## 3 Results
### Organic Pigment Analysis
#### 3.2.2 Fluorescence Microscopy Observation  
Fluorescence microscopy is a specific analytical tool for the recognition and mapping of different mordant and organic binders in a complex layer structure [22]. When observing samples using an UV excitation light, different binders may exhibit characteristic colors and fluorescence intensities, which can provide information about their spatial distribution within the painting layer. On the basis of previous studies and the Py-GC/MS results, it is possible that dry oil and red lac resin were mainly used as the adhesive medium in the gold gilding of Kizil. Furthermore, pyrrole substances were also detected in samples #5, #8, #9, and #10, indicating the presence of proteins. In order to distinguish different kinds of organic materials in the cross-section, fluorescence microscopy observation under UV light was carried out. In this study, mock-up samples were prepared using minium mixed with oil and animal glue, and observed under a UV lamp at the central wavelength of 365 nm after the staining with 0.05 wt.% rhodamine B (RhOB) ethanol solution and 0.06 wt.% fluorescein isothiamate (FITC) acetone solution, which were used in the identification of fat and protein substance [23]. Figure A4 shows that rhodamine staining led to orange-red fluorescence for oil, while pigments with animal glue (protein) showed green/white fluorescence after FITC staining. The UV fluorescence images, as well as the stained UV micrographs, are collated in Figure A5a,b. The fluorescence color of sample #1 was not obvious; the fluorescence in the image after FITC staining indicates where protein may have been used. Orange fluorescence could be observed in the UV images after RhOB staining except for #1 and #9. The yellow fluorescent layer in #2, #3, #4, #6, and #9 was located below the gold foil and mixed with lead red. Samples #3, #7, #8, and #9 had lac resin below the foil. The bright-white fluorescent layer in samples #3 and #10 was particularly evident, indicating the presence of the dry oil used as mordant to bind the foil. In sample #1, gold existed as powder, which may have been mixed using animal glue due to the lack of obvious orange fluorescence material shown in RhOB staining. In addition, the layers above the foil could be seen clearly under fluorescence microscopy in #2, #6, #7, and #9. The upper layers over gold foil of #2 and #7 showed green fluorescence under UV light after FITC staining, indicating possible proteins; the upper layer of #6 was red under UV, indicating lac resin. The upper layer of #9 was orange after RhOB staining, probably suggesting a layer composed of lead red with oil. The results of fluorescence microscopy observation are summarized in Table 4.  
In contrast, as shown in Figure 5, red fluorescence could be observed in samples #3, #7, and #9 (a small amount of fluorescence could be seen in sample #8), most likely originating from the lac resin which presented orange fluorescence under UV light [24,25]. In addition, partial orange fluorescence was identified from the upper film layer of foils #4 and #6. Samples #6 and #8 were not found to be characteristic of lac resin in the Py-GC/MS results, probably because too few residues were present to be detected.