Paint layers
The structure and materials of the Vairocana Statue are very complex, including sandstone substrate, gilding and paintings. The components of non-degraded sandstones of the rock carving are feldspar, quartz, plagioclase, calcite, and a small amount of clay []. OM and SEM results of cross sections show the samples are build up in multilayers including CS 1, CS 5, CS 10, CS 11, CS 13 and CS 16, and visible layers are numbered 1 ∼ 7 or 8 from the surface to inner in Fig. 2. Table 1 summarizes multi-paint stratigraphy and layer composition of these samples examined by Raman and EDS. The other ten samples illustrate that delamination phenomena are not visible i.e. one paint layer close to ground layer. Blue pigment CS 2, CS3 and CS4 show two distinct layers. The top layer is composed of Ultramarine blue and the bottom layer i.e. ground layer composed of gypsum (by Raman), calcite (by Raman), quartz (by Raman) and clay (by SEM-EDS) as the presence of elements e.g. Na, Mg, Al, Si, S, Ca and Fe consistent with XRD analysis results []. Green layer of CS 6, CS 7 and CS 8 is mixture of lavendulan, mimetite, cerussite, barite, whewellite and gypsum, and the ground layer consist of gypsum, calcite, quartz and clay. Red layer of CS 9 and CS12 is composed of vermilion, and black layer of CS 15 contains carbon. Gold foil layer (∼1 µm) of CS 14 is consist of Au content 96.4 wt% and Ag 2.8 wt%. The gold plastic paint used to bond gold foil are defined as the base layer close to the sandstone substrate. The plastic paint is made of Chinese lacquer and addition of tung oil by use of FTIR and GC/MS [] and divided into three layers (similar to layer 2, 3 and 4 in Fig. 2e) e.g. red lacquer layer mixed with vermilion to high gold foil gloss, black lacquer layer, and gray lacquer layer mixed with quartz and clay to smooth the adhered surface of sandstone.
Table 1 Layer stratigraphy and components of cross sections
Code
|
Layer
|
Colour
|
Identified components
|
Code
|
Layer
|
Colour
|
Identified components
|
CS1
Fig.2a
|
1
|
Blue
|
Ultramarine Blue
|
CS2
Fig.2b
|
1
|
Green
|
Emerald green, lavendulan, mimetite, cerussite, gypsum, barite, whewellite
|
2
|
Gray
|
Gypsum, calcite, quartz and clay
|
2
|
Green
|
Lavendulan, mimetite, cerussite, gypsum, barite, whewellite
|
3
|
Blue
|
Ultramarine Blue
|
3
|
Gray
|
Gypsum, calcite, quartz and clay
|
4
|
Gray
|
Gypsum, calcite, quartz and clay
|
4
|
Blue
|
Ultramarine Blue
|
5
|
Blue
|
Ultramarine Blue
|
5
|
Gray
|
Gypsum, calcite, quartz and clay
|
6
|
Gray
|
Gypsum, calcite, quartz and clay
|
6
|
Yellow
|
Orpiment
|
7
|
Red
|
Vermilion
|
7
|
Gray
|
Sandstone substrate
|
8
|
Gray
|
Sandstone substrate
|
|
|
|
CS5
Fig.2c
|
1
|
Red
|
Vermilion
|
CS11
Fig.2d
|
1
|
Red
|
Vermilion
|
2
|
Orange
|
Red lead
|
2
|
Orange
|
Red lead
|
3
|
Gray
|
Gypsum, calcite, quartz and clay
|
3
|
Gray
|
Quartz and clay
|
4
|
Black
|
Carbon, calcite, lead white
|
4
|
Green
|
Lavendulan
|
5
|
Gray
|
Gypsum, calcite, lead white and clay
|
5
|
Gray
|
Quartz and clay
|
6
|
Orange
|
Red lead, Vermilion
|
6
|
Orange
|
Red lead
|
7
|
Gray
|
Sandstone substrate
|
7
|
Gray
|
Sandstone substrate
|
CS13
Fig.2e
|
1
|
Gold
|
Gold foil (Au 99.3 wt%, Ag 0.7 wt%)
|
CS16
Fig.2f
|
1
|
White
|
Cerussite
|
2
|
Red
|
Chinese lacquer addition of tung oil, vermilion
|
2
|
Gray
|
Quartz and clay
|
3
|
Black
|
Chinese lacquer addition of tung oil
|
3
|
Orange
|
Red lead
|
4
|
Gray
|
Chinese lacquer addition of tung oil, quartz and clay
|
4
|
Gray
|
Quartz and clay
|
5
|
Gray
|
Quartz and clay
|
5
|
White
|
Cerussite, gypsum, calcite, quartz and clay
|
6
|
Yellow
|
Orpiment
|
6
|
Green
|
Malachite, Atacamite
|
7
|
Gray
|
Quartz and clay
|
7
|
Gray
|
Sandstone substrate
|
8
|
Red
|
Vermilion, red lead
|
|
|
|
Green copper arsenite pigments
Raman analysis on cross sections demonstrates that lavendulan and emerald green in the samples investigated, and no other copper-arsenic containing pigments are detected. Lavendulan is the main constituent of green pigment (e.g. Figure 3b) mixed with mimetite, cerussite, gypsum, barite and whewellite (e.g. Figure 3c and Fig. 3d) on CS 5 ∼ 8. However, emerald green is only in the bottom of the green layer (marked 2 in Fig. 2b) of CS 5 (Fig. 3a).
Further observations with OM and SEM-BSE for CS 5 show the top of the green layer (layer 1 in Fig. 4a) losing the green coloration comparing with the bottom (layer 2 in Fig. 4) caused by the absence of emerald green. The lavendulan particles show single plates radiating from center to edges with a diameter of ∼10 µm and spherulites formed by the plates overlapping or intersecting at the center in Fig. 4b and c. Although lavendulan particles occur with color of pale blue (Fig. 4b), their general crystal habit and structure is consistent with that of emerald green [13]. Nevertheless, the natural lavendulan forms bright blue crusts, spherules and thin rectangular platelets which are flexible and have a low Mohs hardness (2.5) and excellent cleavage parallel to the platy face []. These results suggest that lavendulan is a degradation product of emerald green. The elemental maps of the green layer corresponding to BSE image (Fig. 4d) show the distribution of copper, arsenic, sodium, calcium and chlorine (Fig. 4e-i). The arsenic is not confined to the green particles, and migrates throughout the whole green layer, while the copper distribution is limited to particles. This is similar to observation of the migration of arsenic from the arsenic-containing pigments to paint layers [,]. Furthermore, the absence of arsenic in the blue, gold, red and black samples and paints excludes the possibility of arsenic introduced from the surrounding environment to the green paint. An explanation for the presence of arsenic in the green layer could be deriving from degraded emerald green. The distributions of Na, Ca and Cl in green layer also suggest these elements introduced from the surrounding environment and then react with Cu and As to form lavendulan.
Distributions of crystalline phases of a sub area in green layer (2 in Fig. 2b) of CS 5 obtained with Raman mapping show further details in Fig. 5. In this subarea, emerald green is confirmed as green pigment and surround by cerussite as shown in Fig. 5a and Fig. 5b. Next to emerald green (point 1), distributions of lavendulan (point 2, 3 and 4) are quite different (see Fig. 5c). Mimetite is found in the vicinity of green particles and shows a higher abundance in the areas around lavendulan particles (especially point 3) than that around emerald green (see Fig. 5d). These results suggest arsenic derived from the degraded emerald green particles must have reacted with cerussite to form mimetite as secondary products.
Green copper hydroxychloride pigments
Regarding the green layer of CS 16, layer structure and composition are investigated by Raman, OM and SEM-EDS.
Raman analysis shows the existence of atacamite and malachite in the green layer (layer 6 in Fig. 2f) of CS 16 (Fig. 6). Figure 7a, b and c mainly show that numerous atacamite particles appear as aggregates or masses, while malachite particals sporadically occur in the layer. The synthetic atacamite particles are found to be spherical with dark spots in the centre under the PLM [,]. Figure 7b, c, d and g reveal nearly spherical particles, typical for synthetic atacamite. Malachite could be obtained as a by-product of preparation of synthetic atacamite. Artificial copper trihydroxychlorides could be the most popular green pigments for wall painting and architecture from North Dynasty (386–581 CE) until late Qing Dynasty (1840–1911 CE) []. In addition, the element maps of chlorine associated with copper (Fig. 7e-f and Fig. 7h-i) show delamination phenomena in the green layer is invisible, suggesting the green layer was painted only one time other than repainted using the two kinds of green pigments.
Discussion on dating
Emerald green is a brilliant green pigment first synthesized by Willem Sattler in 1814. Emerald green particles exist in a great variety of crystalline assemblages that are organized as agglomerates of crystal platelets in the form of rosettes or spherulites [13]. It was widely used in China as watercolor in pith paper works and in scroll paintings from the 1850s onwards []. Ultramarine blue show rounded anisotropic particles of homogeneous sizes (≈ 2 µm), suggesting the artificial origin of the lapis lazuli [] are used in the out layer of the painted statues. Artificial ultramarine blue was invented by Guimet in 1828 and its commercial production evolved soon after 1830 []. Then ultramarine blue was introduced from Europe into China in late Qing Dynasty (1840 ∼ 1912 CE), and the importation lasted until 1927 when Chinese chemists Dai A and Ling Z synthesized this pigment [].
The statues in the Little Buddha Bend at Dazu Rock Carving was excavated during the Southern Song Dynasty (1127 ∼ 1279 CE) []. Religious paintings in China usually undergo heavy repairs and even re-painting in different period. As the layer stratigraphy shown (Fig. 2) shown, for more than 800 years so far, the Vairocana Buddha statue has been painted at least four times. Concerning theses documents above, the most recent painting of the statue may be executed after late 1850s, based on the date of the wide use of emerald green.
Because the date of the invention and wide use in China of emerald green and ultramarine blue were nearing, they appeared co-localized in Chinese paintings. In addition, consistent with the recent literature on this topic [16,17,18], the rare nature of mimetite minerals strongly suggests that they are secondary products. Once more this information suggests emerald green was used as original pigment other than lavendulan (secondary products).
Degradation progress of emerald green
The Vairocana Buddha statue is located in the Holy Longevity Sites, where the air circulation is poor because of the semi-closed building structure. Relative humidity in the Dazu Rock Carvings is very high and can reach above 85% from July to September, even oversaturated to form condensation water with pH scale at 6 on the surface of statues []. The stone statues carving contain more soluble salts, specifically SO42−, Cl−, Na+ and Ca2+ []. When exposed to such conditions, degradation phenomena of pigments can occur, namely transformation from emerald green to lavendulan in the paint surface. It can be explained as follows:
The degradation of emerald green pigment occur under acidic conditions provided by the environment [15]. Emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) releases acetic acid (CH3COOH) together with arsenite (HAsO2) and copper ions (Cu2+) in the slightly acidic environment (Eq. 1). In the next step, the arsenite ions ((AsO2)−) are oxidised to arsenate ions ((AsO4)3−). The acidic environment and the acetic acid produced in the Eq.1 decrease the pH of the environment so that, in exposure to air, the arsenite ions are slowly transformed to arsenate ions (Eq. 2) [14,]. The last step involves the reaction of (AsO4)3− and Cu2+, Na+, Ca2+ and Cl− which triggers the formation of lavendulan (NaCaCu5(AsO4)4Cl·5H2O) (Eq. 3). The formation of lavendulan needs acidity [], in dependence on pH of environment with condensation water and acid rain.
Cu(C2H3O2)2·3Cu(AsO2)2 + 8H+ → 4Cu2+ + 6HAsO2 + 2CH3COOH (1)
2(AsO2)− + 2H2O + O2 → 2(AsO4)3− + 4H+ (2)
5Cu2+ + Na+ + Ca2+ + 4(AsO4)3− + Cl− + 5H2O → NaCaCu5(AsO4)4Cl·5H2O (3)
Cerussite is identified as extender minerals added to the paint, which enwraps the emerald green by using Raman mapping. The dissolution of cerussite (PbCO3) occur in the aqueous phase which can be enhanced in acidic solution and releases the Pb2+ ions. The acidic groups from environment are sufficient to promote the dissolution process [], with consume of free protons and release of CO2 (Eq. 4) [16]. The solvated arsenate ions ((AsO4)3−) that are supplied from the hydrolysis of emerald green migrating toward Pb2+ ions react with Cl− ions to produce the precipitation of mimetite (Pb5(AsO4)3Cl) (Eq. 5).
PbCO3 + 2H+ → Pb2+ + CO2 + H2O (4)
5Pb2+ + Cl− + 3(AsO4)3− → Pb5(AsO4)3Cl (5)
Simultaneously, calcium oxalate, e.g. whewellite CaC2O4·H2O, is also founded together with lavendulan by Raman (Fig. 3d). Oxalate salts are formed by calcium ions reaction with oxalate ions over a wide pH range, and commonly occurr in art objects. The source of oxalate ions can be described as metabolic products of microorganisms living on the surface of paintings. It is especially relevant for objects in outdoor conditions, e.g. sculptures and wall paintings []. More likely, the statue surface of the Little Buddha Bend is proved to grow mould []. Oxalate ions are also suspected to release during degradation of emerald green in Eq. 1.
Furthermore, this dicovery of lavendulan in Chinese painting is seldom reported. It is detected in the Cave 11 of Yungang Grottoe (云冈石窟), Shanxi Province and colored drawing on timber structure of Kumbum Monaster (塔尔寺), Qinghai Provence []. We also find lavendulan on paintings of Maidservant figures in the Saint Mother’s Hall of Jinci Temple (晋祠), Shanxi Province. In this study, emerald green has low permanence in the environment of high moisture and soluble salt. This work may help the scholars draw attention to the existence of degradation phase when investigating pigments in complex environment system, e.g. emerald green in damp and salt containg condtions. It also help scholars rethink about rare minerals as original pigments or degradation products. Because of wide usage of emerald green in China, rare copper-arsenic containing minerals could have a high propensity to be identified as degradation products in the patings after the late Qing Dynasty. Synthetic pigments is crucial for interpreting the historical context of cultural objects, especially for the dating.