A multi-analytical approach for the characterization of materials, manufacturing process and damage mechanisms of wall paintings in Samye Temple, Tibet. (November 2022)

• Record Type: Journal Article
• Authors: He, Jing; Zhou, Weiqiang; Hu, Dongbo; Liu, Shaojun; Otero, Jorge; Rodriguez-Navarro, Carlos
• Is Part Of: Dyes and pigments. Volume 207(2022)
• Publication Date: 2022-11
• Subjects: Acrylic coating -- (poly)alkyl methacrylate -- Consolidation -- Nuclear magnetic resonance -- FTIR-ATR -- Wall painting -- Heritage conservation; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2022.110704
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: This research focuses on the multi-technique characterization and identification of degradation mechanisms of the wall paintings at the Samye Temple in Tibet, which is considered the first built temple of Tibetan Buddhism with an enormous cultural significance to the region. Wall painting general stratigraphy was thoroughly analyzed by Optical Microscopy (OM), Scanning Electron Microscopy – Energy Dispersive X-Ray Spectroscopy (SEM-EDS) and X-ray Powder Diffraction (XRD). Results reveal a clear sequence, from the interior to the surface, consisting on an adobe wall support, a coarse mud plaster, a fine mud plaster, a white ground layer, a surface paint layer, and a coating layer. The identification of natural and/or synthetic pigments and possible re-painted areas was analyzed by polarized light microscope and SEM-EDS, and subsequently studied using Raman spectroscopy. Results reveal the mineral pigments include red cinnabar (HgS) and red lead (Pb3 O4 ), blue azurite (Cu3 (CO3 )2 (OH)2 ), green malachite (Cu2 CO3 (OH)2 ), yellow orpiment (As2 S3 ), and a gold layer (gold powder). White and black pigments are magnesite (MgCO3 ) and carbon, respectively. Additionally, the external surface coating varnish found on the top of the paint layer was also analyzed by Transmission Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance mode (FTIR-ATR), Proton Nuclear Magnetic Resonance (H 1 NMR), Thermogravimetric analysis (TG), Mass Spectrometry (MS) and SEM-EDX analysis. Results show main components of this coating include acrylates and, possibly, silanes, revealing a recent conservation intervention activity. The hydric expansion/contraction of clay minerals in the adobe support and mud plaster layers and the rigid and impervious acrylate coating layer could have accelerated the deterioration and be the responsible for the development of cracks both at the interface of the different support layer and normal to the wall painting surface, resulting in massive losses. These findings are relevant to better understand the traditional manufacturing process of historic wall paintings in Tibet, which is not fully understood yet, and serve to further conservation activities of this historically rich temple. Graphical abstract: Image 1 Highlights: Study of Tibetan wall paintings unveils materials, craftsmanship and decay process. Pigments and surface coatings identified using H 1 NMR/MS/FTIR/Raman and SEM-EDS. A modern polymer (polyacrylate) was the main component of the surface coating. Deterioration due to expansion of clays and rigid (poly)acrylate coating.