*3.1. Pigments*

3.1.1. Blue

Blue painted areas of the examined paintings are very often imaged purple in an IRFC, suggesting a use of ultramarine and/or co-containing blue. The analyses confirmed that the different blue hues primarily involve ultramarine (sulphur-containing sodium aluminium silicate), observed with PLM by particles with low refractive index that appear red with Chelsea filter and by SEM-EDS detection of Na, Al, Si, and S. Admixtures of viridian (hydrated chromium oxide) were confirmed by the detection of Cr and coincident PLM observation of large and rough particles with high refractive index. The presence of ultramarine and viridian is compliant with the purple appearance of the paint mixture in IRFC. Admixtures of cobalt blue (cobalt aluminium oxide) were confirmed with SEM-EDS by the concomitant presence of Co and Al as well with PLM (particles with a high refractive index that appear red with Chelsea filter) in *Boat near the cliff* (sample 26) and *French lady* (sample 20). Very dark blue in *Countryside in France* (sample 5) was probably made by mixing ultramarine with carbon black. It is worth noting that no Prussian blue (hydrated iron hexacyanoferrate) was found in the investigated blue mixtures, despite its discovery in the earlier study [7]. Prussian blue is known for its high tinting strength, which may be easy for artists to handle. It also produces dark greenish-blue shades that Liu Kang probably did not find suitable for painting sky or water. He might also be aware of the Prussian blue's tendency to fade or change in colour over time [23,24].

In almost all examined blue paint samples, except those from *Landscape in Switzerland* and *Boat near the cliff*, a high concentration of Pb seems to point to the consistent use of lead white (lead carbonate) for achieving lighter tints. Meanwhile, the presence of Ba, S, and Zn elements could be related to lithopone (a mixture of barium sulfate and zinc sulfide) and/or zinc white (zinc oxide) and barium white (barium sulfate), a common extender for lead white [25] as well as for viridian [26].
