10.6092/issn.1973-9494/7945
Micro-Raman spectroscopy was used to analyze and identify pigments and determine the palette of color in wall paintings from four churches of the Colonial Period, in the Bolivian highlands. The ancient churches Santiago de Callapa, Curahuara de Carangas, NuestraSeñora de Copacabana de Andamarca and San José de Soracachi were constructed during the 17th and 18th centuries, around the important colonial Ruta de la Plata, between Potosí city and Arica harbor on the Pacific coast of South America. Most pigments used in the wall paintings correspond to inorganic minerals as well as to organic colorants. Fully chlorinated copper phthalocyanine (green), copper phthalocyanine (blue), β-naphthol (yellow) and benzoimidazolone (red), were identified in the corresponding colored areas suggesting subsequent recent interventions or restorations. Ground layer materials were also characterized. The present results allow for discussion of the pictorial techniques used in the Andean highland during the colonial period and allow conservators and restorers to make important decisions regarding the type of intervention to perform in the case of these important historical and patrimonial monuments.
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1973-9494
7945
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Magdalena Pereira
Conservation Science in Cultural Heritage
Raman spectroscopy
Raman Identification of Pigments in Wall Paintings of the Colonial Period from Bolivian Churches in the Ruta De La Plata
Text.Serial.Journal
https://conservation-science.unibo.it/
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Articles
2018-03-28
2018-03-26
2018-03-28
2019-09-06
117-137
CSCH
Magdalena Pereira
2017
10.6092/issn.1973-9494/7945
117
Fundación Altiplano, Arica
1973-9494
Raman spectroscopy
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137
Prieto, G., Wright, V., Burger, R.L., Cooke, C.A., Zeballos-Velasquez, E.L., Watanave, A., Suchomel, M.R., Suescun, L. (2016). The source, processing and use of red pigment based on hematite and cinnabar at Gramalote, an early Initial Period (1500–1200cal. BC) maritime community, north coast of Peru. <em>Journal of Archaeological Science: Reports</em>, 5, pp. 45-60.
Raman Identification of Pigments in Wall Paintings of the Colonial Period from Bolivian Churches in the Ruta De La Plata
https://conservation-science.unibo.it/article/download/7945/7661
17
R    AMAN IDENTIFICATION OF PIGMENTS IN WALL
     PAINTINGS OF THE COLONIAL PERIOD FROM
     BOLIVIAN CHURCHES IN THE RUTA DE LA PLATA

  Carlos Rúa
  Taller Nacional de Patrimonio Muebles, Ministerio de Culturas y Turismo
  La Paz, Bolivia

  Marcela Sepúlveda, Sebastián Gutiérrez
  Laboratorio de Análisis e Investigaciones Arqueométricas (LAIA), Instituto de Alta
  Investigación (IAI), Universidad de Tarapacá
  Arica, Chile

  José J. Cárcamo-Vega*9
  Laboratorio de Análisis e Investigaciones Arqueométricas (LAIA), Instituto de Alta
  Investigación (IAI), Departamento de Química, Universidad de Tarapacá
  Arica, Chile

  Julio Surco-Luque
  Departamento de Química, Universidad de Tarapacá
  Arica, Chile

  Marcelo Campos-Vallette
  Departamento de Química, Universidad de Chile
  Santiago, Chile




                                                                                             CONSERVATION SCIENCE IN CULTURAL HERITAGE
  Fernando Guzmán, Paola Conti
  Facultad de Artes Liberales, Universidad Adolfo Ibáñez
  Viña del Mar, Chile

  Magdalena Pereira
  Fundación Altiplano
  Arica, Chile


  Keywords: Andean colonial churches, wall paintings, pigments, Raman spectroscopy




* Corresponding author: jcarcamo@uta.cl / jjcarcamo@gmail.com

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                                                                                                     Introduction

                                                                                                     This interdisciplinary study was realized with the participation of different scientists
                                                                                                  from various disciplines: chemistry, conservation, art history and archaeology. All the
                                                                                                  specialists contributed in different aspects related to the aim of the project which in-
                                                                                                  cluded the iconographic and stylistic analysis of the visual representation, its historical
                                                                                                  and cultural contextualization, and the microscopic and physical-chemical characteri-
                                                                                                  zation of the materials in the wall paintings of four churches on the Ruta de la Plata,
                                                                                                  Bolivia. This comprehensive approach was realized for the irst time in the South Cen-
                                                                                                  tral Andean region with specialists from different countries: Bolivia, Chile and Argentina
                                                                                                  to exchange different experiences, but also knowledge of the painting practices during
                                                                                                  the colonial period. Materiality of visual art representation from the 16th-17th centuries
                                                                                                  exhibits a particular complexity related to the combination of practices and knowledge
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  from the old world, mixed with European art traditions, including that from the local in-
                                                                                                  digenous peoples of the Andean region. Bringing together all these different elements,
                                                                                                  obliges us to discuss, from an interdisciplinary approach, all physical-chemistry results
                                                                                                  obtained from the spectroscopic analysis to fully understand the original painting pro-
                                                                                                  cess employed inside the churches and possible later interventions, which were not
                                                                                                  always recorded, performed in the past.


                                                                                                     1. Church wall paintings in the Ruta de la Plata

                                                                                                      Between the 16th and 18th centuries, the Ruta de la Plata was one of the main axes
                                                                                                  of trade used by the Spanish colony for the circulation of raw materials and minerals
                                                                                                  throughout the Andes region. These resources were obtained from different mountain
                                                                                                  localities, such as Huancavélica in the Central Andes and Potosí in the Bolivian high-
                                                                                                  lands from the South-Central region and then transported to the Royal Treasury of
                                                                                                  Spain from Arica harbor on the Paciic coast (Figure 1). This route represented, among
                                                                                                  its other uses, a signiicant network of roads for the commerce of silver minerals and
                                                                                                  other raw materials extracted from the Cerro Rico mine of Potosí [1]. However, the
                                                                                                  dynamism of the Ruta de la Plata declined signiicantly in the 18th century as a result of
                                                                                                  the reduced mineral production in Potosí and the preference for Buenos Aires as the
                                                                                                  new port for commercialization [2]. Important colonial cities were founded along the
                                                                                                  Ruta de la Plata, as well as the infrastructures necessary to support the hard displace-
                                                                                                  ment between the Andean highlands and the Paciic coast to the west. In each village,
                                                                                                  in the current territories of Bolivia and Chile, several churches were built to contribute
                                                                                                  to and promote the evangelization of the local indigene population. Inside the church-
                                                                                                  es, murals were important supports for painting, iconography and illustrating various
                                                                                                  scenes from daily life, as they visually conveyed the ecclesiastical doctrine, seemingly
                                                                                                  without ignoring local realities.
                                                                                                      The aim of the present work is to perform a Raman study and to identify pigments
                                                                                                  used in the wall paintings of four Bolivian churches of the Ruta de la Plata belonging
                                                                                                  to the period from the 16th to18th century. With the present results, we intend to deter-
                                                                                                  mine the color palette and to infer about the pictorial techniques used in this historical
                                                                                                  context, in the South-Central Andean region. They are the Santiago de Curahuara de
                                                                                                  Carangas church, with early 17th century paintings, which underwent restoration work
                                                                                                  during the 18th century [4]; the La Sacristía de Santiago de Callapa and San José de


                                                                                            118
Soracachi churches, whose characteristics, observed in the representations and archi-
tecture, allow their wall paintings to be dated to the second half of the 17th century [5];
and lastly, the Copacabana Andamarca church, whose wall paintings were executed in
the late 17th century or beginning of the 18th century [6].




Figure 1. a) εap of the four colonial churches in the South Andean Region; b) Ancient map with
the six parts of Potosí province, 1787. Archivo general de Indias. MP-Buenos Aires, 160 [3].

    When these four churches were irst constructed they belonged to the Archbishopric
of La Plata, in the ecclesiastical jurisdiction of the old Viceroyalty of Peru, and later,




                                                                                                       CONSERVATION SCIENCE IN CULTURAL HERITAGE
from the 18th century, to the Viceroyalty of La Plata [4]. The synod of 1772-1773 shows
clear concern about the role of paintings and sculptures in these ecclesiastic buildings
[4]. After clearly recommending the use of iconographic programs that the faithful could
read like a book, the text refers to the possibility that the meaning of a Christian image
could be altered by the presence, even if hidden, of images or material objects associ-
ated with ancient local indigenous practices. This led to the integration of particularities
into the pictorial program which were characteristic of the various local indigenous
communities so that they contributed to evangelization transfer from the old European
world [7]. The iconographic and stylistic analysis of the images painted on various
programs on the interior walls of the churches, show these variabilities. However, the
analysis of the materiality of these paintings can also complement our knowledge of
the pictorial and symbolic practices of the colonial period in the South Andes Region.
Artists commissioned for the realization of mural paintings might have had access to
products transported and sold throughout the Ruta de La Plata, but they certainly had
access to their own local knowledge [7]. Knowledge of the materials involved in artistic
or cultural heritage can, moreover, contribute to understanding the ancient colonial and
Andean technologies and pictorial techniques. Finally, it can also aid conservators and
restorers in making decisions regarding the planning and restoration of these historical
monuments.
    Information on the color and pigments used in the wall paintings of the Ruta de la
Plata churches can be obtained through several scientiic techniques, including spec-


                                                                                                 119
                                                                                                  troscopic analyses which are the most frequently used in heritage science [8,9]. Ra-
                                                                                                  man spectroscopy is one of the most powerful spectroscopic tools employed due to its
                                                                                                  unique properties; it is a non-destructive technique; no sample preparation is required
                                                                                                  and it displays characteristics of reliability and speciicity [10-15]. Raman spectrom-
                                                                                                  eters have become the instrument of choice when analyzing archaeological materi-
                                                                                                  alities and pigments on art works [16]. It is also amenable to in-situ analysis, based
                                                                                                  on the development of iber optics technology [17-1λ]. εost Raman spectrometers
                                                                                                  are today coupled with microscopes, thus making it possible to observe the sample
                                                                                                  and scan the spectrum from a micrometric area [20, 21]. The use of micro-Raman
                                                                                                  spectrometers gives information on the microstructure of analyzed samples also by
                                                                                                  improving spatial resolution. Nevertheless, its major disadvantage lies in the formation
                                                                                                  of undesirable luorescence, which is an accompanying phenomenon when measuring
                                                                                                  diverse materials and is dificult to forecast. The main applications of Raman in herit-
                                                                                                  age studies concern the identiication of pigments and dyes on different supports. Pig-
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  ments together with binders or agglutinant and extender, generally of mineral origin,
                                                                                                  are added to a paint mixture to improve or slightly modify some of its properties, such
                                                                                                  as diluting it or giving it additional body or, inally, to increase the quantity of paint. The
                                                                                                  analysis of these substances provides important information about materials, artists,
                                                                                                  artistic schools or technological knowledge and evolution [22].


                                                                                                     2. Experimental

                                                                                                     2.1 Sample collection

                                                                                                     Forty-four samples, measuring 0.5-1.0 mm2, were collected in September 2012. They
                                                                                                  were taken from selected wall paintings following internationally accepted procedures
                                                                                                  [23]. Samples were organized according to the different colors displayed in the artworks.
                                                                                                  Figure 2 displays details of the artworks indicating the points where the samples were
                                                                                                  collected. 20 samples were taken from the Cuarahuara de Carangas church (CDC); 5
                                                                                                  from the Santiago de Callapa church (STC), while from the San José de Soracachi church
                                                                                                  (SRC) 11 samples were selected. In total, six colors were identiied. Finally, 8 samples
                                                                                                  were taken from the Nuestra Señora de Copacabana de Andamarca (CPC) church.

                                                                                                     2.2 Microscopy measurement

                                                                                                     Optical microscopy data were acquired with a trinocular stereoscopic microscope
                                                                                                  Optika, model SZε-2 δED, with a total magniication of λ0x. Recorded images (Figure
                                                                                                  3) were calibrated by referencing one rule of 0.10 mm with divisions of 0.01 mm; the
                                                                                                  scaling was performed so that the displayed images were in the 0.01-0.02 mm range.
                                                                                                     Figure 3 shows some of the micro-samples from the churches described above.
                                                                                                  The size of the investigated samples, as well as the distribution of the colored pigment
                                                                                                  in each micro-sample can be observed in Figure 3.

                                                                                                     2.3 Raman measurements

                                                                                                    Raman spectra were recorded on a Raman RenishawInVia Relex apparatus,
                                                                                                  equipped with 532, 633, and 785 nm laser lines for excitation, a Leica microscope and


                                                                                            120
                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE

Figure 2. Details and sampling points of wall painting inside churches: A) Curahuara de Carangas
(CDC); B) Sacristía de Santiago de Callapa (STC); C) San José de Soracachi (SRC) and D)
Copacabana de Andamarca (CPC).


an electrically cooled charge-coupled device detector. The instrument was calibrated
using the 520 cm-1 line of a Si wafer and a 50x objective. The resolution was set to
4 cm-1 and 1-5 scans of 10-50 s each were averaged. Spectra were recorded in the
1800-200 cm-1 region. Spectral scanning conditions were chosen to avoid sample deg-
radation and photo decomposition. The best results were obtained using the 785-nm
laser line and a power range of 10-100 mW on the sample. Data were collected and
plotted using the programs WIRE 3.4, GRAMS 9.0 and OriginLab Pro 2016.




                                                                                                   121
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  Figure 3. Microscopic images of representative coloured samples from: A) Santiago de Curahuara
                                                                                                  de Carangas (CDC); B) Sacristía de Santiago de Callapa (STC); C) San José de Soracachi
                                                                                                  (SRC) and D) Copacabana de Andamarca (CPC).


                                                                                                    3. Results

                                                                                                    3.1 Raman spectrum of pigments

                                                                                                     The artwork, code sample, color and pigments found in the samples are displayed
                                                                                                  in Table 1. The Raman band assignment of the samples is based on general Raman
                                                                                                  data [24-26], for dyes [25-31] and pigments [27, 32]. The spectra are shown as ob-
                                                                                                  tained and are illustrated along with the discussion.




                                                                                            122
Table 1. Sample code, color and pigments in wall paints of Curahuara de Carangas (CDC),
Santiago de Callapa (STC), San José de Soracachi (SRC) and Nuestra Señora de Copacabana
de Andamarca (CPC) churches.




                                                                                                CONSERVATION SCIENCE IN CULTURAL HERITAGE




  3.2 Santiago de Curahuara de Carangas church

    The red sample CDC-M7 in Figure 4A displays a Raman spectrum with bands at
253, 311 and 354 cm-1 which were ascribed to cinnabar (HgS) in agreement with data
from RRUFF [32]. These bands are assigned to nHgS vibration modes and are orien-
tation dependent. Samples CDC-M11, -M16 and-M23 show the same Raman bands
as CDC-M7.
    The black color in sample CDC-ε8, Figure 4B is identiied as carbon material [32].
The same component was identiied in samples CDC-ε12 and CDC-ε14.




                                                                                          123
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  Figure 4. Raman spectrum and sample codes of the: (A) red; (B) black; (C) green; (D) yellow gold
                                                                                                  and; (E) blue samples of the Curahuara de Carangas church wall paint.

                                                                                                     In Figure 4C, the green areas in sample CDC-M9 are dominated by copper phthalo-
                                                                                                  cyanine (CuPc). Bands in the spectrum, mainly those at 1539, 776, 740 and 685 cm-1,
                                                                                                  are consistent with the presence of CuPc. as bands at 1339 and 776 cm-1 involve
                                                                                                  isoindole moiety, while the macrocycle ring deformations are expected in the 300-450
                                                                                                  cm-1 range. The green material itself was determined to be phthalocyanine green, a
                                                                                                  copper-based pigment irst synthesized in 1λ28 [33]. Phthalocyanine green, C32H0-
                                                                                                  2N8Cl14-16Cu, was indexed as CI:74260 in 1971 [33]; it is usually adsorbed on an
                                                                                                  aluminum hydrate base to form a deep vivid green color that is used as a substitute
                                                                                                  for viridian. The permanent pigment is unaffected by light, heat, and chemicals. Sam-
                                                                                                  ples CDC-M10, CDC-M15, CDC-M19 and CDC-M24, show the same Raman bands
                                                                                                  as CDC-M9, clearly indicating a recent intervention on the Curahuara de Carangas
                                                                                                  church wall paintings.
                                                                                                     The Raman spectrum of the sample CDC-M9, (Figure 4C), is highly coincident with
                                                                                                  that reported by Chaplin et al. [34] for a green phthalocyanine. In their work, authors
                                                                                                  concluded that in the case of the screen alone, in a large painted leather screen and
                                                                                                  two illuminated title pages in 17th century books of ordinances of the Worshipful Com-
                                                                                                  pany of Barbers, London, a restoration in the 1980s was carried out with different
                                                                                                  pigments: hematite, phthalocyanine green, rutile, and a mixture of azurite, malachite
                                                                                                  and barium sulphate. No bands of the green perchlorinated copper phthalocyanine
                                                                                                  CI:74250 reported by Scherrer et al. [25] were detected. The present result indicates
                                                                                                  that restoration work was performed on the wall painting including the phthalocyanine
                                                                                                  component in the green areas.
                                                                                                     The yellow gold sample CDC-M13, Figure 4D, displays a Raman spectrum, identi-
                                                                                                  cal to that reported by Scherrer et al. [25] for the PBr 25 CI:12510 monoazo pigment
                                                                                                  benzimidazolone. This compound is accompanied by the vermilion pigment HgS dis-
                                                                                                  playing bands at 253 and 344 cm-1[35]. The bands of the monoazo pigment benzimi-
                                                                                                  dazolone at 1578 and 1553 cm-1 are ascribed to nN=N modes and the bands at 1484,


                                                                                            124
1391 and 1357 cm-1 are nCC ring vibrations, the last two belonging to the naphthalene
ring following Lutzenberger et al. [36]. A deformation of the benzoimidazole ring is as-
signed to the band at 1281 cm-1. Para-disubstituted ring breathing is ascribed to the
band at 1231 cm-1. The band at 1161 cm-1 could be assigned to a CO vibration of the
imidazole ring moiety. A CH deformation is attributed to the band at 1109 cm-1.
    The blue sample CDC-M18 displays a Raman spectrum (Figure 4E) characterized
by a group of weak relative bands at 1574, 1016, 599, 544, 400, 315 and 246 cm-1. The
irst two bands are highly coincident with the equivalent of indigo dye (C16H10N2O2),
following Baran et al. [37]; the band at 1574 cm-1 is assigned to a coupled nC=C/nCO
mode. The very weak band at 544 cm-1 is probably a benzene ring out of plane bend-
ing. The band at 246 cm-1 also observed by δeona et al. [38] is a ive-member ring
out-of-plane bending mode. The band at 1016 cm-1 is an out of plane CH deformation.
Indolic ring modes are expected in the 300-400 cm-1 range.

   3.3 Sacristía de Santiago de Callapa church

   The Raman spectrum of the black pigment in the sample STC-M27, (Figure 5A),
displays large D (defects) and G (graphitic) bands at 1338 and 1582 cm-1, respec-
tively; they are ascribed to nCC modes of a carbon material [32].




                                                                                                       CONSERVATION SCIENCE IN CULTURAL HERITAGE




Figure 5. Raman spectrum and sample codes of the: (A) black; (B) green; (C) red; (D) white and
(E) blue samples of the Santiago de Callapa church wall paint.

   Sample STC-M28 in Figure 5B, displays a green coloration. The Raman spectrum
shows bands at 1172, 1077, 989, 416, 307 and 262 cm-1. These bands are char-
acteristic of copper sulphate salts, probably from minerals antlerite Cu2+3SO4(OH)4
or / plus krohnkite Na2Cu(SO4)2•2H2O and brochantite Cu4SO4(OH)6 [32]. The most
intense band at 989 cm-1 is assigned to a nsymSO4 vibration, while the weak band
at 1077 cm-1 is the corresponding asymmetric mode [39]. The band at 1172 cm-1 is a
nsymSO4 mode, while the band at 416 cm-1 is a doubly degenerate n2symmetric SO4
bending mode. The band at 262 cm-1 is probably a lattice vibration [40].


                                                                                                 125
                                                                                                      The Raman spectrum of the red sample STC-M29 in Figure 5C contains bands at
                                                                                                  1009, 408, 293 and 224 cm-1. The last three bands are highly coincident with vibra-
                                                                                                  tions of hematite [32]; the bands at 408 and 293 cm-1 are assigned to the Eg mode
                                                                                                  while the last is ascribed to an A1g mode [18]. The weak band at 1009 cm-1is ascribed
                                                                                                  to gypsum CaSO4 probably used as iller.
                                                                                                      The white pigment STC-M30 in Figure 5D is probably gypsum CaSO4•2H2O, dis-
                                                                                                  playing Raman bands at 1016, 4λ8 and 3λ0 cm-1. The irst band is the n1SO4 mode,
                                                                                                  while the second is the n2SO4 mode; this assignment is highly coincident with data by
                                                                                                  δiu et al. [41]. The band at 3λ0 cm-1 is a 2 bending mode following Frost et al. [42].
                                                                                                      The intense blue pigment in sample STC-M31, (see Figure 5E), displays character-
                                                                                                  istic Raman bands of a phthalocyanine polycycle. In fact, the most probable compound
                                                                                                  is classiied as PB 16μ6 CIμ 74160μ6 with strong bands at 1530, 1342, 747 and 681
                                                                                                  cm-1; these bands can be ascribed, following Basova et al. [30], to the nCN, nCC,
                                                                                                  nCuN/ CNC and CNC/nCN modes, respectively. According to historical information,
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  phthalocyanines were discovered in the early 1900s [43], thus the only way to explain
                                                                                                  this coloration is related to recent interventions or restoration of the wall painting.

                                                                                                     3.4 San José de Soracachi church

                                                                                                     The Raman spectrum of the black pigment from sample SRC-M32, Figure 6A, is
                                                                                                  quite similar to that observed in the case of the Santiago de Callapa and Nuestra Se-
                                                                                                  ñora de Copacabana de Andamarca church samples; the Raman spectrum with bands
                                                                                                  at 1341 and 1580 cm-1 corresponds to a carbon material. Carbon was also identiied
                                                                                                  in sample SRC-M42.




                                                                                                  Figure 6. Raman spectrum and sample codes of the: (A) black; (B) blue; (C) white; (D) red ochre
                                                                                                  and (E) green samples of the San José de Soracachi church wall paint.

                                                                                                    The blue in samples SRC-M33 and SRC-34, Figure 6B, displays a Raman spectrum
                                                                                                  characterized by bands at 1575 cm-1, a band at 546 cm-1 and a multiple band at 252
                                                                                                  cm-1 plus other weak bands at 1460, 1312, 1228 and 1009 cm-1. This set of bands is


                                                                                            126
highly coincident with an indigo dye (C16H10N2O2), following Baran et al. [37]. The
band assignment has already been discussed in samples CPC-M46 and CDC-M18 for
the indigo pigment.
   The white sample SRC-M36 in Figure 6C displays a Raman spectrum with bands
at 309, 385, 411, 492, 1008 and 1134 cm-1 corresponding mainly to gypsum as in the
case of the previously discussed sample STC-M30.
   The red ochre sample SRC-M37 in Figure 6D contains bands at 300 and 388 cm-1
corresponding to goethite Fe3+O(OH) [32]; the additional band at 1008 cm-1 belongs
to gypsum. The band at 300 cm-1 is a symmetric Fe-OH bending mode and the band
at 388 cm-1 is an antisymmetric stretching Fe-O-Fe/-OH mode [44]. The spectrum of
the sample SRC-M37 is identical to that of the sample SRC-41.
   The Raman spectrum of the green sample SRC-M34 is quite similar to that ob-
served in samples SRC-M39, Figure 6E, in this same church and CPC-M44 of Nuestra
Señora de Copacabana de Andamarca church. The green sample displays an intense
band at 989 cm-1 which is characteristic of the nsymSO4 mode of the sulphate anion.
The band at 1129 cm-1 is a nasymSO4 mode, while the band at 600 cm-1corresponds
to asymmetric deformations of SO4 [45]. The band observed at 416 cm-1 is accompa-
nied by shoulders at 409 and 420 cm-1; these bands were assigned to the n2 bending
modes. The band at 262 cm-1 is probably a lattice vibration [40]. The spectral proile
of the SRC-M39 sample is highly coincident with the Raman spectrum of the antlerite
mineral Cu2+3SO4(OH)4 [32].

  3.5 Nuestra Señora de Copacabana de Andamarca church

   The basic copper sulphate minerals antlerite, brochantite, posnjakite and langite
display characteristic bands in the 986-973 cm-1 range [39]. The green sample CPC-
M44, Figure 7A, displays an intense band at 973 cm-1which is characteristic of the
nsymSO mode of the sulphate anion.




                                                                                              CONSERVATION SCIENCE IN CULTURAL HERITAGE
   The band at 608 cm-1 corresponds to asymmetric deformations of SO4 [45]; the
bands at 478 and 318 cm-1 are ascribed to nCu-O vibrations, following Lane [40]. An
intense Raman band is observed at 415 cm-1 with shoulder bands at 386 and 446
cm-1; these bands were assigned to the n2 bending modes by Frost et al. [42]. Other
bands lower than 250 cm-1at 242 and 318 cm-1, are probably lattice vibrations [40].
The spectral proile of CPC-ε44 is highly coincident with the Raman spectrum of the
mixture of antlerite Cu2+3SO4(OH)4 and brochantite Cu4SO4(OH)6 minerals [32].
   The blue sample, CPC-M46, displays a Raman spectrum in Figure7B, characterized
by an intense band at 1575 cm-1, a band at 543 cm-1 and a large and medium band
at 253 cm-1 plus other weak bands at 1460, 1364, 1313, 1228, 1015, 637, 600, 383,
314 cm-1. This set of bands is highly coincident with the indigo dye (C16H10N2O2),
as reported by Baran et al. [37]. The band at 1575 cm-1 is assigned to a coupled
nC=C/nCO mode, while the band at 543 cm-1 is probably a benzene ring out of plane
bending mode. The band at 253 cm-1, also observed by Leona et al. [38], is probably
a ive-member ring in-plane and out-of-plane bending mode. The band at 1460 cm-1
is an aromatic nCC mode and the band at 1364 cm-1 involves the NH fragment and
CH deformation modes coupled to nCN vibrations. A six-member ring nCC mode is
ascribed to the band at 1313 cm-1. In and out of plane CH deformations are assigned
to the bands at 1228 and 1015 cm-1, respectively. The N-H out-of-plane bending oc-
curs at 637 cm-1. A CCOC deformation mode could be ascribed to the band at 600


                                                                                        127
                                                                                                  cm-1. Indole ring modes are expected in the 300-400 cm-1 range. Indigo is typically
                                                                                                  a natural dyestuff that has been extracted from the leaves of various indigofera spe-
                                                                                                  cies since ancient times; indigo has also been made synthetically since the late 19th
                                                                                                  century [33]. It can therefore not be excluded that the identiied blue color may derive
                                                                                                  from a synthetic origin.
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  Figure 7. Raman spectrum and sample codes of the: (A) green; (B) blue; (C) purple; (D) pale red
                                                                                                  and (E) white samples of the Nuestra Señora de Copacabana de Andamarca church wall paint.

                                                                                                     The Raman spectrum of the purple sample CPC-M48 in Figure 7C shows broad
                                                                                                  bands, in particular, at 226, 293, 410, 499, 610 cm-1 which is coincident with the Raman
                                                                                                  signals of the hematite pigment [32]. Gypsum is also present in this sample displaying
                                                                                                  the characteristic band at 1009 cm-1.
                                                                                                     The sample CPC-M49, pale red in color, displays a set of Raman bands in Figure
                                                                                                  7D attributable to a synthetic azopigment, namely β-naphthol [25] mixed with gypsum
                                                                                                  [32]. In fact, the most intense bands of the azopigment are 1589 and 1338 cm-1, along
                                                                                                  with the medium bands at 1398 and 1127 cm-1. Weak bands are observed at 1487
                                                                                                  and 1266 cm-1. The bands at 1017, 397 and 312 cm-1 correspond to gypsum; analysis
                                                                                                  of these bands was done in the case of the Santiago de Callapa church. Three spec-
                                                                                                  tral features are characteristic of azocompounds: nN=N vibration (1380-1450 cm-1),
                                                                                                  nsymC-N mode (1130-1200 cm-1) and symC-N vibration (1160-1200 cm-1) [46]. The
                                                                                                  characteristic naphthalene band is identiied at 158λ cm-1. The identiication of the
                                                                                                  synthetic pigment suggests that the wall painting had undergone recent interventions
                                                                                                  or restoration; azopigments were originally synthesized in the last two decades of the
                                                                                                  19th century [47].
                                                                                                     Rutile (TiO2) was identiied [32] in the white sample CPC-ε50 (Figure 7E), dis-
                                                                                                  playing bands at 609 and 257 cm-1. Other bands at 1128, 1017, 862, 631, 499 and
                                                                                                  412cm-1 are ascribed to gypsum [32]. The strongest Raman spectrum of gypsum at


                                                                                            128
1017 cm-1 corresponds to the symmetric n1SO4 tetrahedral mode. Bands at 412 and
499 cm-1 are assigned to the n2 symmetric bending, while the peak at 1161 cm-1 cor-
responds to the n3 asymmetric stretching vibration mode. The bands at 631 and 674
cm-1 are attributable to the n4 asymmetric bending vibration modes.

  3.6 Raman spectra of components in the ground layer

   The strong band at 1008 cm-1 is ascribed to the symmetric stretching mode of a
sulphate group. This band appears in various samples. Several minerals containing
the sulphate group could in principle be used as components of the preparatory layer
of the mural paintings: alum-(K), KAl(SO4)2•12H2O displays a band centered at 993
cm-1[32] while leightonite [32], K2Ca2Cu(SO4)4•2H2O, displays a maximum at 1004
cm-1; alunite KAl3(SO4)2(OH)6 shows a band at 1020 cm-1. Gypsum, CaSO4•2H2O,
shows Raman bands at 1136, 1008, 671, 620, 494 and 415 cm-1[48], which is highly
coincident with the observed signals in various present samples. This analysis indi-
cates that the most probable component used in the preparation of the ground layer
was gypsum. No organic binders were identiied in the samples.


  4. Discussion

    All spectroscopic results are summarized in Table 1 and contribute to identifying the
palette of colors from the Colonial Period. Nevertheless, contemporary and modern
paint was also identiied and interpreted as used in possible recent restoration. In all
the churches, the black pigment used to paint was identiied as carbon. This material
was widely used in the Colonial Period since it was easy to obtain [49-51]. Red paints
correspond to hematite or cinnabar (vermillion). Both are historical colonial pigments
identiied in easel painting [52]. Indigo (CDC-ε18, CDC-ε21, CDC-ε22, SRC-ε33,




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
CPC-M46) an important dying pigment was used as blue [53]. Red ochre (SRC-M37)
and purple (CPC-ε48) were separately identiied as goethite and gypsum, and he-
matite, respectively; a combination of different proportions was probably used to ob-
tain various tonalities [54]. The yellow sample (SRC-M41) was analyzed as a mixture
of goethite and gypsum. Green pigments correspond to antlerite, a copper hydroxy-
sulphate. Finally, anhydrite (calcium sulphate) [55] and rutile (titanium dioxide) were
analyzed for the white paint. One case of pale red (CPC-M49) is particularly unusual
and corresponds to a synthetic pigment used since the 19th century. One blue sample
(CDC-M9) shows a phthalocyanine pigment which appeared at the beginning of the
20th century [54]. A similar pigment was identiied in the Santiago de Callapa church
(STC-M31) in the green paint. The yellow gold color (CDC-M13) corresponds to ben-
zimidazolone, a synthetic pigment created in the 20thcentury.
    The different pigments identiied, correspond effectively to the Colonial Period. Art-
ists used pigments commercially available at this time for painting on different sup-
ports, such as wood, easel painting and on church walls [50, 52, 57]. Some of these
mineral and organic pigments have been known since pre-hispanic times, and include
carbon, hematite, goethite, cinnabar, antlerite and indigo, showing some continuity
from the local pre-Columbian pictorial tradition. These different coloring materials were
also known and used in rock art (hematite, goethite, cinnabar and carbon), on vegetal
iber, leather (antlerite and copper mineral pigment) or textiles (for indigo) [55, 56,


                                                                                            129
                                                                                                  58], among others. In contrast, four studied samples correspond to synthetic pigments
                                                                                                  developed in the late 19th century and early 20th century. The identiication of these
                                                                                                  particular pigments allowed us to conclude that paints in the Curahuara de Carangas
                                                                                                  and Santiago de Callapa churches had undergone intervention and repainting, prob-
                                                                                                  ably during past restoration. In the case of the Nuestra Señora de Copacabana de
                                                                                                  Andamarca church, restoration work was performed between 2005 and 2009 by the
                                                                                                  Bolivian and German governments.


                                                                                                        5. Conclusions

                                                                                                      Pigments of the four wall paintings of the Bolivian Andean highland churches San-
                                                                                                  tiago de Curahuara de Carangas, Santiago de Callapa, San José de Soracachi and
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  Nuestra Señora de Copacabana de Andamarca in Bolivia in the Ruta de la Plata, be-
                                                                                                  longing to the Colonial Period, were identiied and analyzed through optic microscopy
                                                                                                  and microRaman spectroscopy. Most of the original pigments correspond to colored
                                                                                                  mineral salts such as green from antlerite / krohnkite, red from hematite and cinna-
                                                                                                  bar, white from rutile and red ochre from goethite; another blue color comes from the
                                                                                                  natural dyestuff indigo; black is obtained from carbon. Other pigments such as fully
                                                                                                  chlorinated copper phthalocyanine in the green areas, cooper phthalocyanine in the
                                                                                                  blue areas, β-naphthol in the yellow areas and benzoimidazolone in the red areas were
                                                                                                  identiied, suggesting old and recent interventions or restoration work. A iller / support
                                                                                                  such as gypsum was found in most samples. No organic binders were identiied in the
                                                                                                  samples, but we cannot discard its use and its future analysis with complementary
                                                                                                  techniques. The present results are coincident with a recent painting analysis per-
                                                                                                  formed on the Copacabana de Andamarca church [50], where authors were able to
                                                                                                  identify organic binders which corresponded to precise recipes used in colonial times
                                                                                                  and painting techniques applied on church walls, namely an a secco technique. The
                                                                                                  present work remains open to other scientiic analyses that complement and allow
                                                                                                  for further studies in other artworks enabling development of additional conservation
                                                                                                  protocols. These results also contribute to a discussion on continuity and innovation
                                                                                                  in relation to the palette of colors used since before the colonial period, an important
                                                                                                  dimension regarding technological knowledge regarding pigment use and production
                                                                                                  in the highlands of the South-Central Andean region.


                                                                                                        Acknowledgements

                                                                                                     Authors acknowledge FONDECYT projects 1140524 and 1150974. We are also
                                                                                                  grateful to the Convenio de Desempeño UTA-MINEDUC.


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                                                                                                     Biographical notes

                                                                                                     Carlos Rúa was born in 1954 in Bolivia. He graduated with a Bachelor of Fine
                                                                                                  Arts from the University of Tomás Frías, Potosí city, Bolivia. He has followed courses
                                                                                                  of specialization in the conservation and restoration of movable heritage, in Mexico,
                                                                                                  Spain and Brazil. He has conducted professional exchanges in the United States and
                                                                                                  Germany. He has participated in conservation meetings, seminars and symposiums in
                                                                                                  Bolivia and abroad. Since 1987, he has been in charge of the National Workshop on
                                                                                                  the Preservation and Restoration of Movable Heritage of the Ministry of Cultures and
                                                                                                  Tourism of Bolivia, performing the management, coordination and administration of
                                                                                                  projects for the conservation of movable cultural property in different regions of Bolivia.

                                                                                                      Marcela Sepúlveda was born in 1976 in France. Today she is an adjunct professor
                                                                                                  at the Instituto de Alta Investigación in the Universidad de Tarapacá. An archaeologist


                                                                                            134
from the Universidad de Chile (2002), she obtained a Doctorate in Prehistory, Ethnol-
ogy and Anthropology in 2006 at the Université Paris 1- Panthéon Sorbonne, France
and a post-doctorate in 2010 at C2RMF-UMR 171-CNRS with Philippe Walter, funded
by Maison des Sciences de l’Homme. She is specialized in rock art studies and mate-
riality of color from the pre-Columbian archaeological context, she has developed an
archaeometric approach, combining different analytical techniques such as SEM-EDX,
XRF, DRX, RAMAN. She created the Analysis and Research Archeometric Laboratory
(LAIA) at the Universidad de Tarapacá in 2014, funded by the Chilean Education Min-
istry. In 10 years of academic experience, she has directed 15 theses of undergraduate
programs in archaeology, 3 masters and 1 doctorate thesis. She has published more
than 60 papers in peer reviewed journals or book chapters, including her own research
or in collaboration with national and foreign colleagues. She has participated in the
editorial board of the Heritage Science Journal.

    José Cárcamo-Vega born in 1976, has a Bachelor degree, MSc and PhD in chem-
istry. From 2012 he has worked as a researcher at the Analysis and research Ar-
chaeometric Laboratory (LAIA) of the University of Tarapacá. His work focuses on the
characterization of archaeological and patrimonial materials, using Infrared, Raman,
Raman enhanced by metal nano-structured surfaces (SERS) and X-ray luorescence
spectrometry (XRF). His research involves the identiication of pigments used by the
pre-Hispanic cultures of northern Chile using Raman spectroscopy and XRF. He has
also developed investigation concerning the identiication of collagens in human tis-
sues through Raman and SERS. His experience in the handling of different techniques
of classical and instrumental physical-chemical analysis, has allowed him to publish in
specialized journals and to take part in national and international congresses.

   Julio C. Surco Luque was born in Chile 1993; he is an undergraduate student in
chemistry, working in research in the analysis laboratory of archaeometric investiga-




                                                                                                 CONSERVATION SCIENCE IN CULTURAL HERITAGE
tions (LAIA) of the University of Tarapacá. He is currently working on the molecular
materiality and pictorial techniques used in wall painting in the churches of the Ruta
de la Plata. He is experienced in the use of vibrational spectroscopy and complemen-
tary techniques such as electronic and optical microscopy, SEM, XRF, and theoretical
calculations.

   Marcelo Campos-Vallette was born in Chile, 1948. He is currently a Full Professor at
the University of Chile. He obtained his PhD at the Université de Bordeaux in 1981 and
postdoc during 1983-84 at the University of Toronto. His research focuses on Molecular
Spectroscopy, developing investigations in Vibrational Spectroscopy; he has published
around 150 papers in ISI journals. He currently uses infrared and Raman techniques in
the study of several biological molecular systems, contaminants and is centered mainly
on heritage aspects. Identiication of pigments and colorants in historical and archaeo-
logical samples is performed by Prof. Campos using additional Raman techniques such
as surface enhanced Raman scattering allowing identiication and characterization of
traces of dyes. Over a period of 20 years he has directed the doctoral thesis of 10 stu-
dents and presented approximately 70 works in international congresses.

   Sebastian Gutiérrez was born 1976 in Santiago. He obtained his professional de-
gree as a chemist from the University of Tarapacá in 2008. Since 2012 he has worked


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                                                                                                  as an associate researcher at the Archaeological Research and Analysis Laboratory
                                                                                                  (LAIA) and University of Tarapacá. He obtained his degree in chemistry with the study
                                                                                                  and characterization of the black coating used in Chinchorro mummies from an archaic
                                                                                                  period, on the northernmost coast of Chile. He is an expert on X-ray Fluorescence
                                                                                                  (pXRF) and Scanning Electron Microscopy (SEM-EDX) techniques. He has a Man-
                                                                                                  agement License for second and third category radiological equipment. Since 2011 he
                                                                                                  has participated in several research projects and he is coauthor of various papers and
                                                                                                  presentations at national and international level about different studies on archaeo-
                                                                                                  logical materials, such as pigments, dyes, ceramics, mineral beads, metals, obsidian,
                                                                                                  among others.

                                                                                                     Fernando Guzmán Ph.D. in Art History, Universidad de Sevilla. He obtained a
                                                                                                  grant from the Andes Foundation to work on numismatic collections (1991-1993). He
C. Rúa et al. - Raman identification of pigments in wall paintings of the colonial period




                                                                                                  was Curator of the Decorative Arts Museum in Santiago of Chile (1994-1999) and
                                                                                                  Director of the Regional Museum of Ancud In Chiloe Island (1999-2000). He has been
                                                                                                  Professor of Chilean Art at the Universidad Adolfo Ibáñez since 2001. In 2012 and
                                                                                                  2014 he obtained a grant from the National Science and Technology Council of Chile
                                                                                                  (CONICYT) to do research on the mural paintings in the churches between Potosi
                                                                                                  and Arica. In 2016, he was a Getty Scholar at the Getty Research Institute. He has
                                                                                                  published: Representaciones del Paraíso, retablos en Chile, siglos 18t y 19t, (2009),
                                                                                                  La Pintura mural de Parinacota en el ultimo bofedal de la Ruta de la Plata (2013) in
                                                                                                  collaboration with other researchers and numerous articles about Chilean Art in the
                                                                                                  18th and 19th centuries.

                                                                                                     Magdalena Pereira Master in History from the Universidad de Sevilla. She is a re-
                                                                                                  searcher at the Fundación Altiplano, she and works on Colonial Latin American Art and
                                                                                                  local history. She is a co-researcher on the project funded by the National Science and
                                                                                                  Technology Council of Chile (CONICYT), to work on the mural paintings in the church-
                                                                                                  es located between Potosi and Arica. She has published: Arica y Parinacota, La iglesia
                                                                                                  en la Ruta de la Plata (2011) with R. Moreno, Guañacagua, restauración de la iglesia
                                                                                                  de San Pedro (2011), Arica y Parinacota, Las huellas de la Ruta de la Plata (2013),
                                                                                                  and La Pintura mural de Parinacota en el último bofedal de la Ruta de la Plata (2013)
                                                                                                  in collaboration with other researchers, and numerous articles about Chilean Art.

                                                                                                     Paola Conti has a Master in History from the Université de Poitiers. She is a Profes-
                                                                                                  sor at the Universidad Adolfo Ibáñez; a researcher in Medieval Art and Colonial Latin
                                                                                                  American Art. She obtained a grant from the Universidad Adolfo Ibáñez in the period
                                                                                                  2006-2008. In 2011, she obtained the grant Stipendien werk ICALA-Lateinamerika
                                                                                                  Deutschland to do research on “Katherinavon Kleve” at the Fakultät für Philosophie
                                                                                                  und Geschichte of the Münster University, and at the Hauptstaats archive of Düssel-
                                                                                                  dorf. She is a co-researcher on the project funded by the National Science and Tech-
                                                                                                  nology Council of Chile (CONICYT), to work on the mural paintings in the churches
                                                                                                  located between Potosi and Arica. She has published: La Pintura mural de Parinacota
                                                                                                  en el último bofedal de la Ruta de la Plata (2013) in collaboration with other research-
                                                                                                  ers, and numerous articles about Medieval and Colonial Art.




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   Summary
   Micro-Raman spectroscopy was used to analyze and identify pigments and deter-
mine the palette of color in wall paintings from four churches of the Colonial Period,
in the Bolivian highlands. The ancient churches Santiago de Callapa, Curahuara de
Carangas, Nuestra Señora de Copacabana de Andamarca and San José de Soraca-
chi were constructed during the 17th and 18th centuries, around the important colonial
Ruta de la Plata, between Potosí city and Arica harbor on the Paciic coast of South
America. Most pigments used in the wall paintings correspond to inorganic minerals as
well as to organic colorants. Fully chlorinated copper phthalocyanine (green), copper
phthalocyanine (blue), β-naphthol (yellow) and benzoimidazolone (red), were identi-
ied in the corresponding colored areas suggesting subsequent recent interventions
or restorations. Ground layer materials were also characterized. The present results
allow for discussion of the pictorial techniques used in the Andean highland during
the colonial period and allow conservators and restorers to make important decisions
regarding the type of intervention to perform in the case of these important historical
and patrimonial monuments.

   Riassunto
   δa spettroscopia εicro-Raman è stata utilizzata per analizzare e identiicare i pig-
menti e determinare la tavolozza di colori dei dipinti murali di quattro chiese risalenti al
periodo coloniale, negli altopiani boliviani. Le antiche chiese di Santiago de Callapa,
Curahuara de Carangas, Nuestra Señora de Copacabana de Andamarca e San José
de Soracachi sono state costruite nel XVII e XVIII secolo, attorno alla coloniale Ruta de
la Plata, tra la città di Potosí e il porto di Arica sulla costa del Paciico, Sud America. δa
maggior parte dei pigmenti utilizzati nei dipinti murali si riferisce a minerali inorganici e
coloranti organici. Il rame clorurato di ftalocianina (verde), il rame di ftalocianina (blu), il
β-naftolo (giallo) e il benzimidazolone (rosso) sono stati identiicati nelle corrispondenti
aree colorate, suggerendo successivi interventi. Sono stati anche caratterizzati i ma-




                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE
teriali dello strato preparatorio. I risultati attuali permettono di discutere le tecniche pit-
toriche utilizzate nell’altopiano andino durante il periodo coloniale, permettendo inoltre
ai conservatori e restauratori di prendere decisioni sul tipo di intervento che interessa
questi importanti monumenti storici.




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