## 4 Results  
### Non-Invasive Investigations  
#### 4.1.1 ED-XRF Analysis  
ED-XRF analysis was carried out on 33 hues of the wall paintings (measurement points from M18 to M45) involving interior walls, eastern vestibule and north lateral niche vaults (Figures 2 and S1). Generally, ED-XRF analysis determines chemical components of investigated area based on X-ray spectra (qualitative analysis). However, we have to consider that in the case of wall paintings with multiple layers, or when a surviving area of pigments is smaller than the irradiated area, the X-ray analyses provide not only the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Code** & **Sampling Area** & **Description** & **Analytical Techniques** \\ \hline MY31 & Vault, lateral niche north & Blackish paint layers & \\  & & and plaster & PLM, ESEM-EDS \\ \hline MY32 & Vault, lateral niche north & Brownish paint layers & GC-MS, PLM, ESEM-EDS \\  & & and plaster & ESEM-EDS \\ \hline MY33 & Vestibule, north-east wall & Reddish paint layer & \(\upmu\)-Raman, GC-MS, PLM, ESEM-EDS, ATR-FTIR \\  & & and plaster & \\ \hline MY44 & Entrance, north-west wall & Blackish paint layer & \(\upmu\)-Raman, PLM, ESEM-EDS \\  & & and plaster & \\ \hline MY45 & Entrance, north-west wall & plaster & \(\upmu\)-Raman, PLM, ESEM-EDS, ATR-FTIR \\  & & & \\ \hline MY45e & Vestibule, north-east wall & Brownish/yellowish & GC-MS, PLM, ESEM-EDS, \\  & & paint layer and plaster & ESEM-EDS, ATR-FTIR \\ \hline MY46 & Vestibule, north-east wall & Blackish paint layer & GC-MS \\ \hline G1 & _Acacia leucoplhoa_ & Acacia gum & GC-MS \\ \hline G2 & _Acacia catechu_ & Acacia gum & GC-MS \\ \hline G3 & _Azadirachta indica_ & Neem gum & GC-MS \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Samples list. compositional information of the targeted pigments but also those of the lower layer or the surrounding and underlying area. ED-XRF investigations confirmed the ubiquitous presence of Ca, As, Hg, S, Mg, Sr, Si, K, Cl and Fe. All areas revealed high amounts of Ca, As and S referable to lime/gypsum plaster and arsenic-based compounds. The content of Si, K, Fe, Ti and Mn can be related to red and brown earth pigments [26]. Low levels of Ni and Zr were also revealed, which can be linked to residual minerals developed by metamorphic rocks [27]. In the black and reddish tones, vermilion could be ascertained for the presence of Hg. Due to a large number of ED-XRF spectra that were collected from different paint layers, a simple observation at the spectra and interpretation of the particular association of elements in the entire dataset was difficult if not impossible. Thus, a dimension reduction approach in observing the dataset was adopted to look at the spectra data. As a multivariate statistical approach, PCA was chosen for this purpose. PCA has already been proven to be a powerful tool for elaborating compositional data of a large number of XRF spectra collected from painted materials [28; 29], and could have been used similarly in order to have a useful look at the dataset obtained by ED-XRF measurements from the pigments at Me-tav-ya. PCA on the log-transformed data from the net area under the peaks of the X-ray characteristic of the elements showing more than 60% of total variance accounted for the first two principal components (PCs), demonstrating a relatively good estimate of data observation with PCA. As Figure 2 shows, the high correlation between Fe, Al and Si, is related to the earth pigments (such as ochres) while the high association of Ca and S suggested the occurrence of calcium sulphate, probably in gypsum form related to restoration interventions. The presence of Hg suggested the occurrence of vermillion and the moderate association of Hg and Cl (the correlation coefficient between Hg and Cl is about +0.50) may be linked to the darkened vermilion due to the well-known vermilion alteration in presence of halides [30].