“A woman clothed with the sun”. The diagnostic study and testing of enzyme-based green products for the restoration of an Early 17th Century wall painting in the Palazzo Gallo in Bagnaia (Italy)

A 17th century wall painting, representing the Virgin between two Saints, in a noble Italian renaissance palace, the Palazzo Gallo in Bagnaia (Viterbo, Italy), was restored in 2021 in the context of a wider restoration campaign involving the main room of the palace built by cardinal Sansoni Riario. Diagnostic analyses performed using traditional characterization techniques (optical microscopy on micro-stratigraphic sections, X-ray fluorescence spectroscopy and Fourier transform infrared spectroscopy) provided the identification of both the original painting and its restoration materials, while imaging investigations using the ultraviolet fluorescence photography, false color images and multispectral mapping provided by the hypercolorimetric multispectral imaging (HMI) technique enabled the evaluation of the state of conservation, the location of restoration interventions and supported the monitoring of the cleaning procedure. An altered protective Paraloid®-based coating dating from the early 2000s had to be removed due to the unpleasant glossy finishing it had given to the painted surface, making the scene barely readable. To pursue a restoration protocol based on environmental sustainability and green chemistry, enzyme-based gels marketed by the Nasier-Brenta© and CTS© companies were tested in different protocols for the cleaning of the mixture (known as beverone) covering the painting. Although some interesting results were observed, the enzymatic cleaning had limited effectiveness, and was more timing-consuming than was reasonable. Traditional chemical solvents such as Dowanol PM (methoxy propanol) and benzyl alcohol were necessary to complete the cleaning of the painting surface.