A Reasoned Diagnostic Procedure to Support the Restoration of the 17th Century Stucco Altar Dedicated to St. Michael the Archangel in Barbarano Romano (Viterbo, Italy)

The 17th-century stucco altar dedicated to St. Michael the Archangel is an interesting, but very damaged, artwork located in the complex of St. Angel in the little town of Barbarano Romano in Central Italy. During the recent and quite necessary restoration carried out by University of Tuscia students on the Conservation and Restoration of Cultural Heritage Master’s program, some problems with the surface coating were encountered in the cleaning phase. Diagnostic and scientific analyses were crucial to better understanding the composition of these materials to perform the safest and most efficient cleaning procedures. The first of many steps required by this approach was an in situ analysis, starting from on-site analysis and diagnostic documentation through X-ray fluorescence spectroscopy and ultraviolet fluorescence photography, followed by laboratory investigations. The latter included μ-Raman and Fourier transform infrared spectroscopies, gas chromatography coupled with mass spectrometry, and scanning electron microscopy equipped with an energy-dispersive detector. Each technique provided useful data to determine the chemical composition of the white surface coating, which was found to be a non-original overpaint containing lead and organic binder. This overpaint had been applied to retouch the white stucco during a previous restoration project. All this new information contributed to achieving the final decision to remove this layer.