Abstract
The spontaneous degradation of pigments in painting may occur depending on environment, especially the surrounding condition of high humidity and soluble salts. Even some of these are well studied, more investigations are still in need for heritage science. Paint cross sections from Vairocana Statue in Dazu Rock Carvings, one of World Cultural Heritage, Chongqing, China are studied by optical microscopy (OM), Raman spectroscopy (Raman) and mapping, and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDS). Ultramarine blue, emerald green, synthetic atacamite, vermilion, red lead, gold foil, orpiment, cerussite, gypsum and barite are identified as pigments. Moreover, lavendulan (NaCaCu5(AsO4)4Cl·5H2O) and mimetite (Pb5(AsO4)3Cl) are degradation products of emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) and cerussite(PbCO3) originally used as green pigment with mixture. These secondary products are formed via multistep progresses as degradation of emerald green and cerussite, oxidation of arsenite ((AsO2)−), migration of arsenate ((AsO4)3−) and precipitation reaction of these ions with other necessary ions from environmental conditions. Based on the pigment and paint layer stratigraphy analysis, the paintings of the statue have been executed at least four times and the most recent may be after 1850s. The formation of lavendulan give rise to a highlight for identification of green copper-arsenic containing pigments and characterization of degradation in complex conditions.
Highlights
Degradation phenomena occur in the paintings where original pigments, binding media, ground layer and substrate to undergo chemical and physical transformations, leading to discolouration or deterioration of materials
Each layers of these samples are analyzed by Optical microscope (OM), micro-Raman spectroscopy (Raman) and scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM–EDS)
Secondary arsenate minerals, namely lavendulan and mimetite, are formed due to the degradation of emerald green and cerussite in the slightly acid and highly humid conditions. This processes appear to: formation of arsenate and copper ions as a result of degradation of emerald green; migration of these ions including partially arsenate ions towards the surrounding area where dissolution of Pb2+ ions from cerussite and Cl− ions from environment to product precipitation of mimetite; copper ions remaining in their original place together with arsenate ions and Na+, Ca2+, Cl− to form lavendulan
Summary
Degradation phenomena occur in the paintings where original pigments, binding media, ground layer and substrate to undergo chemical and physical transformations, leading to discolouration or deterioration of materials. These processes can be initiated by internal factors, for example the characteristics of pigments with organic binders and other materials, or by external factors, which suffer from environmental conditions (relative humidity, illumination, temperature, etc.), biological activity, pollution or human interventions, or several of these together [1, 2]. Azurite (2CuCO3·Cu(OH)2) and malachite (CuCO3·Cu(OH)2), as the best representatives of basic copper salts (carbonates, chlorides, sulphates, etc.), are widely used as mineral blue and green pigments. Arsenic-based pigments such as orpiment (As2S3) and realgar (As4S4) are highly
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