Challenges in the data analysis of Asian lacquers from museum objects by pyrolysis gas chromatography/mass spectrometry
Introduction
Asian lacquers have been used since ancient times in East Asia as coatings for all kinds of surfaces, and lacquered objects have always been appreciated because of the beauty, brightness, toughness, durability and waterproofing of their coatings [1,2]. These objects and artworks are widespread in Oriental art, but they have also become extremely popular in Europe since the 16th century [3], and have been collected since then.
As far as museums are concerned, lacquered objects often enter the collections from very diverse geographical regions and sometimes after multiple stops along the way. Provenance is often attributed based on art historical knowledge, and, even when it is officially recorded, debates and questions easily arise [4]. Given the popularity of lacquered objects, European craftsmen started imitating and paraphrasing them, using locally available materials and techniques to closely reproduce the glossy luxury of their Oriental equivalents [5]. It is therefore extremely important to determine the chronology, manufacture origin or even authenticity of such objects to avoid misleading attributions [4].
The preparation of the lacquer coating is a complex process, in which mixtures of several materials are used in multiple layers [[6], [7], [8]]. The complexity is enhanced by the possible presence of additional substances, such as plant oils and pigments, used to produce the lacquered decoration. The identification of all these materials is the first crucial step to understand the overall composition of an object, solve problems of conservation and authenticity, differentiate between the different Asian lacquers, point towards provenance and distinguish European from Asian productions [[9], [10], [11]]. If the exact preparation process and materials used are identified, hypotheses on specific production centres are also possible [7].
The raw material of Oriental lacquerware is produced from the sap of three lacquer trees: Toxicodendron vernicifluum 2 (China, Japan and Korea), Toxicodendrum succedaneum 2 (Vietnam and Taiwan), and Gluta usitata 2 (Laos, Myanmar, Cambodia and Thailand) [12,13]. Although the geographical distribution of these trees is a more complex topic than what appears in the literature [14], the use of their sap is mostly reported in agreement with the above-mentioned regions. The chemical composition of all saps is a complex mixture of catechol and phenol derivatives (60–65%), proteins (glycoproteins (2%) and a laccase enzyme (1%)), polysaccharides (7%) and water (30%) [15]. The laccase enzyme is the responsible trigger for the polymerisation reaction that leads to the formation of the hardened lacquer film [16]. The catechol/phenol mixture composition is different in the three trees and such difference is maintained in the final polymer, thus enabling the three types of lacquers to be chemically distinguished [17,18]. The main components of the product extracted from Toxicodendron vernicifluum are referred to as urushiol; laccol is obtained from Toxicodendron succedaneum, and thitsiol from Gluta usitata [19]. The main compounds in urushiol and laccol are catechol derivatives with C15 and C17 alkenyl chain, respectively. Thitsiol contains additional catechol derivatives with an ω-phenylalkyl chain of C10 and C12 [15,20].
Relatively abundant research has been undertaken to characterise Asian lacquers, by focusing on finding molecular markers to identify the three different saps [21,22], studying the interactions between plant oils (used as additives in lacquer formulations) and lacquers [23] and investigating the pathways of lacquer degradation [24,25]. Due to the high polymerisation degree and complex chemical composition of oriental lacquers, pyrolysis-gas chromatography coupled with mass spectrometry (Py-GC/MS) is the most suitable analytical approach for their chemical characterisation and for their identification in samples of unknown composition [[26], [27], [28]]. Due to the high number of pyrolysis products obtained and the difficulty of interpretation, a remarkable effort has been made to systematise the identification process by using the AMDIS software in combination with a complex Excel spreadsheet [27]. The polar nature of most pyrolysis products derived from lacquer makes a derivatisation step highly recommended. Both methylation (TMAH) [8,9,27] and silylation (HMDS) [15,20] have been proposed in the literature with good results. Advantages and disadvantages of these derivatisation methods are mostly related to the identification of materials other than lacquer possibly present in a sample, with TMAH being particularly suitable for lipids, and HMDS being more suitable for proteins and polysaccharides [29]. Regardless of the derivatisation method, the strategy to differentiate among lacquers is based on the study of chromatographic profiles of specific pyrolysis products and the identification of molecular markers [9,12,13,15,17,20,22,26,27,[30], [31], [32], [33]]. Data analysis is often quite complex and lacquer identification, although univocal if carried out correctly, is not always straightforward.
To improve and simplify data analysis, a summary of published markers and chromatographic profiles ‒ obtained with and without thermally assisted reactions (silylation and methylation) ‒ is included in this paper. Based on this, a decision-making scheme was established, and proposed to help guide data interpretation. This method of data analysis was then applied to the systematic investigation of a group of lacquered objects from the Asian art collection at the Museum of Zaragoza, Spain (“Federico Torralba” collection) [34], with the aim to characterise the complex mixtures of materials possibly present, assess the provenance of the objects and describe the analytical difficulties arising from the study of these types of objects. Because many of the lacquerwares were coloured or decorated, scanning electron microscopy (SEM) coupled to energy-dispersive X-ray spectrometry (EDS) was used as complementary technique to Py(HMDS)-GC/MS, in order to determine the elemental composition of the coatings and study the artistic technique.
Section snippets
Objects and samples
Eight lacquered objects were selected from the Asian art collection at the Museum of Zaragoza (Spain). The objects include three food containers, three boxes, one plate and one Buddha sculpture. They were selected in order to cover a chronological period from the 16th to the 20th century and represent different possible provenances (China, Japan, Myanmar and Thailand), according to stylistic interpretation. Table 1 includes an image and a description of the objects under investigation. Some of
Pyrolysis data analysis
The pyrolysis data were reviewed with the aim to highlight chromatographic profiles and molecular markers based on available literature [9,12,13,15,17,20,22,26,27,[30], [31], [32], [33]]. A summary of these profiles and markers is reported in Table 2, together with the mass-to-charge (m/z) values used to extract chromatographic profiles of non-derivatised compounds, as well as methylated and silylated derivatives.
Table 2 clearly reflects the complexity of the scenario that must be taken into
Conclusions
In order to identify Asian lacquers in a series of museum objects by Py(HMDS)-GC/MS, a decision-making scheme was established, based on the mass spectrometric features of the chromatographic profiles and molecular makers generated during the pyrolytic process. The scheme was proposed to help guide data interpretation and to distinguish the three Asian lacquers - urushi, laccol and thitsi. It also revealed to be a useful tool for the straightforward identification of the lacquer origin, even
CRediT authorship contribution statement
Diego Tamburini: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Ilaria Bonaduce: Supervision, Writing - review & editing. Erika Ribechini: Supervision, Writing - review & editing, Funding acquisition. Carmen Gallego: Resources. Josefina Pérez-Arantegui: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Visualization, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research leading to these results was supported by a visiting fellowship from the Department of Chemistry and Industrial Chemistry (University of Pisa) awarded to J. Pérez-Arantegui in 2018. This work was also performed within the context of the project “Advanced analytical pyrolysis to study polymers in renewable energy, environment, cultural heritage” funded by University of Pisa (PRA_2018_26). The authors would like to acknowledge the use of the FESEM utilities in the Servicio General de
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Present address: Department of Conservation and Scientific Research, Freer Gallery of Art and Arthur M. Sackler Gallery, National Museum of Asian Art, Smithsonian Institution, 1050 Independence Ave SW, Washington DC, 20560, US.