The Louvre Crucifix by Giotto – Unveiling the original decoration by 2D-XRF, X-ray radiography, Emissiography and SEM-EDX analysis

Abstract

Giotto (1266–1337) and his workshop realized c. 1315 the large Crucifix now in the Louvre Museum. The conservation of this masterpiece in 2010–2013 in the C2RMF studios gave the opportunity for a comprehensive investigation of the execution technique through a characterization of the paint layers. The first examinations revealed an original gilding and decoration surrounding the Christ which was repainted during the 19th C., raising the question of the original decoration appearance. The original decoration still present underneath was unveiled. The different imaging analytical techniques applied brought complementary results allowing us to reconstruct the original decoration. Elemental images of selected areas on the Crucifix were obtained using bi-dimensional X-ray fluorescence imaging technique (2D-XRF) with a prototype XRF spectrometer mounted on a motorized X-Y system. The head of the instrument was moved over a 150 x150 mm2 area with 1 mm steps, using a 1 mm beam spot and acquisition time of 2 seconds per pixel. The XRF spectra of each pixel provided semi-quantitative elemental maps of the layers: gilding and pigments even at low concentrations could be recognised. The XRF maps achieved were completed by X-ray radiographs and emissiographs of the Crucifix. Some micro-samples analyzed with the scanning electron microscope (SEM-EDX) allowed a better understanding of the stratigraphy. The comparison of the direct light and the X-ray pictures enabled us to distinguish the repaint from the original pattern. The visible metallic leaf is a thin gold leaf applied on a mordant gilding while the original one is a double leaf of gold and silver (Zwischgold) applied on a red bole. As for the painting, the visible decoration is composed of brown earth and organic greenish color whereas the original is made of red lake, copper green, azurite and vermillion. The 2D-XRF maps evidenced two levels of copper that could be ascribed to two different copper pigments, a blue and a green one. A complementary methodology combining classical X-ray imaging, sample analysis and 2D-XRF was applied to large areas of the Giotto’s crucifix. This approach allowed us to distinguish the underlying original composition from the 19th C. overpainted decoration.