Analysis of the Mural Pigments in Murong Zhong Tomb of Tuyuhun Khan

The Tang Dynasty tomb murals are the pinnacle of ancient Chinese tomb murals, which also play a pivotal role in the history of painting. Among them, the murals of the tomb of Khan Murong Zhong are an important component of the murals unearthed from the tombs of the Tuyuhun royal family in the Tang Dynasty, which showed the daily life scenes in Tuyuhun people, which are of vital importance to the study of the politics, economy, culture and custom of that era. In order to better protect and display the murals of Murong Zhong's tomb, a comprehensive analysis of the pigments and binding materials was conducted for the first time, serving as the basis for repairing the murals and reliable methods for identifying ancient murals.

Surface microscopic morphology of the pigment layer and the profile of the pigment layer were observed by the Japanese Kerns VHX-5000 ultra-depth of field three-dimensional microscopic system. After the surface of the mural samples was coated with gold, the mineral particles on surface of the pigments were analyzed to identify the elemental composition by using scanning electron microscope/energy spectrometer (SEM/ESM) (EDAX Octane plus /FEI MLA650F, USA). Raman spectra were recorded with HORIBA LabRAM HR Evolution Raman spectrometer equipped with a diode 532 nm laser. For additional verification, the samples were characterized with XRD (D8 DISCOVER, Japan). The phototube power was 2 kW (Cu target), with an angular reproducibility of ±0.001 degrees. Pyrolyzer (EGA/PY-3030D, Frontier, Japan) and GC-QQQ (Agilent 7890 B/7000 B, USA), were used to analyze binding materials in the mural fragments. In the experiment, the temperature was maintained at 100 ℃ for 1 min, at the heating rate of 50 ℃·min⁻¹, then the temperature was raised to 600 ℃ and then maintained for 5 min. Column oven temperature was kept at 40 ℃ for 1 min, at heating rate of 2 ℃·min⁻¹, then the temperature was increased to 100 ℃ and then 290 ℃, maintaining for 30 min.

Through field investigation and analysis, samples of black, blue, crismon, red, orange and white pigments were collected. From the surface and cross-section photomicrographs, it was observed that the thicknesses of the black pigment layer, blue pigment layer, crison pigment layer, red pigment layer and orange red pigment layer were 11–40 μm, 9–30 μm, 9–28 μm, 11–16 μm, and 28–98 μm, respectively. The black pigment samples contain O, Ca, C, and so on, among which the color element of C was relatively high. Blue pigment contains O, Ca, etc., while crismon pigment samples contain O, Ca, Fe etc., with the color element Fe. The red pigment sample contains C, O, Mg, Fe, etc., including the color element Fe. The orange pigment sample contains C, O, Ca, Pb, etc., including the color element Pb. White pigment samples contain C, O, Ca, etc. O, Ca, Al, K, Si, and Mg may come from soil, while Mg and Na come from soluble salt and other substances. To further confirm the main chromophores of the samples, the pigment samples were analyzed by using Raman spectrometer. The spectra of the black pigment present two broad Raman bands at 1305 cm⁻¹ and 1577 cm⁻¹, confirming that the Pigment was carbon black. The orange pigment could be identified as mimium (Pb₃O₄, Raman bands at 120 cm⁻¹, 163 cm⁻¹, 478 cm⁻¹, and 548 cm⁻¹). The blue pigment has Raman peaks at 546 cm⁻¹, 600 cm⁻¹, 1086 cm⁻¹, 1146 cm⁻¹, 1250 cm⁻¹, 1312 cm⁻¹, 1364 cm⁻¹, 1584 cm⁻¹, and 1701 cm⁻¹, which match with the Raman spectrum of indigo. The crison pigment is identified as hematite (Fe₂O₃, 226 cm⁻¹, 293 cm⁻¹, 410 cm⁻¹, 1088 cm⁻¹, and 1317 cm⁻¹). The characteristic Raman bands at 228 cm⁻¹, 295 cm⁻¹, 413 cm⁻¹, and 616 cm⁻¹ indicated that hematite (Fe₂O₃) was used as a red pigment. The Raman bands of the white pigment are at 284 cm⁻¹ and 1087cm^-1, which can be attributed to calcite (CaCO₃). To better verify the Raman data, the pigment samples were analyzed by using XRD, resulting in similar results. The main components of the pigment samples are calcite and quartz. In addition, Pb₃O₄ was detected in the orange pigment sample. The composition of pigment cements in the murals of Murong Zhong tomb was mainly analyzed by thermal cracking instrument. It was found that the samples contain characteristic compounds of protein cements, suggesting that the binding materials in the murals of Murong Zhong tomb are animal glue.

The pigments used in the murals of Murong Zhong's tomb are inorganic pigments and organic dyes. The black pigment is carbon black, the crismon and red pigments contained iron, the orange pigment is minium, the blue pigment is indigo, and the white pigment is calcite. The calcite is in both the ground layer and the pigment layer. In the process of painting, the mural is blended with pigment and animal glue. New information has been obtained for understanding the technical characteristics of the pigments of the tomb murals of the Tuyuhun ethnic group in the Tang Dynasty, thus providing fundamentals for the development of restoration work.