Analysis Of Organic Pigments In Grottoes Murals And Study Of The Mechanism Of Oxidation In The Atmospheric Environment

Grottoes murals are colourful interpretation of grottoes temples and grottoes architecture,and the pigments used to paint them are important vehicle for this colourful interpretation,as well as an invaluable source for the study of ancient Chinese technology,culture,religion and the history of Chinese painting.The grottoes murals have been in existence for a very long time,and the harsh conditions in which they have been preserved have led to oxidation and discolouration of the pigments and the loss of pigment layers,which has resulted in very few grottoes murals surviving intact.The analysis and identification of pigments used in grottoes murals,especially organic pigments that are susceptible to oxidation and discolouration,and the study of the mechanisms of oxidative discolouration in the atmospheric environment,are of great value and significance for the preservation of the historical and cultural heritage of the grottoes murals,which are of great value to all mankind.In this study,13 samples of grottoes murals were analysed and identified by using a scanning electron microscopy-energy spectroscopy analyser,and the existence of pigments such as gold foil,red lead,cinnabar,azurite blue,atacamite,lazurite,lead yellow,realgar,orpiment and yellow kaolinite was detected.It also avoids the interference in the detection of organic pigments in mural samples.This paper established a generally applicable high performance liquid chromatography technique for the analysis of organic pigments and grottoes mural samples and in antiquity using high performance liquid chromatography,and applies it to the detection of organic pigments that may have been used in 13mural samples.Finally,the organic pigment safflower yellow was detected in BDLG mural sample;isatin and indigo were detected in TTS-0816-6 mural sample and TTS-0620 mural sample;safflower yellow and indigo were detected in TTS-0624 mural sample.This study combined quantum chemical theoretical calculations and experimental studies to provide an in-depth study of the derivation mechanism,reaction processes and reaction products of the oxidative degradation of indigo in atmospheric environment polluted by O₃,and the indigo from the TTS-0620 mural sample.We hypothesised that the isatin detected in this mural was caused by the oxidative decomposition of indigo by O₃.In this study,the reaction mechanism of the oxidation of indigo by O₃ was modelled using quantum chemical methods,and the reaction sites of the oxidation and decomposition of the indigo molecule were determined to be at its C=C and N atoms,and the end products of the oxidation and decomposition of indigo by O₃ were identified as isatin,C₁₆H₁₀N₂O₃ and C₈H₆NO₃.These findings provide a theoretical basis and technical support for the subsequent study of the derivation of natural organic pigments in an increasingly ozone-polluted atmosphere and for the better protection of such precious historical and cultural heritage as ancient Chinese grottoes murals.