3.1.5. Pink Gouaches

Two distinct pink gouaches were identified: R6G and CV/MV by SERS in Pink 1 (Pk1), while the presence of CF was detected by p-XRF (Figure S8) though not confirmed by FTIR. The second pink gouache (Pk2) used in P20 is slightly darker and warmer in tone and contained Calcium Lithol Red (PR49:2) as identified by Raman and SERS (Figure S9) [44]. The p-XRF spectra showed significant peaks for Ca and S as expected, the latter relating to the sulfonated group of the beta-naphthol [41].

### 3.1.6. Blue Gouaches

A total of five distinct blue gouaches were identified and all included synthetic ultramarine blue. Blue 1 (B1) was the most prominent blue and contained no additional pigments or fillers while Blue 2 (B2), used in P19 and similar to B1 in tonality, also contained titanium white (TiO2), zinc white (ZnO), and calcium carbonate. The intermediate Blue 3 (B3) used in P6 contained zinc white. Both light Blue 4 (B4), used in P12, and Blue 5 (B5), used in P15, had a light gray-blue tone but no additional pigment was detected aside from ultramarine blue. Ultramarine blue (3Na2O.3Al2O3.6SiO2.2Na2S) was detected in all the blues by p-XRF (Figure S10) through the presence of Al, Si, and S (K and Ca were also present in small quantities) [45]. In μ-FTIR and r-FTIR spectra (with no KK transformation), the characteristic Si-O and Al-O stretch as well as the Si-O main bending mode were readily observed (Figure S11) [30,45]. The pigment was also detected with confocal and p-Raman spectroscopy through the Raman modes of the sulfur radical anions S3 − and S2 − acting as chromophores (Figure S12) [45]. Those bands were weak but still discernible for the light B4 and B5 blues. Titanium white in B2 was detected by p-XRF (Ti), more specifically anatase as confirmed by confocal and p-Raman [46]. Zinc white was detected by p-XRF but could not be confirmed by Raman or infrared spectroscopy, though detectable by the latter [47].
