**4. Conclusions**

The study of Liu Kang's paintings from the Paris phase provides information about the artist's choice of pigments and advances our knowledge of his working practice. Selecting artworks from both the NGS and Liu family collections enabled the tracking of the development of the artist's painting technique during the said period. In addition, the artist's family archives provided insights into his painting process. The imaging techniques, such as IRFC and UVR, played a crucial role in the tentative identification of pigments and guiding sampling of the material for further detailed analyses. RTI, XRR, and NIR provided valuable information about artist's painting process by visualising the hidden compositions underlying *Autumn colours* and *Breakfast*. The MA-XRF scanning of *Landscape in Switzerland* highlighted the distribution of elements that are indicative of certain pigments and their role in the evolution of the painting. Additional PLM, SEM-EDS, and FTIR analyses led to the identification of the paint mixtures. The contemporary colourmen catalogues are a precious resource that supplemented the interpretation of certain materials found in the course of paints' analyses.

The interpretation of the collected data unveiled a restricted palette of colours and the preferential use of ultramarine, viridian, chrome yellow, iron oxides, organic reds, lead white, and bone black. These major pigments were accompanied with the artist's admixtures. For instance, cobalt blue was hardly used and was recorded in some blue, green, and black paint mixtures. Prussian blue was used as a tint for green and black colours. Emerald green had a similar role as an admixture of greens. Additions of cadmium yellow and cobalt yellow are interesting as they were found exclusively in *Landscape in Switzerland*. PLM, SEM-EDS, and FTIR were particularly effective with the detection of three types of organic reds. One could be an organic red on tin and starch substrates. The second is probably a compound related to eosin red on Al- or Pb-containing substrate known as a geranium lake. The third is probably an organic red on an Al-containing substrate. However, more analyses are needed to better characterise the organic red pigments. Although lead white is a predominant white pigment, its role seems to have been reduced in favour of the lithopone and/or barium white and zinc white in *Landscape in Switzerland* and *Boat near the cliff*. Artist also liked to incorporate the white of the ground into the final effect. Carbon black was added to other colours to modify their shade. However, deep blacks usually appear in bold outlines and were achieved by mixing carbon black with ultramarine, Prussian blue, cobalt blue, and viridian.

FTIR analyses additionally confirmed the presence of an oil binder in all investigated paintings from the NGS collection. Moreover, the identification of formation of metal soaps and geranium lake pigment will have some relevance for future conservation diagnostics. This is because metal soaps may contribute to the deterioration process of the paint layers, while geranium lake has a strong fading tendency [59,60].

Studies of the artist's technique revealed that he adopted coloured sketching; however, he was also able to skip this stage and effectively create compositions with rapid and decisive brushstrokes. It can be speculated that he had conducted small-scale compositional studies in different techniques prior to painting on the canvas. The subsequent progression of painting process can be characterised by constant and generally successful experimentation with brushwork. Ultimately, Liu Kang's paintings from the Paris phase are defined by continual exploration and learning of different Modernist painting styles. This very mature artistic approach benefitted him upon his return to China, when he felt a strong need to develop his own painting style.

**Author Contributions:** Conceptualization, D.L.; methodology, D.L.; investigation, D.L., T.K., and B.S.; data curation, D.L.; writing—original draft preparation, D.L.; writing—review and editing, D.L. and T.K; visualisation, D.L. and B.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors would like to acknowledge the National Gallery Singapore for allowing us to investigate the paintings and the Heritage Conservation Centre for supporting this study. The authors are indebted to Gretchen Liu for sharing Liu Kang's family painting collection and the artist's archival photographs and Hanna Szczepanowska at West Virginia University for her critical review, which was essential for further improvement of the content. They also wish to thank Roger Lee (Assistant Painting Conservator from the Heritage Conservation Centre) for his assistance at RTI; Kenneth Yeo Chye Whatt (Principal Radiographer from the Division of Radiological Sciences at the Singapore General Hospital); and Steven Wong Bak Siew (Head and Senior Consultant from the Department of Radiology at the Sengkang General Hospital) for enabling the X-ray radiography. The authors wish to thank Bruker Nano GmbH for lending us the MA-XRF (M6 Jetstream) and facilitating the scanning.

**Conflicts of Interest:** The authors declare no conflict of interest.


**Appendix**

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**Figure 1.** ATR-FTIR spectra of green paint from sample 18, taken from *Autumn colours*, with labelled marker peaks of emerald green and spectra of a reference sample of emerald green.


**Figure 2.** Compositions of pigments, listed in the catalogue of W&N from 1928, used for the manufacturing of oil and watercolours. The highlighted details show carmines, varieties of chrome greens, cinnabar green, cobalt yellow, emerald green, and geranium lake.


**Figure 3.** Oil colours, listed in the catalogue of Bourgeois Ainé from 1929. The highlighted details show carmines, a range of madder lakes, geranium lake, geranium red, varieties of chrome greens, emerald green, Scheele's green, and its variant mineral green.

**Figure 4.** Oil colours, listed in the catalogue of Lefranc from 1931. The highlighted details show carmines, geranium lake, a range of madder lakes, varieties of chromegreens, emerald green, Scheele's green, and its variant mineral green.
