3.1.2. Green

The acquired PLM and SEM-EDS data revealed that viridian was Liu Kang's consistent choice for green. However, it is very often present in combination with other pigments. For instance, positive identification of emerald green was possible in sample 18 from *Autumn colours* with SEM-EDS detection of Cu and As elements and with FTIR by absorption bands at 1555, 1451, 816, 762, 690, and 635 cm−<sup>1</sup> (Appendix A, Figure 1) [27]. However, based only on the concomitant presence of Cu and As, elements in other green paint mixtures, emerald green (copper acetoarsenite), and/or Scheele's green (copper arsenite) were considered.

According to Bourgeois Ainé and Lefranc's catalogues of oil paints, *Vert Veronese* (emerald green), *vert de Scheele* (Scheele's green), and its variant, *vert minérale*, were available during that time in Paris (Appendix A, Figures 3 and 4). Emerald green was also offered by W&N (Appendix A, Figure 2). Although it is known that some grades of emerald green were commercially adulterated with chromium pigments [28,29], MA-XRF distribution maps of Cr and Cu elements from *Landscape in Switzerland* showed only a partial co-location of Cu and Cr in the green areas (Figure 4). Hence, it could be said that the Cu–As-based green was not modified by the manufacturer but deliberately mixed with viridian where it suited the artist.

The analyses of the light and warm green hues of the investigated paintings confirmed a consistent use of chrome yellow (lead chromate) as an admixture of viridian. This can be exemplified by sample 3 from *Village scene* (Figure 5a). In this sample, besides viridian, chrome yellow was detected with PLM by anisotropic, rod-shaped particles with a high refractive index. The SEM-EDS analyses of the green and yellow clusters of not properly mixed paint indicated a varied intensity of Pb-, Ca-, and Cr-peaks contributing to viridian and chrome yellow, probably extended with chalk (calcium carbonate) or calcium chromate [30]. The presence of viridian in the examined paint is in agreement with the purple imaging of the sampling area (Figure 5b,c).

The co-location of Cr- and Cd-signals recorded with MA-XRF of *Landscape in Switzerland* (Figure 4) suggested an addition of cadmium yellow (cadmium sulfide) to viridian. This observation was corroborated with red UV fluorescence of the yellow particles [11], SEM-EDS detection of strong Cd- and S-signals, and PLM observation of sample 21 from *Landscape in Switzerland* (yellow, anisotropic particles with a high refractive index turn green in crossed polarised filters) (Figure 6). However, the presence of S, Ba, and Zn in the investigated mixture may suggest that, instead of pure cadmium yellow with lithopone and/or zinc and barium whites, zinc-modified light cadmium yellow or cadmopone (coprecipitated cadmium sulfide and barium sulfate) was used [31]. A high concentration of Pb in most of the examined light-green samples may suggest concurrent admixtures of chrome yellow and/or lead white to viridian. A combination of viridian and lead white to obtain a light-green tone was detected in *Breakfast* (sample 4). Meanwhile, strong Zn-signals recorded in the light-green brushstrokes of *Landscape in Switzerland* (sample 20) and *Boat near the cliff* (sample 11) suggest an admixture of lithopone and/or barium white and zinc white.

**Figure 4.** Visible light image and MA-XRF maps of *Landscape in Switzerland*, showing the distribution of the major elements. The greyscale corresponds to the intensity of the signal of each element: white equals high intensity, black means low intensity. The colour maps combine distribution of Cu- and Cr-based pigments and Fe- and Sn-based pigments.

**Figure 5.** SEM-EDS spectra of: (**a**) green and yellow areas in sample 3 extracted from *Village scene* and inset microscopy image of the cross-section with marked areas of analyses; (**d**) dark-green paint from sample 2 extracted from the same painting and inset microscopy image of the cross-section with the marked area of analysis. The detail of the painting shows the sampling spots (**b**) and infrared false-colour image of the same areas, revealing the distribution of viridian (purple) and the mixture of Prussian blue with viridian and lead white (tints of violet) (**c**).

**Figure 6.** Microscopy images of the cross-section of sample 21, extracted from *Landscape in Switzerland*, photographed in: (**a**) VIS; (**b**) UV. The corresponding SEM-EDS spectra of the green paint are seen in (**c**). Red fluorescence of yellow particles and a strong Cd-signal suggested the presence of cadmium yellow.

Based on the SEM-EDS and PLM analyses, dark green was very often achieved by mixing viridian with Prussian blue in combination with lead white and/or chrome yellow. This can be exemplified by sample 2 in *Village scene* (Figure 5d). The SEM-EDS analysis of the dark-green paint revealed that it is rich in lead, chromium, and iron, which can be assigned to lead white, viridian, and Prussian blue. The latter was observed with PLM by dark-blue isotropic particles with a low refractive index that appear dark green with a Chelsea filter. This pigment mixture is consistent with the IRFC imaging, as the dark-violet colour is determined by the purple representation of viridian, combined with a dark-blue representation of Prussian blue (Figure 5b,c). A similar pigment mixture was identified with PLM and SEM-EDS in dark-green paint from *Countryside in France* (sample 11). However, FTIR did not depict any peaks attributable to viridian due to overlapping bands of other compounds; the most intensive peaks of this pigment fell in the range of 500–400 cm−1, behind the spectral range of the measurement. Although the artist did not employ Prussian blue in the blue painted areas, he preferred it for his hue modification of the green colours. Interestingly, in *French lady* (sample 15), in addition to Prussian blue, a cobalt blue was added to the green paint to produce the desired hue.

The green colour, obtained by mixing blue and yellow, was found in *Landscape in Switzerland* (sample 20). The green sampling area appears blue in the IRFC, suggesting the presence of Prussian blue. The SEM-EDS detection of Pb, Cr, and Fe elements, combined with PLM and FTIR analyses, confirmed chrome yellow by absorption peaks at 1061, 967, 826, and 626 cm−<sup>1</sup> and Prussian blue by absorption peak at 2086 cm<sup>−</sup>1. The paint mixture also contains yellow iron oxide observed with PLM (anisotropic yellow particles with a high refractive index). However, its FTIR confirmation was inconclusive due to overlapping bands of chrome yellow. Moreover, there is a possibility that the artist used a commercial chrome green composed of Prussian blue and chrome yellow [26,32]. Such composite paint was available from Bourgeois Ainé in three different hues and from Lefranc in five hues under the name of *vert anglais* (Appendix A, Figures 3 and 4). It was also listed by W&N

as chrome green, available in three hues, and as cinnabar green, available in five hues (Appendix A, Figure 2).

#### 3.1.3. Yellow

The analyses of the samples of yellow paints revealed the use of four different yellow pigments, but it is evident that chrome yellow was a prevailing pigment. It was identified as a principal component of sample 9 from *Countryside in France* and sample 8 from *Village scene*. It appears as an ingredient found together with yellow iron oxide in *Autumn colours* (sample 3) and *Breakfast* (sample 3). Nevertheless, it is difficult to ascertain if chrome yellow and iron oxide were deliberately mixed by the artist on the palette or commercially prepared. It is known that the ochres were tinted during the manufacturing process with the addition of chrome yellow [30,33], which also has its own extenders, such as barium white, gypsum, kaolin, and calcium chromate [30].

Analyses of the yellow paint mixture from *Landscape in Switzerland* (sample 23) allowed features consistent with cadmium yellow or its variants—light cadmium yellow or cadmopone—to be detected. As visualised by the MA-XRF Cd-distribution map, a usage of this pigment for painting yellow areas was very limited; however, it occurs extensively as an admixture in light-green passages (Figure 4).

The SEM-EDS detection of Co and K in the samples of yellow paint from *Landscape in Switzerland* (samples 23, 36) allowed their attribution to cobalt yellow (potassium cobaltinitrite), later confirmed with PLM by observation of isotropic, yellow, and dendritic particles with a high refractive index (Figure 7). The use of cobalt yellow by Liu Kang seems to be unusual as the pigment is known for its undesirable low hiding power in oil medium. Therefore, its main application was usually a watercolour technique [34,35]. Cobalt yellow was available from Bourgeois Ainé and Lefranc only as a dry pigment and in watercolour and gouache paints. However, it appears in W&N catalogues of oil paints (Appendix A, Figure 2).

**Figure 7.** Cobalt yellow particles from sample 23, extracted from *Landscape in Switzerland*, photographed in plane-polarised light.

#### 3.1.4. Brown

Brown passages were painted using predominantly brown and yellow iron oxides. Lighter tints were produced with a small addition of lead white and/or Cr-containing yellow(s). Darker-brown brushstrokes in *Boat near the cliff* (sample 14) contain Ca and P elements that can be assigned to bone black admixture, confirmed with PLM (anisotropic

black and grey particles). In addition, the co-location of Fe and Mn recorded with SEM-EDS suggests the presence of umber. The SEM-EDS and PLM of the dark-brown colour from the armrest of the chair in *Breakfast* (sample 10) suggested a mixture of red iron oxide with Prussian blue and organic red pigment (unique low refractive index). FTIR completed this outcome by detecting of some absorption bands indicative of organic red at 1656, 1623, 1545, 1451, and 1270 cm−1. However, more peaks typical for organic red fall in the range from 1200 to 1000 cm−<sup>1</sup> and were interfered by the presence of red iron oxide. In addition, the paint mixture contains a good deal of starch grains with an extinction cross, which were visible in cross-polarised light (Figure 8a–d). However, FTIR measurements detected only one peak at 3270 cm−<sup>1</sup> that is related to starch, while further identification was hampered by the intensive peak of red iron oxide at 1005 cm−<sup>1</sup> and overlapping characteristic bands of starch at 1014 and 995 cm−1. Starch was frequently added to the organic reds during the manufacturing process to improve its handling properties and to obtain a lighter tint [36,37]. The PLM and FTIR analyses of the paint sample correlate with the occurrence of Sn, detected with SEM-EDS, suggesting the presence of a tin-containing substrate for the organic red, which was usually cochineal lake or brazilwood lake pigment (Figure 8e) [38,39]. A composition containing alizarin crimson and brazilwood on starch and tin substrates was reported in the earlier study of Liu Kang's painting [8]. Lefranc listed a tin-containing organic red *laque anglaise* composed of *carmin* and *oxyde d'etain* (tin oxide); it was one of the most expensive, priced at 22 francs for a No. 6 tube and 9.50 francs for tube No. 2, whereas blanc d'argent (lead white) was at 3.75 francs for a tube No. 6. (Appendix A, Figure 4). *Laque anglaise* from Bourgeois Ainé had a comparable price (Appendix A, Figure 3).
