The Pigments’ Characterization of the 15th Century Bas-relief “Madonna col bambino, Santa Caterina da Siena e un priore certosino” by A.G. Amadeo through SEM-EDX and Vibrational Spectroscopic Techniques.

Abstract

The present study deals with spectroscopic analytical techniques and aimed at the characterization of the materials present on the surface of a polychrome terracotta bas-relief dating back to the 15th century entitled “Madonna col bambino, Santa Caterina da Siena e un priore certosino”. The analyzed work comes from the workshop of Giovanni Antonio Amadeo (Pavia, 1447 - Milan, 1522) and is stored at Castello Sforzesco in Milan. Seven samples were taken from different areas of the work for the characterization of the pigments, binders and, possibly, products of degradation by using analytical techniques such as optical microscopy, SEM-EDS, ATR-FTIR and Raman spectroscopies. From the obtained results it was possible to acquire useful information about the materials, especially the chromophores and some organic substances composing the examined samples. By means of the analytical results it was possible to identify the pigments used by the artist on the polychrome terracotta’s surface, namely white, red, blue and ocher, and to detect the presence of gold leaf on the background of the painting. In particular, in the white sample, lead white was recognized, whereas in the blue samples predominantly azurite was found. In the light-ocher sample just lead white could be identified, while, in the dark-ocher one, a reddish pigment based on iron mixed with lead white was recognized. In the bright red sample, the quite valuable cinnabar mixed with lead white was detected. Lastly, in the gilded sample true gold as gold leaf applied directly on white lead was identified. The study also allowed us to state that the color palette used by the artist on the artwork was consistent with the historical information provided by the literature. It is worth mentioning that the Raman spectrometer did not perform very well because of the high fluorescence due to the presence of some organic materials on the samples’ surface. My analytical research helped the restorers in their subsequent restoration work on the terracotta. Indeed, the artwork was subjected to a cleaning operation carried out by Mario Colella and Silvia Marchioron, restorers at the Piccolo Chiostro Study and Conservation Center. This procedure was performed to gradually remove the dark coating that obscured the surface of the bas-relief, which made the colors almost unrecognizable. Therefore, they were able to bring back the original hues of the precious materials used by the artist. As a matter of fact, the cleaning revealed a rich and lively chromatic palette ranging from the bright red of the Madonna’s robe and the Child’s pillow, to the white of the Carthusian’s robe, the veil of Saint Catherine and the Child’s robe. Moreover, it showed the fair skin tones of the characters and the different expressions of their faces, previously completely hidden, making it possible to have a completely different vision and reading of the artwork. According to the museum, however, the real surprise was the discovery of the color with which the Virgin’s veil was made, i.e. green, an unusual chromatic choice compared to the traditional blue of the mantle. Through the analyses and measurements carried out on the work, it was possible to find that the pigments were the original ones in almost all cases, specifically the ones used in the Lombard artistic workshops of the 16th century. However, some synthetic pigments were discovered, and it was therefore possible to ascertain that the artwork underwent previous restoration procedures.