View a Copy of This Licence, Visit Mmons.Org/LicenSes/By/4.0
Total Page：16
File Type：pdf, Size：1020Kb
Li et al. Herit Sci (2020) 8:64 https://doi.org/10.1186/s40494-020-00410-2 RESEARCH ARTICLE Open Access Degradation of emerald green: scientifc studies on multi‑polychrome Vairocana Statue in Dazu Rock Carvings, Chongqing, China Zhimin Li1, Lele Wang2, Huili Chen3 and Qinglin Ma4,5* Abstract The spontaneous degradation of pigments in painting may occur depending on environment, especially the sur- rounding 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 map- ping, 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 identifed as pigments. Moreover, lavendulan (NaCaCu (AsO ) Cl 5H O) and mimetite (Pb (AsO ) Cl) are degradation products of 5 4 4 · 2 5 4 3 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 3 arsenite ((AsO2)−), migration of arsenate ((AsO4) −) 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 identifcation of green copper-arsenic containing pigments and characterization of degradation in complex conditions. Keywords: Dazu Rock Carvings, Pigment, Degradation, Raman mapping, Emerald green, Lavendulan, Mimetite Introduction [1, 2]. Degradation of pigments can be visually perceived Degradation phenomena occur in the paintings where as color changes or confrmed as structural modifca- original pigments, binding media, ground layer and sub- tions by employing modern analytical or micro-analytical strate to undergo chemical and physical transformations, methods. leading to discolouration or deterioration of materials. Studies on degradation processes and products of pig- Tese processes can be initiated by internal factors, for ments inside museum on oil paintings, manuscripts and example the characteristics of pigments with organic in an outdoor environment on polychrome statue and binders and other materials, or by external factors, which wall paintings are well known and documented. Azurite sufer from environmental conditions (relative humidity, (2CuCO3·Cu(OH)2) and malachite (CuCO 3·Cu(OH)2), as illumination, temperature, etc.), biological activity, pollu- the best representatives of basic copper salts (carbonates, tion or human interventions, or several of these together chlorides, sulphates, etc.), are widely used as mineral blue and green pigments. In the case of artifacts, the pub- lished data indicates the transformation of azurite and *Correspondence: [email protected] malachite into green basic Cu chlorides in the presence 4 International Joint Research Laboratory of Environmental and Social Archaeology, Shandong University, Qingdao 266237, Shandong, China of chloride and humidity [3–5]. Arsenic-based pigments Full list of author information is available at the end of the article such as orpiment (As 2S3) and realgar (As 4S4) are highly © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco mmons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Li et al. Herit Sci (2020) 8:64 Page 2 of 12 appreciated in China for yellow and red to red–orange Hall of Jinci Temple (晋祠), Shanxi Province. In these tones. Arsenic sulfde pigments are bound to shift color papers, it is still unclear that lavendulan may be formed exposed to visible light and realgar undergoes photo- by the degradations of other pigments containing Cu and induced polymorphism and becomes friable, assuming As. hues from bright yellow pararealgar (As4S4) to colorless Naturally, taking physical and chemical conditions of arsenolite (As2O3) whereas the photo-oxidation of orpi- their usage and storage into account, pigments in wall ment results in arsenolite [6–8]. Lead pigments e.g. lead paintings and polychrome statues are much more sub- white (2PbCO3·Pb(OH)2), red lead (Pb3O4) are employed jected to the environment, even in complex conditions owing to bright colors and high covering power in paint- including high humidity or dissolved salts. Even some ings. Lead pigments turn black caused by reaction with of these are well studied above, more tracking and new sulphur-containing compounds to black galena (PbS) and investigations are still needed and crucial for the conser- oxidation to the black-brown plattnerite (PbO 2) [9, 10]. vation feld. Vermillion (HgS) is extensively used as bright red pig- In 2016, China’s National Cultural Heritage Admin- ment. It darkens attributed to the formation of a black istration launched a large-scale perseveration demon- compound, metacinnabar (β-HgS) when exposed to light stration projects for the Buddhism Grottoes in Sichuan − and HgCl2 when Cl ions present in the pigments [11, Province and Chongqing Municipality. At that time, a 12]. subproject aiming to conserve and restore the Niche of A popular man-made brilliant green pigment, emerald Sakyamuni Entering Nirvana Statue in Great Buddha 大佛湾 green (Cu(C2H3O2)2·3Cu(AsO2)2), containing copper and Bend (Dafowan, ) and Polychrome Statues in arsenic is found to be unstable in paints. Emerald green Little Buddha Bend (Xiaofowan, 小佛湾) at Dazu Rock darkens as it mixed with sulphur-containing compounds Carvings (大足石刻) was undertaken by the Chinese from the atmosphere to form brown copper sulfdes Academy of Cultural Heritage. Located outdoors in a (CuS) [13]. Emerald green degrades in oil paintings rich long time, the statues have been inevitably sufering from with a relatively high concentration of monoacids, diac- surface weathering, water seepage, rock instabilities, bio- ids and resin acids, forming copper soaps (4Cu(FA)2) logical erosion, salt corrosion, etc., so the scientifc inves- (FA = fatty acids) and arsenic trioxide (As2O3), leading tigation and conservation is becoming urgent. Te frst to transparent discolored brown layers [14]. Emerald step is to investigate the characteristics of the paintings green is also found to transform to mineral lammerite on caved sandstone surface for determining the appro- (Cu3(AsO4)2) in wall paintings as a result of the migra- priate intervention treatment. Terefore, the polychrome tion of arsenic [15]. Recently, rare lead arsenate mineral, statue Vairocana Statue in Little Buddha Bend at Dazu mimetite (Pb5(AsO4)3Cl), is shown to be degradation Rock Carvings of Chongqing municipality was chosen as product of multistep alteration pathway of pigments a demonstration example. 3− orpiment and realgar and arsenate ions ((AsO4) ) to In this study, a coupling of Raman spectrum and map- precipitate with Pb2+ ions [16–18]. ping, OM, and SEM–EDS have been applied to identify Lavendulan (NaCaCu5(AsO4)4Cl·5H2O), an uncom- the composition of original pigments and signs of their mon copper arsenate mineral, was originally described degradation in cross sections of color layers from paint- by Breithaupt in 1837 from Annaberg. It was frst found ings of polychrome statue Vairocana Statue. Te infor- by Vogel in 1853 at Joachimsthal, and then by Goldsmith mation obtained can accelerate a better understanding of in 1877 from San Juan, Chile [19]. Te identifcation of the transformation processes which afect the history and lavendulan specimen is confrmed by single crystal X-ray stability of paints. Material identifcation of layer stratig- difraction techniques [20]. Raman spectroscopy has also raphy can confrm the date of paintings executed. reliable performance to characterise lavendulan, and infrared spectra reported [21]. Tis mineral was found Materials and methods as alteration products in ancient slag heaps and used for Materials colourant efects in cosmetics of ancient Egyptians [21]. Outstanding as the pinnacle of Chinese cave temple art Te discovery of lavendulan in Chinese painting is sel- from the 9th to 13th centuries, Dazu Rock Carvings were dom reported. It is detected in the wall painting of Cave listed as World Cultural Heritage by UNESCO in 1999. 11 of Yungang Grotto (云冈石窟), Shanxi Province and Te sites located in Dazu District, Chongqing, China, colored drawing on timber structure of Kumbum Mon- are comprised of the clifside carvings at Baodingshan, astery (塔尔寺), Qinghai Province [22]. Lavendulan is Beishan, Nanshan, Shizhuanshan and Shimenshan. Dazu also found in paint layers of statues from Anyue Grotto Rock Carvings are remarkable for their aesthetic quality, (安岳石窟), Sichuan Province [23]. We fnd lavendulan their rich diversity of subject matter, both secular and on paintings of Maidservant fgures in the Saint Mother’s religious, and the light that they shed on everyday life in Li et al. Herit Sci (2020) 8:64 Page 3 of 12 China during this period.