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Spectrochimica Acta Part A 68 (2007) 1085–1088
Raman spectroscopic analysis of Mexican natural artists’ materials Peter Vandenabeele a,∗ , Mayahuel Ortega-Avil`es b , Dolores Tenorio Castilleros c , Luc Moens a a
Ghent University, Department of Analytical Chemistry, Proeftuinstraat 86, B-9000 Ghent, Belgium b UAEM, Facultad de Ingenier´ıa, Cerro de Coatepec S/N, C.P. 50000, Toluca, Mexico c ININ, Carretera Federal M´ exico-Toluca km 36.5, Salazar Edo. de M´exico, C.P. 52045, Mexico Received 25 October 2006; accepted 26 January 2007
Abstract This work represents the Raman spectra of 15 natural artists’ materials that were obtained from local market in Mexico. Some of these products are not endemic to the region, but are often used in local conservation practice. Other materials are of local origin and have been used for centuries by local craftsmen. The Raman spectra that are reported here are: Chia oil, linseed oil, Campeche wax, beeswax, white copal, dammar, colophony, mastic, pixoy, chapopote, chucum, aje gum, gutta gum, peach gum and gum Arabic. The sample of pixoy was mixed with TiO2 , although it was not clear whether this was done intentionally or not. The Raman spectrum of chapopote, the local name for bitumen, contained features of carbonaceous and terpenoid matter. The Raman spectra of chapopote and chucum suffered severely from fluorescence, resulting in noisy Raman spectra. Aje gum and gutta gum are not gums, since they are resinous (terpenoid) in nature. Aje is a rare animal resin originating from Coccus axin. © 2007 Elsevier B.V. All rights reserved. Keywords: Art analysis; Raman spectroscopy; Mexican materials; Restoration; Natural artists’ materials; Binding medium; Resins; Gums; Bitumen
1. Introduction In Mexico, conservation scientists are continuously trying to preserve the enormous cultural patrimony of the country. However, one of the important obstacles that they find during their work is the lack on knowledge of the traditional materials that have been used by Mexican artists and craftsmen. Indeed, little is known on which materials that were used, and moreover, there is need on information on the properties of these materials as well as on the way they were applied [1]. When trying to use traditional materials, conservators encounter difficulties to select suitable materials, techniques and procedures to reach a good restoration. Historically, different types of natural materials were mentioned in ancient documents, including rubber, binding media, lacquers and natural adhesives, coming mainly from secretions of insects, trees and local fruits. For conservators, it is vital to be able to identify the different materials of
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[email protected] (P. Vandenabeele).
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which ancient objects and artworks consist, since this determines the methods and materials that will be used during conservation [2]. Raman spectroscopy may be a suitable technique to obtain a fast and adequate identification of these natural materials that were used [3]. Indeed, especially the non-destructive character of the approach is well appreciated. The technique has been used for the investigation of different types of artworks, including manuscripts, polychromes, and paintings, including wall painting fragments from an archaeological site in Mexico. The technique has not been limited to the identification of inorganic artists’ materials; several Raman spectroscopic studies were performed on organic binding media and resins, using Fourier-transform (FT-) Raman spectroscopy [4–7] as well as dispersive Raman spectroscopy [8,9]. In order to be able to identify the local organic materials that were used to elaborate ancient Mexican artefacts, it is necessary to have access to a spectral database, containing local reference materials. This paper examines the Raman spectra of some Mexican organic compounds that conservation scientist think were used by ancient artists or craftsmen, and which could be applied nowadays for restoration purposes.
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2. Experimental 2.1. Samples Fifteen samples were investigated by dispersive Raman spectroscopy and compared with spectra from samples of common products from the restoration practice. Some local samples were obtained from restorers from the National School of Restoration, Mexico, while the other samples were harvested directly from the plants or natural seeps. Similar materials are considered to be used by Native Americans, for the construction of their artworks. An overview of these samples and their main use is given in Table 1. 2.2. Raman spectroscopy Raman spectroscopy was performed by using a Renishaw System-1000 spectrometer (Wotton-Under-Edge, UK). The instrument is equipped with a diode laser with a laser wavelength of 785 nm and an output power of 50 mW. Laser intensity on the sample could be modified up to ca. 5 mW, by using a set of neutral density filters. In order to take possible sample inhomogeneity into account, for each sample at least three Raman spectra were recorded by using the 50× objective lens, allowing for a spectral footprint of ca. 2 m. All spectra were recorded between 400 and 1800 cm−1 for at least five accumulations of 30 s. 3. Results and discussion 3.1. Fatty acid containing samples The group of fatty acid containing binding media consists of two subclasses, namely the waxes and the drying oils [8]. Chia oil is a typical drying oil, which has frequently been used in Mexico. The plant source of this binding medium is Salvia hispanica. Apart from its use as artistic binding medium, the oil is as well used as food source. In Fig. 1a, its Raman spectrum is compared with the Raman spectrum of a sample of linseed oil, as obtained
Fig. 1. Baseline-corrected Raman spectra of (a) Chia oil and linseed oil, and (b) Campeche’s wax and beeswax.
from a Mexican restorer. The Raman bands of both products are observed at the same positions, although some differences in their relative intensities are observed. The chemical composition of Chia oil is very similar to that of linseed oil, which explains the similarity between the spectra of both oils. Fig. 1b shows the Raman spectrum of Campeche’s wax, compared to the reference spectrum from beeswax [8]. It is clear that the spectra are very similar, since Campeche’s wax is a type of beeswax that is produced by the local bee species called Melipona. Beeswax consists mainly of saturated unbranched compounds of high molecular weight. These include esters of
Table 1 Overview of the samples that were investigated in this study and their main applications for artistic purposes Local name
English name
Source
Main use as artists’ material
Ch´ıa Linseed oil Cera de Campeche Beeswax Copal bianco Dammar Colophony Mastic Pixoy Chapopote Chucum Aje gum Gutta gum Peach gum Gum Arabic
Chia oil
Salvia hispanica Linum usitutissimum Apis melipona Apis mellifica Bursera species Dipterocarpaceae species Pinus species Pistacia lentiscus Guazuma ulmifolia
Binding medium Binding medium Binding medium Binding medium Varnish Varnish Varnish Varnish Binding medium Varnish, glue Binding medium Binding medium Colorant Binding medium Binding medium
Campeche’s wax
Bitumen
Gamboge, gummi gutti
Pithecellobium albicans Coccus axin Garcinia handbury Prunus persica Acacia
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Fig. 2. Raman spectra of some resins: white copal, dammar, colophony and mastic.
fatty acids and high molecular weight alcohols, the corresponding free fatty acids and monohydric alcohols and hydrocarbons. The main component is myristyl palmitate ester. This composition is as well reflected in the Raman spectrum: no bands corresponding with C C vibrations are observed, such as the ν(C C) at ca. 1650 cm−1 or the in plane d(CH) of dialkyl ethylenes at ca. 1160 cm−1 . 3.2. Gums and resins In restoration practice, plant exudates are often used as binding medium or as a protective coating on the artefact. Some of these are of polysaccharide nature, whereas others have merely a terpenoid structure. The first group – the gums – are soluble or swell when immersed in water, while the resins are insoluble. Resins are of terpenoid origin and their Raman spectra contain features that can be attributed to ν(C C) stretching vibrations, typical bands between 1800 and 1600 cm−1 . On the contrary, Raman spectra of gums usually do not contain features above 1500 cm−1 , as their polysaccharide structures do not contain unsaturated C C bonds. Fig. 2 represents the Raman spectra of four resin samples, which are often used in conservation practice. These samples were obtained from local markets but dammar, colophony and mastic are not endemic to Mexico. White copal is used as a varnish and is extracted from trees of the Burseraceae family. Before using it as a varnish, the resin is dissolved in turpentine or an oily medium. Dammar is a varnish that became quite popular since the 19th century and its main component is dammarolic acid (C54 H77 O3 (COOH)2 ). After tapping it from Dipterocarpaceae trees, it is dissolved in turpentine and applied as a spirit varnish. Colophony is a resin that is extracted from pine trees (Pinus) and consists mainly of rosin acids (C19 H20 COOH). After harvesting, the volatile components of the pure resin are removed through distillation. Mastic originates from Pistacia lentiscus and has often been used as a solution in drying a oil or in a volatile solvent, such as turpentine. Raman spectra of resins contain distinct Raman bands above 1600 cm−1 , which can be attributed to the ν(C C) stretching vibration. Resins are of terpenoid origin
Fig. 3. (a) Raman spectrum of pixoy; (b) baseline-corrected Raman spectrum of pixoy; (c) Raman spectrum of chapopote; (d) baseline-corrected Raman spectrum of chapopote (different area) and (e) chucum.
and therefore they all contain C C units. Raman bands in the regions between 1480 and 950 cm−1 can generally be attributed to δ(CH), δ(CH2 ) and δ(CH3 ) deformations and ν(CC) stretching vibrations. Fig. 3 presents the Raman spectra of three other local samples (pixoy, chapopote, chucum). These samples are materials that are traditionally used in native Mexican art objects and therefore they are of importance to local conservation scientists. Pixoy is a gum that originates from bay cedar (Guazuma ulmifolia), a local tree in Central America. This material has several local names, such as caulote, chicharr´on, cumulote, gu´acimo, guacimillo, guacimo caulote or gu´acimo de ternero. Chapopote is the Mexican name for bitumen. Natural bitumen seeps occur in pockets along the Mexican Gulf coastal plain. Chapopote was processed by mixing it with mineral or vegetal additives (e.g. resins) to make that it would become rigid after application and that it would not readily melt in the sun [10]. Prehispanic mesoamerican peoples collected, processed and used bitumen for decoration, as a sealant, adhesive, building construction material, chewing gum, inciense, paint, body adornment and fuel [11]. Chucum is tapped from Pithecellobium albicans, a tree which is endemic in 25 states of Mexico. The resin is also known as Chucum blanco, guamoche, piquiche or guam´uchil. The Raman analysis of these endemic samples did not yield high-quality spectra, although long measuring times were applied. Fluorescence was seriously hampering the investigations, and the spectra b, d and e in Fig. 3 are therefore baseline corrected. The high levels of noise are a consequence of the shot noise from the fluorescence background. The Raman spectrum of pixoy (Fig. 3a) is dominated by two intense Raman bands that correspond with those of rutile (TiO2 ). This white mineral may have been added intentionally to give the gum a paler shine or it might be introduced accidentally during harvesting the gum. When examining the small Raman bands above 600 cm−1 (Fig. 3b), some features are observed in the area between 1500 and 1200 cm−1 , typical for δ(CH2 ) deformations
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the Raman spectra of peach gum and gum Arabic do not have features in this region, since the polysaccharides in these spectra do not contain unsaturations. The Raman spectra of these two gums are highly similar, although clear differences can be observed in the spectral region below 900 cm−1 . This similarity is not surprising, since both gums mainly constitute of the same monosaccharide units (e.g. l-arabinose and d-galactose). 4. Conclusion
Fig. 4. Baseline-corrected Raman spectrum of aje gum. Raman spectra of gutta gum, peach gum and gum Arabic.
in polysaccharides. Clearly no Raman bands are observed which can be attributed to C C functional groups (ν(C C) stretch expected in the region above 1600 cm−1 ), typical for terpenoid resins. By mean of GC/MS terpenes were identified in archaeological samples of bitumen [11]. Fig. 3c shows that the Raman spectrum of chapopote is dominated by two intense but broad features, which can be attributed to the presence of carbon, which might originate from the bitumen fraction in the sample. The Raman spectra of another area of the sample of chapopote and chucum are very noisy and are presented in Fig. 3d and e. Only some broad features around 1600 and 1400 and 880 cm−1 can be observed. Fig. 4 presents the Raman spectra of four other specimen that were purchased on a local market, namely aje gum, gutta gum, peach gum and gum Arabic. Aje gum is named a gum, although it merely is a resinous material of animal origin. Aje (as well named axi or axin) is a product of the insect Coccus axin, which lives in the branches of the Jatropha curcas and Spondias mombin trees. The material has been used since the prehispanic era, mainly in Chiapas and Michoac´an. Opposite to the use of the well-known animal resin shellac (product of the Coccus lacca insects), which was used as a spirit varnish, aje is usually used along with Chia oil as an oil varnish. Opposite to what its name suggests, gutta gum (also named gummi gutti or gamboge) is not a gum but a resin. It is the exudate from Garcinia plants and is well appreciated for its yellow colour. Peach gum is a gum, which is often used in tempera systems, sometimes in combination with casein, whereas gum Arabic originates from different Acacia species. The Raman spectra of aje and gutta gum clearly present some features between 1600 and 1700 cm−1 , which can be assigned to the ν(C C) stretch of terpenoid materials. Clearly,
In this paper, several natural conservation materials were examined by means of Raman spectroscopy. These products were purchased on local markets in Mexico, harvested directly from the plants or natural seeps, apart from the materials that are of general use in conservation practice, they also comprehend a range of traditional materials from endemic species. One of the interesting aspects of this research is that several products are often labelled as a gum, whereas they are of terpenoid (i.e. resinous) nature. Apart from plant products, Raman spectra are also presented of Campeche’s wax, a beeswax originating from the local Melipona bee, and of the animal resin Aje. Acknowledgements The authors thank the Research Foundation-Flanders (FWOVlaanderen) for its financial support. P.V. wishes to acknowledge the Research Foundation-Flanders (FWO-Vlaanderen) for his postdoctoral grant. They also thank Rolando Araujo, Cristina Ru´ız and Yareli Jaidar for providing samples of some local natural materials. References [1] F.M. Cort´es, Pegamentos, Gomas y Resinas en el M´exico Prehisp´anico, Resistol S.A, M´exico, 1997. [2] E.C.G. Franco, M.A.F.G. Salas, Investigaci´on de los Adhesivos Empleados en la Conservaci´on en M´exico, Tesis de Licenciatura, M´exico, 1979. [3] Z.N. Guti´errez, T´ecnicas Fisicoqu´ımicas e Instrumentales Utilizadas en la Detecci´on de Falsificaciones de Obra Gr´afica: Alcances y Limitaciones, Tesis de Maestr´ıa, M´exico, 2001. [4] H.G.M. Edwards, D.W. Farwell, L. Daffner, Spectrochim. Acta A 52 (1996) 1639. [5] H.G.M. Edwards, D.W. Farwell, Spectrochim. Acta A 52 (1996) 1119. [6] H.G.M. Edwards, M.G. Sibley, C. Heron, Spectrochim. Acta A 53 (1997) 2373. [7] H.G.M. Edwards, M.J. Falk, J. Raman Spectrosc. 28 (1997) 211. [8] P. Vandenabeele, B. Wehling, L. Moens, H. Edwards, M. De Reu, G. Van Hooydonk, Anal. Chim. A 407 (2000) 261. [9] P. Vandenabeele, D.M. Grimaldi, H.G.M. Edwards, L. Moens, Spectrochim. Acta A 59 (2003) 2221. [10] S. Koob, J. Am. Inst. Conserv. 37 (1) (1998) 49. [11] C.J. Wendt, Bitumen Sourcing in the Olmec Region, Reports submitted to FAMSI: Foundation for the Advancement of Mesoamerican Studies, 1994.
