can be assumed but the state of preservation of the medals does not allow confirmation. The inscription is the beginning of a psalm: ADORAMUS TE, CHRISTE ET BENEDICIMUS TIBI (“We adore Thee, O Christ, and we bless Thee”).

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Left: "Adoration of the Magi" 1636, Nicolas Lagouz. Credit: Bruno Rousseau/CG49, inventory of cultural heritage. Right: 3D reconstitution using phase contrast synchrotron micro tomography at the ESRF. Credit: Paul Tafforeau/ESRF.

The identification of these medals reinforces our knowledge of beliefs and practices of local populations in the 17th century. But beyond this scientific discovery, the quality of the images obtained through non-destructive techniques opens new perspectives for archaeologists. Although synchrotron X-ray imaging has been used frequently for paleontological research since its development at the ESRF in 2000, its use for the study of objects like this box is a novelty that opens new paths and collaborations in the field of archaeology. No doubt this innovative approach in archaeology is destined to reveal many more secrets thanks to synchrotron light.

Top image: The metal box after restoration to stop oxidation. Collection MAG. Credit: Denis Vinçon