Identification of Organic Materials Used in Gilding Technique in Wall Paintings of Kizil Grottoes

Abstract Red and yellow organic resinous layers were observed beneath the foil decoration of Cave 171 (5 th ‐7 th Century A.D.) in Kizil Grottoes, the earliest Buddhist grottoes in ancient Central Asia. Liquid chromatography‐mass spectroscopy and pyrolysis gas chromatography‐mass spectroscopy results demonstrated the presence of lac resin and typical pyrolysis of drying oil. In order to overcome the difficulties in confirming the organic materials in different gilding layers, micro‐Fourier transform infrared spectroscopy (μ‐FTIR) and desorption electrospray ionization‐mass spectrometry (DESI‐MS) scanning were performed on the painting cross‐section. High contents of di‐carboxylic acids, which are the typical markers of degraded drying oil, are detected in the upper yellow layer and lower red ground, indicating the drying oil was mixed with minium (Pb 3 O 4 ) as binder for preparing the substrate as well as the mordant for gilding, and lac resin was painted over minium as the background color.