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Goodall, Rosemary, Hall, Jay, Sharer, Robert, Traxler, Loa, Rintoul, Llew,& Fredericks, Peter(2008)Micro-attenuated total reflection spectral imaging in archaeology: applica-tion to Maya paint and plaster wall decorations.Applied Spectroscopy, 62(1), pp. 10-16.

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https://doi.org/10.1366/000370208783412627

This is the author-manuscript version of this work - accessed from http://eprints.qut.edu.au Goodall, Rosemary A and Hall, Jay and Sharer, Robert J and Traxler, Loa and Rintoul, Llew and Fredericks, Peter M (2008) Micro-ATR spectral imaging in combination with Raman spectroscopy and ESEM-EDX in Archaeology: Application to Maya Paint and Plaster Wall Decorations . Applied Spectroscopy 62(1):pp. 10-16. Copyright 2008 Society for Applied Spectroscopy

1

Micro-ATR spectral imaging in combination with Raman

spectroscopy and ESEM-EDX in Archaeology: Application to Maya

Paint and Plaster Wall Decorations

ROSEMARY A. GOODALL*, JAY HALL, ROBERT J. SHARER, LOA TRAXLER,

LLEW RINTOUL and PETER M. FREDERICKS

School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box

2434, Brisbane, Qld 4001, Australia (R.A.G., L.R., P.M.F.); School of Social Sciences, University

of Queensland, 4067, Brisbane, Australia (J.H.); and University of Pennsylvania Museum,

Philadelphia, PA, 19104-6324, USA (R.J.S., L.T.)

Fourier transform infrared - attenuated total reflection imaging has been successfully used

to identify individual mineral components of ancient Maya paint. The high spatial

resolution of a micro FT-IR-ATR system in combination with a focal plane array detector

has meant that individual particles in the paint have been resolved and identified from

their spectra. This system has been used in combination with micro-Raman spectroscopy

to characterize the paint which was found to be a mixture of hematite and silicate particles

with minor amounts of calcite, carbon and magnetite particles in a sub-micron hematite

and calcite matrix. The underlying stucco was also investigated and found to be a

combination of calcite with fine carbon particles, making a dark sub-ground for the paint.

2

Index Headings: FT-IR-ATR Imaging; Raman microscopy; paint; pigments; stucco; Copan;

Maya

INTRODUCTION

The paints used by ancient people for the decoration of ceramics, buildings and cave

walls, have been studied intensely over the last 20 years.1-4 Knowledge of the mixtures used to

create paint blends can provide insight into, the type of materials utilized, the binding materials

required, where the materials were sourced, and is also of great assistance in the conservation of

the painted surfaces. To understand the mixtures used by ancient artisans in the formulation of

paints the individual components need to be identified. While many analytical techniques can

give valuable information on the elemental composition and structural makeup of the materials,

few have the spatial resolution required to identify the individual particles. Fourier transform

infrared imaging using an ATR attachment can provide both spatial and spectral information

about the sample area.5 When combined with techniques such as micro-Raman spectroscopy and

environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray

microanalysis (EDX), detailed information regarding paint components, their size, composition

and structure can be obtained.

*Author to whom correspondence should be sent.

E-mail: r.goodall@student.qut.edu.au

3

FT-IR imaging couples an FT-IR microscope system with a focal plane array (FPA) detector to

enable the simultaneous collection of multiple spectra across the sampled area. The number of

spectra collected per image is dependent on the size of the FPA; for example, a 32x32 array

allows 1024 independent spectra to be collected in a single measurement. The images generated

from individual spectra are spatially resolved maps providing the location of each component in

the sampled area.5 When operated in reflection or transmission mode FT-IR imaging has a

maximum lateral spatial resolution of around 5-10 µm.6 This spatial resolution is limited by

diffraction and furthermore may not always be achievable because of signal-to-noise ratio

considerations. Micro-ATR imaging offers an increased lateral spatial resolution of around 2-

3µm,5 because of the change in wavelength of the light as it passes through the internal reflection

element, and can be applied to samples that are non-reflective or not suitable for transmission

measurements. FT-IR imaging has so far been applied mainly in polymer7,8 and

biological5,6,9,10,11 studies with a small number of geological studies12 and a single study of an oil

painting fragment.13 For archeological paint samples the particle size is typically in the range

from 1 to 20 µm, making this technique ideal for the identification of individual particles. For

paint mixtures this permits an understanding of the spatial makeup including individual particle

size, shape and distribution. This in turn leads to a better understanding of individual pigments,

and can help to identify if they are natural mineral complexes or deliberate mixtures of minerals

to achieve a definite color or effect. Micro-Raman spectroscopy has also been used in similar

studies14,4 because of the high spatial resolution of the system, allowing spectra to be recorded on

individual particles of 1-2 µm. Raman spectroscopy is particularly useful for the identification

of iron oxide minerals, which are often the main constituent of ancient red and yellow paints,

because of the restricted spectral range of FPA detectors. Generally, FPA detectors are not

4

effective below about 800 cm-1 and hence many important vibrational modes, for example of

oxides, are not seen. However, many minerals found in ancient paints are poor Raman

scatterers, or give rise to fluorescence, and are often more easily identified by infrared

spectroscopy, making the two techniques complementary.

In this work FT-IR-ATR imaging was used to characterize Maya paint samples from an

ancient building at Copan, Honduras. The Maya decorated the external and internal surfaces of

their structures by first coating them with stucco and then applying colored paints. The objective

of the work was to identify as many of the mineral phases in the paint and stucco as possible.

Elemental maps recorded using EDX analysis in an environmental scanning electron microscope

(ESEM-EDX) together with backscattered electron images from the ESEM, were used to

confirm the composition and morphology of each particle. The ESEM and FT-IR-ATR images

were matched for complete characterization of the paint particles.

EXPERIMENTAL

Samples. The sample is from the Sub Jaguar Tomb15 and consists of three separate pieces of

painted stucco which have exfoliated from the wall painting of the tomb. Each sample piece

consists of a thick layer of coarsely ground stucco up to 8 mm thick, covered by a thin fine layer

of stucco and then by a layer of red paint. All pieces were examined initially as received and

then a part of one of the pieces was set in Araldite M resin, and polished to a smooth surface

using diamond paste. This polished cross-section was used in subsequent mapping and imaging

analysis.

5

Attenuated Total Reflection FT-IR Spectral Imaging (FT-IR-ATR spectral imaging). Data

were collected using a Varian FT-IR imaging system. The system consists of a rapid scan

Varian 3100 FT-IR spectrometer, a Varian 600 UMA FT-IR microscope equipped with an ATR

objective and a 32x 32 liquid nitrogen cooled mercury-cadmium-telluride (MCT) focal plane

array detector. A germanium slide-on crystal was used in the ATR objective. The crystal was

pressed onto the sample in various locations within an area defined by the environmental

scanning electron microscopy- energy dispersive X-ray microanalysis (ESEM-EDX)

experiments. The area analyzed each time was approximately 40 x 40 µm. Spectra were

acquired at 8 cm-1 resolution with 512 scans co-added, over a range of 4000-850 cm-1. Spectra

were processed and images created using Varian Resolution pro software. Band positions were

identified where necessary using band fitting of individual spectra with Grams/AI software

(Thermo Scientific, Madison WI). Band fitting was carried out using a Gaussian curve function

with the minimum number of bands used. The iterations were limited to 50 with a squared

correlation of r2 greater than 0.995. Unless otherwise stated, spectra are presented without

processing.

Raman spectroscopy. Spectra were recorded directly from the pigment and stucco samples

using a Renishaw inVia spectrometer (Renishaw plc, Gloucestershire, UK) consisting of a Leica

microscope with a 50x objective attached to a spectrograph equipped with an electrically-cooled

charged coupled device (CCD) detector. Excitation was from a Renishaw diode laser emitting at

785 nm. The laser power was reduced using an optical filter to approximately 0.7 mW at the

sample. The lower laser power was used to avoid inducing thermal changes in the mineralogy of

the iron oxide minerals. Integration time was 50 s with 5 accumulations.

6

ESEM and X-ray microanalysis. Backscattered electron imaging, X-ray microanalysis and X-

ray mapping of the cross-sections set in resin were carried out on an FEI Quanta 200

environmental scanning electron microscope (ESEM). The instrument was operated in

environmental mode using water vapor as the specimen chamber gas, chamber pressures of 1.5-

2.0 Torr (200–267 Pa), an accelerating voltage of 20 kV and a working distance of 10mm. Under

these conditions surface charge cancellation occurs and therefore the sample could be examined

directly without a conductive coating. Since the backscattered electron signal is dependent on

the average atomic number at each sample point, the backscattered images allowed

discrimination of the different phases in the sample cross-section. Elemental analysis of

individual constituents in the cross-section was carried out using an EDAX energy-dispersive X-

ray microanalysis system. X-ray maps (elemental images) were acquired over 256x200 pixels at

a magnification setting of 900x and a chamber pressure of 2.0 Torr. Under high vacuum

conditions, the analytical spatial resolution is limited solely by the electron beam spread within a

solid sample. For Ca X-rays produced in a calcite matrix the X-ray emission volume per analysis

point in the sample surface can be calculated to be 3.4 µm, and for Si in quartz it is 3.6 µm16.

Under ESEM conditions, some scatter of the incident electron beam takes place in the chamber

gas17, and therefore the spectrum at each analysis point will contain a small contribution from the

surrounding material18,19. Modelling of the beam scatter17 shows that 70% of the beam is

unscattered at 2 Torr, hence the extraneous contribution is limited to no more than 30%. The

effect of the beam scatter on the elemental X-ray maps is a slight softening or broadening of

detail such as grain edges.

7

Micro-attenuated total reflection infrared spectroscopy. ATR Spectra were recorded directly

from the polished surfaces of the mounted cross-sections by contacting the ATR crystal onto

selected areas of the samples. The FT-IR spectrometer was a Nicolet Nexus 870 with a

Continuμm™ infrared microscope equipped with an ATR objective incorporating a Si internal

reflection element (Thermo Electron Corp., Madison, WI). The system was equipped with a

liquid nitrogen cooled MCT detector. Spectra were recorded with a resolution of 4 cm-1 and a

range of 4000 - 650 cm-1 for 128 co-added scans. All ATR spectra are corrected for wavelength

dependence of depth of penetration using OMNIC software (Thermo Scientific, Madison, WI).

RESULTS

The stucco: The sample pieces were first examined using a light microscope and an estimate

made of the particle sizes in each layer. The stucco is a pink/buff color, with a large range of

particles 1mm down to 5-10 µm sizes dispersed throughout. Particles vary in color from brown,

red, yellow to white. This large variety of colored material differs from the composition of the

stuccos found on the external surfaces of buildings20, which are white and are predominately

composed of calcite. Raman spectroscopy of both the raw sample and the polished cross-section

identified a range of minerals. These are predominantly calcite and quartz, with small particles

of rutile, hematite, goethite, magnetite, carbon, and some volcanic rocks, possibly feldspars or

plagioclase. The distribution of these small mineral particles and their low number suggests that

they are contamination from the preparation process and not deliberate inclusions. Many of the

minerals are used in other paints and some could be part of the grinding stones. The larger sand

and soil particles could have been included with the calcite as fill for the stucco. FT-IR-ATR

images of this layer (not shown here) confirm the distribution of a range of non-calcite particles

8

in the layer including clay, mica and quartz particles. Above this coarse layer is a finely ground

calcite layer with a smaller number of impurities. This layer is quite dark in comparison to the

lower layer and Raman spectra of the particles revealed that this layer consists of very fine

carbon particles dispersed in the calcite stucco. The fine nature of this amorphous carbon and its

even distribution suggests that the inclusion of the carbon was deliberate to create a dark under-

layer for the paint. Again this contrasts with the fine white, almost pure calcite stucco layer used

under paint on the external surfaces of buildings. Unfortunately, the high levels of carbon from

the setting resin and carbonate in the stucco prevented the observation of the variation in the

carbon levels in the ESEM-EDX maps. Carbon is not easily detected by infrared spectroscopy

and so the distribution was not mapped by this technique either. This type of carbon rich stucco

layer has not previously been reported for Maya stucco material, or in other stucco contexts.

An ESEM-EDX map of the area of stucco immediately under the paint layer was

recorded. The mapped area is predominately Ca, C and O with 5 small grains showing high Fe

levels and a range of grains containing high Si and Al. All other grains are Ca based in a high Ca

background making it difficult to differentiate individual grains. Micro-Raman spectroscopy

was used to confirm that most of the grains were calcite. The small number of silicate grains

suggests that these are accidental rather than deliberate additions. ATR spectral images of 40x40

µm areas of this mapped section show strong calcite spectra for most of the image, with bands at

1400 and 870 cm-1. ATR spectral images generated using the band at 1030 cm-1 show a small

number of grains coinciding with the silicate grains identified in the ESEM maps.

9

Red Paint layer: Surface microscopic observations of this paint identified a coarsely ground

material of uneven texture. Initial Raman spectroscopy of the surface identified that the paint

was predominately made up of hematite particles with some magnetite particles. Contaminating

particles consisted of calcite, carbon, quartz and goethite in order of amount, along with a

number of particles which fluoresced or were Raman inactive. The presence of a large number

of different mineral phases suggested that this was a heterogeneous paint which differs from the

homogeneous paints used in the external building decoration21. Analysis of the paint layer in the

polished cross-section identified a significant number of distinct mineral particles with a wide

range of particle sizes. Mineral types identified were similar to those identified in the surface

analysis, but also included rutile and anatase. Micro-FT-IR-ATR analysis also identified the

presence of calcite and clay minerals in the paint. Preliminary ESEM analysis of the cross-

section confirmed that the paint was made up of a range of particles with sizes from less than 1

µm up to 50 µm (Fig. 1). ESEM-EDX confirmed that the paint was composed predominantly of

Fe and Ca with some Si, Al and low amounts of Mg. The presence of high levels of Ca along

with crystalline calcite in the paint matrix confirmed that calcite stucco was used as a binding

agent in the paint layer.

In order to identify the particle types and morphology, an ESEM micrograph of a section

of the paint layer was recorded (Fig. 1). An ESEM-EDX map of this area was then carried out.

Thirteen elements were mapped, Ca, C, Fe, Al, Si, Mn, O, Na, Mg, K, P, Cl and Ti. The most

relevant elements for this determination are Fe, Ca, Si, Al and to a lesser degree Na and Mg, as

the other elements were not found in the sample or only one particle was identified, as in the case

of Ti. The matrix of the pigment layer is a mixture of Ca and Fe (Fig. 2 Ca and 2 Fe), with

10

individual particles containing high Fe or Ca clearly delineated. The stucco layer lies across the

lower right hand corner of the map and is very high in Ca, low in Si and Al and contains no Fe.

The Si map contains a range of particles with varying intensity, the brighter particles containing

a higher concentration of Si. Some particles are very high in Si and this is confirmed by the

Raman analysis which identified some of the particles as quartz. Al is present in varying

concentrations in some of the silica particles, along with Na and Mg which are present in very

low levels in many of the particles. A small number of silicate particles also contain K and could

be a form of mica. A number of layered particles conforming to the morphology of mica were

identified in the ESEM analysis.

FT-IR-ATR spectral imaging was carried out on three sub-sections of the ESEM mapped

section of the paint layer. These are designated 1, 2 and 3, (Fig. 1) and also marked on the Si

map (Fig. 2). The ESEM image of Area 1 has two large, approximately 22x15 µm grains at the

top. An FT-IR-ATR spectral image generated using the intensity of the 1065 cm-1 band (Fig. 3b)

shows the left hand grain shape clearly differentiated from the matrix material, while the right

hand grain is much less distinct. ESEM-EDX analysis of the left hand grain gives a very high Si

level with no Al, hence the very bright appearance in the Si map (Fig. 2). To better understand

the variations in the silicate minerals present in this area, band fitting was used to clarify the peak

positions. This enabled the discrimination of silicate minerals from alumino-silicate minerals.

The band fitting analysis of a typical infrared spectrum for this particle is shown in Fig. 4a. The

strong band at 1030 cm-1 along with the bands at 1192 and 1154 cm-1 are assigned to Si-O

stretching and are associated with silicate minerals.22 The band at 1068 cm-1 assigned to the Si-

O bond stretching is lower than the 1080 cm-1 normally found for quartz suggesting that this is

11

another form of silicate structure, possibly an amorphous silicate glass.23 This material could be a

mixture of silicate phases, unfortunately the lack of other diagnostic bands in the spectrum

makes it difficult to correctly match the mineralogy of the spectra. The second (right-hand)

particle can be seen more clearly distinguished from the matrix material in Fig. 3c and 3d. These

images were generated using the intensity of the band at 1014 cm-1. The band fitting analysis of

a typical infrared spectrum (Fig. 4b) has an intense Si-O stretching band22 at 1014 cm-1 with a

very weak Si-O-Al band at 942 cm-1 and a weak Al-O-H band22 at 916 cm-1. The Al map (Fig.

2) shows a higher concentration of Al in this particle. The low levels of Al (approx 4%)

compared to Si (approx 24%) and the lack of clay bands present in the spectra suggests that this

is most likely an Al substituted silicate.22 Micro-ATR infrared spectra confirm the presence of

quartz in the paint layer with bands at 1075, 798 and 780 cm-1, silicates with bands at 1055,

1035, 1010 and 798 cm-1 and also the presence of clay silicates with -OH stretching bands at

3695, 3666 and 3618 along with bands at 1030, 1008, 936 and 913 cm-1. The rest of the image is

predominantly matrix material with a broad carbonate band at around 1400 cm-1 and in some

areas a weak broad band at 1033 cm-1. Individual calcite particles are not observed in this image.

The ESEM micrograph, Fig. 3e, of Area 2, shows a mixture of grains imbedded in a

finely ground matrix of sub-micron particles. These grains range in size from 6 µm to 19 µm.

The ATR spectral image (Fig. 3f and 3g) generated using the band at 1010 cm-1 shows distinct

grains with one area, lower left, which has not been fully resolved from the adjoining grain. This

unresolved area is a mixture of finely ground matrix particles. A typical spectrum of this area

contains both calcite, 1407, 870 cm-1 and silicate 1068 and 1032 cm-1 bands (Fig. 5a). The grain

in the bottom left of the image is clearly resolved. The typical spectrum for this grain has an

12

intense band at 1077 cm-1 typical of quartz (Fig. 5b), while some of the peripheral spectra also

have bands at 1010 and 915 cm-1 due to Al substituted silicate on the edge of the grain. ESEM-

EDX analysis of this grain shows high Si, some Al and relatively high Fe. The grain is

surrounded by small iron particles which are detected by the slightly scattered beam in the low

vacuum conditions of the ESEM. The small grain in the bottom right hand corner of the ATR

spectral image has bands at 1115, 1030, 1012 and 905 cm-1 which are indicative of an alumino-

silicate. The high iron levels found in the ESEM-EDX analysis of this grain are due to the

surrounding sub-micron particles of iron (Fig. 2 Fe). The grain in the top centre of the FT-IR-

ATR spectral image (Fig. 3f) is very high in Si with no Al and a typical spectrum shows an

intense band around 1060 cm-1. The FT-IR-ATR spectral image generated using the band at 1400

cm-1 (Fig. 3h) which is the carbonate band in calcite, shows two grains on the right hand side.

These grains both have typical spectra with intense bands around 1400 and at 870 cm-1 indicative

of calcite (Fig. 5c). The Raman spectrum of the top grain confirms that it is calcite. The lower

area appears to be a tightly packed agglomeration of the matrix material, which is high in calcite

stucco. This area is also visible in Fig. 3f which is based on the intensity of the 1010 cm-1 band,

confirming the presence of silicate material in the matrix.

The ESEM image of Area 3 (Fig. 3i) shows two small grains of around 10 µm and 8 µm

respectively in a finely ground matrix of sub-micron particles. The FT-IR-ATR spectral image

generated using the 1012 cm-1 band (Fig. 3j), shows two grains clearly distinguished from the

matrix material. Both grains are visible in the ESEM-EDX Al and Si maps. The bands at 1064,

1046, and shoulder at 950 cm-1 of a typical spectrum (Fig. 6a) for the top left grain, are

suggestive of a disordered silicate material with the weak band at 915 cm-1 due to Al substitution

13

in the silicate22 matrix. The grain in the lower right of the image has a typical spectrum with an

intense band at 1012 cm-1 and a shoulder at 1022 cm-1 which is more suggestive of a layer

silicate (Fig. 6b). The ESEM-EDX analysis confirms higher levels of Al in the mixture and this

combined with a high level of K in the grain indicates a slightly different mineralogy to the

alumino-silicate material found in the other areas. The carbonate band at 1400 cm-1 in this

spectrum is due to contamination from the surrounding matrix as the ESEM-EDX result

confirms only a low level of Ca in this grain. The image generated using the 1400 cm-1 band

(Fig. 3l) shows carbonate containing particles as well as a less defined area which is part of the

finer matrix. The larger intense area at the top of the image appears to be a cluster of small

particles of 1-2 µm each. Raman spectra of the particles confirm that they are calcite. Many

sections of the fine matrix material have spectra with a combination of carbonate and silicate

bands. The matrix in the ESEM-EDX maps contains a mixture of the elements Fe, Ca, Si and Al

and gives Raman spectra with weak hematite and calcite bands.

DISCUSSION

Conventional micro-ATR infrared spectroscopy can only provide a generalized spectrum

of a limited area and the overlapping of the strong Si-O bands make it difficult to identify

individual components. In contrast FT-IR-ATR spectral imaging of this heterogeneous paint

mixture reveals a complex mixture of individual particles of different mineral phases. Individual

spectra can be obtained for each particle allowing the differentiation of each of these mineral

14

types. The main coloring agent in the paint mixture, established using Raman spectroscopy, is

iron oxide, in this case predominantly hematite. This material, in combination with finely

ground calcite, makes up the majority of the very fine sub-micron matrix material. The calcite in

the matrix can also be clearly identified in the FT-IR-ATR spectral images along with calcite

particles distributed throughout. Clusters of this matrix have been identified in the images, and

spectra confirm that they are a mixture of calcite and silicate materials. These areas can be

differentiated from individual grains in the images because they lack the definitive edges and

shape seen in the grains. A small proportion of calcite stucco added to the paint mixture when

wet would aid in the adhesion of the paint layer to the stucco surface. The addition of stucco to

the paint also lightens the color resulting in the softer red observed in this sample. In this

instance there is a clear delineation between the paint layer and the stucco layer (Fig. 2 Ca)

indicating that the paint has been applied to a dry stucco surface. The FT-IR-ATR spectral

images of the stucco layer show a small number of silicate and alumino-silicate grains. Some as

small as 5 µm have been differentiated in a predominantly calcite stucco matrix. Other grains

identified in this layer are calcite. The fine carbon particles throughout this layer are not

detected in the FT-IR-ATR spectral images but are identified using micro-Raman spectroscopy.

The presence of finely ground carbon in the fine stucco layer applied directly under the painted

surface indicates that a different surface appearance was sought. This was an attempt to darken

the walls of the tomb prior to painting and perhaps darken the appearance of the paint layer.

The FT-IR-ATR spectral images of the paint layer obtained here show a selection of

distinct particles distributed across the layer. In each of the areas examined grains ranging in size

from 6 to 22 µm have been resolved from the surrounding matrix material. What makes this

15

paint mixture interesting is the variety of silicate minerals present making up a significant

percentage of the total paint. The components identified using FT-IR-ATR spectral imaging,

include quartz, disordered silicate and alumino-silicates combined with iron rich silicates. The

mixture of particulates and the coarseness of the grains, along with the presence of distinct grains

of high iron containing hematite, indicate that the silicates have been added to a higher grade

hematite as a grinding and filling agent. Clay material in particular can be added as a binding

agent to assist in the adhesion of the paint to the stucco under layer. The inclusion of small

contaminating particles of magnetite and goethite along with carbon particles in the paint,

suggests that little care has been taken to maintain the purity of this paint. This is in stark

contrast with other paints from the external surfaces of buildings examined in an earlier study21.

These paints consist of finely ground hematite with only traces of other contaminant materials.

This matches with what we know about the rest of the tomb. The roughness of the rock walls,

the coarse application of the stucco, and contaminants in all coating materials point to a hurried

preparation and application in this tomb.

CONCLUSIONS

The paper reports the successful application of FT-IR-ATR spectral imaging to layered

archaeological paint samples. The improved spatial resolution of this technique over

conventional micro-ATR infrared, allows the orientation and morphology of individual grains to

be distinguished. The heterogeneous mixture of mineral phases, including a high proportion of

other materials with the pigment, hematite either as a byproduct of the preparation process or to

deliberately extend the paint, was determined. By combining these data with the results from the

16

complementary technique of micro-Raman spectroscopy a comprehensive characterization of the

paint mineralogy has been established. This paint is a deliberate mixture of hematite, silicates,

calcite, carbon and magnetite particles in a fine submicron matrix of hematite and calcite stucco.

The interpretation of the FT-IR-ATR spectral imaging was confirmed with the use of ESEM-

EDX microanalysis, X-ray mapping and backscattered electron micrographs. The use of carbon

to darken the underlying stucco has not been previously reported.

ACKNOWLEDGMENTS

We wish to thank the Instituto Hondureno de Antropologia e Historia for its support in this and

other studies we have carried out. We also thank Dr Thor Bostrom (Analytical Electron

Microscopy Facility, QUT) for carrying out the ESEM-EDX analyses, for assistance in the

interpretation and presentation of the elemental maps, and for his advice and recommendations

on this paper.

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Figure Captions

Fig. 1 ESEM backscattered electron image of paint layer covering the area of elemental mapping

Fig. 2 Elemental maps for Al, Si, Mg, Ca, Fe, Na. The Si map includes the areas imaged using

the FT-IR-ATR spectral imaging system

Fig. 3 ESEM and FT-IR-ATR spectral images of the studied areas. a) ESEM image of area 1, b)

ATR spectral image generated using the intensity of the 1065 cm-1 band, c) ATR spectral image

generated using the intensity of the 1014 cm-1 band, d) ATR 3D spectral image generated using

the intensity of the 1014 cm-1 band, e) ESEM image of area 2, f) ATR spectral image generated

using the intensity of the 1010 cm-1 band, g) ATR 3D spectral image generated using the

intensity of the 1010 cm-1 band, h) ATR spectral image generated using the intensity of the 1400

cm-1 band, i) ESEM image of area 3, j) ATR spectral image generated using the intensity of the

1012 cm-1 band, k) ATR 3D spectral image generated using the intensity of the 1012 cm-1 band,

l) ATR spectral image generated using the intensity of the 1400 cm-1 band

Fig. 4 a) FT-IR-ATR spectrum of particle top left area 1 with band fitting of Si-O region

identifying bands at 1174, 1065 cm-1 of an amorphous silicate, b) FT-IR-ATR spectrum of

particle top right area 1 band fitting of Si-O region identifying bands at 1117, 1014, 942, 916

cm-1 of an alumino-silicate

Fig.5 FT-IR-ATR spectrum of a) the unresolved region, bottom right area 2, containing a

mixture of calcite and silicate bands b) particle of quartz lower centre of area 2, c) calcite

particles right hand side area 2

Fig. 6 FT-IR-ATR spectrum of a) disordered silicate particle, top left area 3, b) layer silicate

particle, lower right area 3

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Figure 1 Figure 2

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Figure 3 Figure 4

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Figure 5 Figure 6