# The Characterization of a Fragment of a Medieval Fresco from Corbii de Piatra Cave Church
## 2 Materials and Experimental Techniques  
The fresco sample was taken from the northern wall of the church. The authors, who are specialists in sacred art and materials engineering, assessed the current state of the fresco painting and analyzed the degradation processes by comparing images from previous years. Based on this analysis, they determined the location for taking samples, the minimum sample size required, and the elements that needed to be highlighted through their characterization. The deontology of the conservation and restoration of historical monuments was respected during the sampling [5; 6]. The mortar sample was taken from the wall of the Corbii de Piatra cave church, especially from the second floor where the fresco support is most exposed to water infiltration processes through the sandstone wall. The sample was taken from an area with high adhesion that was covered with a red paint layer at a height of 1.75 m. The analysis of the pigments used for the red color was performed on the sample taken from this area (Figure 1d).  
A single sample was taken for all the analyses performed. The fresco sample was characterized using various methods, including optical microscopy in polarized light, cathodoluminescence, atomic force microscopy, X-ray diffraction, and scanning electron microscopy with the energy dispersive fluorescence spectroscopy module for elemental chemical analysis.  
Figure 1: Corbii de Piatra Church: (**a**) an overview of 2022; (**b**) the layout of the mural paintings in 2011 [2]; (**c**) the state of the painting in 2022; and (**d**) the place where the fragment of the fresco with the red paint layer was taken.  
The analyses were performed in polarized light with the help of a Zeiss Observer A1 m optical microscope, which allows magnification up to 1000\(\times\), visualization in bright field, dark field, polarized light, and acquisition of images with a Canon camera.  
The cathodoluminescence analysis was carried out with a Nikon E400 optical microscope, a cold cathode cathodoluminescence device (CL 8200 MK 3A), and a COOLPIX 950 digital microphotography device. The parameters required to perform the technique are as follows, according to the standard in use an average vacuum value of 0.5 Torr, a current-voltage on the beam ranging from 15 to 17 kV, and a current intensity on the electron gun ranging from 350 to 400 mA. The mineralogical composition of the mortar components, compared with the geological maps, allows the identification of the original raw materials' provenance.  
Atomic force microscopy (AFM) was carried out with EasyScan II Nanosurf equipment using a non-contact mode, a CTR10 tip, and Nanosurf software to evaluate the scans to use the fresco sample embedded in the resin for flatness, with the surface mechanically polished at a very low speed without removing the external paint layer. The experiments followed the determination of the surface layer morphology of the material constituents and the analysis of the intimate contact between them at their interface.  
The X-ray diffraction analysis (XRD) was used to identify the crystalline phases of the components in the mortar [7, 8]. The analysis was conducted using an XPERT PRO MPD 3060 X-ray diffractions device from Panalytical (Almelo, Netherlands) equipped with a Cu X-ray tube (K\(\alpha\) = 0.154051 nm) and set to scan 2 Theta values between 20\({}^{\circ}\)-70\({}^{\circ}\), with a step size of 0.13\({}^{\circ}\) and a time/step of 51 s, at a scan speed of 0.065651 \({}^{\circ}\)/s.  
The SEM-EDS electron microscopy characterization was carried out using the HITACHI SU5000 electron microscope equipped with a backscattered electron detector and the energy dispersive fluorescence spectroscopy module for elemental chemical analysis.