## 4 Discussion  
On the basis of the above analysis, the gilding decoration of the Kizil Caves murals could be divided into two categories: gold powder and gold foil (Figure 6). Sample #1 and the uppermost layer of #5 belonged to the category of powder. Gold powder was applied to the plaster base with minium, gypsum, and dye, using animal glue as a binding medium, similar to a pigment, which is different to the reported composition of draining and piling gold, i.e., a mixture of gold powder and calcite [26; 27]. In addition, as also seen in Byzantine art (relatively close time), there were only a few occasions of gold powder usage noted. The gold powder was probably transformed from gold-quicksilver amalgams or was a result of gold leaf residue [28].  
The samples analyzed in this study predominantly belonged to the foil category. According to the foil modrant, this category could be divided into two main groups: an oil base, e.g., the modrant of samples #2, #4, #6, and #10, which was composed of drying oil or red pigment mixed with drying oil; lac resin on top of an oil base as foil modrant, e.g., #7, #8, and #9. In addition, there was a more unusual sample, i.e., #3. This sample had a gold foil base layer with a layer of dry oil on top of the oil base + lac resin. Given that this sample was taken from a fine gilding decoration on a red background, it is assumed that the lac resin here was applied over minium to alter its color to dark red, and that dry oil was the binding material for the gold.  
The use of oil for gilding was common in both China and Europe, with tung oil being detected in several polychrome artifacts [17; 29; 30] and siccative oil and lead-based compounds usually employed for oil-based (mainly linseed oil) modrant gilding [31]. Even through the analysis in this paper could not reveal the exact type of drying oil, it was hypothesized to be safflower seed oil or pecan oil, with reference to the oils used in this region in ancient times. The use of lac resin for gilding is a special technique which has not been reported so far. Red lac resin was used as colorant in Asian mural paintings [26; 32; 33], but there have been no records for gilding. This is a reflection of the prosperity of the ancient trade in Kucha (lac was originally produced in South Asia [34; 35]), which contributed to an innovation of techniques.  
Figure 5: Sample with orange-red fluorescence emission observed under UV.  
One of the more striking features of the gilding of the Kizil Grottoes is the presence of a surface coating on the gold foil. Most of the samples analyzed had surface coatings  
Figure 6: The identified result of each layer and a schematic diagram of the layer structure.  
(only samples #8, #3, and #10 did not). This indicates that the ancient painters of the murals in Kizil not only used the golden luster of gold leaf to decorate the murals, but also used it as a ground layer to apply colorant for a special visual effect. This shows the diversity of the decorative effects of gold leaf. The materials used to coat the foil were also quite diverse, with samples #4 and #6 being coated with organic lac resin, samples #2 and #7 being coated with animal glue mixed with red hematite or orange-red lead, and sample #9 being coated with a mixture of dry oil, lead pellets, and gypsum. This diversity not only reflects the various decorative skills of the ancient painters, but is also an important physical manifestation of the ancient Kucha region as an important crossroad on the Silk Road.  
Dyes were also clearly observed with gypsum in the layers under the foil of #4 and #5. Gypsum and indigo were both found in #5 using Raman spectroscopy, and gypsum mixed with some kind of red dye was observed in #4. Caves No. 47 and No. 48 were dated in the fourth century AD; hence, it is possible that the ancient painters had mastered the use of organic dyes and inorganic pigments to show rich colors around this time.