3.1.5. Red

The analytical results showed that Liu Kang employed three types of organic red pigments in most of the examined paintings. They were used as primary reds or in combination with other pigments added to modify hue. An Sn-containing organic red on starch substrate was confirmed with SEM-EDS, PLM, and FTIR in *Village scene* (sample 9) and *Landscape in Switzerland* (sample 35, layer 1) and resembled the aforementioned mixture from *Breakfast* (sample 10).

The FTIR of sample 9 from the *Village scene* showed low intensity peaks at 1562 and 1545 cm−1, which are indicative of organic red; however, an in-depth molecular characterisation was not possible due to overlapping signals of other compounds present in the investigated sample. A similar issue concerns the identification of starch. Although starch was well observed with PLM, only two FTIR peaks, at 3280 and 1649 cm−1, were considered conclusive. Additionally, detection of Fe and co-location of Pb-, Cr-, and Ca-signals, suggested that an organic red was used in a combination with red iron oxide (absorption peaks at 1026, 1007, 935, 911, 798, 777, and 753 cm<sup>−</sup>1), chrome yellow (absorption peaks at 1031, 844, 834, and 624 cm−1), chalk (absorption peaks at 1410, 870, and 712 cm−1), and probably lead white (1410 and 677 cm<sup>−</sup>1) (Figure 9).

The sample 35 from *Landscape in Switzerland* contains two distinguishable layers. Based on the PLM, SEM-EDS, and FTIR, the top red is composed mainly of red iron oxide with an admixture of Sn-containing organic red, detected by absorption peaks at 1620, 1563, 1555, 1529, 1471, 1290, 1265, 1246, and 588 cm−1, and chalk. This top layer was applied over dry paint predominantly consisting of the organic red, detected by peaks at 1621, 1562, 1552, 1531, 1310, 1265, 1247, and 604 cm<sup>−</sup>1. In addition, a presence of starch was confirmed with PLM observation and FTIR by absorption peaks at 3294, 1639, 1370, 1341, 1247, 1204, 1150, 1077, 1016, 931, 861, 759, and 704 cm−1, while Sn-based substrate was confirmed with the SEM-EDS. Moreover, the analysed paint contains some admixtures of lead white and probably Cr-containing yellow(s) (Figure 10). The MA-XRF map of Sn distribution in *Landscape in Switzerland* shows that some passages were initially painted with a heavy use of Sn-containing organic red (Figure 4) and finally covered with different colours as visualised on the cross-section of sample 25 (Figure 11).

**Figure 8.** Microscopy images of the cross-section of sample 10, extracted from *Breakfast*, photographed in: (**a**) VIS; (**b**) UV. Circular, blue-fluorescing particles of the starch substrate are visible in layer 4 and marked with arrows (**b**). The PLM pigment dispersion from layer 4 is photographed in: (**c**) plane-polarised light; (**d**) cross-polarised light. The clumps of starch particles are marked with arrows. The corresponding SEM-EDS spectra of the red paint from layer 4 (**e**) shows a strong Sn-signal, suggesting the presence of tin substrate.

**Figure 9.** Microscopy image of the cross-section of sample 9 extracted from *Village scene*, photographed in VIS and UV, followed by SEM-EDS maps showing the distribution of the detected elements. The greyscale corresponds to the intensity of the signal of each element: white equals high intensity, black means low intensity. A high intensity of Sn can be assigned to the tin substrate of organic red while the co-location of Ca-, Cr-, and Pb-signals in the centre of layer 2 suggests the presence of chrome yellow.

Another organic red was detected in *Boat near the cliff* (sample 29). PLM allowed the observation of red particles with a low refractive index, which is characteristic of organic reds. An intense orange fluorescence in UV light (Figure 12b) suggests natural madder [38]; however, the SEM-EDS detection of bromine, based on Br Lα<sup>1</sup> and Br Kα<sup>1</sup> signals (Figure 12c), indicates that the red may be a compound related to eosin red commercially known as geranium lake—which is also characterised by the orange UV fluorescence [40–42]. A comparison of the FTIR spectra of the investigated red with the reference sample of eosin Y revealed some degree of matching. Typical FTIR features consistent with the organic red were identified by absorption bands at 3335, 1561, 1455, 1345, 1221, 1174, 981, 877, 802, 766, 717, 667, and 634 cm−1. However, the overlapping bands at 1455, 1174, 981, 802, 717, and 634 are also attributable to lithopone and/or barium white, oil, and acrylic resin, the latter of which is considered to be a varnish and applied during the conservation treatments in 2006 (Figure 13) [43]. Other elements present in the sample, such as Pb, Ba, Al, and S, were difficult to interpret. They can be assigned to lead white admixture and barium white extender. However, it is known that eosin was used to produce lake pigments (such as geranium lake), usually precipitated on an Alor Pb-containing substrate [32,44–47]. Thus, Ba and S could be assigned to barium white, which is a common extender of lake pigments. Nevertheless, a presence of lithopone is suggested based on the FTIR detection of characteristic peaks at 1174, 1116, 1077, 981, 634, and 607 cm<sup>−</sup>1.

**Figure 10.** Microscopy image of the cross-section of sample 35 extracted from *Landscape in Switzerland*, photographed in VIS and UV, followed by SEM-EDS maps showing the distribution of the detected elements. The greyscale corresponds to the intensity of the signal of each element: white equals high intensity, black means low intensity. The high intensity of Sn in layer 1 can be assigned to the tin substrate of organic red, which is probably mixed with chrome yellow based on the co-location of Cr- and Pb-signals. Layer 2 reveals strong Fe- and weak Sn-signals, suggesting a mixture of iron oxide with organic red with tin substrate.

**Figure 11.** (**a**) Detail of *Landscape in Switzerland*, showing the sampling spot; (**b**) microscopy image of the cross-section of sample 25 with the marked area of SEM-EDS elemental analyses; (**c**) corresponding SEM-EDS spectra of the analysed area from layer 1, indicating a strong Sn-signal from the tin substrate of organic red.

**Figure 12.** Microscopy image of the cross-section of sample 29 extracted from *Boat near the cliff* photographed in: (**a**) VIS; (**b**) UV. SEM-EDS spectra of the sample and inset backscattered electron (BSE) image with marked area of analyses (**c**). The spectra shows Br peaks which could be indicative of geranium lake, as well as Pb, Ba, Al, and S, which can be assigned to lead white and barium white extender and/or Pb- or Al-based substrate.

Lefranc marketed the *laque geranium* (geranium lake) with information that it is anilinebased pigment (Appendix A, Figure 4), although recent research identified eosin in its sample from 1926 [48]. Bourgeois Ainé listed *laque géranium* and *rouge géranium* (geranium red) without a chemical description, while geranium lake from W&N was derived from coal tar, according to their catalogue (Appendix A, Figures 2 and 3). Although the investigated red paint from *Boat near the cliff* exhibits features intrinsic to geranium lake, more analyses are needed to better elucidate its composition.

The organic red was also observed with PLM in the sample extracted from *Countryside in France* (sample 7). The sample does not fluoresce in UV, and the SEM-EDS detection of Al might be indicative of Al-containing substrate for the organic red detected with FTIR by peaks at 1617, 1576, 1559, 1506, 1499, 1447, 1417, 1397, 1343, 1303, 1276, and 1256 cm−<sup>1</sup> [49]. However, the insufficient suit of FTIR peaks or low intensity of peaks did not allow a conclusive attribution.

In *Autumn colours* (sample 7) and *French lady* (sample 9), red paints are composed mainly of red iron oxide, modified with minor admixtures.

### 3.1.6. White

UVR photography is a powerful tool for a preliminary differentiation of lead white painted areas from zinc white and titanium white (titanium dioxide). Thus, lead white was observed in almost all paintings, based on its unique ability to reflect UV (Figure 14a,b). Additional SEM-EDS analyses showed that lead white occurs with a calcium carbonate, which was probably added by the manufacturer as an extender. It is worth noting that the examined white from the *Countryside in France* (sample 29) is a ground layer intentionally exposed by the artist during the painting process. It is composed predominantly of lead white with admixtures of barium, zinc, and titanium whites [9], suggesting it is of a different grade from the lead white identified in the artist's white paints [28]. Likewise, the white ground skilfully exposed by the artist in *Boat near the cliff* for describing foamy water is composed of mixture of lead and zinc whites [9]. White brushstrokes that turn dark grey and black in the UVR of *Landscape in Switzerland* and *Boat near the cliff* suggested a

use of UV-absorbent titanium white or zinc white (Figure 14c,d). The latter was confirmed by the yellow-green UV fluorescence and SEM-EDS measurements of the white paint cross-sections from both paintings. Additionally, the MA-XRF of *Landscape in Switzerland* visualised a strong Zn-signal, which correlates with the white painted areas, while the Pb distribution map suggests chrome yellow and admixtures of lead white (Figure 4). The SEM-EDS detection of Ba and S in the sample 22 of *Landscape in Switzerland* suggests a common admixture of lithopone and/or barium white [50]. A minor and trace presence of Ti identified only in the colour mixtures may suggest a commercial admixture of titanium white.

**Figure 13.** ATR-FTIR spectra of red paint from sample 29, taken from the *Boat near the cliff*, with labelled marker peaks of organic red and spectra of reference samples, identifying oil, eosin Y, acrylic resin, and lithopone.

**Figure 14.** VIS and corresponding UVR detail images of (**a**,**b**) *Countryside in France*; (**c**,**d**) *Boat near the cliff*. The UVR images indicate that white house in *Countryside in France* and foamed water in *Boat near the cliff* (marked with red arrows) show a strong UV reflectance attributable to lead white. White brushstrokes on the boat and clouds in *Boat near the cliff* appear dark grey and black in UVR (marked with yellow arrows), suggesting a use of UV absorbent zinc white, later confirmed with SEM-EDS.
