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Raman spectroscopic analysis of the Maya wall paintings in Ek’Balam, Mexico
Spectrochimica Acta Part A 61 (2005) 2349–2356
Raman spectroscopic analysis of the Maya wall paintings in Ek’Balam, Mexico P. Vandenabeele a,∗ , S. Bod´e a , A. Alonso b , L. Moens a b
a Ghent University, Department of Analytical Chemistry, Proeftuinstraat 86, B-9000 Ghent, Belgium Subdireccion de Conservacion Arqueologica y Acabads Arquitectonicos, Coordinacion Nacional de Restauracion del Patrimonio Cultural, INAH, Xicotencatl y General Anaya s/n, San Diego Churubusco, 04010. Mexico D.F., Mexico
Received 10 June 2004; accepted 4 February 2005
Abstract Raman spectroscopy has been applied to the examination of wall painting fragments from the archaeological site of Ek’Balam (Yucat´an, Mexico). Thirty-three samples have been studied, all originating from room 23 of the Acropolis, and being representative of the painting technique at Ek’Balam during the late Classic Maya period. Several pigments such as haematite, calcite, carbon, cinnabar and indigo were identified in these samples. The latter pigment was presumed to be present as ‘Maya blue’, which is an intercalation product of indigo and palygorskite clay. The observed Raman spectra are reported and some band assignments have been made. This survey is the first Raman spectroscopic examination of a whole set of pigments in archaeological Maya wall painting fragments. © 2005 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Pigment analysis; Maya wall paintings; Ek’Balam; Maya blue
1. Introduction During recent years, the number of applications of Raman spectroscopy in art and archaeology has constantly been growing [1–5]. Nowadays, in many conservation laboratories all over the world, the method has obtained a position amongst standard techniques, such as infrared (IR) spectroscopy, electron microscopy, X-ray fluorescence (XRF) or chromatographic techniques, this being a consequence of several advantageous features, notably its non-destructive character, the ability to obtain molecular information on a micrometer scale and the possibility to examine inorganic as well as organic substances. Historically, the introduction of new applications in Raman spectroscopic research has always progressed along with the improvements in instrumentation. The introduction of Raman spectroscopy to the examination of artistic and archaeological objects was instrument driven: accessibility ∗
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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of FT-instrumentation, new charge coupled device (CCD) detectors for dispersive instrumentation, new lasers, (confocal) Raman microscopy and recently the introduction of mobile, fibre optics-based instruments have favoured this evolution. Although the first applications of Raman spectroscopy in art and archaeology were often mediaeval manuscripts [6–9], several wall painting fragments have been examined as well. Well-known examples are the examination of mediaeval and Roman wall paintings in churches and chapels [10,11], as well as the study of fragments of prehistoric cave art [12]. In these studies, inorganic pigments have usually been identified by using Raman instruments operating at different laser wavelengths. Recently, mobile Raman instrumentation has been applied for the direct examination of mediaeval wall paintings on the ceiling of a chapel in the Walloon region in Belgium [13]. The study in the current paper uses dispersive Raman instrumentation, operated with a 785-nm laser, for the examination of wall painting fragments from the famous Maya site of Ek’Balam on the Yucat´an peninsula in current Mexico.
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Mesoamerican cultural remains have been studied by using several analytical methods. Among these, Raman spectroscopy has been applied in a limited number of studies of antique objects, mostly focussing on local ancient resins [14–16], such as copals, ambers and other terpenoids. Some minerals, mainly opals and jade, have been studied extensively [17–19]. A paper has been published on the Raman spectroscopic examination of Native American Indian rock art and others on the analysis of pigments on pre-Columbian statues [20–22]. This paper describes for the first time a Raman spectroscopic survey of antique Maya wall paintings. The objectives of this paper are: Firstly the identification of the original pigments on the wall paintings of the famous Maya site of Ek’Balam, located on the Yucat´an peninsula in Mexico. The identification of these materials contributes to a better knowledge of the techniques that have been applied in these wall paintings. This is of assistance to the conservators who preserve these artefacts. Moreover, this knowledge contributes to extending the general knowledge of the Maya culture, especially the late classic period. Secondly this paper intends to demonstrate some of the abilities of Raman spectroscopy for the study of Mesoamerican wall paintings.
2. Experimental Dispersive Raman spectroscopy has been performed using two spectrometers, each operating at 785 nm laser wavelength. The first instrument is a Renishaw System-1000 spectrometer, equipped with a 50 mW laser source (∼5 mW at the sample). The fragments were mounted on microscope slides by means of a piece of plasticine and then positioned on the stage of the Olympus BH-2 microscope. The 5× objective was used for easy positioning, whereas the analysis was performed with the 20× or 50× (Olympus) objective lenses. The Rayleigh line was removed from the spectrum by means of a holographic notch filter, while the Raman light was dispersed on a 1200 grooves/mm grating; the Raman signal was detected on a Peltier cooled CCD detector, allowing us to obtain a spectral resolution of ∼1 cm−1 . All the samples have also been examined with a fibreoptics-based dispersive instrument [23]. This instrument is equipped with a 300 mW laser, accounting for up to 80 mW at the sample; by adjusting the laser current, the power can be reduced in order to avoid possible damage. The spectrometer was operated in the low-resolution mode (∼8 cm−1 ), yielding a spectral window of about 2700 cm−1 . Spectra were recorded with a 20× objective (Mitutoyo). The instrument is calibrated by using a neon lamp and several standards (sulphur, acetonitrile/toluene, cyclohexane, polystyrene and εcaprolactone). An elaborate description of this instrument is given elsewhere [23]. Using two different instruments, it was possible to perform an explorative investigation of each sample using the fibre optics spectrometer first (at low resolution using a more
powerful laser), and then performing a more time consuming analysis on the Renishaw instrument (at a lower laser power, at higher resolution and using confocal properties of the instrument). In general, the Renishaw instrument yielded better spectra.
3. Maya culture and the site of Ek’Balam Maya culture is one of the most important and sophisticated ancient cultures from the Mesoamerica cultural area of pre-Columbian times. Since the year 500 b.c. these native cultural groups inhabited the region of southern Mexico and the higher lands of Central American countries such as Guatemala, El Salvador, Belize and Honduras. A few Maya people still survive, and dwell this region in small villages, preserving some of the ancient Maya traditions, such as their language and ritual festivities. The ancient Mayas developed in a hierarchical society. Political, religious and economical powers were concentrated in the form of kings and their families, ruling different urban centres. The big Maya cities concentrated local power and contributed to it by setting up a society consisting of well-differentiated groups. Maya cities were used by regional states to manage the agrarian production of an immense peasant population. Military force and religious authorities were part of that state force and were also organised in a hierarchic way. Some Maya cities, like Cop´an (Honduras) or Yaxchil´an (Mexico) were the ideological support of a population up to 20,000 people. Their architectural centres were recognized as the core of the central power [24]. From the first millennium b.c. onwards, the Maya began to build public buildings, creating vast facades with a stage where spectacles were performed in front of plazas where people congregated [24,25]. The buildings and squares were profusely decorated with paintings, carved stones and stucco reliefs. In all the Maya cities, pyramidal temples were built around the plazas; sacred shrines were placed in the upper levels, dedicated to ancestors. Maya architects, artists and craftsmen were part of the upper class. Every Maya king and his family gathered and employed a group of artists to perform the building and decorative program of their power centre. This group of specialists was supposed to design an innovative architectural plan to create a remarkable place, distinguishing their town from the surrounding Maya cities. Besides the ancient main Maya cities of Chich´en Itz´a, Cob´a and Tul´um, three urban centres from different periods, the city of Ek’Balam was one of the few important settlements on the western edge of the Yucat´an peninsula, 40 km from present Valladolid (Mexico) (Fig. 1a). Ek’Balam was a medium size city with a long span of human occupation from the Pre-Classic (400 b.c.) to the late Classic (900 a.d.) periods [26]. Archaeologists of the site believe that Ek’Balam was an important city in its flourishing time, with an urban area of approximately 12 km2 . A stone fortress at its perimeter confined the political centre, while the site has a typical Maya
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Fig. 1. (a) The archaeological site of Ek’Balam is located on the Yucat´an peninsula, Mexico. The acropolis (b) is a large pyramidal construction, with beautiful stucco decorations (c and d).
arrangement of quadrangular plazas enclosed by pyramidal structures. During archaeological excavations in 1997–1998, marvellous stucco decors and mural paintings were discovered in what the archaeologists consider ‘the Acropolis’ of the site (Fig. 1b–d). The Acropolis, being an enormous building of 160 m × 70 m and 31 m tall, is a peculiar and complex structure with several construction phases in order to enlarge the small initial edifice located underneath later constructions [26]. The Acropolis has stone and modelled sculptures and reliefs in its facades. Moreover, a large number of rooms displayed fine and delicate wall painting decorations and modelled stucco elements. The marvellous stucco decorations necessitated carrying out a conservation program in order to preserve most of the elements in situ (Fig. 1c and d). An innovative project was proposed to run in parallel with the archaeological activities since 2001. Experimental research has been carried out to solve some of the main problems detected in stucco and mural paintings preservation, to understand the artistic program of the site and to differentiate its handcraft tradition from some other Maya sites.
4. Results and discussion Thirty-three samples of the Ek’Balam wall paintings have been studied extensively by means of Raman spectroscopy. Fragments of wall and stucco paintings were selected from
the interior of room 23 of the Acropolis. These samples consist of small fragments of different sizes, on an average less than 1 cm across. The room interior has been painted with a profuse decoration of several colours and designs. Most of the wall render was detached from its support because of ancient partial destruction of the room when the builders of the Acropolis filled the room with rocks and earth in order to give it sufficient strength to support the subsequent level of construction. Colours are present in different shades of red, yellow, green, blue and black (Fig. 2). The supporting ground layer behind has been examined as well. The Conservation Project considered the variety of the painting samples from this area as representative to determine the painting technology applied by the Ek’Balam artists in the late Classic Period. 4.1. Ground layer Raman analysis of the ground layer yielded the characteristic Raman bands of calcium carbonate which were found in every sample. In particular the band at ∼1086 cm−1 , which can be attributed to the symmetric stretch of the CO3 2− ion, is easily observed, and the Raman band at ∼712 cm−1 can be assigned to the in-plane bending vibration of calcite. Calcium carbonate has often been applied as stucco in wall paintings. Usually limestone was ground and heated (calcination), a process in which the calcium carbonate decomposes into calcium oxide and CO2 . Later, the CaO is mixed with water, leading to the formation of Ca(OH)2 . This lime mortar was applied on the walls where, in reaction with atmospheric CO2 , calcite was formed. It is remarkable that in these samples no gypsum
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Fig. 2. Overview of some of the Ek’Balam wall painting fragments that have been examined in this study. Different colours are observed: yellow, green, blue, red and black.
(CaSO4 ·2H2 O) has been identified, as this material is often encountered as a weathering product of calcium carbonate. This could be an indication of the absence of SOx , sulphur compounds and thiobacillus in the environment in which they have been conserved in these centuries. In most of the paint layers, the characteristic Raman bands of calcite (1086 cm−1 and 712 cm−1 ) have been observed, but it is not clear whether this is caused by the ground layer shining through the tiny pigment layer or whether the painters used a mixture of the pigment with calcium carbonate to obtain a brighter shade. A third interpretation of the ever-present characteristic Raman bands of calcite in the spectra could suggest that lime carbonised forming a protective coating (cf. fresco technique). It is known that some pre-Columbian people in Central Mexico synthesized a white pigment, dubbed teti¸catl or tizate, by heating and grinding of limestone [27]. This material was probably also used as a pigment by the Maya. Limestone was quarried by the Maya to build their monuments and carve their sculptures. It was also used to prepare thin layers of stucco to decorate floors and walls with white and painted plasters. The Maya used burned and slaked lime (calcium carbonate) mixed with ground stone or sand to produce stucco. Calcium carbonate was prepared in a form of mouldable cement to be applied directly over the surfaces of walls and floors, both indoors and outdoors of the main buildings. Some ornaments were also modelled and painted in stucco to decorate friezes and columns. Several elements of stucco were painted with mineral pigments suspended in an organic binder (a kind of a tempera technique) or applied directly to the fresh plaster made of stucco.
When examining the ground layer, black spots were observed. Sometimes black fibrous materials, obviously plant remains, were encountered. These fibres could have been added to the stucco to provide additional mechanical strength. When recording a Raman spectrum of this material, a spectrum of carbonised matter was obtained (mostly ν(C C), ν(C C), δ(CO) vibrations were observed). This material may originate from a contamination from plants, present in the soil used to fill the room, before constructing the next level of the Acropolis. 4.2. Red fragments When looking at the red samples, different shades could be distinguished, ranging from orange to deep red. From visual inspection, different pigments could be identified in these samples. Many samples show the characteristic Raman spectrum of haematite (Fe2 O3 , Fig. 3b) (Raman bands were observed on Raman shifts of 292 cm−1 with a shoulder around 299 cm−1 , 410 cm−1 , 496 cm−1 , 610 cm−1 and 1304 cm−1 (broad)) being the principal component of red ochre. This pigment can be found in many places all over the world. This pigment has frequently been applied, probably because it is very stable to light and different chemical environments and because it is easily available. Beside haematite, on some brightly coloured samples, the characteristic Raman spectrum of cinnabar (HgS) was observed (Fig. 3a). The pigment was found as a mineral, and could be applied directly, after grinding. The pigment could also be synthesized from its components, in order to obtain a pure pigment [28], but it is not known if the Mayas used this
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Fig. 3. Raman spectra obtained from Ek’Balam wall painting fragments: (a) cinnabar on a red area; (b) haematite and CaCO3 on a red area; (c) carbon in a black region and (d) a mixture of Maya blue, haematite and CaCO3 on a purple area. Spectra (b) and (d) are baseline corrected.
technique. Unfortunately, as both materials have the same chemical composition and structure it is impossible to discriminate between both materials by Raman spectroscopy. This pigment is very Raman active and the most intense band, at 254 cm−1 , can be assigned to the ν(Hg–S) stretching vibration. This is the first time that cinnabar has been identified in Maya mural paintings. It has been identified earlier to cover bones, ritual objects or stucco decorations [25], but never in wall paintings. In some red areas, it was impossible to identify the red colorant. Red crystals were observed, when observing the specimen through a microscope, but due to excessive fluorescence, no proper Raman spectrum could be obtained using a 785-nm laser. When focusing the laser beam on these crystals for several hours, the amount of fluorescence background was reduced, resulting in distinguishable bands of calcite but still no evidence of a red pigment. It may be suggested that a fluorescent red dye had been precipitated on CaCO3 crystals, to provide a different shade. 4.3. Blue fragments The blue areas from the wall painting fragments showed spectra like those in Fig. 4. These spectra show bands similar to those of the spectrum of indigo, but several bands are shifted. The material that is analysed is probably Maya blue, a famous pigment that has been identified in several Mesoamerican art objects [29–31]. This is an intercalation product of indigo in a palygorskite [(Mg,Al)2 Si4 O10 (OH)·4H2 O] matrix, the latter being a layered clay mineral of a white colour. In order to obtain purple shades, Maya blue was combined with haematite (Fig. 3d).
Fig. 4. Raman spectra of Maya blue as obtained from different coloured areas on the Ek’Balam wall painting fragments. Spectra (a–c) blue crystals, (d–f) turquoise crystals, (g) green crystals, (h–j) yellow crystals (baselinecorrected spectra). The band at 1086 cm−1 in some of the spectra is attributed to CaCO3 .
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One of the most remarkable properties of this pigment is its stability towards light and acids. It has been discussed for several years whether this material was of mineral or organic origin. In the 1960s Gettens demonstrated with Xray diffraction that the main component of this pigment is palygorskite, while Shipard mentioned the possibility of an organic component in this material [29]. In 1966 van Olphen found several possible synthesis routes to synthesize this material [30]. Experimental archaeologists tried to find the traditional way of synthesising this pigment: leaves and twigs of indigofera species were soaked overnight in a suspension of clay in water. Then the coarse material was removed by filtration and the clay suspension was intensively mixed, in order to ventilate the mixture; the indigo molecule became oxidised. After filtration, the clay was heated in a furnace, resulting in a blue, turquoise or green pigment. Another way of synthesising this pigment is heating the clay together with the indigo pigment. Good results were obtained with a 10% mixture of indigo, heated at 190 ◦ C, for 5 h [31]. The presence of indigo and the fact that the blue fragments were very good preserved (after probably more than 1000 years), gives us a very good indication, that effectively “Maya blue” is observed. Most of the Raman bands in the spectrum of Maya blue can be attributed to the indigotin molecule [32]. In comparison with the Raman spectrum of indigo, most bands have
shifted to higher wavenumbers: sometimes a wavenumber shift of ∼10 cm−1 is encountered, although some are not significantly shifted or shifted to lower wavenumbers. An overview of the Raman band positions of our spectra is given in Table 1. The Raman band assignments are based on the work by Tatsch and Schrader [33]. It is observed that in general the stretching vibrations are more sensitive to shifting than deformations are. In some cases, new bands arise, which are not observed in the Raman spectrum of indigo. In addition to the evident possibility that these bands can be assigned to palygorskite, it is also possible that they are due to the symmetry of the cavity in the clay matrix or due to a distortion of the indigo molecule, Raman inactive bands become Raman active. Indeed, in some cases the band positions correspond with the band positions in the infra-red spectrum of indigo, as observed by Tatsch and Schrader. 4.4. Green fragments The green areas in the Ek’Balam samples all resulted in spectra similar to those of the Maya blue samples. Although the colours of the blue and green areas are clearly different, the Raman spectra are identical. From the archaeological literature [31] it is known that the indigo/clay ratio determines the colour: if this ratio is small, then the pigment that is obtained has a more greenish shade [34]. This
Table 1 Overview of the Raman band positions of Maya blue, as determined in the Ek’Balam samples, compared with the band positions of indigo as referred [21,35] Blue samples Blue part of a Mayan statue [21] Indigo according to [36] Indigo according to [21] Assignment of the vibration Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) 1703 (−16) 1628 (+4) 1581 1573 (0)
1690 (−3) 1635 (−3)
ν(C O) ν(CC), δ(CH)
1577 (−4)
1462 (+1)
1459 (+4)
ν(CC), ν(C C), ν(C O) ν(CC), δ(CH) ν(CC), δ(CH)
1362 (+6) 1322 (−4) 1254 (0) 1221 (+3)
1364 (+4) 1310 (+8) 1250 (+4) 1224 (0)
1365 (+3) 1310 (+8) 1252 (+2) 1219 (+5)
1146 (0) 1107 (+1) 1023 (+3)
1148 (−2) 1096 (+10)
1147 (−1)
1687 1632
1680 (+7) 1631 (+1)
1573 1483 1463 1411 1368 1318 1254 1224 1191 1166 1146 1108 1026 1017 1008 930 862 811 758 673 600 586 555
1573 (0) 1493 (−10) 1466 (−3)
1013 (+4) 1005 (+3) 945 (−15) 861 (+1)
940 (−10)
754 (+4) 669 (+4) 599 (+1)
762 (−4) 674 (−1) 599 (+1)
(+) 599 (+1)
553 (+2)
545 (+10)
550 (+5)
(+)
δ(NH), δ(CH) ν(CC) δ(CH), δ(C O) δ(CH), ν(CN) ν(CC)ring δ(CC) δ(CC) δ(CC), γ(CH) γ(CH) δ(CH) δ(C CO C C) γ(CH) ν(CN) γ(CC)5-ring δ(CH), δ(N C C) δ(CC) δ(C O), δ(CH), δ(C NH C) γ(CH), γ(NH) δ(C C CO C)
Assignments are based on the vibrational analysis of the indigo molecule by Tatsch and Schrader [33]. Values between brackets indicate the shift (cm−1 ) compared to the spectra of the blue samples in this work. The (+) symbol indicates that in the spectra a weak band or shoulder was observed, although the band position was not reported by the authors.
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effect can be obtained either by using small amounts of indigofera or by short soaking times. When examining the green wall painting fragments, in some cases yellow crystals were observed. When recording a Raman spectrum of these crystals, the spectrum of Maya blue was obtained. One can suggest that in these cases the aeration of the indigo/clay mixture was incomplete and that the indigo molecule is trapped in the reduced form (leuko-indigo) into the crystal lattice. Raman spectra of different shades of Maya blue, ranging from blue to green and yellow crystals, are shown in Fig. 4. 4.5. Yellow fragments The yellow fragments of the Ek’Balam wall paintings showed a high amount of fluorescence, thus hampering the recording of the Raman spectra. It has been suggested that an organic dye might have been applied. Another yellow pigment that has often been used is yellow ochre, with limonite or goethite as its main mineral components. These minerals usually show a weak Raman spectrum, when working with a 785-nm laser, such that these will presumably be hidden when strong fluorescence occurs. 4.6. Black fragments The black pigment in these wall painting fragments is clearly carbon (Fig. 3c). This material has frequently been used since prehistoric times. This pigment is produced as soot of the incomplete combustion of organic matter, such as plants, oil, bones, ivory, etc. Pre-Columbian people generally used charcoal as black pigment in paint and in ink [35]. The Raman spectrum of semi-amorphous carbon shows two broad bands, in which the bandwidth and the relative intensity are measures of the crystalline character of the material.
5. Conclusions The pigments on the Maya wall painting fragments from the archaeological site of Ek’Balam have been identified by Raman spectroscopy. Haematite, cinnabar and carbon have been positively identified. The blue pigment in these fragments is Maya blue, an intercalation product of indigo in a palygorskite clay matrix. Moreover, in the green fragments the same spectrum was obtained and also some yellow crystals in these samples resulted in a similar Raman spectrum. In general, the pigments identified in this work confirm that the natural environment of the Maya offered a wealth of materials to fashion their works of fine quality [24].
Acknowledgments The authors would like to thank the Fund for Scientific Research – Flanders (FWO-Vlaanderen) and the Flemish Gov-
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ernment – Cultural Department (museumdecreet) for their financial support of the research projects. P.V. is most grateful to the Fund for Scientific Research – Flanders (FWOVlaanderen) for his postdoctoral grant.
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Raman spectroscopic analysis of the Maya wall paintings in Ek’Balam, Mexico P. Vandenabeele a,∗ , S. Bod´e a , A. Alonso b , L. Moens a b
a Ghent University, Department of Analytical Chemistry, Proeftuinstraat 86, B-9000 Ghent, Belgium Subdireccion de Conservacion Arqueologica y Acabads Arquitectonicos, Coordinacion Nacional de Restauracion del Patrimonio Cultural, INAH, Xicotencatl y General Anaya s/n, San Diego Churubusco, 04010. Mexico D.F., Mexico
Received 10 June 2004; accepted 4 February 2005
Abstract Raman spectroscopy has been applied to the examination of wall painting fragments from the archaeological site of Ek’Balam (Yucat´an, Mexico). Thirty-three samples have been studied, all originating from room 23 of the Acropolis, and being representative of the painting technique at Ek’Balam during the late Classic Maya period. Several pigments such as haematite, calcite, carbon, cinnabar and indigo were identified in these samples. The latter pigment was presumed to be present as ‘Maya blue’, which is an intercalation product of indigo and palygorskite clay. The observed Raman spectra are reported and some band assignments have been made. This survey is the first Raman spectroscopic examination of a whole set of pigments in archaeological Maya wall painting fragments. © 2005 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Pigment analysis; Maya wall paintings; Ek’Balam; Maya blue
1. Introduction During recent years, the number of applications of Raman spectroscopy in art and archaeology has constantly been growing [1–5]. Nowadays, in many conservation laboratories all over the world, the method has obtained a position amongst standard techniques, such as infrared (IR) spectroscopy, electron microscopy, X-ray fluorescence (XRF) or chromatographic techniques, this being a consequence of several advantageous features, notably its non-destructive character, the ability to obtain molecular information on a micrometer scale and the possibility to examine inorganic as well as organic substances. Historically, the introduction of new applications in Raman spectroscopic research has always progressed along with the improvements in instrumentation. The introduction of Raman spectroscopy to the examination of artistic and archaeological objects was instrument driven: accessibility ∗
Corresponding author. Tel.: +32 9 264 66 23; fax: +32 9 264 66 99. E-mail address: [email protected] (P. Vandenabeele).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.02.034
of FT-instrumentation, new charge coupled device (CCD) detectors for dispersive instrumentation, new lasers, (confocal) Raman microscopy and recently the introduction of mobile, fibre optics-based instruments have favoured this evolution. Although the first applications of Raman spectroscopy in art and archaeology were often mediaeval manuscripts [6–9], several wall painting fragments have been examined as well. Well-known examples are the examination of mediaeval and Roman wall paintings in churches and chapels [10,11], as well as the study of fragments of prehistoric cave art [12]. In these studies, inorganic pigments have usually been identified by using Raman instruments operating at different laser wavelengths. Recently, mobile Raman instrumentation has been applied for the direct examination of mediaeval wall paintings on the ceiling of a chapel in the Walloon region in Belgium [13]. The study in the current paper uses dispersive Raman instrumentation, operated with a 785-nm laser, for the examination of wall painting fragments from the famous Maya site of Ek’Balam on the Yucat´an peninsula in current Mexico.
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Mesoamerican cultural remains have been studied by using several analytical methods. Among these, Raman spectroscopy has been applied in a limited number of studies of antique objects, mostly focussing on local ancient resins [14–16], such as copals, ambers and other terpenoids. Some minerals, mainly opals and jade, have been studied extensively [17–19]. A paper has been published on the Raman spectroscopic examination of Native American Indian rock art and others on the analysis of pigments on pre-Columbian statues [20–22]. This paper describes for the first time a Raman spectroscopic survey of antique Maya wall paintings. The objectives of this paper are: Firstly the identification of the original pigments on the wall paintings of the famous Maya site of Ek’Balam, located on the Yucat´an peninsula in Mexico. The identification of these materials contributes to a better knowledge of the techniques that have been applied in these wall paintings. This is of assistance to the conservators who preserve these artefacts. Moreover, this knowledge contributes to extending the general knowledge of the Maya culture, especially the late classic period. Secondly this paper intends to demonstrate some of the abilities of Raman spectroscopy for the study of Mesoamerican wall paintings.
2. Experimental Dispersive Raman spectroscopy has been performed using two spectrometers, each operating at 785 nm laser wavelength. The first instrument is a Renishaw System-1000 spectrometer, equipped with a 50 mW laser source (∼5 mW at the sample). The fragments were mounted on microscope slides by means of a piece of plasticine and then positioned on the stage of the Olympus BH-2 microscope. The 5× objective was used for easy positioning, whereas the analysis was performed with the 20× or 50× (Olympus) objective lenses. The Rayleigh line was removed from the spectrum by means of a holographic notch filter, while the Raman light was dispersed on a 1200 grooves/mm grating; the Raman signal was detected on a Peltier cooled CCD detector, allowing us to obtain a spectral resolution of ∼1 cm−1 . All the samples have also been examined with a fibreoptics-based dispersive instrument [23]. This instrument is equipped with a 300 mW laser, accounting for up to 80 mW at the sample; by adjusting the laser current, the power can be reduced in order to avoid possible damage. The spectrometer was operated in the low-resolution mode (∼8 cm−1 ), yielding a spectral window of about 2700 cm−1 . Spectra were recorded with a 20× objective (Mitutoyo). The instrument is calibrated by using a neon lamp and several standards (sulphur, acetonitrile/toluene, cyclohexane, polystyrene and εcaprolactone). An elaborate description of this instrument is given elsewhere [23]. Using two different instruments, it was possible to perform an explorative investigation of each sample using the fibre optics spectrometer first (at low resolution using a more
powerful laser), and then performing a more time consuming analysis on the Renishaw instrument (at a lower laser power, at higher resolution and using confocal properties of the instrument). In general, the Renishaw instrument yielded better spectra.
3. Maya culture and the site of Ek’Balam Maya culture is one of the most important and sophisticated ancient cultures from the Mesoamerica cultural area of pre-Columbian times. Since the year 500 b.c. these native cultural groups inhabited the region of southern Mexico and the higher lands of Central American countries such as Guatemala, El Salvador, Belize and Honduras. A few Maya people still survive, and dwell this region in small villages, preserving some of the ancient Maya traditions, such as their language and ritual festivities. The ancient Mayas developed in a hierarchical society. Political, religious and economical powers were concentrated in the form of kings and their families, ruling different urban centres. The big Maya cities concentrated local power and contributed to it by setting up a society consisting of well-differentiated groups. Maya cities were used by regional states to manage the agrarian production of an immense peasant population. Military force and religious authorities were part of that state force and were also organised in a hierarchic way. Some Maya cities, like Cop´an (Honduras) or Yaxchil´an (Mexico) were the ideological support of a population up to 20,000 people. Their architectural centres were recognized as the core of the central power [24]. From the first millennium b.c. onwards, the Maya began to build public buildings, creating vast facades with a stage where spectacles were performed in front of plazas where people congregated [24,25]. The buildings and squares were profusely decorated with paintings, carved stones and stucco reliefs. In all the Maya cities, pyramidal temples were built around the plazas; sacred shrines were placed in the upper levels, dedicated to ancestors. Maya architects, artists and craftsmen were part of the upper class. Every Maya king and his family gathered and employed a group of artists to perform the building and decorative program of their power centre. This group of specialists was supposed to design an innovative architectural plan to create a remarkable place, distinguishing their town from the surrounding Maya cities. Besides the ancient main Maya cities of Chich´en Itz´a, Cob´a and Tul´um, three urban centres from different periods, the city of Ek’Balam was one of the few important settlements on the western edge of the Yucat´an peninsula, 40 km from present Valladolid (Mexico) (Fig. 1a). Ek’Balam was a medium size city with a long span of human occupation from the Pre-Classic (400 b.c.) to the late Classic (900 a.d.) periods [26]. Archaeologists of the site believe that Ek’Balam was an important city in its flourishing time, with an urban area of approximately 12 km2 . A stone fortress at its perimeter confined the political centre, while the site has a typical Maya
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Fig. 1. (a) The archaeological site of Ek’Balam is located on the Yucat´an peninsula, Mexico. The acropolis (b) is a large pyramidal construction, with beautiful stucco decorations (c and d).
arrangement of quadrangular plazas enclosed by pyramidal structures. During archaeological excavations in 1997–1998, marvellous stucco decors and mural paintings were discovered in what the archaeologists consider ‘the Acropolis’ of the site (Fig. 1b–d). The Acropolis, being an enormous building of 160 m × 70 m and 31 m tall, is a peculiar and complex structure with several construction phases in order to enlarge the small initial edifice located underneath later constructions [26]. The Acropolis has stone and modelled sculptures and reliefs in its facades. Moreover, a large number of rooms displayed fine and delicate wall painting decorations and modelled stucco elements. The marvellous stucco decorations necessitated carrying out a conservation program in order to preserve most of the elements in situ (Fig. 1c and d). An innovative project was proposed to run in parallel with the archaeological activities since 2001. Experimental research has been carried out to solve some of the main problems detected in stucco and mural paintings preservation, to understand the artistic program of the site and to differentiate its handcraft tradition from some other Maya sites.
4. Results and discussion Thirty-three samples of the Ek’Balam wall paintings have been studied extensively by means of Raman spectroscopy. Fragments of wall and stucco paintings were selected from
the interior of room 23 of the Acropolis. These samples consist of small fragments of different sizes, on an average less than 1 cm across. The room interior has been painted with a profuse decoration of several colours and designs. Most of the wall render was detached from its support because of ancient partial destruction of the room when the builders of the Acropolis filled the room with rocks and earth in order to give it sufficient strength to support the subsequent level of construction. Colours are present in different shades of red, yellow, green, blue and black (Fig. 2). The supporting ground layer behind has been examined as well. The Conservation Project considered the variety of the painting samples from this area as representative to determine the painting technology applied by the Ek’Balam artists in the late Classic Period. 4.1. Ground layer Raman analysis of the ground layer yielded the characteristic Raman bands of calcium carbonate which were found in every sample. In particular the band at ∼1086 cm−1 , which can be attributed to the symmetric stretch of the CO3 2− ion, is easily observed, and the Raman band at ∼712 cm−1 can be assigned to the in-plane bending vibration of calcite. Calcium carbonate has often been applied as stucco in wall paintings. Usually limestone was ground and heated (calcination), a process in which the calcium carbonate decomposes into calcium oxide and CO2 . Later, the CaO is mixed with water, leading to the formation of Ca(OH)2 . This lime mortar was applied on the walls where, in reaction with atmospheric CO2 , calcite was formed. It is remarkable that in these samples no gypsum
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Fig. 2. Overview of some of the Ek’Balam wall painting fragments that have been examined in this study. Different colours are observed: yellow, green, blue, red and black.
(CaSO4 ·2H2 O) has been identified, as this material is often encountered as a weathering product of calcium carbonate. This could be an indication of the absence of SOx , sulphur compounds and thiobacillus in the environment in which they have been conserved in these centuries. In most of the paint layers, the characteristic Raman bands of calcite (1086 cm−1 and 712 cm−1 ) have been observed, but it is not clear whether this is caused by the ground layer shining through the tiny pigment layer or whether the painters used a mixture of the pigment with calcium carbonate to obtain a brighter shade. A third interpretation of the ever-present characteristic Raman bands of calcite in the spectra could suggest that lime carbonised forming a protective coating (cf. fresco technique). It is known that some pre-Columbian people in Central Mexico synthesized a white pigment, dubbed teti¸catl or tizate, by heating and grinding of limestone [27]. This material was probably also used as a pigment by the Maya. Limestone was quarried by the Maya to build their monuments and carve their sculptures. It was also used to prepare thin layers of stucco to decorate floors and walls with white and painted plasters. The Maya used burned and slaked lime (calcium carbonate) mixed with ground stone or sand to produce stucco. Calcium carbonate was prepared in a form of mouldable cement to be applied directly over the surfaces of walls and floors, both indoors and outdoors of the main buildings. Some ornaments were also modelled and painted in stucco to decorate friezes and columns. Several elements of stucco were painted with mineral pigments suspended in an organic binder (a kind of a tempera technique) or applied directly to the fresh plaster made of stucco.
When examining the ground layer, black spots were observed. Sometimes black fibrous materials, obviously plant remains, were encountered. These fibres could have been added to the stucco to provide additional mechanical strength. When recording a Raman spectrum of this material, a spectrum of carbonised matter was obtained (mostly ν(C C), ν(C C), δ(CO) vibrations were observed). This material may originate from a contamination from plants, present in the soil used to fill the room, before constructing the next level of the Acropolis. 4.2. Red fragments When looking at the red samples, different shades could be distinguished, ranging from orange to deep red. From visual inspection, different pigments could be identified in these samples. Many samples show the characteristic Raman spectrum of haematite (Fe2 O3 , Fig. 3b) (Raman bands were observed on Raman shifts of 292 cm−1 with a shoulder around 299 cm−1 , 410 cm−1 , 496 cm−1 , 610 cm−1 and 1304 cm−1 (broad)) being the principal component of red ochre. This pigment can be found in many places all over the world. This pigment has frequently been applied, probably because it is very stable to light and different chemical environments and because it is easily available. Beside haematite, on some brightly coloured samples, the characteristic Raman spectrum of cinnabar (HgS) was observed (Fig. 3a). The pigment was found as a mineral, and could be applied directly, after grinding. The pigment could also be synthesized from its components, in order to obtain a pure pigment [28], but it is not known if the Mayas used this
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Fig. 3. Raman spectra obtained from Ek’Balam wall painting fragments: (a) cinnabar on a red area; (b) haematite and CaCO3 on a red area; (c) carbon in a black region and (d) a mixture of Maya blue, haematite and CaCO3 on a purple area. Spectra (b) and (d) are baseline corrected.
technique. Unfortunately, as both materials have the same chemical composition and structure it is impossible to discriminate between both materials by Raman spectroscopy. This pigment is very Raman active and the most intense band, at 254 cm−1 , can be assigned to the ν(Hg–S) stretching vibration. This is the first time that cinnabar has been identified in Maya mural paintings. It has been identified earlier to cover bones, ritual objects or stucco decorations [25], but never in wall paintings. In some red areas, it was impossible to identify the red colorant. Red crystals were observed, when observing the specimen through a microscope, but due to excessive fluorescence, no proper Raman spectrum could be obtained using a 785-nm laser. When focusing the laser beam on these crystals for several hours, the amount of fluorescence background was reduced, resulting in distinguishable bands of calcite but still no evidence of a red pigment. It may be suggested that a fluorescent red dye had been precipitated on CaCO3 crystals, to provide a different shade. 4.3. Blue fragments The blue areas from the wall painting fragments showed spectra like those in Fig. 4. These spectra show bands similar to those of the spectrum of indigo, but several bands are shifted. The material that is analysed is probably Maya blue, a famous pigment that has been identified in several Mesoamerican art objects [29–31]. This is an intercalation product of indigo in a palygorskite [(Mg,Al)2 Si4 O10 (OH)·4H2 O] matrix, the latter being a layered clay mineral of a white colour. In order to obtain purple shades, Maya blue was combined with haematite (Fig. 3d).
Fig. 4. Raman spectra of Maya blue as obtained from different coloured areas on the Ek’Balam wall painting fragments. Spectra (a–c) blue crystals, (d–f) turquoise crystals, (g) green crystals, (h–j) yellow crystals (baselinecorrected spectra). The band at 1086 cm−1 in some of the spectra is attributed to CaCO3 .
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One of the most remarkable properties of this pigment is its stability towards light and acids. It has been discussed for several years whether this material was of mineral or organic origin. In the 1960s Gettens demonstrated with Xray diffraction that the main component of this pigment is palygorskite, while Shipard mentioned the possibility of an organic component in this material [29]. In 1966 van Olphen found several possible synthesis routes to synthesize this material [30]. Experimental archaeologists tried to find the traditional way of synthesising this pigment: leaves and twigs of indigofera species were soaked overnight in a suspension of clay in water. Then the coarse material was removed by filtration and the clay suspension was intensively mixed, in order to ventilate the mixture; the indigo molecule became oxidised. After filtration, the clay was heated in a furnace, resulting in a blue, turquoise or green pigment. Another way of synthesising this pigment is heating the clay together with the indigo pigment. Good results were obtained with a 10% mixture of indigo, heated at 190 ◦ C, for 5 h [31]. The presence of indigo and the fact that the blue fragments were very good preserved (after probably more than 1000 years), gives us a very good indication, that effectively “Maya blue” is observed. Most of the Raman bands in the spectrum of Maya blue can be attributed to the indigotin molecule [32]. In comparison with the Raman spectrum of indigo, most bands have
shifted to higher wavenumbers: sometimes a wavenumber shift of ∼10 cm−1 is encountered, although some are not significantly shifted or shifted to lower wavenumbers. An overview of the Raman band positions of our spectra is given in Table 1. The Raman band assignments are based on the work by Tatsch and Schrader [33]. It is observed that in general the stretching vibrations are more sensitive to shifting than deformations are. In some cases, new bands arise, which are not observed in the Raman spectrum of indigo. In addition to the evident possibility that these bands can be assigned to palygorskite, it is also possible that they are due to the symmetry of the cavity in the clay matrix or due to a distortion of the indigo molecule, Raman inactive bands become Raman active. Indeed, in some cases the band positions correspond with the band positions in the infra-red spectrum of indigo, as observed by Tatsch and Schrader. 4.4. Green fragments The green areas in the Ek’Balam samples all resulted in spectra similar to those of the Maya blue samples. Although the colours of the blue and green areas are clearly different, the Raman spectra are identical. From the archaeological literature [31] it is known that the indigo/clay ratio determines the colour: if this ratio is small, then the pigment that is obtained has a more greenish shade [34]. This
Table 1 Overview of the Raman band positions of Maya blue, as determined in the Ek’Balam samples, compared with the band positions of indigo as referred [21,35] Blue samples Blue part of a Mayan statue [21] Indigo according to [36] Indigo according to [21] Assignment of the vibration Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) Raman wavenumber (cm−1 ) 1703 (−16) 1628 (+4) 1581 1573 (0)
1690 (−3) 1635 (−3)
ν(C O) ν(CC), δ(CH)
1577 (−4)
1462 (+1)
1459 (+4)
ν(CC), ν(C C), ν(C O) ν(CC), δ(CH) ν(CC), δ(CH)
1362 (+6) 1322 (−4) 1254 (0) 1221 (+3)
1364 (+4) 1310 (+8) 1250 (+4) 1224 (0)
1365 (+3) 1310 (+8) 1252 (+2) 1219 (+5)
1146 (0) 1107 (+1) 1023 (+3)
1148 (−2) 1096 (+10)
1147 (−1)
1687 1632
1680 (+7) 1631 (+1)
1573 1483 1463 1411 1368 1318 1254 1224 1191 1166 1146 1108 1026 1017 1008 930 862 811 758 673 600 586 555
1573 (0) 1493 (−10) 1466 (−3)
1013 (+4) 1005 (+3) 945 (−15) 861 (+1)
940 (−10)
754 (+4) 669 (+4) 599 (+1)
762 (−4) 674 (−1) 599 (+1)
(+) 599 (+1)
553 (+2)
545 (+10)
550 (+5)
(+)
δ(NH), δ(CH) ν(CC) δ(CH), δ(C O) δ(CH), ν(CN) ν(CC)ring δ(CC) δ(CC) δ(CC), γ(CH) γ(CH) δ(CH) δ(C CO C C) γ(CH) ν(CN) γ(CC)5-ring δ(CH), δ(N C C) δ(CC) δ(C O), δ(CH), δ(C NH C) γ(CH), γ(NH) δ(C C CO C)
Assignments are based on the vibrational analysis of the indigo molecule by Tatsch and Schrader [33]. Values between brackets indicate the shift (cm−1 ) compared to the spectra of the blue samples in this work. The (+) symbol indicates that in the spectra a weak band or shoulder was observed, although the band position was not reported by the authors.
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effect can be obtained either by using small amounts of indigofera or by short soaking times. When examining the green wall painting fragments, in some cases yellow crystals were observed. When recording a Raman spectrum of these crystals, the spectrum of Maya blue was obtained. One can suggest that in these cases the aeration of the indigo/clay mixture was incomplete and that the indigo molecule is trapped in the reduced form (leuko-indigo) into the crystal lattice. Raman spectra of different shades of Maya blue, ranging from blue to green and yellow crystals, are shown in Fig. 4. 4.5. Yellow fragments The yellow fragments of the Ek’Balam wall paintings showed a high amount of fluorescence, thus hampering the recording of the Raman spectra. It has been suggested that an organic dye might have been applied. Another yellow pigment that has often been used is yellow ochre, with limonite or goethite as its main mineral components. These minerals usually show a weak Raman spectrum, when working with a 785-nm laser, such that these will presumably be hidden when strong fluorescence occurs. 4.6. Black fragments The black pigment in these wall painting fragments is clearly carbon (Fig. 3c). This material has frequently been used since prehistoric times. This pigment is produced as soot of the incomplete combustion of organic matter, such as plants, oil, bones, ivory, etc. Pre-Columbian people generally used charcoal as black pigment in paint and in ink [35]. The Raman spectrum of semi-amorphous carbon shows two broad bands, in which the bandwidth and the relative intensity are measures of the crystalline character of the material.
5. Conclusions The pigments on the Maya wall painting fragments from the archaeological site of Ek’Balam have been identified by Raman spectroscopy. Haematite, cinnabar and carbon have been positively identified. The blue pigment in these fragments is Maya blue, an intercalation product of indigo in a palygorskite clay matrix. Moreover, in the green fragments the same spectrum was obtained and also some yellow crystals in these samples resulted in a similar Raman spectrum. In general, the pigments identified in this work confirm that the natural environment of the Maya offered a wealth of materials to fashion their works of fine quality [24].
Acknowledgments The authors would like to thank the Fund for Scientific Research – Flanders (FWO-Vlaanderen) and the Flemish Gov-
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ernment – Cultural Department (museumdecreet) for their financial support of the research projects. P.V. is most grateful to the Fund for Scientific Research – Flanders (FWOVlaanderen) for his postdoctoral grant.
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