## 2 Materials and Methods
### Sampling and Laboratory Analyses  
Before the restoration intervention, preliminary in situ investigations were carried out using a portable video microscope in order to support the choice of the most appropriate sampling points. Sampling activities were performed with the authorization of the Super-intendency of Archaeology, Fine Arts and Landscape for the metropolitan city of Reggio Calabria and the province of Vibo Valentia. After a careful observation of the surface, carried out by the restorers together with conservation scientists, seven micro-samples were chopped from the surface of the painting using the most suitable stainless-steel tools, such as lacets and small chisels, generally used in restoration works. The sampling points were chosen considering the presence of alteration products, substances superimposed on the surface and the morphological characteristics of the pictorial film. Each sampling point was marked with a letter and a number and properly documented by photography. The size of the collected samples is approximately 1 cm \(\times\) 1 cm with a thickness that ranges from 0.5 cm to 0.2 cm. Once sampled, the fragments were analyzed through complementary laboratory-based investigation methodologies. The list of samples taken from the surface and the diagnostic methods performed are briefly described in Table 1, while the sampling locations are mapped in Figure 3. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample ID** & **Description** & **Employed Analyses** \\ \hline S\_1 & \begin{tabular}{c} Stratigraphic sample, including both \\ plaster and the pinkish pictorial layer \\ \end{tabular} & OM; SEM-EDS \\ S\_2 & \begin{tabular}{c} Pictorial layer \\ \end{tabular} & FT-IR \\ S\_3 & \begin{tabular}{c} Sample, including a substance having a \\ resinous aspect \\ \end{tabular} & FT-IR \\ S\_4 & \begin{tabular}{c} Stratigraphic sample, including both \\ plaster and the light grey pictorial layer \\ \end{tabular} & OM; SEM-EDS \\ S\_5 & \begin{tabular}{c} Sample of the whitish veil on the painting \\ surface \\ \end{tabular} & FT-IR \\ S\_6 & \
\begin{tabular}{c} Plaster \\ \end{tabular} & IC \\ S\_7 &
\begin{tabular}{c} Mortar of the support \\ \end{tabular} & IC \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of analyzed samples, description and employed techniques. Figure 3: Mapping of the sampling points on the left part of the apse. Two cross-sections were prepared according to traditional techniques. Samples S_1 and S_4 were dipped in epoxy resin and let polymerize. Subsequently, the samples were perpendicularly cut and then polished to allow the stratigraphic observation at higher magnification. Optical Microscopy (OM) was then conducted on the two stratigraphic micro-samples (S_1 and S_4) to identify the main morphological features of both the plaster and the pictorial film. OM analysis was performed using a Zeiss AxioLab microscope equipped with a digital camera to capture images. In addition, high magnification images were taken on samples S_1 and S_4 by means of the portable digital microscope Dinolite(r) AM411-FVW, 1.3 Mpx (1280 \(\times\) 1024), 10-70\(\times\) and 200\(\times\) magnification, UV + white led light source connected to a portable computer. The scanning electron microscope (SEM) coupled with a spectrometer EDS provides information on the morphology and the chemical composition of both the binder and the pigments [25]. For this reason, Ultra-high-resolution SEM (UHR-SEM)--ZEISS CrossBeam 350 equipment, coupled with the spectrometer EDS-EDAX OCTANE Elite Plus-Silicon drift type, was employed on samples S_1 and S_4 coated with a thin and highly conductive graphite film. Instrumental conditions set for EDS analysis were HV: 15 keV and probe current: 100 pA.  
Infrared spectroscopy (FT-IR) was used to investigate the nature of organic and inorganic compounds.