Investigation of the painting materials in Zhongshan Grottoes(Shaanxi, China)
© Egel and Simon; licensee Chemistry Central Ltd. 2013
Received: 21 February 2013
Accepted: 21 August 2013
Published: 11 September 2013
This paper reports on the characterization of paint samples from polychrome sculptures in the main cave of the Zhongshan Grottoes, China. Optical Microscopy (OM), Environmental Scanning Electron Microscopy in combination with Energy Dispersive X-ray analysis (ESEM/EDX), Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy were carried out in order to study the stratigraphy of the sculptural polychromy and to determine the painting materials.
Minium Pb3O4 and mercury sulphide HgS, cinnabar or its synthetic form vermilion were found as red pigments. Two mixtures were used to produce a rose color: lead white Pb3 (CO3)2 (OH)2 with minium and hematite added to gypsum. Yellow was attributed to an ochre. The green paint layer has been identified as botallackite [Cu2(OH)3Cl], an isomer of atacamite and paratacamite. Copper oxalate was also found in this green paint layer and calcium oxalate were detected in a numerous of paint layers without restriction to any specific colors.
Pigments and their use as mixture or as overlapping different paint layers in Zhongshan Grottoes were identified on a selected number of samples. Over painted areas could be identified and two reasons could explained the blackening of the paintings: a loss of the fragilized colored paint layer, which make the underneath black paint layer visible and a darkening of the upper paint layer, due to the burning of the candles in the temple.
Furthermore, FTIR analysis performed on the samples give the indication of an oily binder.
Zhongshan Grottoes have an important historical and artistic value and has been listed among the key cultural sites of China since 1988. A partial copy is shown in the Shaanxi Historical Museum in Xi’an.
The investigation concentrates on the identification of pigments, using complementary analytical techniques for a full characterization of the constituents. Optical Microscopy (OM), Environmental Scanning Electron Microscopy in combination with Energy Dispersive X-ray analysis (ESEM/EDX), Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy were carried out in order to study the stratigraphy of the sculptural polychromy.
Results and discussion
Analytical results from the samples described in this work
Paint layers/colors (max. thickness in μm)
ESEM/EDX (main elements)
L3: Brownish (20)
L3: Si, Al, Ca
L2: Green (90)
L2: Cu, Cl
L2: Botallackite, atacamite, calcium oxalate
L1: White + orange (90)
L1: Lead white
L3: Green (55)
L3: Cu, Cl
L3: Botallackite, atacamite, copper oxalate
L2: Black (25)
L1 + L2: Si, Al, Ca, S, Fe, K, Mg
L2: Silicate, oxalate, quartz
L1: Brown (180)
L1: Silicate, calcium oxalate
L3: Black (3)
L2: Si, K, Al, Ca, S, Fe
L2 + L3: Silicate, gypsum, quartz, calcium oxalate, probably oil
L3: Carbon black
L2: Rose (36)
L1: Si, Al, K, Fe, Ca (Pb locally)
L1: Kaolinite, calcium oxalate, quartz
L2: Hematite, gypsum
L1: Yellow (63)
L4: White + orange (66)
L4: Lead white
L3: Transparent (8)
L3: only embedding material (FTIR imaging)
L2: White - translucent (55)
L2: Si, K, Al, Ca
L2: Silicate, calcium oxalate
L1: Yellow (120)
L1: Fe, Si, Al, Ca (Pb locally)
L1: Kaolinite, calcium oxalate
L3: White + orange (88)
L3: Lead white
L2: Rose (66)
L2: Si, Al, K, Ca
L2: Silicate, calcium oxalate, gypsum
L2: Hematite, gypsum
L1: Yellow (22)
L1: Si, Fe, Al, Ca, K
L1: Kaolinite, calcium oxalate
L5: Red (22)
L5: Hg, S, Ca, Pb, Si, K, Al
L5 + L4: Silicate, probably oil
L5: HgS, minium
L4: Orange (33)
L4: Pb, Hg, S, Si, K, Al
L4: Minium, litharge, massicot
L3: Brown (310)
L3: Si, Al, K, Fe, Ca
L3: Silicate, calcium oxalate, quartz
L2: Orange (33)
L1: Brown (66)
L1: Silicate, calcium oxalate, quartz
L5: Orange (33)
L5: Pb, Fe
L4: Red (66)
L4: Si, Al, Fe, Ca
L4: Silicate, calcium oxalate, probably oil
L3: Red (11)
L3: Silicate, calcium oxalate
L2: White-brown (70)
L2: Si, K, Al, Ca
L2: Silicate, calcium oxalate
L1: Brown (500)
L1: Ca, Si, Al, K, Fe
L1: Silicate, calcium oxalate
L8: Gold (1)
L8-L4: Silicate, quartz, calcium oxalate, probably oil
L7: Preparation layer (20)
L7: Si, Al, K, Fe
L6: Gold (1)
L5: Preparation layer (20)
L5: Si, Al, K, Fe
L4: Gold (1)
L3: Beige (35)
L3: Si, Al, K
L2: White (70)
L2: Pb, Si, Al, K, Fe
L1: Light gray (66)
L1: Si, Al, K
These two compounds are together with paratacamite and clinoatacamite isomers and have the same chemical composition Cu2(OH)3Cl . Nevertheless, due to their different crystal system, it is possible to distinguish them by their different absorption pattern in the region 3400–3200 cm-1 (hydroxyl stretching vibrations) and the fingerprint region 1100–700 cm-1 by means of FTIR [5, 6]. Atacamite is not confirmed as only one band at 3353 cm-1 suggests its presence and Raman spectroscopy did not revealed the corresponding evidence. Moreover, the FTIR spectra of the green paint layer of samples S1 and S2 show vibrational bands at 1650–1620 cm-1, 1360/1320 cm-1 and 1320 cm-1, which correspond to the carbonyl stretching vibrations of copper and respectively calcium oxalate .
While the use of atacamite is well known in Chinese sculptural polychromies, wall paintings and other works of art [8–11], botallackite was rarely found as a painting pigment. However, it was identified in wall paintings from the Bingling Temple near Lanzhou, Ganzu Province  as well as in a painted sculpture from Yulin Grottoes . In the context of degradation and corrosion of bronze objects, the presence of botallackite was also reported . The rare cases in which botallackite has been characterized could be explained by its lower stability in comparison to the other copper trihydroxychlorides and its rare naturally occurrence [4, 15]. The spherical form of the crystals, as seen in Figure 4b, suggests a synthetic origin of the pigment, rather than a source in the natural mineral .
The three green paint samples, analysed by Cauzzi et al. were identified as a mixture of atacamite and malachite. The samples were taken from the lotus-leaves basement up-holding one of the Buddhas . It can therefore be assumed that the green areas of the main cave of Zhongshan Grottoes were painted with different green materials: either a mixture of atacamite with malachite (lotus leaves) or with the pigment botallackite (robe of the Buddha statues).
Samples S1, S4 and S5 (L1, L4 and L3 respectively, Table 1) show a mixture of a white and orange pigments, which, depending on the proportion of the orange pigment can result in a slight rose paint layer (Figures 4b, 6e and 6f). This rose tone consists of a mixture of lead white Pb3 (CO3)2 (OH)2 with minium Pb3O4, determined by means of FTIR and Raman spectroscopy respectively.
The similar stratigraphical structure of samples S3 and S5 (L1 and L2, Figure 6c and 6f) and the thin dirt layer observed in the cross-section of sample S5 between L2 and L3, let suggest that L3 in sample S5, which is made of lead white and minium, is an over paint. The area, in where sample S5 was taken, was insofar originally made of the rose tone composed by gypsum and hematite, lying on a yellow ochre layer, which gives to the rose a warmer tone (L1, Figure 6c). Likewise, sample S4 was probably first yellow consisting in a yellow ochre overlayed with a translucent paint layer (L1 and L2 of sample S4, Figure 6e), made of a silicate (Table 1). The thin transparent layer, named as L3 in the cross-section of sample S4 is possibly due to a mechanical separation of the layers L2 and L4. In fact, no other absorption bands as those from the embedding resin were registered by means of ATR-FTIR imaging. This separation between the layers is also evidenced by the backscatterd electron image in Figure 9.
Red and orange colors
Degradation of the paintings
Two reasons can explain the darkening of the polychromic surface. On the sampling location of a sample S2, a loss of the powdery upper green layer (L3) could be observed, which let appear the black paint layer L2 underneath (Figure 4c). On the other side, the cross-section of sample S3, shown in Figure 6c, presents as upper layer an extremely thin (approximately 3 μm) black layer (L3). Raman spectroscopy could detect carbon black, which origin can be attributed to the fumes generated by the use of candles and incense in the temple (Figure 6a).
The formation of metal oxalates as degradation products
Oxalates and particularly calcium oxalate were detected in a numerous of paint layers without restriction to any specific colors (ground layer, yellow, green, rose red). Copper oxalate was also found in one green sample consisting of botallackite. Calcium and copper oxalates can easily be distinguished through FTIR spectroscopy with their typical vibrational bands between 1320 and 1360 cm-1. Calcium oxalate presents only one carbonyl stretching vibration at 1320 cm-1, while copper oxalate shows two absorption bands at 1320 and 1360 cm-1. The identification of metal oxalates has been often reported in literature over the past decades [18–21]. Their formation could be the result of the attack of sensitive pigments, such as calcium carbonate by oxalic acid. Several sources for the presence of oxalic acid are conceivable: it is a metabolism product of lichens and fungi, it can be produced by many plants  and the photo-oxidation of organic binding media can conduct to the formation of low molecular-weight dicarboxylic acids, which can again convert in the lowest unit of oxalate acid .
The origin of oxalates in the paint layers from Zhongshan Grottoes is unclear. Nevertheless, they are probably products formed during alteration and degradation processes caused by oxalic acid.
The analysis of binding media was not the issue of this work. Nevertheless, FTIR analysis of some paint layers give some indication for oil as a binder, which is in accordance with the results presenting by Cauzzi et al..
After microscopic observations and documentation of the sample particles using a digital microscope Keyence VHX-500FD, cross-sections were prepared by embedding the samples in epoxy resin (EpoFix® Resin and Hardener supplied by Struers) and further by polishing them with Micromesh® abrasive cloths of decreasing granulometry (from 1500 to 12000 mesh size).
Optical microscopy on the cross-sections revealed the different layers of the stratigraphy. For this purpose, visible (VIS) and ultraviolet photomicrographs (UV, 365 nm excitation wavelength) were taken using an Olympus BX 50 microscope or an Axio Imager A2m microscope (Zeiss).
The elemental distribution through the cross-sections were determined by ESEM/EDX analysis, performed at 20 kV by Quanta 200 ESEM (FEI) equipped with a backscattered electron detector and energy dispersive X-ray analyzer XFlash 4010 (Bruker).
The identification of pigments was carried out by means of FTIR and Raman spectroscopy. FTIR analysis of pigment particles were performed with a Paragon 1000 spectrometer from Perkin Elmer equipped with a microscope. FTIR spectra were recorded in transmission mode in the 4000 – 500 cm-1 range, with a resolution of 4 cm-1. FTIR spectra were compared with spectra from the IRUG (Infrared and Raman Users Group) database and from the own database of the Rathgen Research Laboratory.
The extraction in trichlormethan was made as follow: a little volume (<1 mL) of solvent was added to the paint layer and slightly warmed up at ca. 40°C. The solvent was then evaporated on a cavity microscope glass slide and the residue was measured by FTIR.
Raman spectroscopy was performed on particles or cross-sections. Raman spectra were obtained using a HORIBA XploRA Raman-microscope, equipped with three excitation light sources: 532 nm, 638 nm and a 785 nm laser and compared with HORIBA’s spectral databases.
The painting technique of the polychrome sculptures from Zhongshan Grottoes, in northern Shaanxi province is unwell studied so far. Only one publication could be found in relation to this Chinese cultural site . Eight samples, taken from the main cave were investigated using several complementary techniques, including microscopy, FTIR and Raman spectroscopy and ESEM/EDX, in order to understand the stratigraphy and to characterized the materials used.
Usual pigments as earth pigments (yellow and red ochre), lead pigments (lead white and minium), cinnabar, gypsum and kaolinite were found. However, the green pigment was identified as botallackite, a more rarely indentified material, which is an isomer of the better known atacamite and paratacamite. The discordance in the results obtained between the three green samples examined by Cauzzi et al., all of them characterized as a mixture of malachite and atacamite and the two green samples analyzed here (both identified as botallackite) can be explained by different sampling locations: the lotus-leaves basement up-holding one of the Buddha statues  and the right pillar behind the central Buddha altar respectively. Nevertheless, further examinations should be carried out in order to understand if one pigment would correspond to a latter application. Over painted areas could namely be detected comparing the stratigraphy of three samples, taken in different rose tone regions. A mixture of hematite and gypsum, as well as a yellow ochre overlayed by a translucent paint layer (silicate) were both over painted with a mixture of lead white and minium, resulting in a slight rose color.
Prof He Ling from Xi’an Jiaotong University and Prof. Jiang Baolian for the collaboration during the sampling in situ.
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