The synergy of indoor air quality assessment and artwork materials characterisation: The santuario della Beata Vergine in Saronno as a case study

Time and environmental conditions are among the main factors related to the degradation of artwork and historical buildings. It is, well-known that artistic materials exposed to light, humidity, variation in temperature, and pollution undergo chemical physical characteristics changing and, as a consequence, are subject to various kinds of corruption processes. Therefore, the study of degradation mechanisms and possible strategies for their prevention has become of main importance in the field of cultural heritage. In this research, several microclimatic parameters, the presence of particulate matter and/or pollutants such as nitrogen dioxide and BTEX, were monitored to assess the indoor and outdoor characteristics of the Santuario della Beata Vergine dei Miracoli in Saronno (VA). In addition, in-situ measurements for the characterisation of pigments, binders, and products of degradation on several works of art in the sanctuary were taken. Special attention was paid to Bernardino Luini’s frescoes’ colour palette, with the main aims of establishing the state of conservation of the paintings and taking part in the possible authentication of the fresco located outside the church. 2. Results and Discussion The microclimates surrounding the beautiful frescoes cycle by the master of the Renaissance Bernardino Luini and the valuable wooden sculptures representing the Deposition and the Last Supper were monitored during different periods of the year, including important religious events like Easter Week. Several dataloggers for temperature, relative humidity, and illuminance detection and an optical particle counter (OPC) for particulate matter (PM) continuous evaluation were placed around the area of interest. Moreover, some diffusive, passive samplers to determine the mean concentration of NO2 and BTEX were also used. As part of the research, the outdoor parameters were also monitored and a comparison with the indoor values was made with the purpose of finding possible correlations. The preliminary results of this part of the research indicate that, notwithstanding the indoor concentration of pollutants and changes in thermal and humidity parameters are on average lower with respect to the outdoor values, the microclimate inside the Sanctuary was often above the threshold outlined by the Italian legislation and technical standards [1]. Several non-invasive measurement campaigns were performed in the meantime, with the aim of identifying pigments, binders and possible products of degradation on the “Marriage of the Virgin” by Bernardino Luini (Figure 1), the “Adoration of the Christ Child” (which is located outside the church and is thought to be an original Bernardino Luini as well), and some of the wooden painted sculptures. This study was performed by means of totally-non-invasive spectroscopic and imaging complementary techniques such as Raman, FTIR, visible reflectance and XRF spectroscopies, hyperspectral imaging and colourimetry. The resulting palettes of the two frescoes are both characterised by the presence of calcium carbonate (used both in white details and to lighten other colours), yellow ochre and Naples yellow, red ochre and/or umber, and Fe-based green. Nevertheless, some differences were also evidenced, especially with regard to the blue areas. Indeed, while smalt was used to paint Joseph’s mantle in both works, Mary’s mantle showed the presence of valuable lapis lazuli only in the “Marriage of the Virgin”. Azurite, and traces of smalt were instead identified in the “Adoration of the Christ Child”. Some products of degradation like gypsum and oxalates were detected here and there on both frescoes. Furthermore, some in-situ XRF and visible reflectance analyses were carried out on the wooden sculptures placed in the two main chapels of the Sanctuary. Apart from the identification of cinnabar (HgS) as the main pigment for the red clothes of the characters, the occurrence of the phenomena of degradation was also observed. Still under study is the possible correlation of the detachment of some of the painted film with unsuitable air quality and microclimate of the environment. The study of the possible correlation between environmental conditions/pollution and degradation on all the investigated objects is ongoing, and some modeling experiments are being performed to delve deeper into this issue. 3. Conclusions This research highlights the importance of the synergic application of environmental monitoring and artistic material analysis in museum contexts. Indeed, the monitoring of air quality allowed assessing the environmental conditions inside and outside the Sanctuary over time, even covering important liturgic events. On the other hand, the analyses performed on the frescoes and sculptures highlighted several products of degradation that are probably correlated with the environmental conditions. The knowledge about the mechanisms behind the corruption of works of art is one the most challenging topics nowadays, and this makes this study a pioneer in interdisciplinary research for it is being performed in real conditions and over a long time, which is not so common to find in the literature.