## 3 Materials and Methods  
The sampling campaign was conducted at the end of July 2020 in collaboration with Dr. [PERSON], an archaeologist from Pandora Archeologia (Veruno, Italy). Mortar samples and one charcoal sample within a mortar were collected from various sampling areas in the lower chapels of the San Salvatore complex. According to the recommendations of the archaeologists and architects overseeing the restoration and recovery project of the complex and given the significance of the samples and their macroscopic characteristics, a selection of samples was made, as shown in Figure 3 and in Table 1, where the types of analyses performed for each sample are reported. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample Code** & **Area** & **Material** & **Note** & **XRPD** & **OM** & **SEM-** & **SG** & **XRPD\_SG** & **14C** \\ \hline MV\_SS\_01 & A & Mortar/Plaster & Deeper layer, scratch coat & x & x & x & x & x & x \\ \hline MV\_SS\_01\_C & A & Charcoal in the mortar & & & & & & & x \\ \hline MV\_SS\_02 & A & Mortar/Plaster & \begin{tabular}{c} Finish coat layer of the scratch \\ coat (white in color) \\ \end{tabular}  & x & x & x & x & x & x & x \\ \hline MV\_SS\_03 & A & Mortar/Plaster & & & & & & & \\ \hline MV\_SS\_04 & A & Mortar/Plaster & & & & & & & \\ \hline MV\_SS\_05 & B & Mortar/Plaster & 1 st layer & & & & & & \\ \hline MV\_SS\_06 & B & Mortar/Plaster & 2 nd layer & & & & & & \\ \hline MV\_SS\_07 & B & Mortar/Plaster & 1 st preparatory layer & x & x & x & x & x & x \\ \hline MV\_SS\_08 & C & \begin{tabular}{c} Plaster \\ \end{tabular}  & \ \begin{tabular}{c} Fresco dated 1450 \\ \end{tabular}  & x & x & x & x & x & x & x \\ \hline MV\_SS\_09 & C & Mortar/Plaster & \begin{tabular}{c} White surface layer of \\ the dome \\ \end{tabular}  & \The analytical methodology consisted of several distinct phases: (i) chemical-mineralogical characterization; (ii) purification procedures; (iii) characterization of extracted fine powder; (iv) sample treatment; and (v) final radiocarbon dating [14; 15; 22]. The characterization and separation procedures were applied at the CIRCe Centre in Padua, Italy, in the department of Geosciences of the University of Padova, while graphitization and AMS measurements were performed at the CIRCE Laboratory, Department of Mathematics and Physics, Universita degli Studi della Campania, in Caserta, Italy. Petrographic analyses were carried out using optical microscopy (OM) on 30 um thin-sections, utilizing a Nikon Eclipse ME600 microscope (Tokyo, Japan) equipped with a Canon EOS 600D Digital single-lens reflex camera (Tokyo, Japan). Thin sections coated with graphite were microstructurally and microchemically characterized using a CamScan MX2500 scanning electron microscope (SEM) (Electron Optic Services, West Orange, NJ, USA) equipped with an EDS system. Mineralogical quantitative phase analyses (QPAs) were conducted by X-ray powder diffraction (XRPD) on fine sample powders obtained through micromicronization. XRPD analyses were performed using a Malvern PANalytical X'Pert PRO diffractometer (Malvern, Worcestershire, UK), and data interpretation was carried out using X'Pert HighScore Plus 3.0 software by Malvern PANalytical for qualitative phase recognition, and Topas 4.1 software by Bruker (Billerica, MA, USA) for quantitative phase analysis through Rietveld refinement [23]. Purification of the mortar samples involved manual cleaning and disaggregation, followed by ultrasonic baths and wet sedimentation to separate particles [10; 24]. Fine-grained particles were filtered using a vacuum pump system and 0.1 um filters. The filtered particles underwent XRPD analysis to assess the presence of contaminants. Samples containing LDHs and Mg-hydrates underwent thermal treatment at 550 \({}^{\circ}\)C for 30 min to break down the crystal structure of the mineral phases, potentially inducing errors in age determination.