Study on the technique of the Roman age mural paintings by micro-XRFwith Polycapillary Conic Collimator and micro-Raman analyses

Giovanni Paternoster a,*, Raffaele Rinzivillo a, Felice Nunziata a, Emilio Mario Castellucci b,Cristiana Lofrumento b, Angela Zoppi b, Anna Candida Felici c, Gabriele Fronterotta c,

Chiara Nicolais c, Mario Piacentini c, Sebastiano Sciuti c, Margherita Vendittelli c

a Dipartimento di Scienze Fisiche dell’Università “Federico II” and INFN-Sezione di Napoli Complesso Universitario di M.S.Angelo via Cintia,80126 Napoli, Italy

b Dipartimento di Chimica, Polo Scientifico, Università di Firenze Via della Lastruccia, 350019 Sesto Fiorentino, Italyc Dipartimento di Energetica dell’Università “La Sapienza” Via A. Scarpa 14 00161 Roma, Italy

Received 24 October 2003; accepted 20 October 2004

Abstract

XRF and micro-Raman stratigraphic microanalyses of fragments of some mural paintings, belonging to the Archaeological Site of Oplonti

(Napoli) and the Vigna Barberini site in the Palatino (Roma), were performed. In order to collimate the fluorescence X-rays emitted by the

samples, an X-ray polycapillary conic collimator (PCC) has been used in front of the detector. This device arrangement is compact, versatile,

and portable. The nature of the pigments, the compositional elements, and the thickness of the fragment layers have been studied. The

stratigraphic analysis partially confirms the preparation techniques described by Plinius and Vitruvius; moreover it confirms the hypothesis

that the artifacts are not fresco paintings. This work has been conducted within the context of a wider research on the Roman age mural

paintings.

© 2005 Elsevier SAS. All rights reserved.

Keywords: X-Ray collimator; ED-XRF; Portable Instruments; Micro-Raman; Mural paintings; Roman age

1. Aim of the research

The high quality and the good conservation status of the

paintings found at the beginnings of the Pompeii excavations

rise the problem of the techniques of their production. It has

been suggested that they were either encaustics, or based on

fresco, tempera or even on more elaborate techniques.

For identifying such techniques with a non-destructive

method (or better, with a quasi non-destructive method, based

on the analysis of the fragments and/or deteriorated paintings

at the archaeological sites), a very compact apparatus for

stratigraphic XRF analyses, to be used also in situ, has been

set up. We tested this instrument on several fragments of mural

paintings coming from Oplonti (Napoli) and Vigna Barberini

(Palatino, Roma). Some fragments were analysed also with a

micro-Raman device to determine the number and the thick-

ness of the paintings layers, the nature of the used pigments

and to check for the presence of organic compounds.

2. Introduction

In the field of Cultural Heritage, the Energy Dispersive

X-Ray Fluorescence (EDXRF) technique reached a great

development thanks to its numerous advantages. It allows non-

destructive, fast, versatile, sensitive, and in situ multi-

elemental analyses [1–4]. In studying the techniques of ancient

mural paintings and the pigments employed, these advan-

tages are well suited for recognising the compounds used for

the pigments, detecting the presence of organic compounds,

separating the matrix effect, and the signals coming from over-

lapping layers of painting [5–7]. In order to overcome some

of these difficulties, an EDXRF apparatus has been set up,

* Corresponding author.

E-mail addresses: paternoster@na.infn.it (G. Paternoster),

emiliomario.castellucci@unifi.it (E.M. Castellucci),

archeometria@uniroma1.it (A.C. Felici).

Journal of Cultural Heritage 6 (2005) 21–28

http://france.elsevier.com/direct/CULHER/

1296-2074/$ - see front matter © 2005 Elsevier SAS. All rights reserved.

doi:10.1016/j.culher.2004.10.003

which permits to carry out stratigraphic analyses with high

spatial resolution.

The Roman paintings’ stratigraphy and production tech-

niques were firstly described by Vitruvio [8] and Plinius [9]

the senior. A mural painting is composed of a preparatory

layer (tectorium) made of three or more (until six) layers of

plaster: starting from the raw wall, at first there is the arric-cio, constituted of three layers of lime and sea or volcanic

sand (or pozzolana) and then the plaster, constituted of three

layers of lime and marble powder. When the plaster is still

wet, the pigments, mixed with suited binders and diluents,

are laid over the tectorium. To preserve the paintings from

the environmental ravages, a wax mixture is laid, heated, and

smoothed so that the painting surface appears translucent.

At the beginnings of the 50s the chemist and archaeologist

Selim Augusti conducted many precise chemical analyses on

lots of paintings from Ercolano and Pompeii; his results seem

to prove the existence of a particular technique, already pro-

posed at the beginning of the XIX century, based on the use

of soaped water mixed with lime, clay (or chalk), and wax

[10–12]. This particular technique has recently been found

on several paintings of the roman age [13,14]. More recently,

Mora et al. [15] proposed the use of a fresco technique for the

background, and a limed water technique for the decorative

part. However, in agreement with other recent studies, it seems

that more then one single technique was used [16–19].

3. Experimental

3.1. Samples

The study was conducted on a total of 15 fragments with

an average surface area of about 15 cm2. Eleven samples come

from the Oplonti Archaeological Complex near Naples and

below they will be referred to as “Oxx”, “xx” indicating a

reference number. These fragments had not been restored and

they had no context so we have been allowed to make some

micro-destructive analyses on them. A stratigraphic analysis

has been conducted on the fragments O01 and O15. The

former fragment has a compact and resistant blue back-

ground; probably it belongs to the II Pompeian style, second

half of the I century BC; the latter belongs to the IV style,

second half of the I century AD. It is white, smooth, and as

compact as marble. Both fragments show a shining surface,

due to the presence of many crystalline inclusions.

The other four fragments come from a section of the

Palatino archaeological site in Rome called Vigna Barberini

Complex. One fragment is blue and we label it the Barberini

Blue fragment (BB); a second one shows a geometric deco-

ration in different colours, and we call it the Barberini Deco-

rated fragment (BD). Both fragments are restored but not pre-

served with any protective varnish. The third and the fourth

fragment are similar to each other, probably coming from the

same painting. We will refer to them as a single sample: the

Barberini Protected fragment (BP). They are black with a pink

stripe and have been restored and protected with Paraloidvarnish.

3.2. Micro-XRF

A small area of the samples is irradiated with an X-ray

beam and the emitted fluorescence photons are collected and

analysed. By determining the number and the energies of the

fluorescence X-ray lines, it is possible to trace up the ele-

ments in the sample [20]. With our micro-XRF device we

were able to analyse areas of the sample as small as 140 µm

in diameter. The stratigraphic distribution of the elements was

obtained by scanning along the lateral sections of the frag-

ments [21].

The measurements have been taken with a portable device

for EDXRF realized at the Laboratory of the University of

Naples. The X-ray generator is the E.I.S. air-cooled Pd anode

generator with a focus of 200 × 200 µm2 and a 250 µm thick

Be window; it can operate with a maximum voltage of 30 kV

and a maximum current of 0.6 mA. The X-ray beam, colli-

mated with tantalum diaphragms to a 1 mm diameter spot

size, is incident on the sample at 45°.

The fluorescence photons are collected along the normal

to the sample surface through a Polycapillary Conic Collima-

tor (PCC) mounted in front of the Si-PIN detector (Amptek

model XR-100CR, with 165 eV resolution at 5.88 keV).

The PCC has been supplied by the Institut für Gerätebau

GmbH [22, 23]. Its entrance is at 11.5 mm from the sample

surface (Fig. 1) and allows a resolution of 140 µm FWHM.

The X-ray tube and the detector are mounted on a three micro-

metric axes stand. The positioning is obtained with two lasers

and with a cross of two 20 µm golden tungsten wires on the

sample. A colour micro camera was used for the positioning

remote control and for the storage of the analysed point image.

The electronic chain is made up of an Amptek amplifier,

the Amptek multi-channel analyser MCA8000A and a por-

table computer for set-up control and for storing and analys-

ing the spectra [24].

Fig. 1. Lateral view of the PCC pointing to a fragment section of a mural

painting.

22 G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

Taking into account the detector efficiency, the energy

transmission of the PCC and the device geometry, we can

detect the K-lines from S to Zr and the L-lines from Pd to U.

The EDXRF spectra have been analysed with WinAXIL

software [25], inserting data about the X-ray source, the detec-

tor, and the device’s geometry. These parameters have been

independently measured and fixed to reduce the error on the

determination of the areas of the peaks from degenerate lines.

As the samples are constituted by a wide and inhomoge-

neous matrix, we prefer to present the results as relative rates

(i.e., percentage ratios between the single peak net rate and

the total net rate) instead of quantitative percentages.

3.3. Micro-Raman

Micro-Raman measurements were carried out at the Chem-

istry Department of Florence using a Renishaw RM2000 in-

strument operating with an Argon air cooled laser source and

a diode laser, with excitation wavelengths at 514.5 and

785.2 nm, respectively. A 50× magnification objective was

employed to focus the laser beam onto the sample providing

a spatial resolution of about 1–2 µm; a wide area could be

probed by way of consequential point-by-point analyses.

Raman spectra were acquired with an irradiating laser power

measured under the microscope of about 2 mW, while lower

power values were required by highly light sensitive samples,

for which neutral density filters were used. The spectral reso-

lution was 4–6 cm–1 depending on the experimental condi-

tions; the investigated spectral range spanned from about

200 to 1600 cm–1 where the main diagnostic signals of inor-

ganic materials (pigments and minerals) fall, while the high

frequency region between 2800 and 3100 cm–1 was mainly

inspected for the CH stretching vibrations due to organic com-

ponents.

Paintings were analysed with both laser line excitations,

while the presence of organic materials was mainly checked

using the diode laser line.

All fragments were observed with a microscope, without

any preparation. Only for the O01 sample a cross-section was

prepared.

4. Results and discussion

4.1. Pigment recognition

The painted surfaces have been analysed by XRF and

micro-Raman techniques to determine the pigments used. In

several samples the emitted fluorescence was so high as to

hide the Raman lines.

Regarding the elemental analysis, we consider only those

elements having a net area of the XRF peak greater then three

standard deviations and a relative rate greater then 0.1%.

Not all the elements found in a spot are characteristic of

the pigment used because of the contribution to the XRF lines

coming from the preparation layers and also because the

colours are often laid one over the other. It is possible to rec-

ognise the characterising elements by correlating their peak

intensities with the colours and the fragments: those ele-

ments, which are present in every analysed point of one par-

ticular colour with a high relative rate, are considered char-

acterising the pigment (Table 1).

Some considerations on the detected elements are neces-

sary:

• traces of phosphorus and sulphur, spread over all the

colours, were found only in some fragments: particularly

in the O03, O05, and O17 ones. They probably arise from

ashes and dust contamination that the painting samples had

been suffering until their recovery;

• potassium is always present, showing higher intensity

(more than five times its averaged relative rate) in the green

colours, whilst lower intensities were found in the whites;

• calcium is always present, but it is dominant only in white

and black colours;

• titanium is present in the yellow colours and in some green,

brown, and red ones;

• chromium characterizes the green colours;

• manganese is particularly evident in the browns, yellows,

greens, and in some reds;

• iron is characteristic of all the earth colours; it is present in

traces in the other ones;

• nickel characterizes the green colours and it is signifi-

cantly present in the blues;

• traces of copper are present in all colours, but it is domi-

nant in the blues.

• zinc is a natural impurity of the earth colours;

• arsenic is present in some red colours;

• strontium was detected in all colours with quite different

rates;

• lead is particularly evident in some red colours and in the

Barberini fragments.

As for Raman measurements, the compositional results were

obtained averaging over a wide set of data points selected

within the area to be characterised. The visible and near-IR

laser excitation lines were both used for pigments’ identifi-

cation, according to the paint colour: in general red, brown,

yellow, and white shades were investigated through the diode

laser line, while for green and blue pigments the green laser

line was preferred.

Raman spectra were recorded irradiating directly the sur-

face of the painted layers giving rise in a lot of cases to a

strong fluorescence probably due to surface organic contami-

nation. The fluorescence effect was that of originating a rather

strong background signal, leading to a complication in the

detection of the weaker Raman signals, thus in the identifi-

cation of the pigments employed. This was particularly evi-

dent with the argon laser line, while the diode laser excitation

at higher wavelength very unlikely gives rise to absorption

and then to emission of fluorescence.

Due to the lack of fluorescence, the diode source was also

preferred for checking the presence of organic materials

through the detection of CH stretching vibrations modes.

23G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

Unfortunately no Raman signals were observed in that region

and no organic substances were identified.

In the O15 sample the white painting layer gave solely the

spectrum of a carbonate compound that was attributed to cal-

cite.

The BB and BD samples coming from “Vigna Barberini”

were quite well characterised without requiring any type of

preparation. From the BB sample, showing a light blue colour,

the spectrum of Egyptian blue was obtained, while calcite

was attributed to the paint calcareous matrix. The BD sample

showed a poli-chromatic painting for which it was possible

to investigate each region separately. Except the green and

brown drawings, which exhibited a relevant fluorescence sig-

nal, it was possible to identify the white pigment as calcium

carbonate, while hematite was identified in red and violet

areas.

Most of the samples analysed showed a rather noisy lumi-

nescence background, induced even by the near-infrared diode

laser line. This fluorescence emission could be at first related

to the presence of some organic components, but this assign-

ment could not be confirmed by Raman measurements since

its intense signal in the high frequency spectral region pre-

vents to reveal any Raman peak that could be undoubtedly

attributed to such chemical components.

Therefore, we can conclude that the pigments used are [15,

26, 27] (Table 1):

• Atramentum, amorphous C, for the black;

Table 1

Pigment analysis results

24 G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

• Egyptian blue for all the analysed blue points;

• ochre and red earth for the brown colours;

• green earth (Creta Viridis), iron and potassium silicate, for

the green colours;

• ochre for the yellow;

• some variety of red earth and ochre for the red/pink/violet

colours, characterised by the ratio Mn/Fe and by small

quantity of As and Ti [27]; in four points we detected a

higher rate of lead due to the presence of minium;

• paretonium or melinum (white clays) for the white pig-

ment in fragments O08 and BD; lime and calcareous pow-

der for the white background in the remaining ones;

• white lead was probably used in the preparation layer of

the Barberini paintings.

4.2. Stratigraphy

Strontium is present in all colours. It reaches a high level

in the O03, O08, and BD fragments, which have a painted

white background, because it is probably present in the white

clays. This can be argued from the absence of strontium in

the analyses done on the points where the pigment is missing

and on the rear side of the fragments (Table 2).

To confirm this hypothesis 0.25 cm2 of the O01 fragment

has been milled to a depth of 600 µm by steps of 100 µm,

performing an XRF analysis at each step. In Fig. 2 the

obtained relative rates are reported.

It is evident that iron and strontium decrease with depth to

a small constant value, reached at –300 µm level.

Before discussing the stratigraphic microanalysis, some

comments on the visual inspection of the lateral sections of

the fragments should be done.

At the microscope with 6× magnification, the O01 blue

fragment shows a pictorial layer about 200 µm thick, well

distinguished from the preparation layers (Fig. 3). The plas-

ter, about 4 mm thick, is made of at least two separated layers

with different granulation of the calcite crystals. Inside, just

below the pictorial layer, it is possible to note a little spheri-

cal black inclusion, similar to those found in the arriccio; it

is probably part of the volcanic sand used in the tectorium.

At the same magnification the O15 white fragment shows

a very similar stratigraphy, except for the absence of the pic-

Table 2

Elements relative rate measured on points of missing colour and in the rear

parts (plaster) of the Vigna Barberini fragments

Fig. 2. Relative rate of the elements vs. depth for the Oplonti 01 blue fragment, measured on successive milled layers. The “X” stays for elements heavier than

Cu, apart the Sr.

Fig. 3. Transversal section of the Oplonti O01 blue fragment seen with a

stereo-microscope (6×).

25G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

torial layer and for the calcite crystals, that can reach a size of

about 2 mm in the innermost plaster layer (Fig. 4).

The dimension and the structure of the calcite crystals con-

firm theAugusti’s hypothesis that crushed limestone was used

instead of marble powder.

The two plaster layers have a thickness of 1–2 and 2–4 mm,

respectively, depending on the wall and the arriccio rough-

ness. This stratigraphy does not agree with that described in

the introduction, but it is coherent with that found by Augusti

[10].

4.3. Micro-XRF stratigraphy

Few millimetres on the lateral side of two fragments from

Oplonti, the O01 (blue) and the O15 (white), were gently

smoothed and polished. On that region a series of micro-

XRF analyses has been conducted.

The O15 white fragment shows a very high content of cal-

cium with traces of lighter elements until a depth of about

5 mm; from there on, iron and strontium are also present. In

the O01 blue fragment section, four regions, of different

elemental content, can be distinguished (Figs. 5 and 6):

• the pictorial layer, about 200 µm thick, containing Ca and

Cu as major elements due to the Egyptian blue pigment. It

contains also Fe and K, but no lighter elements. These

results agree with those found in previous analysis;

• an intermediate region, less then 150 µm thick, where only

Cu disappears while the other elements remain constant in

their relative rate;

• a third region, 4 mm thick, similar to the one found in the

O15 white fragment. The elements identified are: calcium

(major element) with traces of potassium and iron, but no

evidence of strontium. This region corresponds to the plas-

ter made with milled limestone, as recognised at the opti-

cal microscope;

• the arriccio layer containing iron, potassium, strontium

and traces of Zn, Zr, and Pb, in addition to the calcium.

4.4. Micro-Raman stratigraphy

A cross-section embedded in a resin matrix was prepared

to investigate better the nature of the blue painting layer. This

layer was about 80–100 µm deep, made of minute blue grains

that were identified to be Egyptian blue, a copper silicate

(CaCuSi4O10) widely used since antiquity [28, 29]. The under-

neath layer, about 170 µm thick, appeared with a light tone

due to carbonate crystals of calcite (CaCO3), wherein spo-

radic small orange grains (<2 µm) could be observed but were

Fig. 4. Transversal section of the Oplonti O15 white fragment seen with a

stereo-microscope (6×).

Fig. 5. The relative rate of the majority and minority elements vs depth measured in a lateral section of the Oplonti O01 blue fragment. The “X” stays for

elements heavier than Cu, apart the Sr. On the upper line are indicated the layers boundaries.

26 G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

not analysed. Going down, also the plaster was investigated

where calcite was mainly identified, with traces of silicate

components (such as diopside CaMgSi2O6), while no iron

minerals were detected. The inferior part of the section, the

arriccio, was black brown in colour. Here, Raman spectra

revealed the presence of hematite (a-Fe2O3) [28] and some

feldspars; a high signal at 667 cm–1 in the hematite spectrum

could be related to magnetite content.

A final consideration should be made with respect to the

hypothesis that the strontium element detected by XRF could

be an evidence of its presence as a carbonate. Actually, stron-

tium carbonate, namely strontianite mineral, shows a promi-

nent signal, due to the CO3– anion symmetric stretching vibra-

tion, that falls about 12 cm–1 below the calcite one. If

strontium had been present as SrCO3, according to the minor

content with respect to calcium element, a medium band

should have been detected in the calcite spectrum, while it

has been observed a signal around 1083 cm–1 free from neigh-

bouring features; in many other cases the calcite spectrum

appeared of such poor intensity that even the detection of the

weaker SrCO3 spectrum was complicated, leaving the ques-

tion unresolved.

5. Conclusion

A portable apparatus with a PCC for micro-XRF analyses

has been set up for studying some fragments of mural paint-

ings of Roman age. After having identified the elements

present in the pictorial layer, we did a stratigraphic analysis,

determining the number and the thickness of the preparation

layers. The Raman analyses on the samples confirmed the

XRF results, giving clear indication on the pigments used.

The preparation of the mural paintings follows the scheme

described by Augusti [11]. On the rough wall there is a layer

of lime and (volcanic) sand, named Arriccio, followed by two

layers of lime and crushed limestone, made of particles of

different grain size, and a preparation layer of lime and white

clay. More recent study on Roman mural paintings seems to

confirm the presence of a thin calcite layer under the pictorial

one [30].

Strontium is present only in the pictorial and preparation

layers, thus coming from the compounds used to lay them.

Its presence is also compatible with the use of gypsum, which

can explain the poor firmness of the colours on some frag-

ments. It must be noticed that gypsum has never been used in

the fresco technique; however it has been found in the plaster

of Egyptian and Middle Eastern ruins [10]. Moreover the

analyses do not reveal the presence of sulphates both in the

pictorial layers and in the tectorium.

We could not identify any particular organic compound to

confirm the use of soaped water, suggested by Augusti [11],

because of the high fluorescence background. However, the

greasiness of the pictorial and preparation surfaces could be

due to the presence of wax that, according to Vitruvio [8],

was used to polish the painting.

In any case, we think that the analysed fragments were not

made using a real fresco technique for several reasons. Firstly,

the painting was made with a superposition of the different

colour pigments over a uniform preparatory layer. Secondly,

even in the fragments with a white background, which seems

to correspond to the topmost layer of the plaster, the colour

pulverises and has a poor endurance, leaving only light traces

once taken away from the surface. This is not consistent with

a fresco technique or with the mere use of limewater. The

presence in the pictorial layer of a certain percentage of stron-

Fig. 6. The relative rate of the majority and minority elements vs. depth measured from 1 to 8 mm in a lateral section of the Oplonti O01 blue fragment. The “X”

stays for elements heavier than Cu, apart the Sr. On the upper line are indicated the layers boundaries.

27G. Paternoster et al. / Journal of Cultural Heritage 6 (2005) 21–28

tium leads to guess the use of a calcium compound for the

dilution and the drawing up of the pigments, namely white

clay [15].

There appears no great technical difference between the

two groups of fragments from Oplonti and from Rome. Some

differences have been highlighted in the pigments and com-

pounds used in the preparation layers: this is the case, for

instance, of the preparation layer of the Barberini fragments

in which evident traces of lead and stronger strontium peaks

have been found, while the Oplonti fragments contain a

greater amount of pure lime and calcareous compounds.

Acknowledgments

We wish to thank Prof. Piero Guzzo, Superintendent of

Pompei and Ercolano, for the authorisation and the collabo-

ration given. A particular acknowledgement to Dr. Lorenzo

Fergola, Director of the archaeological site of Oplonti, for

having kindly given us the fragments we studied. We have to

thank also the Archaeological Superintendence of Rome (and

in particular Dr. Maria Antonietta Tomei), which gave us the

opportunity to study the fragments from Vigna Barberini in

Palatino, property of the Ècole Française de Rome but kept

in Museo Nazionale Romano. They were kindly placed at

our disposal by the Director of the “Laboratori del Restauro”

of Museo Nazionale Romano, Dr. Giovanna Bandini.

This research has been partially supported by the “Con-

siglio Nazionale delle Ricerche”.

For the Raman measurements, the financial support of the

EU, contract No. G6RDCT2001-00602, is kindly acknowl-

edged.

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