Page 1 of 16

Y:

C ,. the $aman effect, the differences in energy between the incident and the

scattered photons are quantized, corresponding to the energy differences

within the vibrational or the rotational levels of the molecule. In Figure 10.1

3: the fundamental and the first excited vibrational energy levels of the mole- M.

!4d having energy hvo collides with a molecule, two different

ce. In one case the frequency of the radiation is suitable to

e the transition of the molecule toward one stationary excited state

onic absorption). On the other hand, when the frequency of the radi- riate to induce this effect. the molecule itself will occuov a

energy. If the scattered photon has the same energy as the exciting photon:

the Rayleigh scattering can be observed.

However, in a different situation, the molecule does lose energy, reaching

the state with u = 1 and not returning to the vibrational state v = 0. In this

case the scattered photon energy is hvo - hvl = h(v0 - v,), where v, is the

Modem Anolytieol Melhods in Arc and Archncolug.~, Edited by Enrico Cilikrto and Giueppe

Spoto. Chemical Analysis Series, Vol. 155

ISBN 0-471-29361-X 0 2000 John Wiley & Sons, Inc.

255

Page 2 of 16

Page 3 of 16

10.2. RAMAN INSTRUMENTATION AND TECHNIQUES

10.2. RAMAN INSTRUMENTATION AND TECHNIQUES

recorder

ngure 10.2. schematic representation of Raman spectrophotometer.

Page 4 of 16

RAMAN SPECTROSCOPY 10.3. FOURIEK TK

Figure 10.3. Scheme of Raman microspectraphotometer with dide array detector.

Raman .h11

and to record the weak Raman signals. A Raman spectrophotometer can be

coupled with an optical microscope, as shown in Figure 10.3.

Raman microscopy can be applied either in transmission or in reflection,

depending on the type of problem to be solved or the type of sample. In

Figure 10.4 the micro-Raman spectrum of a red stain on the Carrara marble

In a more expensive version, the Raman microspectrophotometer is

equipped with a confocal microscope that presents two pinholes (see Fig.

10.5). With the confocal microscope, only the radiation scattered by the 0~t.d tocUs PIU,

focused area goes through the second pinhole and reaches the detector, (dotted ~i,

eliminating the radiation scattered by a not perfectly focused zone.

Figure 10.5.

10.3. FOURIER TRANSFORM NEAR-INFRARED RAMAN

SPECTROSCOPY to analyze the scdtterc

when the photon exc

As mentioned above, the Raman bands are usually rather weak. Therefore: induce the electronic e

they can be hidden by any resonant effect such as fluorescence, either coming All these draxr-back

from the sample or from some extraneous compounds. Usually the fluores- which allows us to el

cence yields results four to eight times greater in intensity than Raman scat- composition. This lase

tered radiation. The intensity of a fluorescence signal can be suitably reduced is in the NIR region of

using an exciting source in the near infrared (NIR) and an interferometer transform (FT) NIR 1

Page 5 of 16

has value lower than that required to

Page 6 of 16

. I? 1: tion products. BO 2.~ removed from a ,

I -

is a nondestructive

for other measure)

Depending on

spectra m order t

suffice or a detaile,

a vibrational spect

l of the examlned cc

In colored art11

detailed knowledge

informat~on useful

and authent~cat~on

attnbut~on and ide

lI!@111 Figure 10.6. Schematic drawing of FT-NIR Raman spectrophotometer. Presence of He-Ne duction of certain aser is indispensable to control optical alignment of apparatus. on a pre-eighteentt

twentieth centurv.

10.4. RESONANCE RAMAN SPECTROSCOPY

The resonance Raman (RR) effect, or RR scattering, consists in the excita- tion of a molecule by using a radiation characterized by a frequency within

those belonging to one of its absorption bands. As a consequence, the bands

related to the vibrations of the chromophore group, responsible for the

electronic absorption, are enhanced.

The conditions required to obtain strong Raman bands vary from com- pound to compound. In some cases the same experimental conditions as for

normal Raman spectra are utilized. In other cases it is necessary to operate

(i) in backscattering or (ii) at low temperatures and by using low laser power.

The main advantages of RR are a strong enhancement of some Raman

bands and the effect selectivity.

The record of RR spectra usually requires a normal Raman instrumen- tation. The more used sources are argon or krypton ion lasers, which allow

use of up to 20 different wavelengths. Obviously the availability of dye lasers

greatly increases the number of available exciting lines.

10.5. RAMAN SPECTROSCOPY AS SUPPORT FOR RESTORATION AND

CONSERVATION

Raman microspectroscopy is a precious technique for the chemical charac- terization of materials used in works of art or, eventually, of their degrada-

, .

from the nineteenth

used on a work oi :

hence to its authent

In the field of ar

identified, then spec

can be made. For I

taining the light-ser

exhibition conditior

works on paper the :

establishing the app

When aqueous o

lize the cellulose in 1

dation mechanisms:

affected by these tre;

or chrome yellow. w

Raman rnicroscol

(Clark 1995) of artif

butes of reliability a

immune from inteie~

above, Raman micrc

size, usually adoptin;

lyze different layers c

in a polyester resin. F

in the visible rezion,

Page 7 of 16

10.5. RAMAN SPECTROSCOPY AS SUPPORT 261

tion nroducts. Both small areas of relativelv large artifacts or microsam~les

is a nondestructive technique, the same sample can be subsequentiy employ~d

for other measurements.

Depending on the particular purpose, a simple comparison between 4 I

i:

4

f; : a vibrational spectrum, the Raman spectrum provides an unique fingerprint

of the examined comvound.

& attribution and identifying forgeries as there are known dates for the intro- F. from the nineteenth century. Consequently, the identification of the pigments .I

b. used on a work of art may give an indication as to the date of the work and 1 bta- hence to its authenticity.

In the field of art conservation, if pigments with known susceptibility are "thin -i L,,, identified. then soecific recommendations for storaee. disvlav. and treatments i

h- [ exhibition conditions of low lux levels and controlled time on display. For

b for works on paper the identification of pigments with certain sensitivities aids in

bte establishing the appropriate conservation treatment.

'her. When aaueous or deacidifvine treatments are beine considered to stabi-

@n- B affected by these treatments, such as Prussian blue, which could turn brown,

Bow . or chrome yellow, which converts to orange.

above, Raman microscopy can work on nonmodified samples of very small

. ,

rda- in a polyester resin. Furthermore, since the probe laser wavelengths used are .. . .. i in the visible region, usually 0.5 to 0.7 pm, the spatial resolution of the

Page 8 of 16

RAMAN SPECTROSCOPY

experiment is of this order, and so individual pigment grains of size 20.7 ~m As me.

may be identified. These advantages are unique to Raman spectroscopy and frequency

are of enormous importance when, for example, the work of art has under- resonancc

gone conservation treatments at different stages of its history or when a by lapis I

particular color is made up from a mix of pigments. mineral 1;

It is worth noting that the Raman spectrum is sensitive to both composi- sential cc

tion and crystal form: as is demonstrated, for example, by the results obtained pound de

for titanium dioxide, which is known to exist in three crystal modifications sulfur rac

(mtile, anatase, and brookite), all occurring naturally (Best et al. 1992). A Hassan e

considerable part of the following treatment will be devoted, as a conse- respo~isib

quence of what was stated above, to use of Raman spectroscopy in the defi- of the pi:

nition of the so-called palette of a work of art. trutn wit!

Nevertheless, the capability of this technique of yielding a fingerprint of seen. The

inorganic and organic compounds, together with the opportunity of apply- citinz lint

ing it to manufacts or samples of different shape and size without particular can be at

pretreatments, makes it important in the investigation of a lot of materials and Fran

of either artistic or archaeological interest. Therefore, a subsequent part of is due to

this chapter will be devoted to the use of Raman spectroscopy in the char- 610 nm a

acterization of various materials; ranging, for example, from minerals to 1969) thc

resins or glasses or organic residues in burial sites. Finally, the technique has (Morton

found application in the study of deterioration processes of different sub- As me

strates, mainly frescoes and limestone; and examples of such application will of a pigr

also be reported.

10.6.2.

10.6. APPLICATION OF RAMAN SPECTROSCOPY TO THE STUDY OF

PIGMENT3 IN ARTISTIC HAND-WORKS I The illun

10.6.1. Introduction

The Raman spectra of a large number of pigments, both inorganic and

organic, have been reordered and now form part of a data bank. Some years

ago, Guineau (1987) and recently Bell et al. (1997) published two interesting

spectroscopic libraries. Of particular relevance is the work by Bell et al.,

which presents the Raman spectra of over 60 pigments, both natural and most of \

synthetic, known to have been in use before about A.D. 1850. script. W

It is worth noting that the choice of the exciting laser line is extremely transferri

important to obtain a good result because absorption of the scattered light Ra~na

by the sample may reduce the quality of its spectrum. In such cases the ex- citing line is chosen to fall outside the contour of the electronic absorption

bands of the pigment. For example: vermilion fails to give a significant

Raman spectrum using green excitation but gives a strong Raman spectrum

when excited with red radiation.

Page 9 of 16

10.6. APPLICATION OF RAMAN SPECTROSCOPY

As mentioned above, when a molecule is excited with a laser line whose

frequency corresponds to the maximum of an allowed electronic transition, a

resonance Raman spectrum may be obtained. A classic example is provided

by lapis lazuli, a complex rock mixture containing the valuable, deep-blue

mineral lazurite of approximate formula Nag[Al~Sis024]S,, which is the es- sential constituent of the pigment. The aluminosilicate cage of this com- pound describes a truncated octahedron and contains, in the holes, trapped

snlfur radical anions (notably S;, but also some S;) (Tarling et al. 1988;

Hassan et al. 1985). The internal electronic transitions of these radicals are

responsible for the color. Although accounting for less than 1% of the mass

of the pigment, the S; anion gives such an extremely intense Raman spec- trum with 500 to 600 nm excitation that no hands due to the host lattice are

seen. The Raman spectrum of lazurite, obtained by using the 647.1-nm ex- citing line of a kripton ion laser, shows a very intense band at 548 cm-' that

can be attributed to the totally symmetric stretching of the S; anion (Clark

and Franks 1975; Clark and Cobbold 1978). The great intensity of this band

is due to the fact that lazurite shows a very broad band with a maximum at

As mentioned before, a possible problem when collecting Raman spectra

of a pigment is that of fluorescence occasionally due to supports and binders.

10.6.2. In Situ Analysis of Inorganic Pigments from nluminated Books and

Manuscripts

The illuminator of books and manuscripts usually preferred to use inorganic

pigments since these colors were known to be less fugitive and more stable

than organic ones. Many were available as minerals or by way of simple

syntheses. The more common inorganic minerals and synthetic compounds

or glasses that have been used as pigments at different periods of time are

listed by Clark (1995).

The pigment analysis of manuscripts was not usually performed in the

past owing to the inappropriateness of the established analytical techniques,

most of which required removal of pigment samples directly from the manu- script. Moreover, it may be possible to remove the pigment from offsets

transferred onto the opposite page of a manuscript.

Raman microscopy is the only technique that allows an in situ analysis of

pigments from illuminated books and manuscripts. In fact, in these cases the

object to be analyzed can be stretched on a sufficiently wide translation stage

of the optical microscope and excited by a laser source. As already described,

the use of an aperture at the secondary focus of the scattered radiation has

been found to reduce significantly any interference from this source, with the

Page 10 of 16

RAMAN SPECTROSCOPY

exception of the fluorescence arising from the pigment itself. In the last case,

the most effective solution is to change the excitation wavelength, usually to

The presence of different pigments of essentially the same color on the

same manuscript indicates how important the concept of hierarchy was. For

example, the illuminators often used azurite for the capital letters and pre- ferred to use the more precious and expensive lapis lazuli for the garments of

the Virgin Mary or the most important saints. An illustration of the close

association that was made in the Middle Ages between aesthetic value and

intrinsic worth of materials has been suggested by the fifteenth-century

Italian painter Cennino Cennini. In other cases: to create an interesting effect

and minimize the use of the more expensive lapis lazuli; the technique of

layering lapis lazuli over azurite was adopted (Clark 1995).

The use of Raman microscopy in the field of illuminated manuscript

analysis has been dominated by the aim of characterizing the artist's palette,

providing critical information for conservation, and dating the manuscript.

Furthermore, Raman microscopy can be useful to describe the story of a Due to stro~

pigment. A good example of this comes from the studies of Coupry (Hughes

1990), who used Raman microscopy to identify lapis lazuli in an illuminated

manuscript from the Auxerre Abbey. The mineral lapis lazuli originates

from Afghanistan and, as the manuscript dates from ca. the year 1000, it

was established that the pigment was introduced in Europe from its coun- cross section

try of origin almost two centuries before first being mentioned in written The encit;

the Raman 5

Many manuscripts have been studied by Raman spectroscopy at the allow to ob~

University College of London (Clark 1995; Clark et al. 1995; Best et al.

1992, 1993). These include a thirteenth-century north Italian antiphonal spectrum \v;

displaying lapis lazuli and malachite; a thirteenth-century north Italian showed tlic r

choir hook displaying the technique of layering lapis Iazuli over azurite; a

sixteenth-century German choir book displaying white lead, carbon, azurite,

vermilion, red lead, massicot, and lead tin yellow; a Paris bible, ca. 1275, to obtain ad'

displaying white lead, vermilion, red lead, lapis lazuli, azurite, orpiment,

realgar, and malachite; the Skard copy of the Iceland Book of Law, ca.

1360, displaying vermilion, orpiment, realgar, red ochre, azurite, hone white, tified as the

and verdigris or basic verdigris mixed with green earth (Best et al. 1995); fourteenth cel

medieval Gennan manuscripts displaying vermilion, iron oxide, brown

Siena, azurite, malachite, amorphous carbon, white lead, lead tin, and

azurite in admixture with lead tin yellow (Burgio et al. 1997a); a thirteenth- century ByzantinejSyrian Gospel lectionary displaying orpiment, realgar,

and pararealgar (Clark and Gibbs 1997a); a large Qur'an section, an illumi- nated Arabic manuscript of the thirteenth century, displaying white lead,

lapis lazuli, and vemilion (Clark and Huxley 1996); and three Latin manu- been used in

Page 11 of 16

. .

similar studies have been carried out by other research groups. For ex- ample, J. Corset et al. (1989) studied a commentary on St Paul's epistles by

Corbie Abbey, twelfth century, displaying lapis lazuli; Bussotti et al. (1997) I

studied an illuminated parchment of the fifteenth century displaying lapi;

lazuli, vermilion, and minium. Finally it is worth noting that blue pigments

used bv a nineteenthcenturv French artist were analvzed bv Raman micrns- F I 10.6.3. In Situ Analysis of Inorganic Pigments from Pottery, FBiences, and

Glasses

: through the glaze. Therefore, because the pigmentation can-be analyzed by

Raman spectroscopy only by focusing the laser on the. oiement mains ex-

----- .B. Et,: I. the Raman spectra of blue and red pigments, respectively, did not generally Mm

S ., @a1 g spectrum wis-dwayi obtained (Clark and Gibbs '1997b) This spectrum

dian showed the signal of the silica in the glaze which obscured any signal from

E excellent s&tra. Nevertheless. it is worth notide that lazuli was iden-

. .

own et al. 1997b) by using an argon ion laser.

and The presence of the expensive mineral lapis lazuli in the blue glaze of

nth- ceramic shards, albeit in low concentration, has been established by Raman

Igar, microscopy also on medieval pottery fragments from the south of Italy

mi- (Clark et al. 1997~). Probably such pottery was destined for a very important

ead, person or for religious purposes. It is well known that various pigments have

mu- been used in the past in order to obtain blue decoration. These include

Page 12 of 16

RAMAN SPECTROSCOPY

Egyptian blue, cobalt oxide, all its mixtures with iron, manganese and zinc -

oxides, a combination of oxides of cobalt, copper, and chromium, and cop- per compounds.

A study of eight red and eight yellow ancient Egyptian faience fragments

dated to the eighteenth dynasty uncovered at El-Amarna revealed that all the

red fragments were colored with red ochre or red earth (Fe(II1) oxide plus

clay and silica] and the color of the yellow fragments was due to lead an- timonate yellow [Pb(II) antimonate] (Clark and Gibbs 1997a). The pigments .- used in red on painted ceramic ware, both glazed and unglazed, from dif- ferent medieval archaeological sites in the south of Italy have been identified.

The Raman spectra of various iron-based commercial pigment powders

have been used for reference purpose. The research led to the identification

of red pigments as Fe(II1) oxide (e.g., Indian red, red ochre, or Venetian red) 70 :

and yellow pigments as hydrated Fe(II1) oxyhydroxide (e.g., yellow ochre

and Mars yellow) (Clark and Curri 1998). Figure 10.7. F Two samples of decorated stained glass (fourteenth and nineteenth cen- of red ochre ii

turies) have been examined with the use of FT-NIR Raman microscopy.

The mediaeval sample consisted of a pale yellow glass substratum with an

applied deep red-brown cloverleaf motif. The Victorian glass sample was a

bright apple-green color with a deep red-brown applied geometric pattern.

The pigment used in both cases has been identified as red ochre (Edwards ence of azu

+ and Tait 1998). The FT

iorated nii

10.6.4. Analysis of Inorganic Pigments for Wall Paintings and

Easel Paintings ments ha\:e

the identifi'

In recording Raman spectra of pigments from wall paintings done a fresco

or u tempera, two different types of samples are used. The first is a powdered

sample, usually available in limited amount only, that has been scraped from

the mural painting. The second is a fragment of the painting that has been

previously embedded in a polyester resin and subsequently cut in order to

evidence its cross section. In this case, by using Raman microsGopy, it is

possible to obtain, in a selective manner, the spectra of pigment grains or of

different painted layers.

Publications regarding the Raman spectra of pigments from wall paint- ings are not numerous. Some of these have been reported by B. Guineau

(1987) relative to the identification of hematite in a fragment of a mural Raman m

painting from Pompei (Italy) and of red ochre in a fragment of a mural

painting from Karnak. In a study by Ferrer (Turrell and Corset 1996 and

references therein) red-colored samples were taken from decorations of the

ninth century: the pigments hematite and vermilion were present. Besides, has show1

Raman microscopy of the mural paintings decorating the church of S. Andrea

Page 13 of 16

10.6. APPLICAT~ON OF RAMAN SPECTROSCOPY

Figure 10.7. FT-NIR Raman spectrum of red pignenl from frescoes ofpalazzo Famese. Bands

of red ochre round: 226, 296, 412, 501, and 620 cm-'.

(fifteenth centyry) situated in Melzo near Milan (Italy) has shown the pres- ence of azurite, vermilion, and hematite (Bmni et al., submitted).

The FT-NIR Raman microscopy was used to characterize biodeter- rated nineteenth-century Italian Renaissance frescos by Zuccari at the

palazzo Farnese (Capriola). Whereas yellow, red, and violet/maroon pip

ments have been attributed to red and yellow ochre (see Figs. 10.7 and 10.8),

the identification of the white pigments used on the frescoes remained ques- tionable (Edwards et al. 1997a).

The FT-NIR Raman microscopy has been used for the identification of

red pigments, vermilion and red iron oxide on two fragments of wall paint- ings representing two of the major mediaeval decorative schemes, one (late

twelfth/early thirteenth centuries) from the Sherborne Abbey, Dorset (United

dom), and the other (twelfth century) from the Winchester Cathedral,

pshice (United Kingdom) (Edwards et al. 1997b).

is interesting to note that Raman microscopy was used to characterize

of S. Ambrogio (Cantu, Italy). Examination of gilded fragments by

microscopy established that under the gold leaf a layer composed of

d oxides (PbO, Pb304) and lead sulfate was present.

Raman microscopy has been used to study ease paintings. The exami- ation of contemporary paintings in order to propose an authentication test

as shown the capability of Raman microscopy in this field (Turrell and

orset 1996 and references therein).

Page 14 of 16

Page 15 of 16

10.6. APPLICATION OF U%AN SPECTROSCOPY 269

/i S>

:S_

S. prone to some limitations, namely fluorescence and/or photochemical deg- radation. Moreover, such pigments often scatter very weakly, because of the

fine grain size or the dilution of the viement itself in a mordant to obtain a

mmated Arabrc manuscript. ~hotocheiical degradation has been observed. 11

m investigation without special precautions. An example is given by the ace- /

;. the spectrum of purpurin, which is, with alizarin, a chromophore of the

i: madder dye, while the spectrum of alizarin was obtained by Guineau (1987)

ts taken from alizarin as such and also on an ancient bookbinding. Among yellow

oiments. Bell et al. (1997) list berberine. eamboee Indian vellow [all soectra

a water mng a real artwork, namely a mediaeval Latin manuscript, is presented by

cut was B Burgioetal (1997bi

pigment quercitron. As regards indigo, however, it should be noted that its

spectrum has been reported by Guineau (19871, both for the dye as such and

fixed in the structure of a clay mineral (palygorskite) in the so-called Maya

z then, blue from a fraement of a mural minting (Fig. 10.91: the excitation wave- mg tne I. , , cases. One is resonance Raman spectroscopy, which allowed, for example, ined as ill Bussotti et al. (1996): to obtain very good spectra of alizarin from two red l l!

I paint- lakes in a fifteenth-century painting. In order to observe the resonance should 1 t

Raman effect, the line at 457.9 nm of an argon laser was used: due to the 1 I;

i olren & noth her important technique in the identification of organic pigments

Page 16 of 16

WAN SPECTROSCOPY

-

Figure 10.9. Identification by Raman spectroscopy in a fra~nent of a Maya blue mural painting

of the dye indigo fusd in a clay mineral (a); for comparison, Raman spectrum of a sample of .P

indigo is (b) also shown. (Reprinted, by permission, from Guineau, 1987. Copyright 0 1987 by

-

and dyes, even if at present not yet fully exploited, is FT-Raman spectros- copy. An example is offered by a Raman study (Edwards et al. 1997~) of the Figure 10.10. Raman sp

resin known as "dragon's blood," used for its fine red color in pottery, en- head and (h) jadeire jad

amels, and illuminations. The use of FT-Raman spectroscopy should avoid 1997. Copyright c, 1997

many'of the above-cited problems connected with the fluorescence back- ground, due both to the organic molecules themselves and to their dispersing

nioles 1997); and to

of two ceremonial a

erally as "greenston<

10.7. PECULIAR APPLICATIONS OF RAMAN SPECTROSCOPY IN ART also a semiquantitar

AND ARCHAEOLOGY their Raman wavenu

The other main ap

Besides the considerable amount of work concerning the use of Raman ology concerns the us

spectroscopy in the identification of pigments employed in the field of figu- about organic subjtat

rative arts, two other main kinds of studies exploit this technique to 'harac- biomaterials and org;l

terize samples of artistic or archaeological importance. and handcrafted objel

The first is based on the use of Raman microprobe to identify the mineral useful for this investis

phases in archaeological objects without having to destroy or prepare them. avoids the rise of fluoi

For example, it was possible to identify as chalcedony the main constituent

of three early Roman Empire intaglios found in Paris (Smith and Robin As regards human

1997); to to make a hypothesis concerning the temperature of fusion of the tance, a rather entensi.

walls of a Celtic so-called vitrified fort, a hypothesis allowed by the finding been supplied a cotnp.

of a-cristobalite as the main component besides glass itself (Smith and Ver- teinaceous and inorglr