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J.C. Frade and M.J. Oliveira, Uncovering the Decoration Techniques of a Southeast Asian Lacquered Buddha Sculpture, e-conservation Journal 2, 2014, pp. 79-93
Available online 15 August 2014



Uncovering the Decoration Techniques of a Southeast Asian Lacquered Buddha Sculpture

José Carlos Frade




Abstract
In this work, a Southeast Asian lacquered Buddha sculpture attributed to the 19th century was studied using different micro-analytical tools, namely optical microscopy, micro-FTIR, Py-GC/MS and micro-XRD. The several analyses carried out permitted the identification of the materials applied in the sculpture as well as to determine the alloy that constitutes the sculpture’s support. The sculpture is made of a copper-zinc alloy substrate that was lacquered and decorated. The two major decoration techniques in this artefact are gold leaf application and glass mirror inlay, and the different examinations performed revealed how they were executed. Also, the lacquering technique was assessed through the identification of the materials applied in the different layers of the lacquer coating, and the type of lacquer applied in the sculpture was identified by Py-GC/MS as being obtained from the Melanorrhoea usitate tree. 


1. Introduction

The first evidences on the use of lacquer in Southeast Asia are Chinese documents dated from the 9th century where lacquer is mentioned as being used as coating material for monasteries and palaces in Burma [1]. Nevertheless, it is possible that its use is somewhat earlier, dating from the time of the introduction of Buddhism, as it seems that there have been a more or less close relation between the Buddhist religious practice and the use of that material [1, 2]. It is known that Buddhism was introduced in Burma around the 5th century [1], and probably the art of lacquering soon followed.

Many are the substrates on which Asian lacquer has been applied as a protective and decorative coating, being wood and bamboo the most common materials [3-9]. Usually, the production of a lacquered object involves a great number of work sequences of applying lacquer and polishing [8, 9]: the first layers of lacquer – ground layers – are commonly constituted by clay, raw lacquer and another material that improves the adhesive power of the mixture to the substrate (eg. starch glue made from rice); the middle and final layers are, in general, only made of lacquer (usually lacquer that has undergone some kind of refining or processing) [8-10].

Concerning the decoration techniques of lacquered objects, it is believed that they all had their origins in China and that some techniques were more practiced than others in certain periods and/or regions [8, 9, 11]. In Southeast Asia, along with others, two of the most frequent decoration techniques are the application of gold leaf (lai rot nam, in Thailand; shwei-zawa, in Burma) and the inlay of coloured glass mirrors which may be combined with the inlay of mother-of-pearl or other materials (kruang muk kam bua, Thailand; and hman-zi shwei-cha, Burma) [1].

Asian lacquer is a very durable coating material that can be obtained from various species of trees: Rhus vernicifera (China, Japan and Korea), Rhus succedanea (Vietnam and Taiwan) and Melanorrhoea usitate (Burma and Thailand) [2, 4, 6, 7, 9, 12]. Composed of a complex mixture of compounds that includes a series of catechol derivatives and the enzyme laccase, Asian lacquer hardens through a polymerization process catalyzed by this enzyme [6, 7, 9]. After curing, lacquer films are very hard and insoluble, being extremely difficult to analyse with many of the conventional analytical techniques [6, 7, 13-17]. Actually, the only technique capable of distinguishing the lacquer produced by each species is pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), though Fourier transform infrared spectroscopy allows an easy distinction from other coating materials [12, 17-19]. Using Py-GC/MS, the distinction of Asian lacquers can be attained through the detection of specific pyrolysis products, which may be difficult when using a filament-type pyrolyser, or by means of the extraction of mass chromatograms and comparison of their profile with those from lacquer references. The mass chromatograms of the ions m/z 108 and m/z 104, corresponding respectively to a series of alkylphenols and alkenylbenzenes, are particularly suitable for this purpose [2, 12, 19].

In this work, a Southeast Asian lacquered Buddha sculpture (figures 1 and 2) was studied during its conservation intervention at the Laboratory José de Figueiredo, from the General Directorate of Cultural Heritage, performed by conservator-restorer Belmira Maduro, with the purpose of uncovering the decoration and lacquering techniques employed during its production. The sculpture is attributed to the 19th century (or first half of the 20th century), and belongs to a private collection. It has about 50 cm high and is made of a metal substrate that was lacquered and decorated mainly with gold and coloured glass mirrors. Analytically, little is known about the materials and procedures used in the production of this type objects, and this study was a fine opportunity to access them. The actual knowledge about these lacquered artefacts is described in generic terms in the literature, frequently referring to Chinese or Japanese artefacts, and in many exhibition catalogues [2, 4, 5, 8, 9, 20].

Left to right:
Figure 1. Southeast Asian lacquered Buddha sculpture.
Figure 2. Upper part of Buddha’s crown, which is separated from the rest of the sculpture.


In order to characterize the sculpture in terms of its execution techniques, it was necessary to analyse the sculpture materials and to understand how they were applied through the use of several micro-analytical tools, namely by optical microscopy (OM), Fourier transform infrared micro-spectroscopy (micro-FTIR), pyrolysis–gas chromatography/ mass spectrometry (Py-GC/MS) and X-ray micro-diffraction (micro-XRD). Results gave relevant information on the nature of the materials, and permitted to have a glance on the complexity of what is to make an artefact like this.



2. Experimental

This study began with the collection of micro-samples from the lacquer coating for cross-section examination by optical microscopy and for analysis by micro-FTIR and Py-GC/MS. Sampling was carried out using a surgical scalpel on fissures or detachment areas. The results obtained by these techniques were complemented by micro-XRD non-destructive analyses to the metallic support of the sculpture, to corrosion/degradation products on the surface, and to the decorative elements of the sculpture.

Cross-sections from the lacquer coating were prepared for optical microscopy examinations, through the inclusion of the sample in epoxy resin (Epofix, Struers). OM observation of the cross-sections was carried out under visible light using a Leitz WETZLAR optical microscope coupled to a Leica DC500 digital camera.

Micro-FTIR analyses were performed using a Thermo Nicolet Nexus 670 FTIR spectrometer coupled to a Continuμm IR microscope. Samples were compressed in a micro-compression diamond cell and analysed in transmission mode. Each spectrum is the result of 254 scans acquired at 4 cm-1 resolution over the region 4000 cm-1 to 650 cm-1.

Micro-XRD analyses were made in a Bruker D8 Discover diffractometer with Cu Kα radiation and a GADDS detector. The EVA code was used for phase identification.

Py-GC/MS experiments were carried out with a CDS Pyroprobe 2000 coil filament pyrolyser, attached to an Agilent 6890N gas chromatographer equipped with a 5975N mass spectrometer. Analytical conditions were as described elsewhere [12]. Pyrolysis products were identified by interpretation of mass spectra and by comparison with NIST and Wiley libraries.


3. Results

The combined use of micro-FTIR, micro-XRD and Py-GC/MS in the study of this lacquered Buddha sculpture allowed not only to identify the materials applied during its execution, and those formed as a consequence of their deterioration, but also permitted to understand how the sculpture was lacquered and decorated. The crown in the head  of Buddha was damaged and its upper part was physically separated from the rest of the sculpture (figure 2) so it was possible to perform all micro-XRD analyses in a non-destructive way in this part of the crown since it was small enough to be placed inside the diffractometer’s chamber.
 
3.1. Support and Corrosion/Deterioration Products

In the superior part of the crown, the metal support of the sculpture was exposed in some areas due to the lacquer detachment allowing thus its direct analysis by micro-XRD without the interference of other materials such as lacquer itself or the gold leaf covering most of the sculpture’s surface.

Micro-XRD analysis allowed to verify that the metal support of the Buddha sculpture is a copper and zinc alloy, as well as to identify some corrosion products present on its surface (table I and figure 3). Peaks in the diffractograms of figure 3 show copper chlorides and oxides as being the major corrosion products: atacamite (Cu2(OH)3Cl), paratacamite (Cu3(Cu,Zn)(OH)6Cl2), nantokite (CuCl) and cuprite (Cu2O). These compounds were also detected in the greenish material deposited in some areas of the sculpture’s surface.

Weddellite (calcium oxalate) was also identified in the surface of the object, which may be related with some biological activity as a consequence of the conservation conditions to which the sculpture was submitted over time. This material was detected in most of the points analysed by micro-XRD (table I).

TableI

In fact, almost all the sculpture’s surface was covered by a brownish material deposit that was analysed by micro-FTIR and micro-XRD (figure 4). FTIR spectrum reveals the presence of calcium oxalate (1647, 1323 and 780 cm-1), silicates (1028, 952 and 780 cm-1), traces of calcium carbonate (1419 and 874 cm-1) and a sulphate (1114 cm-1) [21-24]. Micro-XRD confirmed these results, except for calcium carbonate, and allowed to determine that the sulphate was gypsum.

Left ro right:
Figure 3. X-ray diffractograms obtained from the greenish corrosion material in the surface of the sculpture (a) and from the metal that constitutes the substrate of the sculpture (b).

Figure 4. X-ray diffractogram (a) and FTIR spectrum (b) of the brownish material deposited in the surface of the sculpture.
Figure 5. Cross-section of the sculpture gold lacquer coating.



3.2. Gold Lacquer Coating

Samples collected from the gold lacquer coating were examined by OM and analysed by micro-FTIR and Py-GC/MS. Cross-sections (figure 5) reveal that the lacquer coating is constituted by three layers, the first and the third being made of lacquer and the second is a mixture of lacquer and clay.

FTIR spectra of layers 1 and 3 (figure 6) show the characteristic absorptions of Asian lacquer: 3370 cm-1 (O-H stretching), 2928 and 2856 cm-1 (C-H stretching), 1713 cm-1 (C=O stretching), 1628 cm-1 (C=C stretching in aromatics and alkenes + COO– asymmetric stretching in polysaccharides), 1449 cm-1 (C-H deformation + COO symmetric stretching in polysaccharides), 1275 cm-1 (C-O stretching in phenol groups), 1080 cm-1 (C-O stretching), 780 cm-1 (out-of-plane C-H deformations in aromatics), 755 cm-1 (out-of-plane C-H deformations in aromatics), 726 cm-1 (out-of-plane C-H deformations in aromatics + CH2 rocking), and 693 cm-1 (out-of-plane C-H deformations in   aromatics) [12, 18, 21, 22].

FTIR spectrum of layer 2 (figure 6) exhibits some additional bands (some superimposed with those of lacquer) that correspond to the presence of kaolin: 3699 cm-1 (O-H stretching), 3634 cm–1 (O-H stretching), 1034 cm–1 (Si-O-Si stretching), 914 cm–1 (O-H deformation in Al-OH groups), 799 cm-1 (Si-O stretching) and 699 cm–1 (Si-O stretching) [24].

Left ro right:
Figure 6. FTIR spectra of the 3 lacquer layers applied on the sculpture.

Figure 7. Pyrogram (a) and extracted mass chromatogram of ion 104 (b) obtained in the analyses of the lacquer by Py-GC/MS.
Figure 8. X-ray diffractogram of the golden lacquer surface.


The lacquer of the sculpture was analysed by Py-GC/MS with the purpose to identify the type of lacquer that was applied during its execution. The pyrogram and correspondent mass chromatogram of ion 104 are depicted in figure 7. The alkenylbenzenes profile presented by the mass chromatogram shows that the most abundant benzene derivative is undecenylbenzene (eluting at 32.18 min) and is very similar to the profile presented by reference lacquer obtained from Melanorrhoea usitate species [12].

When examining cross-sections, it is not quite perceptible a golden layer corresponding to the gold leaf applied on top of the final lacquer layer. Nevertheless, the direct examination of the sculpture surface using magnifying lenses allowed understanding that gold leaf was applied directly on the last layer of lacquer, without the previous application of a bole layer which might be constituted by vermillion or orpiment mixed with lacquer [9]. The analysis of the golden lacquer coating by micro-XRD confirms this observation since no traces of those materials were detected. Actually, the diffractogram obtained (figure 8 and table I) reveals peaks due to the presence of gold, a copper-zinc alloy and weddellite. In this case, the analysis was sensitive enough that the metal support of the sculpture was detected. Then, in case a bole layer would have been applied, its inorganic materials would have also been detected.

3.3. Sculpture Decoration

Besides the gold that covers almost entirely this lacquered Buddha sculpture, it presents some other decorative elements: green and transparent mirrors and a reddish polychromy.

Figure 9 depicts a detail of the reddish polychromy, and the FTIR spectrum and diffractogram obtained in its analysis. Micro-XRD analysis reveals that the red colour is due to the use of vermillion, and FTIR spectrum shows the characteristic absorptions of beeswax [22].

Left ro right:
Figure 9. Detail of the reddish polychromy in Buddha’s crown (a), and respective FTIR spectrum (b) and x-ray diffractogram (c).


In what concerns to the green and transparent mirrors, it is easy to appreciate that these mirrors were inlayed in the lacquer coating (figure 10) when visually examining the sculpture. The inlay was made through the use of an adhesive material (a black paste) to which the mirror would first adhere and then lacquer was applied covering the mirror completely. Finally, a polishing step would be made until the surface of the mirror appears underneath the lacquer [4].

The material used as adhesive for the mirrors could be analysed with no problems by micro-FTIR, as in many cases the mirrors have already been detached from its place, leaving the black paste exposed (figure 10). It is important to mention that the black paste surface presented a silver coloured material on its surface corresponding to some remains of the mirrors’ reflecting material. The bands in the black paste FTIR spectrum (figure 11) are due to the presence of lacquer and kaolin, meaning that the black paste is a mixture of clay and lacquer. The mirrors’ reflecting material was analysed by micro-XRD in two of the detached mirrors (table I and figure 12), and it was possible to comprehend that the reflecting material of the mirror consists of lead. Also, some lead corrosion products were detected in the XRD analyses, namely lead and sodium basic carbonate (NaPb2(CO3)2(OH)), hydrocerussite (Pb3(CO3)2(OH)2), lead oxide sulphate (PbSO4.PbO). The detection of lead and some of its corrosion products is rather unusual since the most common material used for the production of flat glass mirrors between the 16th and 20th centuries was a tin-mercury amalgam, although there are some accounts on the use of lead, tin and “marcasite” mixed with mercury [25].

Left ro right:
Figure 10. Detail of the inlayed glass mirrors.
Figure 11. FTIR spectrum of the black paste used for the mirrors inlay
Figure 12. X-ray diffractogram of the reflecting material of two detached mirrors: a) green mirror; b) colourless mirror.


4. Conclusions

The combination of several micro-analytical tools allowed the characterization of the lacquer and decoration techniques (gold leaf application and mirror inlay) from a lacquered Buddha sculpture. The sculpture’s support, an alloy of copper and zinc, was finished with lacquer produced by the Southeast Asian species Melanorrhoea usitate. The lacquering technique employed is slightly different from that usually applied in the production of bamboo or wooden artefacts. In this case, probably due to the metallic nature of the substrate, the first layer applied on it was composed just of lacquer instead of the more common mixture of clay and lacquer. This mixture was found to be applied only after this first layer, which was then covered by another coat of lacquer that was simultaneously used as an “adhesive” for the gold leaf decorating the most part of the sculpture‘s surface. The mirror inlay found in the sculpture was made through the use of a paste composed by a mixture of clay and lacquer to which colourless and green mirrors were glued.


5. Acknowledgments

Authors would like to acknowledge M. J. Oliveira (Laboratório José de Figueiredo, LJF) for the XRD analyses and B. Maduro (Conservator at Laboratório José de Figueiredo) responsible for the sculpture’s conservation treatment.


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José Carlos Frade

Chemist
School of the Art, Portuguese Catholic University (Porto, Portugal)
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José Frade, PhD, is Professor at the Portuguese Catholic University and vice-director of CITAR - Research Centre for Science and Technology of the Arts in Oporto, Portugal.


Maria José Oliveira
Physicist
Laboratory José de Figueiredo, General Directorate of Cultural Heritage (Lisbon, Portugal)
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Maria José Oliveira is researcher at the Laboratory José de Figueiredo, from the General Directorate of Cultural Heritage in Lisbon, Portugal.


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