## 2 Materials and Methods
### Raman Spectroscopy  
Raman spectra were collected using a bench spectrometer available at the Earth Sciences Department of the University of Milano (MI). The instrument was a LabRAM HR Evolution (Horiba(r), Turin, Italy equipped with a cooled CCD detector sensor and an Nd-Yag 532 nm/100 mW laser, which generated a spot of 2 \(\upmu\)m diameter on the sample surface. The excitation source and collection optics were arranged in a coaxial configuration and the scattered light was collected by a 50x objective (NA = 0.5; resolution = 0.65 \(\upmu\)m) in backscattering geometry (Figure 4). The Schott KL 1600 LED lamp (colour temperature = 5600 \({}^{\circ}\)K) allowed sample illumination during the preliminary surface observation. The main peak of a pure silicon chip at 520.63 cm\({}^{-1}\) was used to calibrate the system before data acquisition. Sample positioning under the objective was enabled by a motorised sample plate, which was guided using a 4-axis remote joystick (Marzhauser Wetzlar(r), Wetzlar, Germany) connected to the computer via TANGO 4 Desktop controller (Marzhauser Wetzlar(r), Wetzlar, Germany). For each point of interest on the surfaces of the samples, a spectrum was collected in the fingerprint range of 100 to 1200 cm\({}^{-1}\) and in the range of 3200-3600 cm\({}^{-1}\) to detect the hydroxyl group vibrational modes of water molecules within the investigated minerals ([PERSON], 2016). A minimum of 30 spectra were collected for each archaeological artefact, except for the MFP sample, due to the irregularity of its surface (see Figure A1 in Appendix A). Three accumulations and a variable exposure time from 5 to 15 s, depending on the entity of the background fluorescence and the Raman scattering efficiency, were adopted to obtain a significant signal-to-noise ratio. Spectra were first visualised and stored using LabSpec6 Spectroscopy Suite software (v1) while the subsequent spectral processing was carried out using OriginLab. The processing was performed with manual polynomial baseline subtraction and the Lorentzian fitting of the spectral curves. The first qualitative study was conducted for both the leucocratic and the melanocratic fractions by direct comparison of the acquired spectra with the international Raman database RRUFF using CrystalSleuth software (v1.0). The discrimination between amphib and pyroxene was also performed by observing the positions of the Raman peaks in the 3.200-3.600 cm\({}^{-1}\) interval, which are related to the presence of the hydroxyl group (-OH). For the spectra identified as belonging to the plagioclase fraction, a quantitative analysis was performed using the method described by [PERSON] et al. (2018) ([PERSON] et al., 2018). In the present study, the selected peaks for anorthite content (An\({}_{\text{(mol\%)}}\)) evaluation were \(\upnu_{\text{b}}\) at about 478 cm\({}^{-1}\) and \(\upnu_{\text{a}}\) at 507 cm\({}^{-1}\). To preliminarily individuate the position of the investigated mineral in the plagioclase solid solution, the centroid distance (\(\Delta_{\text{ab}}\)) between \(\upnu_{\text{a}}\) and \(\upnu_{\text{b}}\) was calculated. Then, the An(mol%) was obtained using Equation (1) if the mineral has a \(\Delta_{\text{ab}}\) less than 27 cm\({}^{-1}\), while Equation (2) was used if \(\Delta_{\text{ab}}\) exceeds 27 cm\({}^{-1}\), considering the \(\upnu_{\text{a}}\) peak width \(\Gamma_{\text{a}}\) as a reference parameter. Equations (1) and (2) result from the linear regression of data derived from plagioclase minerals with known An(mol%) content. They clearly express the relationship between the An(mol%) content and the measurable \(\Delta_{\text{ab}}\) and \(\Gamma_{\text{a}}\) for the selected \(\upnu_{\text{a}}\) and \(\upnu_{\text{b}}\) peaks, and thus the equations enable the characterisation of unknown plagioclase by their An(mol%) content.