## 2 Materials and Methods
### Methods  
In Table 1, the analytical methods applied to the samples are reported for identifying the stratigraphy and petrographic characteristics of the plaster, the composition of the binder, the pigments, and the alteration products. Cross-sections for the microstratigraphic analysis were prepared from sixteen samples (SG9A, SG9B, SG10, SG11, SG12, SG13, SG14, SG15, SG16, SG17, SG23A, SG25, SG39B, SG40, SG46, SG47) at the Department of Earth Sciences at the University of Pisa. The observation of the thin sections was performed using a ZEISS AxioPlan polarized light microscope (OM). For the preparation of the cross-sections, the samples were first observed under a ZEISS STEMI 305 stereo microscope in order to decide the position and direction of the cut and highlight all the layers of the samples, from the surface to the deepest part. A first set of eight samples (SG9A-B, SG10, SG11, SG12, SG13, SG14, SG15) was investigated at the Department of Geo-environmental and Earth Sciences of the University of Bari Aldo Moro, with the following instrumentation and analytical conditions. Scanning electron microscope (SEM) observations were made on thin sections, previously fixed on aluminum specimen holders and coated with graphite. An SEM EVO-50 XVP (LEO), equipped with an Oxford Instruments AZTEC EDS microanalysis system with SD X-MaxN detector (80 mm2), was used. The accuracy of the analytical data was verified using various standards produced by Micro-Analysis Consultants Ltd. (St. Ives, UK). The working voltage was 15 kV and the beam current between 250 and 400 pA. Spectra acquisitions lasted 50 s, with counts ranging from 25,000 to 30,000. Chemical maps were acquired with a dwell time of 100 us, counting time of 10 min, and a resolution of 2048. A second set of ten samples was analyzed at the University of Padova, eight at the Department of Geosciences (SG16, SG17, SG23A, SG25, SG39B, SG40, SG46, and SG47), and two, without any preliminary preparation, at the Department of Chemical Sciences (SG35 and SG36). \begin{table}
\begin{tabular}{c c c c} \hline
**Sample** & **Panel** & **Description** & **Investigation Technique** \\ \hline SG9A & A & \multicolumn{1}{c}{Plaster with pale ochre paint} & OM, SEM-EDS (t.s.) \\ SG9B & A & \multicolumn{1}{c}{Plaster with pale ochre paint and decay products (?)} & OM, SEM-EDS (t.s.) \\ SG10 & A & \multicolumn{1}{c}{Plaster with violaceous paint} & OM, SEM-EDS (t.s.) \\ SG11 & B & \multicolumn{1}{c}{Plaster with dark-brown paint} & OM, SEM-EDS (t.s.) \\ SG12 & B & \multicolumn{1}{c}{Plaster with blue–gray paint and makeover (?)} & OM, SEM-EDS (t.s.) \\ SG13 & C & \multicolumn{1}{c}{Plaster with white paint} & OM, SEM-EDS (t.s.) \\ SG14 & E & \multicolumn{1}{c}{Plaster with dark paint and protective coating (?)} & OM, SEM-EDS (t.s.) \\ SG15 & E & \multicolumn{1}{c}{Plaster with blue layer} & OM, SEM-EDS (t.s.) \\ SG16 & A & \multicolumn{1}{c}{Pale-green paint and plaster (rewoked area?)} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG17 & A & \multicolumn{1}{c}{Porphyry-like paint and plaster} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG23A & A & \multicolumn{1}{c}{Plaster} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG25 & D & \multicolumn{1}{c}{Pale-blue paint on a red–brown layer (morellone?) and} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG35 & D/E & \multicolumn{1}{c}{Green layer} & EPR (f.) \\ SG36 & D/E & \multicolumn{1}{c}{Light-blue layer} & EPR (f.) \\ SG39B & Vault & \multicolumn{1}{c}{Blue paint on a red–brown layer (morellone?) and plaster} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG40 & Vault & \multicolumn{1}{c}{Filling mortar} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG46 & C & \multicolumn{1}{c}{Plaster with remains of a brown preparation layer} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ SG47 & A & \multicolumn{1}{c}{Plaster with remains of a brown preparation layer} & OM, CL, SEM-EDS, \(\mu\)-Raman (t.s.) \\ \hline \end{tabular}
\end{table}