Scientific investigation of Asian lacquerware

Abstract

Asian lacquerware, renowned for the durability, lustre and cultural significance of its decorative layers, often needs to be studied in terms of its composition, production techniques and degradation processes. In traditional recipes, lacquer is applied as a coating on the surface of a substrate, and is often mixed with additives such as pigments or overlaid with metal powders and inlays. As a result of this intrinsic complexity, the technical examination and scientific analysis of Asian lacquerware encompasses techniques that target both the organic matrix and the inorganic components. Although the techniques mentioned here can be and are used to analyse organic coatings in general, this Technical Brief focuses on Asian lacquerware due to its particular complexity.

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1. Introduction

The word ‘lacquer’ is a general term used to describe lustrous and shiny surfaces historically obtained using a broad range of substances. In this Technical Brief, we are discussing specifically lacquers obtained from the sap of trees belonging to the Anacardiaceae family, indigenous to East and Southeast Asia (Fig. 1).

Fig. 1 Example of lacquerware (Sutra box, Japan, early 17th century, object number 2015.300.292a–c, Mary Griggs Burke Collection, Gift of the Mary and Jackson Burke Foundation, 2015; © The Metropolitan Museum of Art).

In the 17th century, the exceptional aesthetic and mechanical properties of the natural East Asian material became known to the Western world, gaining sudden popularity. However, the material was poorly understood, and European craftspeople began to reproduce the glossy surfaces using local resins and oils, such as dammar, mastic, linseed oil, etc. These imitations, known as japanning, vernis Martin and lacca povera, for example, can be difficult to distinguish from original Asian productions without chemical analysis.

2. Chemistry of Asian lacquers

Asian lacquers are historically categorised into three main types based on the trees from which they are derived. The lacquer from Toxicodendron vernicifluum, mostly distributed in China, Japan and Korea, is referred to as urushi in Japanese or qi in Chinese. Toxicodendron succedaneum, mostly distributed in Vietnam and Taiwan as well as parts of China, Japan and Thailand, produces the so-called ‘Vietnamese lacquer’ or laccol. Gluta usitata is native to Southeast Asia, especially Myanmar, Laos, Cambodia and Thailand, and the lacquer is commonly referred to as thitsi. The chemical composition of the saps includes a complex mixture of catechol and phenol derivatives (60–65%), which contain different components in the three types of lacquer, thus enabling their chemical differentiation (Fig. 2). In addition to the phenol–catechol mixture, the sap contains proteins (notably glycoproteins, ∼2%, a laccase enzyme, ∼1%, and stellacyanin, ∼0.02%), polysaccharides (5–7%) and water (∼30%). The spatial arrangement of the chemical compounds is described as a water-in-oil emulsion, where the water-soluble components, such as laccase, polysaccharides and glycoproteins, form water reverse micelles dispersed in the phenol–catechol matrix.¹

Fig. 2 Main alkylcatechol components of (a) urushiol, (b) laccol and (c) thitsiol.

In the past, it had been assumed that objects would be made of the type of lacquer locally available. However, it has been recently shown that lacquer travelled widely and crossed national borders, and some objects contain more than one type of sap.

3. A myriad of materials and decorative techniques

Lacquer’s natural colour is brown, but black and red lacquers are commonly encountered in objects. These are obtained by mixing lacquer with pigments, such as carbon black, red ochre and vermilion, although modern lacquer artists use synthetic pigments for bright, non-traditional colours. Iron powder is also used in the traditional Japanese kurome process, which is the initial refinement of the raw lacquer by constant stirring to obtain a black colour. Yellow lacquer is traditionally mixed with orpiment. Blue, green, orange and pink shades are also found, especially in contemporary lacquerwares from Southeast Asia.

Other materials are also reported to be occasionally mixed with lacquer to possibly adjust its rheological or adhesive properties. These include oils, proteins, starch, rape seeds, tofu, egg white and animal glue. The preparation layers can also contain pigs’ blood, textile fibres, charred sawdust and starch, among other materials. The decorative layers can also be decorated with metals and mother-of-pearl inlays, and gold, silver and tin leaf and powder.²

4. Analytical approaches

The investigation of lacquered objects is a challenge for scientists, and there is not a single technique that can answer all questions. Each methodology provides information on individual aspects of lacquerware, either non-invasively by in situ measurement or by means of physical sampling followed by ex situ measurement in the laboratory.

4.1 Non-invasive in situ measurement approaches

• Optical and digital microscopy are useful to study the condition of lacquer surfaces and gather information on the decorative techniques used.

• X-ray radiography and computed tomography (CT) are valuable for studying the entire construction of lacquered objects, providing insights into their sub-structures, shedding light on the manufacturing process and revealing possible hidden damage, structural issues and repairs.

• X-ray fluorescence (XRF) provides information on the elemental composition and spatial distribution of the inorganic components of lacquer decorations, for example, pigments, mother-of-pearl and metal inlays.

• Spectroscopic techniques, such as fibre optic reflectance spectroscopy (FORS) and Raman spectroscopy, are valuable tools for the identification of the pigments and other materials that are mixed with the surface lacquer layers.

4.2 Invasive and/or destructive ex situ measurement approaches

• Optical and digital microscopy are useful to study the layer structure of lacquerware samples, which is key to providing information on the lacquer decoration techniques and methods of application. In fact, lacquer is often applied in thin layers followed by polishing – some of the most valuable objects can include more than 50 lacquer layers. The use of UV illumination provides additional information on layers characterised by autofluorescence (Fig. 3).

Fig. 3 Cross section viewed under visible, near UV and far blue illumination, ×250, showing multiple layers of black and red lacquer, as well as European interventions (Chinese screen V&A:W.37-1912, photography ©Victoria and Albert Museum/Lucia Burgio/Valentina Risdonne).

• Scanning electron microscopy (SEM) of samples provides additional information on the technology of lacquer decoration and shows the features of layers in cross sections in a way that is complementary to that provided by optical microscopy (Fig. 4). When coupled with energy dispersive X-ray spectrometry (EDX), SEM-EDX also provides information on the elemental composition of the inorganic materials possibly present in the lacquer layers, such as additives and pigments.

Fig. 4 SEM image overlayed with an optical photomicrograph of a cross section showing the stratigraphy of a lacquer coating sample from a Japanese chest from the collection of the Museum of Zaragoza (Spain). Reprinted from D. Tamburini, et al., J. Anal. Appl. Pyrolysis, 2020, 151, 104905, with permission from Elsevier.³

• Raman spectroscopy can be used on cross sections for the identification of pigments, inorganic materials and some dyes (such as indigo) mixed within the lacquer and foundation layers (Fig. 5).

Fig. 5 Raman spectra of pigments used in admixture with lacquer in various Burmese lacquerwares from the British Museum collection. Adapted from D. Tamburini, et al., Heritage Sci., 2019, 7(1), 28, published under CC-BY-4.0 (https://creativecommons.org/licenses/by/4.0/).⁴

• Fourier-transform infrared (FTIR) spectroscopy can be used to identify some of the materials in lacquerware but does not easily differentiate among the three Asian lacquers. However, it can help in differentiating ‘true’ Asian lacquer from European imitations.

• X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), field desorption mass spectrometry (FD-MS) and enzyme-linked immunosorbent assay (ELISA), have occasionally been used to investigate the hardening process or identify Asian lacquers, but are not widespread.

• Analytical pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC-MS) is the most powerful method for the characterisation and identification of lacquer and other organic materials at a molecular level. It will be discussed more in detail below.

5. Py-GC-MS

The characterisation of the organic composition of lacquers can be achieved only with a limited number of techniques able to investigate polymerised materials. Py-GC-MS, described in detail in a previous Technical Brief,⁵ uses thermal energy (usually in an inert atmosphere) to selectively break the inter-monomeric bonds in the polymer network of the lacquer, so that smaller molecules, referred to as pyrolysis products, are released. These can be more easily studied than the original polymer, yielding valuable information on the polymer structure, properties and preservation state, in addition to enabling its identification. The amount of sample needed is in the range of 50–100 μg and the time of analysis is generally in the range of 30–45 min. These characteristics make the technique particularly suitable to the analysis of cultural heritage samples.

The main classes of pyrolysis products obtained from Asian lacquers can be broadly summarised as alkylcatechols (CT), alkylphenols (Ph), alkylbenzenes (B) and aliphatic hydrocarbons (C). These categories of pyrolysis products display specific molecular profiles, which are consistently obtained regardless of the pyrolytic conditions adopted. Their presence and molecular distribution not only enable chemically distinguishing the three main Asian lacquers but also evaluation of the structure and condition of the polymeric network.

A common challenge in Py-GC-MS applications is the complexity of data interpretation, as complex mixtures of organic materials can produce a high number of pyrolysis products. Specifically, lacquer pyrolysis products do not commonly appear as main chromatographic peaks, due to a general low pyrolysis yield of the alkylphenols and catechols compared with other materials, such as fatty acids in lipids. As a result, data processing strategies often use extracted ion chromatograms (EICs) to highlight the presence of specific molecular markers among the numerous pyrolysis products.

Within the framework of the RAdICAL (Recent Advances in Characterizing Asian Lacquer) workshop series, Michael Schilling and co-workers have proposed an elegant system to standardise the interpretation of data obtained by Py-GC-MS of Asian lacquer samples. The workflow combines the Automated Mass spectral Deconvolution and Identification System (AMDIS), freeware software for chromatogram interpretation, and a customised Excel workbook.⁶ By using a skilfully built spectral library of more than 1500 compounds, the workflow rapidly identifies lacquer pyrolysis products and exports them in the Excel workbook, where each compound is assigned to the material from which it derives. Diagnostic graphs are automatically generated, showing the distribution of the various pyrolysis products present in the analysed sample (Fig. 6).

Fig. 6 Gestalt graph of pyrolysis markers of a Vietnamese lacquer sample containing laccol. Adapted from V. Pintus, et al., Sci. Rep., 2019, 9, 18837, published under CC-BY-4.0 (https://creativecommons.org/licenses/by/4.0/).⁷

The approach has inter-laboratory applicability and enables lacquer formulations to be distinguished and studied, so that researchers who are not necessarily pyrolysis experts can identify the materials potentially present in lacquer samples.

6. Degradation

Asian lacquers are relatively stable but are vulnerable to degradation caused primarily by light and moisture. Degradation often occurs at the grain boundaries within the lacquer’s dense, three-dimensional structure. UV radiation weakens these boundaries, while excessive dryness causes brittleness by removing water’s plasticising effect.⁸ Molecular analysis of aged Asian lacquers also reveals chemical alterations, especially the formation of acid compounds upon oxidation. Special attention has been dedicated to the oxidation of thitsi due to its unique composition, particularly the presence of ω-phenylalkylcatechols, which include a benzylic position highly prone to oxidation. This leads to the formation of alkylphenylketones, detectable as pyrolysis products with profiles resembling alkylbenzenes, as well as additional oxidation products characteristic of thitsi. These compounds actively contribute to lacquer degradation and can help identify thitsi in historical samples.

7. Future perspectives

The current key research gaps include challenges in analysing mixtures of lacquers from different trees and understanding the co-curing of lacquers with other polymers such as oils and proteins. Studies have focused on urushi/qi, with less emphasis on other lacquers or regional variability in molecular composition. Analytical improvements, such as refining derivatisation conditions in analytical pyrolysis and exploring innovative sampling methods, are areas for future development. High-resolution mass spectrometry (HRMS) and portable mass spectrometry instruments also hold potential for advancing the field.

Finally, a cross-disciplinary approach is essential to enhance our understanding of these materials and their use in historical contexts, combining insights from scientists, conservators, art historians and manufacturers. Language barriers and terminological ambiguity, particularly the broad use of the word ‘lacquer’, underscore the need for precise scientific analysis to confirm the presence of Asian lacquers and distinguish them from imitations or alternative materials.

 

Diego Tamburini (British Museum, London, UK) and Lucia Burgio (Victoria and Albert Museum, London, UK).

 

This Technical Brief was prepared for the Analytical Methods Committee by the Heritage Science Working Group and approved by the AMC on 3rd April 2025.

References

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2. https://www.metmuseum.org/essays/lacquerware-of-east-asia, accessed 23 April 2025.
3. D. Tamburini, I. Bonaduce and E. Ribechini, et al. , J. Anal. Appl. Pyrolysis, 2020, 151, 104905.
4. D. Tamburini, V. Kotonski and A. Lluveras-Tenorio, et al. , Heritage Sci., 2019, 7(1), 28 , published under CC-BY-4.0 (https://creativecommons.org/licenses/by/4.0/.
5. Analytical Methods Committee, Analytical pyrolysis in cultural heritage, AMC Technical Brief No. 85, Anal. Methods, 2018, 10, 5463,  10.1039/c8ay90151a.
6. https://www.getty.edu/projects/recent-advances-characterizing-asian-lacquer/, accessed 23 April 2025.
7. V. Pintus, A. J. Baragona and K. Wieland, et al. , Sci. Rep., 2019, 9, 18837 , published under CC-BY-4.0 (https://creativecommons.org/licenses/by/4.0/.
8. C. McSharry, R. Faulkner and S. Rivers, et al., The chemistry of East Asian lacquer: A review of the scientific literature, Stud. Conserv., 2007, 52(sup1), 29–40,  DOI:10.1179/sic.2007.52.Supplement-1.29.

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