Marble chromatic alteration study using non-invasive analytical techniques and evaluation of the most suitable cleaning treatment. The case of a bust representing Queen Margherita di Savoia at the U.S. Embassy in Rome

In spite of the application of different cleaning procedures, the marble used for the portrait bust of Queen Margherita di Savoia continued to show permanent discoloration, consisting of an unevenly distributed grayish alteration, mainly on the front part. In this work, a multi-analytical, noninvasive approach was proposed using spectrocolorimetry, reflectance spectroscopy andmultispectral imaging. The initial assumption, suggesting the presence of altered protective materials based on organic products (such as waxes or oils,) applied in the past according to traditional practices, was excluded, revealing instead the presence of deposits of particulate matter, which penetrated inside the crystalline structure of the marble, leading to a variation in its shade. Cleaning tests were also carried out to define the best product, using sustainable chemicals such as Polar Varnish Rescue®, alkoxyde surfactant, disodium EDTA, GLDA and Politect® Base in order to identify the best methodology and materials for sustainable cleaning, respecting the integrity of the original matter. Politect® Base demonstrated better action in comparison to the other products tested, and similar results were obtained with GLDA, which could be applied in areas where the Politect® is less efficient (e.g., lace).