*2.6. PCA Analysis*

Principal component analysis (PCA) was used as an exploratory data analysis method to group colors with very similar p-XRF, RS, and r-FTIR spectral signatures and in this way help determine the number of paints used to reproduce *Jazz*. PCA was carried out separately on the p-XRF, RS, and r-FTIR spectra using the SOLO-MIA (Eigenvector) software package. Preprocessing of the spectra included Poisson scaling (for the p-XRF spectra in the 1–15 KeV range), first derivative (for RS), and SNV (for r-FTIR), followed by mean centering.

#### **3. Results and Discussion**

#### *3.1. Pigment Identification*

*Jazz* was printed on Velin d'ARCHES paper as specified on the last page of the *Jazz* books. Analysis of the paper support with r-FTIR and p-Raman only produced bands related to cellulose. No additives or coatings were detected with the techniques in use. Analysis with p-XRF detected small amounts of Al, Si, P, S, K, Ca, Fe, and Zn. This small mineral contribution from the paper support was taken into account when evaluating the p-XRF results obtained for the gouaches. The r-FTIR, p-Raman, and p-XRF spectra obtained

for the paper support of the twenty MoMA plates and the twenty MNAM pages were very similar, confirming that the same paper was used for both reproductions.

PCA was used as an exploratory method to distinguish gouaches of a similar color but potentially different composition by comparing their p-XRF, RS, and r-FTIR spectra. The contribution of the paper support was the same for all the spots analyzed and thus had no impact on the PCA analysis. The example provided in Figure 3 illustrates how PCA was used to distinguish three groups of yellow gouaches based on their p-XRF spectra. The scores and loading plots (Figure 3a,b respectively) extracted by PCA differentiate gouaches richer in either calcium sulfate (Ca, S) or barium sulfate (Ba, S, Sr) from gouaches containing neither extender. The same procedure was repeated to examine the data obtained with r-FTIR and with RS. Ultimately, seven different yellow gouaches were distinguished based on the similarities between their XRF, r-FTIR, and RS spectral signatures. The same analysis was carried out on the rest of the colors, leading to the final conclusion that 39 different gouaches had been used to reproduce *Jazz*.

**Figure 3.** Results of PCA analysis carried out on the p-XRF spectra taken on 23 yellow spots across the 20 MoMA *Jazz* plates. The scores plot (**a**) distinguishes three different types of yellows. The loading plot (**b**) indicates that the distinction is based on the presence or not of CaSO4 or BaSO4 extenders.

This information was used to guide further non-invasive analysis by p-Raman and MFT for each color. It was also used to guide sampling for complementary analysis by μ-FTIR, Raman, and SERS. Table 1 summarizes the pigments identified in each gouache. Tables S1 and S2 in the SI provide a more detailed account of the results obtained for each gouache with the different analytical methods and the prints where each of these gouaches was used. p-XRF, IR, and Raman spectra obtained for the black gouaches are provided in Figure 4 (also included in the Supplementary Materials as Figure S1 in the Supplementary Materials), Figure 5 (Figure S2), and Figure 6 (Figure S3) to illustrate the findings. The spectra for the other colors are provided in the Supplementary Materials (Figures S1–S31).


**Table 1.** Pigments and auxiliary compounds detected and identified across three copies of *Jazz* using a multi-analytical approach (PR49:2: Calcium Lithol Red; PG7: Phthalocyanine green, PY3: Hansa Yellow 10G; PY5: Hansa Yellow 5G; PY6: Hansa Yellow 3G; PY10: Hansa Yellow R; PR3: Toluidine Red; PR4: Permanent Red R) \* *not analyzed*.

**Figure 4.** p-XRF spectra for the black gouaches—(red) Bk1 and (gray) paper support for plate P1 and (green) Bk2 for plate P8. Elements present in both gouaches: Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, and Zn (small amounts of Al, Si, S, Ca, Fe, and Zn are also present in the paper).

**Table 1.** *Cont.*

**Figure 5.** (Non-KK transformed) r- and μ-FTIR spectra of Black 1 (Bk1) and Black 2 (Bk2) alongside IRUG [17] reference spectra for cellulose, (†) Bone black (875, 962, 1038, 1087 and 2013 cm−1) and (\*) Prussian blue (2094 cm<sup>−</sup>1); (‡) silicates/aluminosilicates were also observed (1001, 794 cm−1).

**Figure 6.** Confocal and p-Raman spectra of Black 1 (Bk1) and Black 2 (Bk2) alongside reference spectrum for Bone black [17,25] (D and G broad bands (‡) at 1367 and 1590 cm−1—shifted to 1320 cm−<sup>1</sup> in p-Raman). (†) Quartz (468 cm−1) and silicates (272 cm−1) [17,26] were also observed (355 cm−<sup>1</sup> unassigned \*).

#### 3.1.1. Medium, Extenders, Additives, or Impurities

Traditionally, gouache paints are prepared with a gum rabic binder, white fillers for opacity, and organic or inorganic pigments [27]. A detailed medium analysis was beyond the scope of this work, but gum, though generally difficult to distinguish from cellulose was detected in several samples by μ-FTIR (Figure 7) by the presence of the characteristic O-H bending band at 1650–1620 cm−<sup>1</sup> [28]. The analyses also revealed the presence of a variety of white inorganic compounds in the gouaches acting as extenders, lake substrates, and other potential additives. All are easily detected by p-XRF (key elements indicated in Table S1) and in most cases confirmed by infrared and Raman spectroscopies. Barium sulfate (BaSO4) was the most prevalent extender and was identified by XRF (Ba, S, Sr) [29], FTIR [30], and Raman [31] spectroscopies. Calcium carbonate (CaCO3) was identified by FTIR [32] and Raman [31] as well. Calcium sulfate was present in the gypsum form (CaSO4·2H2O) based on the presence of sulfate vibration bands and OH stretching and deformation bands in the IR [33] and Raman [31] spectra. Alumina hydrate was identified by μ-FTIR through hydroxyl deformation vibration bands [34]. In the magenta, violet, and red gouaches, and in the yellow and orange gouaches containing PY5, a significant amount

of aluminum was detected by XRF, but the source could not be identified. A small but varying amount of zinc was detected in all the gouaches by XRF, possibly as an additive or contamination.
