## 5 Conclusions  
Our findings highlight the potential of radiocarbon dating for evaluating magnesium mortars, an aspect that has previously been overlooked in archaeological investigations. The multi-analytical study conducted on mortar and plaster samples selected from the complex of the church of San Salvatore--Massino Visconti (NO) has allowed the characterization of their main compositional, mineralogical, and microstructural characteristics. Furthermore, by successfully isolating and purifying the binder fractions, we achieved reliable radiocarbon ages that align with the historical timeline of the San Salvatore complex. The mortar samples MV_SS_01, MV_SS_07, and MV_SS_08 appear to be slightly magnesian air limes with a mineralogical composition compatible with locally available raw materials. The magnesian components (magnesite and hydromagnesite) identified in the examined samples contribute to ensuring greater homogeneity between binder and aggregate, resulting in mortars with improved mechanical strength. Similarly, the samples from the white finishing layers (MV_SS_02 and MV_SS_10) are made with a fat lime mortar based on magnesian lime. The binder used is therefore an air lime of calcareous and magnesian composition, and the matrix consists of calcium carbonate, magnesium, and elements (such as silicon and aluminum) indicative of a partial hydraulic reaction with the formation of secondary phases (LDH and M-S-H), favored by the occurrence of large amounts of magnesium derived from both the lime itself and the magnesium-rich clay fraction. The gravimetric separation of the binder fraction and subsequent mineralogical characterization allowed the selection of the datable binder fraction and the identification of any contaminating phases for radiocarbon dating. LDH phases capable of trapping carbon dioxide over time, and thereby introducing younger carbon into the system, were identified. The purification process carried out by thermal treatment resulted in the collapse of the LDH structure and the elimination of trapped CO\({}_{2}\). Consequently, the radiocarbon dates obtained are scientifically reliable and consistent with the historical interpretations provided by the studies conducted, except for the MV_SS_08 sample of the fresco inside the chapel of Santa Margherita, as that sampling likely included part of subsequent restorations. The samples selected in the oldest chapel of the San Salvatore complex, the chapel of San Quirico, show radiocarbon ages ranging from 1150 to 1300 (MV_SS_01, MV_SS_01_C, and MV_SS_02). The sample in the entrance wall of the chapel of Santa Margherita (MV_SS_07), built about a century after the adjacent chapel, is dated between 1300 and 1400. Finally, the sample of the finishing layer inside the same chapel, consistent with the date of the assignment to the Augustinians (1450) and preceding an inscription on the same (1541), is likely dated to between 1440 and 1511. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14219899/s1](https://www.mdpi.com/article/10.3390/app14219899/s1)]([https://www.mdpi.com/article/10.3390/app14219899/s1](https://www.mdpi.com/article/10.3390/app14219899/s1)). Figure S1. XRPD patterns of the bulk sample MV_SS_01. The main diffraction peaks of the mineral phases are highlighted. Figure S3. XRPD patterns of the bulk sample MV_SS_02. The main diffraction peaks of the mineral phases are highlighted. Figure S4. XRPD patterns of the bulk sample MV_SS_07. The main diffraction peaks of the mineral phases are highlighted. Figure S4. XRPD patterns of the bulk sample MV_SS_08. The main diffraction peaks of the mineral phases are highlighted. Figure S5. XRPD patterns of the bulk sample MV_SS_10. The main diffraction peaks of the mineral phases are highlighted. Figure S6. XRPD patterns of the binder-concentrated sample MV_SS_01_SG. The main diffraction peaks of the mineral phases are highlighted. Figure S7. XRPD patterns of the binder-concentrated sample MV_SS_01_SG.