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Available online at www.sciencedirect.com 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 TiO 2 , 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. How- ever, 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, lit- tle 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 men- tioned in ancient documents, including rubber, binding media, lacquers and natural adhesives, coming mainly from secre- tions of insects, trees and local fruits. For conservators, it is vital to be able to identify the different materials of Corresponding author. Tel.: +32 9 264 66 23; fax: +32 9 264 66 99. E-mail address: [email protected] (P. Vandenabeele). which ancient objects and artworks consist, since this deter- mines 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 char- acter 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 Mex- ico. The technique has not been limited to the identification of inorganic artists’ materials; several Raman spectroscopic stud- ies 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 Mex- ican organic compounds that conservation scientist think were used by ancient artists or craftsmen, and which could be applied nowadays for restoration purposes. 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.01.031

Raman spectroscopic analysis of Mexican natural artists’ materials

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Page 1: Raman spectroscopic analysis of Mexican natural artists’ materials

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Available online at www.sciencedirect.com

Spectrochimica Acta Part A 68 (2007) 1085–1088

Raman spectroscopic analysis of Mexicannatural artists’ materials

Peter Vandenabeele a,∗, Mayahuel Ortega-Aviles b,Dolores Tenorio Castilleros c, Luc Moens a

a Ghent University, Department of Analytical Chemistry, Proeftuinstraat 86, B-9000 Ghent, Belgiumb UAEM, Facultad de Ingenierıa, Cerro de Coatepec S/N, C.P. 50000, Toluca, Mexico

c ININ, Carretera Federal Mexico-Toluca km 36.5, Salazar Edo. de Mexico, C.P. 52045, Mexico

Received 25 October 2006; accepted 26 January 2007

bstract

This work represents the Raman spectra of 15 natural artists’ materials that were obtained from local market in Mexico. Some of these productsre not endemic to the region, but are often used in local conservation practice. Other materials are of local origin and have been used for centuriesy local craftsmen. The Raman spectra that are reported here are: Chia oil, linseed oil, Campeche wax, beeswax, white copal, dammar, colophony,astic, pixoy, chapopote, chucum, aje gum, gutta gum, peach gum and gum Arabic. The sample of pixoy was mixed with TiO , although it was not

2

lear whether this was done intentionally or not. The Raman spectrum of chapopote, the local name for bitumen, contained features of carbonaceousnd terpenoid matter. The Raman spectra of chapopote and chucum suffered severely from fluorescence, resulting in noisy Raman spectra. Ajeum 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.

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eywords: Art analysis; Raman spectroscopy; Mexican materials; Restoration;

. Introduction

In Mexico, conservation scientists are continuously trying toreserve the enormous cultural patrimony of the country. How-ver, one of the important obstacles that they find during theirork is the lack on knowledge of the traditional materials thatave been used by Mexican artists and craftsmen. Indeed, lit-le is known on which materials that were used, and moreover,here is need on information on the properties of these materialss well as on the way they were applied [1]. When trying tose traditional materials, conservators encounter difficulties toelect suitable materials, techniques and procedures to reach aood restoration.

Historically, different types of natural materials were men-ioned in ancient documents, including rubber, binding media,

acquers and natural adhesives, coming mainly from secre-ions of insects, trees and local fruits. For conservators, its vital to be able to identify the different materials of

∗ Corresponding author. Tel.: +32 9 264 66 23; fax: +32 9 264 66 99.E-mail address: [email protected] (P. Vandenabeele).

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386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2007.01.031

ral artists’ materials; Binding medium; Resins; Gums; Bitumen

hich ancient objects and artworks consist, since this deter-ines the methods and materials that will be used during

onservation [2].Raman spectroscopy may be a suitable technique to obtain

fast and adequate identification of these natural materialshat were used [3]. Indeed, especially the non-destructive char-cter of the approach is well appreciated. The technique haseen used for the investigation of different types of artworks,ncluding manuscripts, polychromes, and paintings, includingall painting fragments from an archaeological site in Mex-

co. The technique has not been limited to the identification ofnorganic artists’ materials; several Raman spectroscopic stud-es were performed on organic binding media and resins, usingourier-transform (FT-) Raman spectroscopy [4–7] as well asispersive Raman spectroscopy [8,9].

In order to be able to identify the local organic materials thatere used to elaborate ancient Mexican artefacts, it is necessary

o have access to a spectral database, containing local reference

aterials. This paper examines the Raman spectra of some Mex-

can organic compounds that conservation scientist think weresed by ancient artists or craftsmen, and which could be appliedowadays for restoration purposes.

Page 2: Raman spectroscopic analysis of Mexican natural artists’ materials

1 ica Acta Part A 68 (2007) 1085–1088

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086 P. Vandenabeele et al. / Spectrochim

. Experimental

.1. Samples

Fifteen samples were investigated by dispersive Raman spec-roscopy and compared with spectra from samples of commonroducts from the restoration practice. Some local samples werebtained from restorers from the National School of Restoration,exico, while the other samples were harvested directly from

he plants or natural seeps. Similar materials are considered toe used by Native Americans, for the construction of their art-orks. An overview of these samples and their main use is given

n Table 1.

.2. Raman spectroscopy

Raman spectroscopy was performed by using a Renishawystem-1000 spectrometer (Wotton-Under-Edge, UK). The

nstrument is equipped with a diode laser with a laser wave-ength of 785 nm and an output power of 50 mW. Laser intensityn the sample could be modified up to ca. 5 mW, by using a setf neutral density filters. In order to take possible sample inho-ogeneity into account, for each sample at least three Raman

pectra were recorded by using the 50× objective lens, allowingor a spectral footprint of ca. 2 �m. All spectra were recordedetween 400 and 1800 cm−1 for at least five accumulations of0 s.

. Results and discussion

.1. Fatty acid containing samples

The group of fatty acid containing binding media consists ofwo subclasses, namely the waxes and the drying oils [8]. Chia oils a typical drying oil, which has frequently been used in Mexico.

he plant source of this binding medium is Salvia hispanica.part from its use as artistic binding medium, the oil is as wellsed as food source. In Fig. 1a, its Raman spectrum is comparedith the Raman spectrum of a sample of linseed oil, as obtained

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able 1verview of the samples that were investigated in this study and their main applicatio

ocal name English name

hıa Chia oilinseed oilera de Campeche Campeche’s waxeeswaxopal biancoammarolophonyastic

ixoyhapopote Bitumenhucumje gumutta gum Gamboge, gummi guttieach gumum Arabic

ig. 1. Baseline-corrected Raman spectra of (a) Chia oil and linseed oil, and (b)ampeche’s wax and beeswax.

rom a Mexican restorer. The Raman bands of both products arebserved at the same positions, although some differences inheir relative intensities are observed. The chemical compositionf Chia oil is very similar to that of linseed oil, which explainshe similarity between the spectra of both oils.

Fig. 1b shows the Raman spectrum of Campeche’s wax, com-ared to the reference spectrum from beeswax [8]. It is clear

hat the spectra are very similar, since Campeche’s wax is aype of beeswax that is produced by the local bee species called

elipona. Beeswax consists mainly of saturated unbranchedompounds of high molecular weight. These include esters of

ns for artistic purposes

Source Main use as artists’ material

Salvia hispanica Binding mediumLinum usitutissimum Binding mediumApis melipona Binding mediumApis mellifica Binding mediumBursera species VarnishDipterocarpaceae species VarnishPinus species VarnishPistacia lentiscus VarnishGuazuma ulmifolia Binding medium

Varnish, gluePithecellobium albicans Binding mediumCoccus axin Binding mediumGarcinia handbury ColorantPrunus persica Binding mediumAcacia Binding medium

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P. Vandenabeele et al. / Spectrochimica Acta Part A 68 (2007) 1085–1088 1087

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ig. 2. Raman spectra of some resins: white copal, dammar, colophony andastic.

atty acids and high molecular weight alcohols, the correspond-ng free fatty acids and monohydric alcohols and hydrocarbons.he main component is myristyl palmitate ester. This compo-ition is as well reflected in the Raman spectrum: no bandsorresponding with C C vibrations are observed, such as the(C C) at ca. 1650 cm−1 or the in plane d(CH) of dialkylthylenes at ca. 1160 cm−1.

.2. Gums and resins

In restoration practice, plant exudates are often used as bind-ng medium or as a protective coating on the artefact. Some ofhese are of polysaccharide nature, whereas others have merelyterpenoid structure. The first group – the gums – are solubler swell when immersed in water, while the resins are insoluble.esins are of terpenoid origin and their Raman spectra contain

eatures that can be attributed to ν(C C) stretching vibrations,ypical bands between 1800 and 1600 cm−1. On the contrary,aman spectra of gums usually do not contain features above500 cm−1, as their polysaccharide structures do not containnsaturated C C bonds.

Fig. 2 represents the Raman spectra of four resin samples,hich are often used in conservation practice. These samplesere obtained from local markets but dammar, colophony andastic are not endemic to Mexico. White copal is used as a var-

ish and is extracted from trees of the Burseraceae family. Beforesing it as a varnish, the resin is dissolved in turpentine or an oilyedium. Dammar is a varnish that became quite popular since

he 19th century and its main component is dammarolic acidC54H77O3(COOH)2). After tapping it from Dipterocarpaceaerees, it is dissolved in turpentine and applied as a spirit varnish.olophony is a resin that is extracted from pine trees (Pinus) andonsists mainly of rosin acids (C19H20COOH). After harvesting,he volatile components of the pure resin are removed throughistillation. Mastic originates from Pistacia lentiscus and has

ften been used as a solution in drying a oil or in a volatileolvent, such as turpentine. Raman spectra of resins contain dis-inct Raman bands above 1600 cm−1, which can be attributed tohe ν(C C) stretching vibration. Resins are of terpenoid origin

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ig. 3. (a) Raman spectrum of pixoy; (b) baseline-corrected Raman spectrumf pixoy; (c) Raman spectrum of chapopote; (d) baseline-corrected Ramanpectrum of chapopote (different area) and (e) chucum.

nd therefore they all contain C C units. Raman bands in theegions between 1480 and 950 cm−1 can generally be attributedo δ(CH), δ(CH2) and δ(CH3) deformations and ν(CC) stretchingibrations.

Fig. 3 presents the Raman spectra of three other local sam-les (pixoy, chapopote, chucum). These samples are materialshat are traditionally used in native Mexican art objects andherefore they are of importance to local conservation scien-ists. Pixoy is a gum that originates from bay cedar (Guazumalmifolia), a local tree in Central America. This material haseveral local names, such as caulote, chicharron, cumulote,uacimo, guacimillo, guacimo caulote or guacimo de ternero.hapopote is the Mexican name for bitumen. Natural bitumen

eeps occur in pockets along the Mexican Gulf coastal plain.hapopote was processed by mixing it with mineral or vege-

al additives (e.g. resins) to make that it would become rigidfter application and that it would not readily melt in the sun10]. Prehispanic mesoamerican peoples collected, processednd used bitumen for decoration, as a sealant, adhesive, build-ng construction material, chewing gum, inciense, paint, bodydornment and fuel [11]. Chucum is tapped from Pithecellobiumlbicans, a tree which is endemic in 25 states of Mexico. Theesin is also known as Chucum blanco, guamoche, piquiche oruamuchil. The Raman analysis of these endemic samples didot yield high-quality spectra, although long measuring timesere applied. Fluorescence was seriously hampering the inves-

igations, and the spectra b, d and e in Fig. 3 are therefore baselineorrected. The high levels of noise are a consequence of thehot noise from the fluorescence background. The Raman spec-rum of pixoy (Fig. 3a) is dominated by two intense Ramanands that correspond with those of rutile (TiO2). This whiteineral may have been added intentionally to give the gum a

aler shine or it might be introduced accidentally during har-esting the gum. When examining the small Raman bands above00 cm−1 (Fig. 3b), some features are observed in the areaetween 1500 and 1200 cm−1, typical for δ(CH2) deformations

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1088 P. Vandenabeele et al. / Spectrochimica A

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ig. 4. Baseline-corrected Raman spectrum of aje gum. Raman spectra of guttaum, peach gum and gum Arabic.

n polysaccharides. Clearly no Raman bands are observed whichan be attributed to C C functional groups (ν(C C) stretchxpected in the region above 1600 cm−1), typical for terpenoidesins. By mean of GC/MS terpenes were identified in archaeo-ogical samples of bitumen [11]. Fig. 3c shows that the Ramanpectrum of chapopote is dominated by two intense but broadeatures, which can be attributed to the presence of carbon, whichight originate from the bitumen fraction in the sample. Theaman spectra of another area of the sample of chapopote andhucum are very noisy and are presented in Fig. 3d and e. Onlyome broad features around 1600 and 1400 and 880 cm−1 cane observed.

Fig. 4 presents the Raman spectra of four other specimen thatere purchased on a local market, namely aje gum, gutta gum,each gum and gum Arabic. Aje gum is named a gum, althought merely is a resinous material of animal origin. Aje (as wellamed axi or axin) is a product of the insect Coccus axin, whichives in the branches of the Jatropha curcas and Spondias mom-in trees. The material has been used since the prehispanic era,ainly in Chiapas and Michoacan. Opposite to the use of theell-known animal resin shellac (product of the Coccus lacca

nsects), which was used as a spirit varnish, aje is usually usedlong with Chia oil as an oil varnish. Opposite to what its nameuggests, gutta gum (also named gummi gutti or gamboge) isot a gum but a resin. It is the exudate from Garcinia plants ands well appreciated for its yellow colour. Peach gum is a gum,hich is often used in tempera systems, sometimes in combina-

ion with casein, whereas gum Arabic originates from differentcacia species. The Raman spectra of aje and gutta gum clearlyresent some features between 1600 and 1700 cm−1, which cane assigned to the ν(C C) stretch of terpenoid materials. Clearly,

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cta Part A 68 (2007) 1085–1088

he Raman spectra of peach gum and gum Arabic do not haveeatures in this region, since the polysaccharides in these spec-ra do not contain unsaturations. The Raman spectra of thesewo gums are highly similar, although clear differences can bebserved in the spectral region below 900 cm−1. This similaritys not surprising, since both gums mainly constitute of the same

onosaccharide units (e.g. l-arabinose and d-galactose).

. Conclusion

In this paper, several natural conservation materials werexamined by means of Raman spectroscopy. These productsere purchased on local markets in Mexico, harvested directly

rom the plants or natural seeps, apart from the materials that aref general use in conservation practice, they also comprehendrange of traditional materials from endemic species. One of

he interesting aspects of this research is that several productsre often labelled as a gum, whereas they are of terpenoid (i.e.esinous) nature. Apart from plant products, Raman spectra arelso presented of Campeche’s wax, a beeswax originating fromhe local Melipona bee, and of the animal resin Aje.

cknowledgements

The authors thank the Research Foundation-Flanders (FWO-laanderen) for its financial support. P.V. wishes to acknowledge

he Research Foundation-Flanders (FWO-Vlaanderen) for hisostdoctoral grant. They also thank Rolando Araujo, Cristinauız and Yareli Jaidar for providing samples of some localatural materials.

eferences

[1] F.M. Cortes, Pegamentos, Gomas y Resinas en el Mexico Prehispanico,Resistol S.A, Mexico, 1997.

[2] E.C.G. Franco, M.A.F.G. Salas, Investigacion de los Adhesivos Empleadosen la Conservacion en Mexico, Tesis de Licenciatura, Mexico, 1979.

[3] Z.N. Gutierrez, Tecnicas Fisicoquımicas e Instrumentales Utilizadas en laDeteccion de Falsificaciones de Obra Grafica: Alcances y Limitaciones,Tesis de Maestrıa, Mexico, 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, Spec-

trochim. 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.