The Louvre Crucifix by Giotto – Unveiling the original decoration by 2D-XRF, X-ray radiography, Emissiography and SEM-EDX analysis
© Eveno et al.; licensee BioMed Central Ltd. 2014
Received: 14 April 2014
Accepted: 5 August 2014
Published: 23 August 2014
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.
KeywordsGiotto Crucifix X-ray fluorescence 2D-XRF SEM-EDX X-ray radiography Emissiography Gilding Zwischgold Pigment
Prior to the conservation treatment, the painting was extensively documented by means of non-invasive methods, including photography (visible, UV and infrared imaging), infrared reflectography imaging, microscope examination, X-radiography and X-ray fluorescence analysis (XRF), in order to assess its conservation state. Examinations showed that the wood support is incomplete but original, while the frame surrounding the cross is modern. They also permitted to highlight presence of several pieces of canvas embedded in the ground on top of the wood panels. X-ray radiography and infrared reflectography imaging revealed underdrawings and incisions. XRF allowed to identify pigments like lead white, vermilion, earth pigments, azurite and lead-tin yellow and the presence of gold and silver in the decorations . To complement these investigations, several micro samples collected during the conservation campaign and prepared in cross-sections were studied by scanning electron microscope with X-ray energy dispersive analysis (SEM -EDX). Organic components were not investigated. Early in the study, extensive repaints were evidenced, particularly in the decorations on both sides of the Christ (Figure 1). The analysis of some samples showed the presence of the original decoration beneath the 19th C. repaints. The aim of the present work is to determine the extent of the repaints and to unveil the original decoration patterns and materials of this masterpiece by combining different analytical techniques, especially the 2D-XRF imaging recently developed at the C2RMF. The investigation methods were applied to the entirety of the cross while the 2D-XRF, very time-consuming, was restricted to the areas presented here.
Results and discussions
Four cross-sections have been taken in the various areas of the decoration: in the green glaze (1 in Figure 2b), in a reddish-brown area (2 in Figure 2c), in a black area (3 in Figure 2c), and in the uncovered gilded area (4 in Figure 2d). In all samples the original ground, red bole and the Zwischgold leafs are covered by the 19th C. gilding, except for the sample taken in the black line, where the Zwischgold is not covered. This observation is important for the understanding of the applied repainting process, and discussed below.
Under the black layer composed mainly of earth pigments and bone black, the SEM-EDX analysis showed the presence of a green layer containing copper, likely verdigris.
These X-ray images revealed several features of the patterns. The visible one closely follows the hidden original one and no additional elements have been observed. As for the hidden motives, flowers and narrow lines appear more finely designed and the decoration under the visible black lines was painted with three distinct materials.
Bi-dimensional X-ray fluorescence
The silver map shows the presence of silver from the original Zwischgold. The areas with higher intensity correspond to an overlapping of the square foils of 8.5 cm wide.
As can be seen in the gold map, the gilding covers a large part of the pattern. It is however impossible to distinguish between the original and the 19th C. gildings. Few areas produce a low gold response, appearing in blue. They correspond to three situations: the Au X-rays of the original underlying foil are absorbed by an overlapping original paint layer (likely copper-based pigments), the modern foil gold signal is attenuated by the overlying lead white motif (white dots in corners), the gold foil was not applied (e.g. on the copper motif).
Calcium seems correlated to the visible black lines, probably coming from bone black.
Iron is correlated to reddish-brown lines of the repainted decoration, corresponding to earth pigments.
Lead, coming from lead white is correlated with the white dots of the repainted decoration and also with the original flowers in the centre of the pattern.
The copper map is particularly informative, as it shows two intensity levels that were ascribed to two distinct copper-based pigments (Cu levels < 2000 counts and > 2000 counts, respectively). The first one, corresponding to the lower level, is a pigment of green colour that was previously evidenced under the black line in a cross section taken from the corner which likely corresponds to verdigris. The full identification of the pigment would have required complementary analysis, such as FTIR. The second one, corresponding to higher copper contents, was found near the foliage, and identified as azurite, since a blue pigment was observed by optical microscopy in cracks in this area. Azurite was also identified by SEM-EDX in a cross-section taken from another area (under the arm of the Christ). These lines now repainted in black were originally green and blue, respectively. The two pigments correspond to the ones with intermediate atomic number discussed above (pigments labelled 1 and 2 in Figure 7).
Mercury map corresponds to a vermillion decoration. The contour of the flower was thus originally lined with red and not black as repainted today. The dots appearing in corners are an experimental artifact corresponding to a ghost image of the lead map induced by an excessive count rate (Figure 7).
The presence of red lake was evidenced through microscope examinations, in particular in cracks. Red lake locations were estimated on the basis of these observations and of cross-sections results. Through the combination of the red glaze interlacing pattern and the previous results on inorganic materials (Figure 8b), a reconstruction of the original decoration might be suggested (Figure 8c). One important conclusion is that the original interlacings strongly differ from the present one. The regions represented in red in Figure 8c appear in gold on the visible image and vice versa. Moreover, this interpretation avoids the clumsy interruption of the visible reddish-brown interlacings. Of course, details too small to be imaged and additional organic materials undetectable with the techniques employed could have been omitted in our proposition.
The present study relies on an integrated experimental approach combining several imaging techniques at microscopic and macroscopic scales, ranging from optical and electron microscopy to X-ray and elemental imagings. The principle, implementation and performances of the various techniques employed are reviewed by M. Alfeld .
Scanning electron microscopy
The SEM analyses have been carried out using the scanning electron microscope Philips XL30 (20 kV), equipped with the energy-dispersive X-ray detector from Oxford Instruments. Data were acquired and processed using the INCA software. The cross-sections were embedded in resin, cut, polished and coated with carbon. The SEM-FEG images have been obtained using the scanning electron microscope Jeol JSM-7800 F operated at 5 kV. To improve image quality, cross-sections were argon ion polished  and coated with platinum/iridium.
X-ray radiography and Emissiography
2D-XRF chemical imaging
The system records a full spectrum at each point every two seconds. The total acquisition time of an image is about 21 hours. The XRF spectrum of each pixel is processed with the PYMCA program developed by the synchrotron community . It allows to generate fitted peak area maps for different elements. The processing of an image takes about a quarter of an hour and produces semi-quantitative elemental maps for various elements (K, Ca, Mn, Fe, Cu, Au, Hg, Pb). The elemental distribution images are visualized using a home-made program called DataImaging 3D .
Combination of various scientific imaging techniques provided a precise picture of the hidden decorative side fields of the Crucifix from the Louvre museum. The results achieved by these analytical techniques, all performed at the C2RMF and fully complementary, bring to light that the original composition was repainted in the first half of the 19th C.
The first and invisible composition is composed of a Zwischgold gilding applied on a red bole, on which a geometric decoration was applied using bright pigments such as azurite, vermillion, verdigris, lead white and red glaze.
The 19th C. repainting was executed with a restricted palette, containing earth pigments, organic green, bone black applied on a gold foil attached to a gilding mordant containing chromium.
The visible patterns generally follow the original ones which appear to be more refined. The very close similarity of the shapes and the use of a gilding mordant for the repaint allow us to understand the employed technique. A gilding mordant layer was applied following and skirting the original pattern on which pieces of gold leaves were deposited, enabling to reproduce the previous design. The original green, blue and red patterns were subsequently repainted in black. Interlacings first painted in red glaze were awkwardly reproduced in red-brown and the blue foliage repainted in pale green and white dots added to the composition.
The knowledge of original and repainted colours provided by this study gives us a better insight of the extensive repaint. The fact that the brilliant original colours were covered by black lines could be due to alteration. The browning of copper pigments is a well known process, so as the blackening of vermillion which was also observed in other parts of the cross and the degradation of the silver contained in the Zwischgold. Thus, the entire decorative side fields might have turned dark with time and motivated its repainting. The colours selected for this intervention might be a misunderstanding and merely reflect the degraded state of the original materials.
Considering the bad conservation state of the original composition, in particular of the silver degradation of the Zwischgold, the repaint was not removed during the restoration treatment.
The materials identified in the original composition, in particular the azurite pigment instead of lapis lazuli and the gilding in Zwischgold instead of pure gold testify the modest status of this artwork in comparison with other Giotto Crosses, such as the crucifix of the Santa Maria Novella . For this reason, the Louvre cross is attributed to Giotto’s workshop under the master supervision .
We thank Rosaria Motta and Claudia Sindaco, in charge of the conservation treatment undertaken in the C2RMF studio from 2010 to 2013, for their precious observations and valuable discussions.
We are grateful to Thiphaine Fabris and Daniel Abriou for the ion polishing preparation of the painting cross-sections at Saint-Gobain research EMC2 laboratory and Patrice Lehuédé for the SEM-FEG examinations.
We warmly thank Dominique Thiébaut curator of the Louvre museum for having initiated and encouraged the study of Giotto’s cross. We are indebted to Vincent Pomarède director of the Louvre painting department for his constant support.
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