Monitoring calcium carbonate crystallizations on mural painting mockups in Roman Catacombs by ultra close-range photogrammetry

The use of non-destructive techniques is mandatory for monitoring of the conservation status of artistic surfaces. The ultra-close range photogrammetry (UCRP) is a particular improvement of the classical technique of the close-range photogrammetry, and has been used in the past to assess the effectiveness of laser cleaning of artistic surfaces and other kind of conservative interventions (e.g., consolidation). In this paper, the UCRP technique is applied to monitor the formation of calcium carbonate crystallizations affecting mural paintings and rock surfaces within the Roman Catacombs of Saints Mark, Marcellian and Damasus in Rome, Italy. This deterioration process can create thick coverage and incrustations, even in some cases speleothems. The chemical reaction involved relies on the availability of calcium sources, which can be also of anthropogenic origin (e.g., lime-based mortars). Microclimate parameters also represent environmental forcing factors, on which the morphology and the degree of crystallinity of the precipitated carbonates depend. Temperature, relative humidity, CO2 concentration and surface temperature were continuously measured within the cubicle of the Twelve Apostlesby means of a multi-sensor monitoring network properly designed. The registered parameters were related with the results of the periodical surfaces survey performed by UCRP on mural painting mockups. These were the test areas purposely realized by applying fresh lime mortars simulating the ancient techniques of lime-based mural painting. The UCRP technique is used to generate the RGB points cloud and the Digital Elevation Model (DEM) of the area of interest. Setting a suitable reference plane through the seeding points and using a dedicated routine to align the obtained models the metric information can be extracted. The same procedure is applied at each step of the monitoring campaign. By comparing the surface patterns in terms of metric and of colorimetric information it is possible to detect the calcite crystallizations from the first formation. The calcite crystallizations were fully characterized in terms of compositional, textural and mineralogical properties by a combination of standard investigations including FT-IR spectroscopy, X-ray diffraction, thin section observations under polarized light microscope, and Scanning Electron Microscopy. This experimental research has been undertaken in the framework of the co-operation between the Institute for the Conservation and Valorization of Cultural Heritage and Pontifical Commission for Sacred Archaeology, Vatican, during the project HYPOGEA. This project is devoted to design of a multidisciplinary methodological approach for the instrumental monitoring of the microphysical environment of the Catacombs of Saints Mark, Marcellian and Damasus, and such as to be potentially exported to similar contexts.