3.1.2. Black Gouaches

Two distinct black gouaches were identified. Both contained a mixture of bone/carbon black, Prussian blue, and silicates with lesser amounts of metal oxides, including possibly iron oxides. The relative concentrations of these pigments were however different: the more predominant Black 1 (Bk1) was slightly richer in Prussian blue and silicates; Black 2 (Bk2), found only in three prints, was richer in bone/carbon black. This pigment is an impure black carbon pigment prepared from charring animal bones containing mainly calcium hydroxyapatite (Ca10(PO4)6(OH)2) and small amounts of magnesium phosphate (Mg3(PO4)2) and calcium carbonate (CaCO3) [27], which is why it could be detected by the presence of P and Ca in the p-XRF spectra (Figure 4), especially in Bk2 where it was more concentrated. Bone black in Bk2 was also confirmed by FTIR through a characteristic band at 2013 cm−<sup>1</sup> assigned to degradation products of the pigment synthesis [35]. The characteristic main phosphate band (ν<sup>3</sup> at 1036 cm<sup>−</sup>1) associated with the hydroxyapatite overlaps with the cellulose and silicate bands in the μ-FTIR and (non-KK transformed) r-FTIR spectra [36] and neither 962 nor 875 cm−<sup>1</sup> ν<sup>2</sup> bands were detected (Figure 5). The presence of the characteristic D and G broad bands in the Raman spectra [25] confirmed the presence of carbon black, either as a constituent of bone black and/or as a pigment (Figure 6). The additional phosphate band at 960 cm−<sup>1</sup> in the Raman spectrum is generally weak [37] and was not observed.

Prussian blue is a synthetically produced ferric ferrocyanide blue pigment (KFe[Fe(CN)6]) [38]. Fe and K were detected by p-XRF (these elements can also indicate the presence of natural iron oxides). Further confirmation was provided by r- and μ-FTIR with the characteristic CN asymmetric stretch [30]. The pigment was not observed in either Raman modalities, although it is detectable [31]. The p-XRF spectra also suggested the presence of silicates (Al, Si, K) and metal oxides (Ti, Cr, and possibly Fe). The presence of silicates and aluminosilicates was confirmed by μ- and r-FTIR [33], although several bands overlapped with bone black and cellulose. Confocal Raman spectroscopy confirmed the presence of quartz and silicates [26].

#### 3.1.3. Violet Gouache

The violet gouache (V) was only used in pochoir P20. SERS (Figure S4) identified Rhodamine 6G (R6G) [39] and methyl violet and/or crystal violet (MV/CV) [40]. The weak Cu and Fe peaks in p-XRF (Figure S5) suggested the presence of copper ferrocyanide (CF), also confirmed by the characteristic CN asymmetric stretch in the μ- and r-FTIR spectra. CF could be the precipitating anion for either R6G (PR169) or for MV (PV27) [41].

#### 3.1.4. Magenta Gouache

The composition of the magenta gouache (M) was the same across nine prints. Reference to this gouache appears in published correspondence between Matisse, Tériade, and Gaut, the then director of the Linel company [42]. The source pigment for this gouache was manufactured in Germany and became unavailable after WWII. In the letter, Matisse mentions the importance of a "Violet Fixe" and the large quantity needed to reproduce a large work, indubitably *Jazz*. SERS spectra indicated the presence of R6G, and confocal Raman also indicated the presence of Rhodamine 3B (R3B) [43] (Figure S6). The p-XRF analysis suggested that these two pigments are precipitated with CF—also confirmed by r-FTIR—and possibly phospho-tungsto-molybdic acid (PTM) since P and W were also detected (Figure S7). The presence of these constituents could be related to the presence of PV2, an R3B-PTM pigment, and PR169, an R6G-CF pigment [41].
