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Article

Spectroscopic Investigation of a Color Painting on an Ancient Wooden Architecture from the Taiping Heavenly Kingdom Prince Dai’s Mansion in Jiangsu, China

by
Kezhu Han
1,
Hong Yang
1,2,
Gele Teri
1,
Shanshuang Hu
1,3,
Jiaxin Li
1,
Yanli Li
1,
Ersudai Ma
1,
Yuxiao Tian
1,
Peng Fu
4,*,
Yujia Luo
1,* and
Yuhu Li
1,*
1
Engineering Research Center of Historical Cultural Heritage Conservation, Ministry of Education, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China
2
The Imperial Palace Museum, Beijing 100009, China
3
The First Historical Archives of China, Beijing 100031, China
4
Shaanxi Institute for the Preservation of Culture Heritage, Xi’an 710004, China
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(2), 224; https://doi.org/10.3390/min13020224
Submission received: 12 January 2023 / Revised: 29 January 2023 / Accepted: 30 January 2023 / Published: 3 February 2023
(This article belongs to the Special Issue Archaeological Mineralogy)
Browse Figures
Versions Notes

Abstract

:
This research sheds light on the analysis of pigments and adhesives applied on a color painting on wooden architecture in Taiping Heavenly Kingdom Prince Dai’s mansion, located in Changzhou, Jiangsu Province in China. Four samples were collected from the painting above the building beam in the mansion, and the samples were analyzed and identified using a series of techniques, including polarized light microscopy (PLM), scanning electron microscope coupled with an energy-dispersive X-ray spectroscopy (SEM-EDS), micro-Raman spectroscopy (m-RS) and Fourier-transform infrared spectroscopy (FTIR). The results indicate that the red, black, blue, and green pigments were identified to be cinnabar, ivory black, indigo, and phthalocyanine green, respectively. The green pigment was inferred to be a lately repainted pigment based on its production age, suggesting that this ancient building had been refurbished or repaired. Given the good stability and visual effect of this green pigment, it is suggested to be used in future conservation processes. The pyrolysis-gas chromatography/mass Spectrometry (Py-Gc/Ms) results indicate that glue containing protein was used as a binder for the pigment samples, and that walnut oil might have been applied to the wooden architecture as a primer before painting. Our findings can well inform curators and conservators of the selection of appropriate restoration materials if necessary, and also provide data support for conservation of similar ancient buildings in southern China.

1. Introduction

Taiping Heavenly Kingdom Prince Dai’s mansion is located in Jintan County, Changzhou, Jiangsu Province (Figure 1a,b). The mansion was built in around 1862 as the residence for Prince Dai, named Huang Chengzhong (1826–1865). He was one of leaders for the Taiping army during the rebellion that was waged in China between the Qing dynasty and the Taiping Heavenly Kingdom from 1851 to 1864. After the reform and opening up, the mansion was opened to visitors and has become a famous tourist attraction in Changzhou. There are a large number of paintings of great significance in the mansion, and, according to the site survey, the color paintings were directly executed on the wooden beams (Figure 1c).
Color painting, i.e., application of various pigments on ancient Chinese wooden architecture, is an important tangible heritage asset of historical, artistic, and cultural value. Such application can decorate wood texture and inhomogeneous wood color and can also prevent the wood from eroding as a result of an undesirable environment [1]. The use of pigments on wooden objects could be traced back as early as the Spring and Autumn period (approximately 770 to 476 BC). Various painting patterns were gradually invented and adopted, such as motifs of dragons and clouds, as well as brocade patterns used in the Qin and Han dynasties. It is reported that repeated motifs of animals and iridescent clouds were discovered on the coffins in the Mawangdui Han Dynasty Tomb [2]. More painting styles were created in the Song dynasty, e.g., the stacked halo painting method. The development of painting styles and methods had not reached their peak until the Ming and Qing dynasties.
It is known that pigments are categorized into inorganic and organic pigments. Inorganic pigments refer to mineral substances, e.g., the most common red pigments include cinnabar (HgS), hematite (α-Fe2O3), and red lead (Pb3O4); common blue pigments include azurite [2CuCO3·Cu(OH)3], ultramarine [(Na,Ca)8(AlSiO4)6(SCl)2], and Chinese blue (BaCuSi4O10); common green pigments include malachite [CuCO3·Cu(OH)2], emerald [Cu(CH3COO)·3Cu(AsO2)2], and atacamite [Cu2(OH)3Cl); the most-used yellow pigments are orpiment (As2S3) and realgar(As4S4); common white pigments include lead white [2PbCO3·Pb(OH)2] and chalk (CaCO3); and the most frequently used black pigments are graphite (C) and iron black (Fe3O4). The identified pigments on wooden architecture in China are summarized in Table 1. These mineral pigments were often used for paintings on buildings, artifacts of tombs, etc. For example, it has been found that cinnabar was used on artifacts in royal tombs at the Yinxu site, built in the late Shang dynasty as early as 1319–1046 BC [3], and Egyptian blue, yellow ochre, and red ochre were discovered on the Egyptian fresco of the Karnak temples (2040–1782 BC) [4]. The widespread use of the mineral pigments is due to their good stability and durability even in harsh environments such as high-humidity environments, e.g., tombs.
Organic pigments are generally derived from plants and marine animals, such as alizarin, orchids, and insects. Fuchsia, discovered by accident by William Henry Perkin during synthesis of quinine, is the first artificially synthesized dye. Indigo blue (C16H10N2O2), a blue pigment extracted from orchids, is one of the most commonly used fabric dyes over time [5]. The synthesized phthalocyanine pigment in the early 20th century has greatly enriched the blue–green organic pigment variety [6]. Although natural organic dyes are bright, non-toxic, and affordable [7], they tend to be relatively unstable compared with the inorganic pigments, especially when exposed to light, fluctuating conditions, or an acidic or alkaline environment.
Pigments can be identified mainly from three perspectives: morphology observation, elements determination, and composition identification. Firstly, polarized light microscopy (PLM) [8] is used to observe pigment morphology, including particle sizes and optical properties, in order to distinguish artificial and natural mineral pigments, during which a small amount of samples is needed. Secondly, elements in pigments can be determined using methods such as scanning electron microscope coupled with an energy-dispersive X-ray spectroscopy (SEM-EDS) [9], energy-dispersive X-Ray spectroscopy (XRF) [10], Fourier-transform infrared spectroscopy (FT-IR) [11], and X-ray photoelectron spectroscopy (XPS) [12]. Finally, X-ray diffraction (XRD) [13] and micro-Raman spectroscopy (m-RS) [14] are used to further identify crystalline structures and phase composition for pigments, and are non-destructive and highly sensitive. All these methods have been reported in several studies, as summarized in Table 1.
Organic adhesives are another important component to adhere the pigment particles to wood, and thus the adhesives are required to be nearly colorless, to minimize the effect on the original color of the pigments. Water-based pigments are commonly mixed with animal glues, such as fish glue and bone glue [15]. Oil-based pigments were found to be mixed with castor oil, linseed oil, and tung oil [16], and emulsion pigments are mixed with eggs and egg whites [17]. It has been reported that the adhesives can be used alone or mixed with others. For instance, the use of both egg white and animal glue was found in the painting of Longju Temple in Sichuan [18]. Some pigments on the painting have been nearly peeling off, probably due to degradation of the adhesives. In this study, we used pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to detect ageing products and identify the compositions of the adhesives, such as animal glue, egg yolk and glass glue, tung oil and casein [19], etc.
In recent decades, most of the literature has focused on paintings on murals and clay sculptures, while the color paintings on ancient wooden architecture, e.g., at the Taiping Heavenly Kingdom Prince Dai’s mansion, whose painting process is rarely applied in other countries except Asia, has not been fully studied or reported before. Small amounts of samples were taken from the painting in this study, and the composition of the pigments and adhesives used in the samples were analyzed.
Table
Table 1. Summary of identified pigments for paintings on ancient wooden buildings, sampling points, techniques used for pigment identification, and corresponding literature.
Table 1. Summary of identified pigments for paintings on ancient wooden buildings, sampling points, techniques used for pigment identification, and corresponding literature.
ColorDate (Century)PigmentsSampleTechniquesLiterature
Red9thCinnabar (HgS)Foguang Temple HallSEM-EDX, Raman, FI-IRFu et al. (2015) [20]
15thLongju TempleSEM-EDS, RamanChen et al. (2019) [18]
16thDagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
16thYanxi HallSEM-EDS, RamanLiu et al. (2018) [22]
9thRed lead (Pb3O4)Foguang Temple HallSEM-EDX, Raman, FI-IRFu et al. (2015) [20]
18thAltar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
16thHematite (α-Fe2O3)Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
16thYanxi HallSEM-EDS, RamanLiu et al. (2018) [22]
Green15thEmerald [Cu(CH3COO)·3Cu(AsO2)2]building in the Jiangxue PalaceEDX, Raman, PLMFu et al. (2020) [24]
15thWuying Hall of the Imperial PalaceEDX, m-RSShen et al. (2006) [25]
16thDagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
19thSummer PalaceSEM-EDS, XRDMa et al. (2019) [26]
9thAtacamite [Cu2(OH)3Cl]Foguang Temple HallSEM-EDX, Raman, FI-IRFu et al. (2015) [20]
15thLongju TempleSEM-EDS, RamanChen et al. (2019) [18]
16thYanxi HallSEM-EDS, RamanLiu et al. (2018) [22]
18thAltar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
White15thLead white [2PbCO3·Pb(OH)2]building in the Jiangxue PalaceSEM-EDS, Raman, PLMFu et al. (2020) [24]
18thAltar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
15thChalk (CaCO3)Wuying Hall of the Imperial PalaceEDX, m-RSShen et al. (2006) [25]
16thHydrocerussite [2PbCO3·Pb(OH)2·H2O]Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
16thGypsum (CaSO4)Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
9thKaolinite [Al2Si2O5(OH)4]Foguang Temple HallSEM-EDX, Raman, FI-IRFu et al. (2015) [20]
Blue15thUltramarine [(Na,Ca)8(AlSiO4)6(SCl)2]Wuying Hall of the Imperial PalaceEDX, m-RSShen et al. (2006) [25]
16thDagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
16thYanxi HallSEM-EDS, RamanLiu et al. (2018) [22]
19thSummer PalaceSEM-EDS, EDS, XRDMa et al. (2019) [26]
9thAzurite [2CuCO3.Cu(OH)3]Foguang Temple HallSEM-EDX, Raman, FI-IRFu et al. (2015) [20]
15thLongju TempleSEM-EDS, RamanChen et al. (2019) [18]
16thYanxi HallSEM-EDS, RamanLiu et al. (2018) [22]
18thIndigo [C16H10N2O2]Altar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
18thTemple of ConfuciusRamanLi (2015) [27]
Black15thGraphite (C)Wuying Hall of the Imperial PalaceEDX, m-RSShen et al. (2006) [25]
15thLongju templeSEM-EDS, RamanChen et al. (2019) [18]
16thDagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
18thAltar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
Yellow18thLitharge (γ-PbO)Altar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
16thChrome (PbCrO4)Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
18thAltar of AgricultureRaman, ED-XRFLi et al. (2021) [23]
16thRealgar (As4S4)Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]
16thOrpiment (As2S3)Dagaoxuan TempleSEM-EDS, Raman, PLMLei et al. (2017) [21]

2. Materials

Samples were collected from the flaking pigments on the color painting on the northern beam of the main hall in the mansion. In order to keep the painting as intact as possible, small amounts of samples were taken from four areas (Figure 2). The pigment particles were firstly observed using a polarized-light microscope, then elemental analysis was carried out using SEM-EDS, and pigment compositions and structures were finally identified using m-RS and FTIR spectroscopes. The organic adhesive in the pigment samples was characterized using Py-GC/MS.

3. Experimental Methods and Instrumentation

3.1. Energy Dispersive Spectrometry (EDS)

Elemental analyses were conducted using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS, HITACHI SU3500, Japan) with a high performance silicon drift detector and X-ray tube (Rh target); the test range was 11NA–92U. The corresponding results are shown in Table 2.

3.2. Micro-Raman Spectroscopy (m-RS)

Raman spectra were recorded using a LabRAM HR Evolution Raman Microscope (inVia Qontor Renishaw, UK) with an 1800 grooves/mm grating and a coupled m-RS charge detector. The excitation wavelengths were 532 nm and 785 nm. A 50× objective lens was used to observe the pigments, and the acquisition range was from 100 to 2000 wavenumbers (cm−1). Spectra of red, black, and green pigments were obtained using 532 nm, and the Raman spectrum for the blue pigment was collected using 785 nm.

3.3. Polarized Light Microscopy (PLM)

Polarized light microscopy (Olympus BX53M, Shinjuku, Japan) was used to observe pigment morphology under 5–100× objectives, before which the samples were pre-processed. The pigment samples were soaked in ethanol for 30 min and then placed onto the microscope slide. After ethanol evaporation, the cover glass was placed over the samples, and then the heated and melted resin was dropped through the gap between the slide and cover glass to immobilize the sample particle.

3.4. Fourier-Transform Infrared Spectroscopy (FT-IR)

KBr was placed in an oven at 180 °C and dried for 24 h; 150 mg of dried KBr was ground as a blank sample and mixed with 2 mg of ground pigment samples. The sample powders were pressed and then scanned using the FTIR spectroscope (Thermo Scientific Nicolet iS10, Waltham, MA, USA) for detection in the wavelength range of 450–4000 cm−1 with the spectral resolution at 1 cm−1.

3.5. Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS)

About 50 μg of pigment sample was placed into a thermal pyrolyzer, with 3 μL of 20% w/w tetramethylammonium hydroxide (TMAH) added for pre-treatment. The sample was then placed in an autosampler and pyrolyzed at 600 °C after resting for 1 h. After the pyrolysis, the generated products were identified by using GC-MS.
The Py–GC/MS consists of two parts. Parameters of the first part for pyrolysis (Frontier Labs EGA/PY−3030D Koriyama, Japan) include thermal cracking temperature, thermal cracking time, syringe temperature, and interface temperature of the syringe and chromatograph, which were set at 600 °C, 10 s, 250 °C, and 320 °C, respectively. The other part includes GC/MS (Shimadzu QP2010 Ultra, Kyoto, Japan) and a SLB−5MS chromatography column (5% diphenyl–95% dimethylsiloxane). The initial temperature of the oven containing the column was set to 50 °C, and this was held for 5 min. The temperature was then increased at a rate of 3 °C/min to a final temperature of 292 °C that was held for 3 min. The pre-column pressure, the flow rate, and the separation ratio were set at 15.4 kPa, 0.6 mL/min, and 1:100, respectively, and the ionization voltage, scan rate, and mass-to-charge ratio of the mass spectrometer were set at 70 eV, 0.5 s, and 50 to 750, respectively.

4. Results and Discussion

4.1. Pigments

4.1.1. Red

PLM was used to examine the red pigment samples, showing dark reddish color under horizontal polarized light (Figure 3a) or reddish orange under perpendicular polarized light (Figure 3b), suggesting that cinnabar might be present. This was further explored using other techniques. The Raman spectrum of the red pigment sample shows peaks at 254 cm−1 (vs), 285 cm−1 (w), and 349 cm−1 (w) (Figure 4). Comparing the Raman spectrum of the red sample with the standard spectrum, the peak at 254 cm−1 is attributed to the stretching vibrational band of Hg-S and the bands at 285 cm−1 (w) and 349 cm−1 (w) belong to the degenerate E modes, which can be assigned to the normal modes ELO and ETO [28], confirming the presence of cinnabar, which is consistent with the EDS results (Table 2). The lack of interference from the fluorescent background of the Raman spectrum indicates the high purity of the red pigment.
Cinnabar, the most widely used red pigment, is commonly used alone in a number of Chinese paintings in tombs and clay sculptures, e.g., at Xialu Temple [29], The Temple of Venus [30], a Roman temple [31], the polychrome arhat at the Lingyan Temple [32], the Song Dynasty Polychrome Statue-Making of Sage Mother Hall of the Jinci Temple [33], and a polychrome clay sculpture of Hua Yan Temple of the Liao Dynasty [11]. Due to its toxicity, it is able to control the growth of most of pests and mold. The discovery and use of mercury ore in China can be dated back to 6000 years ago [34], and objects containing cinnabar have been found in the Hemudu Site (5000–3300 BC) and the Longshan Site (2500–2000 BC). Excavations at the Yinxu site, built during Shang dynasty, revealed that cinnabar was used to paint Shang oracle bones [35], and in the Song dynasty a small amount of cinnabar was used in the production of medicine [36].

4.1.2. Black

A few types of black pigments are available for paintings, such as carbon black or ivory black (C), iron black (Fe3O4), lead dioxide (PbO2), etc. Black pigment has long been used in historic colored paintings, as summarized in Table 1. EDS analysis results (Table 2) show that carbon occupies the highest proportion of the black pigment sample at 65.56%, so it is highly possible that the pigment is mainly composed of carbon black or ivory black. However, the presence of Ca (9.23%) and P (14.16%) indicate that the black pigment might be ivory black, whose composition includes calcium phosphate, calcium carbonate, and carbon. This was then further confirmed by determining the composition of the sample using a Raman spectroscope (Figure 4). The standard Raman spectrum of ivory black shows bands at 964 cm−1, 1316 cm−1, and 1593 cm−1 [37] for our black pigment sample (Figure 4b); the peak at 964 cm−1 can be attributed to the biomineral apatite [Ca3(PO4)2] [38], the peak at 1591 cm−1 was attributed to the G-band (C=C) [39], while the coordination state at 1312 cm−1 is still controversial. Currently, the most accepted concept is that this peak belongs to C-C bonds with sp3 hybridization (mode A1g) in diamond [40]. Additionally, using PLM, under the horizontal polarized light (Figure 3c), the black pigment sample is irregular and has no visible edge, and under perpendicular polarized light (Figure 3d), no imaging is present, verifying the presence of ivory black. Ivory black is a fine black pigment made by calcining ivory. Due to protection of elephants, the raw materials have been gradually substituted by animal bones, e.g., carbonized cow or sheep bones [41].

4.1.3. Blue

Given that element analysis for the blue pigment shows the presence of N, O, Ca, C, and Si (Table 2), it was initially speculated that the blue pigment might be smalt (potassium glass containing cobalt: 65% SiO2, 16%–21% K2O, and 6%–7% CoO with impurities Ba, Ca, and Fe), ultramarine [(Na,Ca)8(AlSiO4)6(SCl)2], or indigo (C16H10N2O2). The detected Ca (27.29%) and Si (0.5%) might be from impurities such as dust deposition on the painting surface. Additionally, the origin of Ca might be from the addition of calcium carbonate to the pigment, which is a common technology that increases the coverage rate. In order to further investigate the composition of the blue pigment sample, the Raman spectrum and FT-IR spectrum were collected, as shown in Figure 5a,b.
The peaks at 1339 cm−1 (m) and 1539 cm−1 (vs) in the Raman spectrum demonstrate that the blue pigment sample might be indigo, as the spectrum is consistent with the standard and the Raman spectra of the other blue pigments do not show characteristic bands at the two positions, in which the peak at 1339 cm−1 (m) is an in-plane curved N-H stretching band and 1539cm−1 (vs) is attributed to symmetric C=C stretching [42]. In addition, the FT-IR spectrum in Figure 5b further confirms the presence of indigo, where the bands at 3430 cm−1, 1626 cm−1, and at both 1462 cm−1 and 1319 cm−1 belong to asymmetric stretching of N-H, the C=O stretching band, and stretching vibrations of C-C as well as in-plane deformation of C-H, respectively [43]. It can be seen from the PLM images that the blue pigment changes from light bluish to dark bluish under horizontal polarized light (Figure 3e) and is fully extinguished under perpendicular polarized light (Figure 3f).
Indigo is an organic pigment extracted from plants throughout a complicated production process and is relatively stable among organic pigments [44]. In 1880, the German scientist Bayer achieved synthesis of this dye, leading to indigo production beginning to be industrialized in 1897. The use of indigo can be traced back to 6000 years ago by the discovery of several textile fragments at the site of Huaca Prieta, Peru, in 2009 [45]. Indigo was rarely applied in most of ancient Chinese architectural paintings, where mineral pigments such as azurite and ultramarine were more frequently used [25]. It is commonly found in excavated silk weavings in China, e.g., indigo-dyed silks were excavated from the Mawangdui Han tomb of the Western Han Dynasty, from about 2200 years ago [46], and is found on other types of objects in other countries, e.g., funerary objects from the Chupicuaro culture [47] and the pre-classic and classic monumental architecture of the ancient pre-Columbian city of Calakmul [48].

4.1.4. Green

Using PLM, under horizontal polarized light (Figure 3g), the green pigment particles gathering in clusters are bright and fine, without the characteristics of typical mineral pigments, and the particles show light extinction under the perpendicular polarized light (Figure 3h). These properties indicate that the green pigment might be organic. As shown in Table 2, the green pigment sample contains N, Cl, P, Ca, and Cu, in which Cu is the characteristic element and Ca is rarely present in the green pigment. It is assumed that the Ca was also probably from impurities such as dust deposition and that the green pigment might be malachite [CuCO3·Cu(OH)2], atacamite [Cu2(OH)3Cl], Paris green [Cu(CH3COO)−3Cu(AsO2)2], or phthalocyanine green (CuC32N8Cl16).
The Raman spectrum demonstrates that the peaks at 677 cm−1 (s), 1439 cm−1 (s), and 1533 cm−1 (s) in Figure 5c are consistent with the standard spectrum of phthalocyanine green (CuC32N8Cl16). In addition, this pigment shows a unique FT-IR spectrum (Figure 5d) where the band at 2919 cm−1 belongs to the C-H asymmetric stretching vibrations, quite a few sharp peaks [49] in band range of 1700–500 cm−1 are attributed to the stretching deformation of the C-C in the aromatic ring system and the in-plane and out-of-plane bending of the C-H bond of the ring system, and the peak at 949 cm−1 belongs to out-of-plane bending modes of N-Cu and C-Cl [50].
Phthalocyanine green is a synthetic organic pigment which has good stability under UV light and high temperature. Given that phthalocyanine green was found in 1936 [51] and the local government did not give attention to the mansion until 1976 [52], this indicates that the painting was probably repaired or refurbished in mid-20th century (1936–1976). The use of this pigment in paintings of ancient Chinese buildings is rarely recorded, and, to our knowledge, phthalocyanine green was only used in conservation of the paintings in Longju Temple in Sichuan [18] and in early 20th century Russian painting [53].

4.2. Adhesive

It is difficult to directly execute mineral and organic pigment particles on color paintings; therefore, adhesives are used. In this study, the red pigment sample was analyzed using Py-GC/MS to identify the adhesive, the total ion chromatogram of the sample was obtained (Table 3 and Figure 6), and the corresponding pyrolysis by-products are shown in Table 3. Due to the application of the methylation technique, most of generated products were determined to be methylation-related. The results demonstrate that many products were pyrolyzed from proteins [54,55], such as 1H-Pyrrole, 1-methyl- (peak 5), pyridine (peak 6), valine (peak 10), alanine (peak 12), glycine (glue, egg white) (peak 14), and methyl ester (peak 7/21/23/25/26) [56]. Over long-time natural aging, contents of these products are decreased compared with that of the fresh glue. The peaks at No. 13 and 15 indicate the presence of glue marker (fish glue) and protein, suggesting the use of glue mainly containing protein [15]. Over long-time aging, the contents of protein decreased compared with that of the fresh glue, indicating the glue degradation on the flaking area of the painting. As many studies suggest [16], the adhesive used for different pigments are often the same on the same painting; we therefore carried out adhesive identification for the red pigment sample only in this study.
The contents of fatty acids were also detected by Py-GC/MS for the red pigment sample (Figure 7), including monocarboxylic and dicarboxylic fatty acids, both of which are typical pyrolysis products of dry oil [57]. The ratio of palmitic acid to stearic acid (P/S) is often used to differentiate various types of dry oils, i.e., P/S values are 0.9–1.1 for boiled tung oil, 1.3–1.6 for raw tung oil, 1.2–1.5 for linseed oil, 1.6–1.8 for poppy oil, and 1.8–2.0 for walnut oil [58]. The determined P/S value of the dry oil in our sample is at 1.96, indicating that the red pigment sample might contain walnut oil. This assumption is further verified by the presence of methylamine and N, N-dimethyl- (peak 2) in the obtained pyrolysis products (Table 3) [59]. The use of the walnut oil might be related to the craftsmanship of the color paintings on the wooden architecture, where oil is applied in wood surface for better painting before executing pigments. As reported in many studies [60], tung oil is the most widely used, indicating that the use of the walnut oil is a unique painting process for paintings in the mansion for Price Dai.

5. Conclusions

In this study, a comprehensive analysis was performed for four pigment samples collected from a flaking area on a color painting on ancient wooden architecture in the Taiping Heavenly Kingdom Prince Dai’s mansion. A series of techniques were used, mainly including EDS, PLM, m-RS, and FT-IR.
The results show that the red and black pigments, identified to be cinnabar (Hg-S) and ivory black (C), respectively, are consistent with the most-used traditional Chinese pigments as applied in a variety of heritage objects. In addition, interestingly, the blue and green pigments were determined to be organic, specifically indigo (C16H10N2O2) and phthalocyanine green (CuC32N8Cl16) respectively. However, indigo is hardly used in most traditional Chinese architectural paintings, which is one of the unique features of these painting materials. Since phthalocyanine green was invented in 1936, it is inferred that the green pigment was repainted later; we therefore speculate that this ancient wooden architecture has been refurbished. It is worth noting that this green pigment was rarely used in traditional Chinese color painting on ancient wooden architecture.
Using Py-Gc/Ms analysis, the presence of protein demonstrates that glue mainly containing protein was used as adhesive for adhering pigment particles. Over long-time natural aging, the contents of protein decreased, indicating glue degradation on the flaking area of the painting. It is found that walnut oil was used on the wood before painting as a primer, which is different from the traditionally used oil, i.e., tung oil.
This study has expanded our understanding of the main pigments and adhesive used for the color painting in the Taiping Heavenly Kingdom Prince Dai’s mansion, in which several uncommon painting materials were applied, giving data support and evidence for future conservation and better management. More research is needed to comprehensively evaluate the condition of other paintings in the mansion for better preventive and intervention conservation treatment, such as prevention of the deterioration caused by microorganisms, which will be one of our future works.

Author Contributions

Conceptualization, K.H. and Y.L. (Yuhu Li); resources, H.Y.; methodology, G.T.; investigation, S.H.; software, J.L.; formal analysis, Y.L. (Yanli Li); data curation, E.M.; writing—original draft preparation, Y.L. (Yujia Luo) and Y.T.; visualization, P.F.; writing—review and editing, Y.L. (Yujia Luo); supervision, project administration and funding acquisition, Y.L. (Yuhu Li). All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Cultural Heritage Administration of China, Mercedes Benz and Beijing Foundation for the Protection of Cultural Relics of the Forbidden City [Study on Conservation Effect Evaluation of Ancient Architecture Painting Technology].

Acknowledgments

This work was supported by the project of the Jinhua Cultural Heritage Administration of Taiping Heavenly Kingdom Prince Dai’s mansion in Jiangsu, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 00224 g001 550
Figure 1. (a) The location of the Taiping Heavenly Kingdom Prince Dai’s mansion (the red dot), situated in Jintan County, Changzhou, Jiangsu Province; (b) Satellite image of the location of Dai Taiping Heavenly Kingdom Prince Dai’s mansion (The red dot represents the location of the mansion); (c) The color painting executed on the wooden beams in Taiping Heavenly Kingdom Prince Dai’s mansion.
Figure 1. (a) The location of the Taiping Heavenly Kingdom Prince Dai’s mansion (the red dot), situated in Jintan County, Changzhou, Jiangsu Province; (b) Satellite image of the location of Dai Taiping Heavenly Kingdom Prince Dai’s mansion (The red dot represents the location of the mansion); (c) The color painting executed on the wooden beams in Taiping Heavenly Kingdom Prince Dai’s mansion.
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Figure 2. Four pigment samples (left) collected from the color painting (right) on the northern beam of the main hall in the mansion (a) red, (b) black, (c) blue, and (d) green pigment), as labeled with yellow dots.
Figure 2. Four pigment samples (left) collected from the color painting (right) on the northern beam of the main hall in the mansion (a) red, (b) black, (c) blue, and (d) green pigment), as labeled with yellow dots.
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Figure 3. PLM images observed under horizontal and perpendicular polarized light for (a,b) red, (c,d) black, (e,f) blue, and (g,h) green pigments. All samples were observed under 20× magnifications. The sampling positions are shown in Figure 2.
Figure 3. PLM images observed under horizontal and perpendicular polarized light for (a,b) red, (c,d) black, (e,f) blue, and (g,h) green pigments. All samples were observed under 20× magnifications. The sampling positions are shown in Figure 2.
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Figure 4. Raman spectrum of the (a) red and (b) black pigment samples.
Figure 4. Raman spectrum of the (a) red and (b) black pigment samples.
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Figure 5. (a) Raman spectrum and (b) FT-IR spectrum for the blue pigment sample (indigo,C16H10N2O2); (c) Raman spectrum and (d) FT-IR spectrum for the green pigment (phthalocyanine green, CuC32N8Cl16).
Figure 5. (a) Raman spectrum and (b) FT-IR spectrum for the blue pigment sample (indigo,C16H10N2O2); (c) Raman spectrum and (d) FT-IR spectrum for the green pigment (phthalocyanine green, CuC32N8Cl16).
Minerals 13 00224 g005aMinerals 13 00224 g005b
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Figure 6. The total ion chromatogram (TIC) of the red pigment samples.
Figure 6. The total ion chromatogram (TIC) of the red pigment samples.
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Figure 7. The relative concentrations of fatty acids for the red pigment sample obtained by Py-GC/MS; carboxylic acid with carbon numbers of n.
Figure 7. The relative concentrations of fatty acids for the red pigment sample obtained by Py-GC/MS; carboxylic acid with carbon numbers of n.
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Table
Table 2. A summary of techniques used for pigment analyses and elemental compositions for corresponding pigment samples.
Table 2. A summary of techniques used for pigment analyses and elemental compositions for corresponding pigment samples.
SampleColorExperimental MethodsElements
1RedEDS, m-RS, PLM,
Py-GC/MS		Hg (50.20%), N (32.8%), P (10.24%), Ca (3.09%), C (1.98%)
2BlackEDS, m-RS, PLMC (65.56%), P (14.16%), Fe (9.57%), Ca (9.23%), K (1.98%)
3BlueEDS, m-RS, PLM, FT-IRN (35.47%), O (27.42%), Ca (27.29%), C (9.3%), Si (0.5%)
4GreenEDS, m-RS, PLM, FT-IRN (55.13%), Cl (17.19%), P (12.09%), Ca (5.91%), Cu (4.91%)
Table
Table 3. Component compositions of the red pigment sample.
Table 3. Component compositions of the red pigment sample.
Peak NumberRetention Time (min)Area (%)Compound
11.53630.18Furan, 3-methyl-
21.817326.97Methylamine, N, N-dimethyl
32.10370.30Benzene
42.8160.77Acetic acid
53.30730.481H-Pyrrole, 1-methyl-
63.50670.45Pyridine
73.5430.13Propanoic acid, 2-methoxy-, methyl ester
84.7172.56Cyclotrisiloxane, hexamethyl
95.57970.07Dimethyl sulfate
107.87970.01Valine
118.4950.143-Methylanisole
129.91470.25Alanine
1310.2370.18Glue marker (fish glue 108/139)
1410.7550.02glycine (glue, egg white)
1511.75730.17Protein
1612.1130.05phenol formaldehyde resin (TMAH)
1714.12836.27Schellmannose
1814.87630.191,2,3,4-Tetramethoxybenzene
1915.08670.05Diarylide, RI = 2338
2015.34870.85Benzaldehyde, 3,4-dimethoxy
2116.8230.12orsellinic acid, 2,4-di-O-methyl, methyl ester
2220.01431.28hexadecenoic acid, methyl ester. FA-C16:1
2320.23633.59Hexadecanoic acid, methyl ester
2421.77870.59Benzene, 1,1′-(1-methylethylidene) bis [4-methoxy-]
2521.93670.909-Octadecenoic acid (Z)-, methyl ester
2622.161.74Octadecanoic acid, methyl ester
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Han, K.; Yang, H.; Teri, G.; Hu, S.; Li, J.; Li, Y.; Ma, E.; Tian, Y.; Fu, P.; Luo, Y.; et al. Spectroscopic Investigation of a Color Painting on an Ancient Wooden Architecture from the Taiping Heavenly Kingdom Prince Dai’s Mansion in Jiangsu, China. Minerals 2023, 13, 224. https://doi.org/10.3390/min13020224

AMA Style

Han K, Yang H, Teri G, Hu S, Li J, Li Y, Ma E, Tian Y, Fu P, Luo Y, et al. Spectroscopic Investigation of a Color Painting on an Ancient Wooden Architecture from the Taiping Heavenly Kingdom Prince Dai’s Mansion in Jiangsu, China. Minerals. 2023; 13(2):224. https://doi.org/10.3390/min13020224

Chicago/Turabian Style

Han, Kezhu, Hong Yang, Gele Teri, Shanshuang Hu, Jiaxin Li, Yanli Li, Ersudai Ma, Yuxiao Tian, Peng Fu, Yujia Luo, and et al. 2023. "Spectroscopic Investigation of a Color Painting on an Ancient Wooden Architecture from the Taiping Heavenly Kingdom Prince Dai’s Mansion in Jiangsu, China" Minerals 13, no. 2: 224. https://doi.org/10.3390/min13020224

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