A Multianalytical Investigation to Preserve Wall Paintings: A Case Study in a Hypogeum Environment

In the present study, a diagnostic approach was used to analyze the wall painting in the apse of the Sotterra church at Paola, in the province of Cosenza, Italy. The Sotterra church is nowadays located 6 m under the ground level. The presbytery area houses valuable pictorial evidence attributable to different phases. The oldest painting, visible in the apse area, dates back to the 11th–12th centuries, and it represents the subject of the present study, while the later decorations are placed in a chronological range from the 14th to the 15th centuries. Due to the peculiar environmental conditions, the conservation of subterranean sites represents a debated issue and must be properly investigated. For this reason, in this research, noninvasive analysis and laboratory-based methods were planned to obtain information about both the composition of original materials and the mechanisms and causes of alterations affecting the wall painting in the apse. Simultaneously, an environmental monitoring campaign of the indoor climate for the duration of nine months was conducted. The results highlighted the use of natural mineral pigments such as ochres and earths. The analysis of the painting stratigraphy revealed that the mural painting consists of two plaster layers characterized by lime-based binder. Moreover, the presence of a high amount of calcium sulfate has been discovered; this latter result, combined with the monitoring of the microclimate, allowed for the establishment of the crystallization and the condensation risks which occur on the investigated surfaces.