## 4 Results
### Preparatory and Priming Layers  
PLM observations and ESEM-EDX carried out on the _Martyrdom of Saint Sebastian_ panel and the _Visitation_ altarpiece (Tables S1-S3) showed the presence of one or two ground layers. The inner one contains coarse grain size gypsum particles (derived from the so-called \"gesso grososo\" or burnt gypsum [29], along with a few large quartz crystals and silicates, followed by a gypsum layer made of fine particles (obtained from \"gesso sotile\" or slaked burnt gypsum with an excess of water). Impurities of Si, Mg, Fe, Al and K are related to silicates (sample GSM1: Figure S3a-c; samples GSM5, GSM7: Table S2). The maximum thickness of the ground layer is around 450-480 \(\upmu\)m (Tables S2 and S3). The FTIR spectrum of almost all the ground samples shows the characteristic signals of both calcium sulphate dihydrate (gypsum, CaSO\({}_{4}\cdot\)2H\({}_{2}\)O) and anhydrous (anhydrite, CaSO\({}_{4}\)) (Table S2). Indeed, structured bands are evident in the ranges 1160-1130 cm\({}^{-1}\), corresponding to the asymmetric stretching of SO\({}_{4}\)\({}^{-2}\); and 590-680 cm\({}^{-1}\) (\(\
u_{4}\)(SO\({}_{4}\)\({}^{2}\) -) mode). In particular, the peaks at 1138, 1117, 670 and 602 cm\({}^{-1}\) of respective samples GSM1, GSM4, GSS1a and GSM6 in Figure 2a are assigned to gypsum, whereas those at 1150, 1112, 675, 611 and 594 are, respectively, assigned to anhydrite. The two bands between 3552 and 3406 cm\({}^{-1}\) and two peaks at 1685 and 1622 cm\({}^{-1}\) could be respectively assigned to the -OH stretching and bending of the water molecules of gypsum [30] (Figure 2a). The presence of anhydrite is consistent with the use of _gesso grososo_ in the inner ground layers. Regarding the organic material (binding materials), the FTIR spectra of all ground samples show a complex pattern in which several compounds can be present at the same time. Overall, wax is present in many samples, as well as esters of carboxylic acids and carboxylates, calcium oxalates and proteinaceous materials (Table S2). Only in one sample, GSM6 corresponding to the priming layer, cerusite (CO\({}_{3}\)\({}^{2-}\) asym stretch at 1438 cm\({}^{-1}\) and 1405 cm\({}^{-1}\), CO\({}_{3}\)\({}^{2-}\) sym stretch at 1052 cm\({}^{-1}\), and in plane CO\({}_{3}\)\({}^{2-}\) bending at 680 cm\({}^{-1}\)) [31] and lead white (OH stretch at 3538 cm\({}^{-1}\), CO\({}_{3}\)\({}^{2-}\) asym stretch at 1397 cm\({}^{-1}\) CO\({}_{3}\)\({}^{2-}\) sym stretch at 1046 cm\({}^{-1}\), and out-of-plane and in plane CO\({}_{3}\)\({}^{2-}\) bending at 840 and 683 cm\({}^{-1}\), respectively) [32] were detected together with esters of carboxylic acids, lead carboxylates and traces of gypsum (Figure 2a). Esters of carboxylic acids (oils) and lead carboxylates can be inferred from the signals at 2926-2929 and 2850\({}^{-2}\) cm\({}^{-1}\) (CH\({}_{2}\) stretch), 1728 cm\({}^{-1}\) (C=O stretch of esters of carboxylic acids) [33], and 1524 cm\({}^{-1}\) (asym stretch of carboxylates) [34]. However, due to the non-homogeneity of the substrate and the selectivity of the sampling,  
Figure 2: _Martrydom of Saint Sebastian_ panel and _[PERSON] and the [PERSON]_ canvas: (**a**) FTIR spectra of the ground layer of samples GSM1, GSM4, GSM6, GSS1a (light brown ground), GSS1b (dark brown ground); (**b**) Detail of the spectra in the range 3000–2800 cm\({}^{-1}\). The dotted lines are meant to show the shift of the signals in the region of interest for different samples. the substances identified in each spectrum can be slightly different, both in composition and concentration. According to the spectrum, GSM1 consists of a mixture of a waxy material mixed with low amounts of lipidic compounds and carboxylates, calcium oxalates (diagnostic signal of the CO sym stretch in oxalate anion at 1318-1325 cm\({}^{-1}\)[35] and proteinaceous material (amide I at 1650 cm\({}^{-1}\) and amide II at about 1543 cm\({}^{-1}\)) [36].