Non-destructive pigment analysis of artefacts by Raman microscopy

Non-destructive pigment analysis of artefacts by Raman microscopy Stephen P. Best, Robin J. H. Clark and Robert Withnall When integrated with a microscope, Raman spectroscopy provides a uniquely specific and sensitive means of identifying pigment grains of sub-micrometre dimensions in a broad range of artefacts. This article reviews the methodology of Raman microscopy and its application to the study of mediaeval manuscripts.

The scientific study of artists’ painting pigments dates back at least as far as 1800 when John Haslam, a London doctor, analysed samples from mid 14th-century wall paintings discovered in St Stephen’s chapel, Westminster. Further studies followed soon thereafter; thus in 1807 Jean Chaptal, a French industrial chemist, published an investigation of pigment samples found in a colour merchant’s shop in Pompeii, and in 1815 Sir Humphry Davy, the eminent English scientist, reported his analytical studies of pigments from excavations in Rome and Pompeii. The pigments in use before 1700 were of natural origin, either in the form of minerals or of dyes or stains mostly derived from plants. Owing to their ease of purification and superior colour-fast properties, inorganic pigments were preferred. A list of those which have been in common use at some period of time, their common names, and their chemical formulae is given in Table 1. In some cases the colour of the pigment, coupled with the Stephen

P. Best, Ph.D.

After graduating in chemistry at the University of Sydney, he joined University College London, initially as a Ramsay Fellow and later, since 1986, as a Lecturer in Chemistry. He is particularly interested in structure and bonding in coordination compounds as determined by spectroscopic investigation. Robin J. H. Clark, D.Sc., Hon.F.R.S.N.Z.,

F.R.S.

After graduating in chemistry at the University of Canterbury, Christchurch, New Zealand he joined University College London in 1962 as successively assistant lecturer, lecturer, reader, professor, dean of science, and (since 1989) Sir William Ramsay professor and head of the Department of Chemistry. His research interests span most aspects of physical inorganic chemistry, especially spectroscopy. Robert Withnall,

Ph.D.

Is a graduate of the University of Cambridge and East Anglia and is currently a Leverhulme Research Fellow at University College London. His research interests embrace many aspects of spectroscopy, in particular infrared and Raman spectroscopy.

Endowour, New Beries, Volume 16, No. 2.1992. 016s9327/w 66.66 + 0.66. 01992. Pergamon Press Ltd. Printed in Great Britain.

66

identification of one or two key elements, would be sufficient to permit identification. Indeed, this was the approach adopted by the early experimenters. The quantities of pigments required for these early analyses were considerable and, although Davy’s claim to have experimented with ‘mere atoms of the colour’ was wildly exaggerated, it was perhaps indicative of a recognition that one criterion of such tests is high sensitivity [l]. Raman microscopy is a technique which, while still falling short of Davy’s claim, nevertheless makes possible significant advances in the analysis of pigments on manuscripts. The Raman effect is concerned with the phenomenon of light scattering by matter, the frequency distribution of the scattered radiation being uniquely characteristic of the composition and structure of the scattering material. Thus when a sample in any state of matter is irradiated by a monochromatic beam of light, frequency vO, it was found (by C. V. Raman in 1928) [2] that the light scattered by the sample contains not only radiation of the original frequency v, (referred to as Rayleigh scattered radiation) but also that of frequencies v0 + vi (referred to as the inelastic, or Raman scattered radiation) where each yi is the frequency of an internal (electronic, vibrational, or rotational) transition of the constituent molecules. Since the values of y are unique to each chemically and structurally different molecule, analysis of the scattered radiation - particularly of the vibrational spectrum by use of a socalled Raman spectrometer - provides an excellent basis for the identification of even sub-nanogram grains of any material. As typically practised, Raman spectroscopy involves laser excitation in the visible region (400-700 nm, 0.4-0.7 ym) whereas infra-red spectroscopy involves broadband excitation, with absorption occurring in the range 400@ 20 cm-’ (2.5-500 pm). Since the diameter of the diffraction-limited focus of a parallel beam of light is proportional to its wavelength, visible radiation has

typically a 20 times smaller focus than infra-red radiation. Further, the dimensions of the focus also depend on those of the source. In this context the lasers used for Raman spectroscopy produce radiation with an effective zero source dimension and may be focused to the diffraction limit, whereas sources of infra-red radiation are finite and therefore give a much larger focus. Raman spectroscopy is thus a highly specific and non-destructive analytical technique with excellent spatial resolution and, accordingly, is uniquely suited to the analysis of grains of pigment in manuscripts and artworks for which destructive and sampling techniques are not permitted. The identification of pigments on mediaeval manuscripts gives detailed insights into the technical and social development of civilisations and can be a valuable means of establishing both the nature of pigments traded and the first dates of such trade. Already, from a very limited number of studies, the application of these techniques has revealed inconsistencies in established thinking. The extension of this work to encompass a wider range of pigments and to include a much broader group of manuscripts is of major importance to our understanding of technical, social, and trading history. Resume of methods currently used in pigment analysis Many analytical techniques exist which can be used to give an unambiguous identification of pigments. Some are based upon elemental analysis (e.g. Xray fluorescence (XRF), inductively coupled plasma (ICP) analysis, atomic absorption spectroscopy (AAS), electron microprobe analysis); some on identification of ‘molecular’ fragments (e.g. u.v./visible reflectance, Raman and infra-red spectroscopy, mass spectrometry, X-ray diffraction); and some on thermal methods (e.g. differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA)) which are sensitive to the bulk properties of the sample. A number of these techniques can be used in conjunction with

TABLE 1

COMMONLY

USED INORGANIC

PIGMENTS

Colour

Common

Name

Chemical

Black

Charcoal

or carbon

Carbon

Blue

Azurite Cobalt

Green

Orange/brown

Red

White

Yellow

Name

Cobalt-doped

blue (from

Prussian

blue

lapis lazuli)

Formula

C

Basic copperfll)

Lazurite

Chemical

alumina

glass

Sulphur radical anions aluminosilicate matrix Iron(lll)

~CUCO&U(OH)~

carbonate

Co0.Al,03

in a sodium Na81A16Si60241S,

hexacyanoferratefll)

Fe4[Fe(CN&.14H20

Verdigris

Basic copperfll)

Green earth - a mix of celadonite and glauconite

Hydrous aluminosilicate of magnesium, iron and potassium

Va:iatio;s on K[ (AI”‘,Fe”‘) iEHiMg )I. (AISi3.Si4)0,0 2

Malachite

Basic copper

CuCO,.Cu(OH),

Cadmium

carbonate

Cadmium(ll)

orange

Ochre (goethite)

Iron(lll)

Cadmium

Cadmium(ll)

red

acetate

selenosulphide

oxide

+ clay selenide

Cd(S,Se) Fe203.H20

Leadfll)

Realgar

Arsenic(h)

Red lead (miniurn)

Lead(ll,

Vermilion

Mercuryfll)

sulphide

HgS

TitaniumflV)

oxide

Ti02

Anatase

+ clay

CdSe

Litharge

(cinnabar)

oxide

CU(O~CCH,),.PCU(OH)~

PbO

sulphide

IV) oxide

AS& Pb&

Barytes

Bariumfll)

Bone white

Calciumfll)

phosphate

Ca3(PO&

Chalk (whiting)

Calcium(ll)

carbonate

CaC03

Gypsum

Calciumfll)

sulphate

Kaolin

Layer aluminosilicate

A12(OH)4Si205

Lead white

Leadill)

PbC03

Lithopone

Zincfll) sulphide sulphate

Rutile

Titanium(lV)

Zinc white

Zincfll)

Cadmium Chrome

carbonate

Leadfll)

Cobalt yellow

oxide

yellow

Lead tin yellow

CaS04.2H20

ZnS + BaSO, TiOZ ZnO

sulphide

chromate

Potassium

BaSO,

and barium(ll)

oxide

Cadmium(h)

yellow yellow

Lead antimonate yellow)

sulphate

cobaltinitrite

CdS PbCr04 KcKo(NOJ~1

(Naples Lead(h) antimonate

Pb2Sbz0,

Lead(h) stannate

Pb2Sn04

Massicot

Lead(h) oxide

PbO

Orpiment

Arsenic(lll)

As2S3

chromatographic separation - gas chromatography (GC), thin layer chromatography (TLC), and high-performance liquid chromatography (HPLC). The criteria for assessingthe applicability of methods for the analysis of pigments are: (a) In situ application (b) Specificity (c) Sensitivity (d) Spatial resolution (e) Immunity to interference Specificity relates to the certainty with which a particular measurement

sulphide

leads to the unambiguous identification of the pigment being analysed. In general, analytical techniques give excellent specificity for pigments with different compositions but none in cases where pigments have the same chemical composition but different structures. Often this presents only minor problems since, when coupled with the colour of the pigment, the identification of the latter becomes unambiguous. In a few cases, however, such pigments are not distinguishable by their colour either: e.g. the rutile and anatase forms of titanium dioxide. By contrast, spec-

or Pbs(SbO&

troscopic techniques are generally sensitive to the structure of the material but are less quantitative in terms of composition. When dealing with mixtures it is imnortant that a techniaue gives results from a single grain of pigm&t (typically ca 5 x lo-’ mm3. 1 x 10d9 e) and. further, that signals from materi: near: by (the binder, substrate, or other pigments) do not interfere with the identification. Spatial resolution is determined by the cross-sectional area and the penetration depth of the radiation or electron beam used to interrogate the

67

TABLE 2 Techniquea

STRENGTHS

AND WEAKNESSES

In situ

OF THE MAIN TECHNIQUES

AVAILABLE

FOR PIGMENT ANALYSIS

Specificity

Sensitivity

Spatial Resolution

Immunity to Interference

badb good good good excellent good poor bad

good goodd fair* good‘ excellent good good good

excellent good poor poor excellent fair fair good

good good poor good goods bad fair good

SEM XRF XRD PIXE/PIGE Raman IR UV/VIS Optical Microscopy

a SEM scanning electron microscopy; XRF X-ray fluorescence; XRD X-ray diffraction; PIXE/PIGE particle-induced X-ray emission, particle-induced gamma-ray emission; IR infra-red reflectance spectroscopy; UV/VIS ultraviolet/visible diffuse reflectance spectroscopy. b Except where used in conjunction with an EDAX (energy dispersive X-ray analysis) attachment. c With appropriate modifications in situ studies may be performed, but with a loss of spatial resolution. d Elements heavierthan potassium. * Increases with atomic number. ‘ Simultaneous analysis of all elements with atomic number > 9; Li, Be, B and N can also be detected with high sensitivity. Q Fluorescence can be an interference, see ‘Limitations’ section. ’ Polarisation studies require samples to be removed.

sample. When pigment analysis is performed on a sample removed from the artwork then spatial resolution and immunity to interference is less problematic since separation or masking techniques may be applied to remove potential interferences. The advantages and disadvantages of the major techniques for pigment analysis are summarised in Table 2. Apart from Raman spectroscopy, which is considered in the next section, nothing scores highly in all respects. For example, although infra-red spectroscopy is highly specific, it has comparatively poor spatial resolution and suffers from a high level of interference, particularly from the binder and support. The electron microprobe on the other hand has good spatial resolution but cannot be applied in situ without coating the pigment with a good electrical conductor Ultraviolet/visible such as carbon. reflectance has fair spatial resolution, but the spectra in general consist of a few broad bands which may prove insufficient for unique identification of a pigment. XRF spectroscopy is an important technique for pigment analysis: although it is usually conducted on samples removed from the artwork, it may be used in situ, generally in an atmosphere of helium. The spatial resolution normally achieved is good, although not to the level required for the analysis of single grains of pigment. A further limitation arises from difficulty in the analysis of elements lighter than potassium, a difficulty which precludes XRF analysis of all organic pigments. Thermal and chromatographic methods by their nature can be applied only when the pigment is physically removed from the artwork.

68

Description of Raman microscopy In the Raman experiment a beam of monochromatic light is focused on the sample and the scattered radiation is analysed [2]. Most of the scattered radiation has the same frequency as the incident radiation (Rayleigh scatterin$ and only a small proportion (ca 1 in 10 ) is inelastically (Raman) scattered. This causes problems both in terms of observing the weak Raman scattered radiation and in separating it from the intense Rayleigh scattering. These problems have been overcome by use of intense sources of monochromatic sophisticated radiation (lasers), monochromator designs, and sensitive detectors. A variety of sampling configurations may be employed, including the coupling of a microscope into the experiment as is done in the technique of Raman microscopy [2]. The key details of the Raman microscope are given in figure 1. The incident laser beam is passed through beamsplitter, bi (usually a 50 per cent transmitting mirror); is converted into a parallel beam by lens Ii, and is then focused on the sample using a microscope objective. The Raman scattered radiation retraces the path of the incident laser beam as far as the beamsplitter, bi, where half of the radiation is directed into the spectrograph or monochromator. Two aspects of the optical configuration are of particular importance for pigment analysis. First, collinear with the final leg of the path of the incident laser beam, is the whitelight beam of a conventional microscope. Thus selection of the particle to be examined is achieved in exactly the same way as in optical microscopy. Selection of either white-light or laser

illumination is accomplished using the swing-away mirror. Second, in the optical train between the microscope and the spectrometer is a secondary focus. The accurate location of an aperture at the focal point improves the spatial resolution of the experiment and this has allowed the collection of Raman spectra at different depths within the sample (depth profiling) in favourable cases. In circumstances where the Raman signal due to the pigment is swamped by fluorescence from either the binder or the substrate an enormous reduction in the fluorescent signal can be achieved by use of the aperture at the secondary focus. The microscope attachment facilitates the study of particles of diameters as small as 0.5 pm. The importance of the technique thus stems from the fact that it is the only microanalytic method available today by use of which it is possible to identify, or at least to characterize, small particles of micrometre dimensions in situ. It is important to recognize that there are no fundamental differences between Raman microscopy and Raman spectroscopy, the terms merely identify the different sampling techniques. Raman spectra of pigments White pigments. White pigments present some of the most difficult problems in pigment analysis. They mainly comphoscarbonates, prise sulphates, phates, and oxides which give characteristic spectra, as exemplified by those given in figure 2 of (a) lead white (PbCOs) (b) chalk (CaCOs), and (c) bone white (Cas(PO&). The symmetric stretching vibration of the anion gives rise to an intense band at ca 1000 cm-r

microsc\ope \ +:q-1---q

ocular mirror --+! I

coppllng

( *,~~~~L~~~ b

beamsplitter

lens

*/““it’

‘ight

II ,,

saJrce

beamsplitter

, b,

h V

500

1000 Wavenumber

lens,I, --

aperture

I cm-’

Figure 3 Raman spectra of the (a) rutile and lb) anatase forms of titanium (IV) oxide.

of Art, the identification of the pigment being required for restorative purposes.

/ x-y-z translation

spectrograph or monochromator

stage

detector Y

Figure 1

a) Lead

Schematic

of a Raman microscope.

whltc

,

I-

B I 5 2

diagram

b) Chalk

6 I (L

L 1000

1200 Wovenumber

600

I cm-’

Figure 2 Raman spectra of various white pigments: (a) lead white (PbCO,); (b) chalk (CaCO,); (c) bone white

PMPO,M.

for sulphates; ca 1050 cm-’ for carbonates; and ca 960 cm-i for phosphates. The exact wave number of these bands is also sensitive to the cation (1050 cm-’ for PbC03 and 1085 cm-’ for CaCOs) as shown in figure 2(a) and (b). Thus, these spectra not only permit the identification of the anion but also of the exact salt. The Raman spectrum is sensitive to both composition and crystal form, as is demonstrated by results obtained from titanium dioxide, the most important white pigment in use today. Titanium dioxide is known to exist in three crystal modifications - rutile, anatase, and brookite - all of which occur naturally. Anatase and rutile have been used as painting pigments since 1923 and 1947, respectively, as is evident from the patent literature [3]. Whereas the composition of the two pigments is identical, the crystal symmetries are different and accordingly their vibrational spectra are different (figure 3). The spectrum of anatase was obtained from a tiny particle (2 pm diameter) sent to us for analysis by the Courtauld Institute

Coloured pigments. Where the pigment is coloured the choice of exciting line for Raman spectroscopy is extremely important because absorption of the scattered light by the sample may reduce the quality of its spectrum. In such cases the exciting line is chosen to fall outside the contour of the electronic absorption bands of the pigment. Vermilion (HgS) and red ochre (Fe20s) fail to give any significant Raman spectrum using green excitation but give strong Raman spectra when excited with red radiation. An interesting example of the effects due to absorption of laser radiation is provided by the Raman spectra of red lead (Pb304) obtained using 514.5 and 632.8 nm excitation (figure 4). In each case welldefined Raman spectra are obtained, but only with 632.8 nm excitation is the spectrum that of the genuine material; that obtained with 514.5 nm excitation matches that of massicot (PbO). These

al h,=514.5

nm

bl +632.a

nm

Figure 4 Raman spectra of red lead (Pb,O,) taken with (a) 514.5 nm and (b) 632.8 nm excitation.

69

observations are easily rationalized, because red lead may be converted into massicot by the application of heat. Red lead absorbs 514.5 nm radiation strongly, and this leads to localized heating in the laser beam which may lead to conversion of the irradiated particle or particles into massicot. Since 632.8 nm radiation is not absorbed by the red lead, there is minimal local heating with this exciting line and thus no decomposition. Resonance Raman scattering In some cases selection of an exciting line with a wavelength which falls within the contour of an absorption band of a sample results in enhancement of the Raman scattering, a phenomenon known as the resonance Raman effect [4]. This enhancement vastly improves the sensitivity of the Raman experiment and permits the collection of spectra from highly dilute samples. The classic example is provided by lapis lazuli, an ill-defined mineral containing the valuable, deep-blue pigment lazurite of approximate formula Na8[A16Si60z4]Sn. The aluminosilicate cage describes a truncated octahedron and contains, in the holes, trapped sulphur radical anions (notably Ss-, but also some S-) internal transitions of which are responsible for the colour [5]. Although accounting for less than 1 per cent of the mass of the pigment, the Saion gives such an extremely intense Raman spectrum with X&600 nm excitation that no bands due to the host lattice are seen [5]. Most other techniques cannot distinguish an uncoloured aluminosilicate from lapis lazuli. Limitations A possible problem arising when collecting Raman spectra of a pigment when present on a painted manuscript is that of fluorescence from supports and binders, because this may swamp the Raman signal. However, fluorescence can usually be minimized by making use of the excellent spatial resolution which the Raman microscope affords. 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. Fluorescence arising from the pigment itself has the same spatial characteristics as the Raman scattering and cannot be removed by the aperture. In this case, the most effective solution is to change the excitation wavelength, usually to longer wavelength (lower frequency). As the excitation wavelength is increased, non-radiative decay processes compete more effectively with fluorescence, leading to a diminution in the latter. We have found that 752.5 nm radiation has a sufficiently long

70

wavelength to enable the collection of the Raman spectrum of indigo, a plantbased pigment well-known to fluoresce when irradiated with exciting lines of shorter wavelength. A wide range of organic pigments is known to give good Raman spectra. This list includes common synthetic organic dyes, including those containing the triarylmethane group [6]; vat pigments, e.g. indigo, the perylenes, aryl amides, and quinacridones [7]; organometallic pigments, e.g. phthalocyanine blue and phthalocyanine green; through to natural pigments e.g. saffron [8], pcarotene [9], astaxanthin [lo], ovorubin [ 1l] and malvidin [12] (the main pigment of grapes). Dyes are, in general, less colour fast than inorganic pigments, so much so that extensive investigations of their fading rates have been made [ 131. In Situ analysis of pigments from books and manuscripts Compared with easel painting or polychrome, comparatively little is known about manuscripts. This is because pigment analysis of manuscripts has not normally been performed in the past owing to the inappropriateness of the established analytical techniques, most of which require pigment samples to be removed from the manuscripts. Where such studies have been performed, new and valuable insights into the exchange and spread of technology and culture have been gained. The occurrence of a particular pigment in a work of art can not only give historians new ideas, it can alter the conventional wisdom. A good example of this comes from the studies of C. Coupry in France who used Raman microscopy to identify lapis lazuli in an illuminated manuscript, Haymon’s Commentary on Ezekiel, from the Auxerre Abbey [14]. The mineral, lapis lazuli, originates from the Hindu-Kush mountains in present-day Afghanistan and, as the manuscript dates from very near the year 1000, it was established that the pigment was introduced into Europe from its native Afghanistan almost two centuries before being first mentioned in written sources. Further studies are required in order to find out when lapis lazuli was first used in other Western manuscripts. H. RoosenRunge and A. E. A. Werner have apparently identified it in the Lindisfarne gospels, dating from the 8th century, by comparison of pigment samples with a known sample using polarized light, supplemented by visual observation in ultraviolet light [15, 161. Our studies of four Islamic Korans from different parts of the East seem to indicate that the use of lapis lazuli is related to the proximity of its provenance. In many instances, identification of pigments and dyes can be used to fix an earliest date on a manuscript. For ex-

ample, the synthetic pigment Prussian blue (also known as Berlin blue or Paris blue) was first made by J. Diesbach in or around 1704, its chemical preparation being too involved for it to have been made before that date [17, 181. Synthetic dyes were prepared much later, the first artificial dye being mauveine, prepared by William Perkin in 1856 [19]. The hierarchy of pigments varies across civilisations and is dynamic. For example, in the Commentary on Ezekiel, the abbot’s blue garments were painted with indigo, probably in the form of woad, while the saint’s more glorious raiment was in precious lapis lazuli [ 141. The occurrence of different pigments of essentially the same colour on the same manuscript indicates how important the concept of hierarchy was. An illustration of the close association which was made in the Middle Ages between aesthetic value and intrinsic worth of materials was given by the 15th-century Italian painter, Cennino Cennini, who argued ‘. . . it is good business to be liberal with precious materials, as well as becoming to one’s dignity as a painter, and acceptable in the eyes of God and the Virgin’ [20]. The pigment hierarchy, then, will shed light on social and economic factors. In addition to the identification of the pigment itself, it is possible to learn about the method of its application. For example, the technique of layering lapis lazuli over azurite was employed in the illuminated initial ‘0’ from a 13thcentury choir book shown in figure 5. Recent optical microscopic studies of samples taken from 13th and 14-century manuscripts have confirmed the use of the technique [21], and these have been supported by our in situ Raman studies with C.A. Porter. The technique had always been considered peculiar to Flemish (panel) paintings of the mid to late 15th century, although it is now known that it was widely adopted by Italian painters - Venetian, or Venetian influenced - in the late 15th to late 16th centuries. Case study The in situ use of Raman microscopy to identify pigments used in a manuscript is exemplified by the following study of an historiated initial ‘R’ from a 16thcentury German choir book (figure 6). While the blue garments of the Virgin and the deeper blue robe which covered her garments appear to be due to different pigments both were, in fact, painted with azurite. Raman spectra of the two shades of blue indicate that the illuminator simply used less azurite and proportionately more binder to produce the lighter shade without choosing to add any white pigment, such as lead white, to gain the same effect. The azurite used in the deep blue robe

pages of the book; vermilion, which has been used on the canopy, and on the bed-sheet. Carbon has been used in the picture outline. The identification of grains of these pigments has been made, in each case, by comparison of their Raman spectra with reference spectra on our database and literature spectra (Table 3) [26]. The pink pigment used to illuminate the angel’s robe and the yellow pigment used for the top of the bed canopy have not yet been identified. The pink pigment proves problematic due to fluorescence when using 514.5 and 632.8 nm excitation, and it seemsnecessary to use a longer excitation wavelength in order to eliminate this. We are also hopeful of identifying the yellow pigment, which we know is not orpiment, realgar, massicot, Naples yellow, or lead tin yellow, since the spectra of these yellows do not match that obtained. Concluding

Figure 5 technique

Historiated of layering

initial ‘0’ from a 13th-century of lapis lazuli over azurite.

consists of coarse grains (cu 30 pm diameter) whereas that used in the under-garment is finer (ca 3 ym diameter); fine grinding causes azurite to become pale and weak in tinting strength [22]. The effect of the particle size on the depth of colour of a powder is a common feature of powdered materials, whose colour is determined by diffuse reflectance. As the particle size is reduced, the average depth to which radiation penetrates before being scattered is also reduced and hence the depth of colour is reduced [23, 241. The angel’s wing and podium are painted in malachite, which is a basic copper carbonate similar to azurite but containing more combined water. Both azurite and malachite have a similar provenance as they are both associated with secondary copper ore deposits. The dark grey pillar top consists of a mixture of lead white, carbon, and azurite as well as smaller amounts of vermilion, red lead, massicot, and lead tin yellow (form I) [25]. This mixture is shown under x 100 magnification in figure 7. The use of a mixture of red, blue, and yellow pigments has enabled the illuminator to produce (by colour subtraction) a range of other colours

choir book displaying

the

which could not be obtained by use of any single pigment alone; mixing was a common practice owing to the restricted range of pigments available, especially to mediaeval artists. Other pigments identified are lead white, which has been used extensively, appearing in the highlighting of the ‘R’, the grey pillar, the dove, and in the

TABLE 3 PIGMENTS MANUSCRIPT

IDENTIFIED INS/WON

remarks

It is clear that Raman microscopy can be used to perform in situ identification of a wide range of pigments; it is therefore a valuable addition to the existing range of techniques currently used for this purpose [27]. Indeed, the capabilities of the technique are unique when dealing with mixtures. Further, and contrary to even our own expectations, an extremely high proportion of pigments on manuscripts yield to this form of analysis. An important development in the study of further pigments will be the use of exciting lines of longer wavelengths (the near infra-red) for Raman microscopy. It is, nevertheless, clear that the technique is already at the forefront of those which may be used to perform in situ (and non-destructive) analysis of pigments; it is thus of help towards the possibility of sympathetic, effective, and valid restoration of our priceless cultural heritage [28].

THE 16TH-CENTURY

GERMAN

Part of Painting

Colour

Pigment

Angel’s wing and podium Robe covering virgin’s dress Virgin’s robe Pillar top Bed canopy Top of bed canopy Picture outline Highlights on ‘R’ Angel’s robe

green. grey/blue blue dark grey red yellow/brown black white pink

malachite azurite azurite mixturea vermilion b carbon lead white ?

a Mostly lead white, carbon, and azurite, with smaller amounts of vermilion, red lead, massicot, and lead tin yellow (form I). ’ Orpiment, realgar, massicot, Naples yellow and lead tin yellow can be ruled out, since these give unambiguous Raman spectra under the experimental conditions employed.

71

t 1” 2 3 85 E g a

Lead

tin yellow

I

I

-

-

-

1500

Azurite

600

1000

400

1000 500 Wavenumber I cm-’

1200

i

Le :ad WIiite

-

800

10

Vermilion

Malachite

800

200

Figure 6 Historiated initial ‘R’ from a 16th-century German choir book, indicating the locations probed by Raman microscopy and the spectra obtained therefrom. The spectrum of lead tin yellow (type I) was obtained from yellow grains in the grey archway (amongst other grains of different colour - a similar mix to those in figure 7). All spectra were taken with 514.5 nm excitation except that of vermilion, for which 632.8 nm excitation was used.

Figure 7 Magnified (x 100) portion of the top of the grey column in figure 6. Individual grains of pigment have been identified by Raman microscopy as specified in Table 3.

Acknowledgments We are indebted to Cheryl Porter for providing the artworks for analysis and for providing background knowledge and enthusiasm for the project. We thank Sam Fogg for access to the illuminated manuscripts shown in figures 5 and 6, Doretta Meshiea for the opportunity to study the Islamic Korans, and John Cresswell for the artwork. We the Wolfson gladly acknowledge Foundation and the Leverhulme Trust for the award of a fellowship (to R.W.) and the ULIRS for support. References [l] Rees-Jones, S. G. Studies in Conservation, 35, 93, 1990. [Z] The original paper on the Raman effect was: Raman, C. V. and Krishnan, K. S. Nature, 121, 501, 1928. For a general text on Raman spectroscopy, see ‘Raman Spectroscopy’, D. - A. Long, McGraw-Hill. New York. 1977. and for a review series containing articles

embracing all aspects of the subject, see ‘Advances in Spectroscopy’, R. J. H. Clark and R. E. Hester (eds), Wiley, Chichester, Vols. 1-19, 1975-1991. [3] Corset, J., Dhamelincourt, P. and Barbillat, J. Chem. Bit., 25, 612, 1989. [4] Clark, R. J. H. and Dines, T. J. Angew. Chem. Internat. Ed. Engl., 25, 131, 1986. [5] Clark, R. J. H. and Franks, M. L. Chem. Phys. Len., 34,69, 1975; Clark, R. J. H. and Cobbold, D. G. Inorg. Chem., 17, 3169, 1978; Clark, R. J. H., Dines, T. J. and Kurmoo, M. Inorg. Chem., 22, 2766, 1983. [6] Guineau,

B. Studies in Conservation,

34, 38, 1989. [7] Binant, C. Pigments et Colorants (editions du CNRS), 1.53,1990; Binant, C. and Lautie, A. Appl. Spectrosc., 43, 851, 1989; Binant, C., Guineau, B. and Lautie, A. Spectrochim. Acta, 45A, 1279, 1989. [8] Merlin, J. C. Pigments et Colorants (Editions du CNRS), 41, 1990. [9] Rimai, L., Heyde, M. E. and Gill, D. J. Am. Chem. Sot., 95, 4493, 1973.

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