Raman Spectroscopic Library of Natural and Synthetic Pigments

SPECTROCHIMICA ACTA

PART A

Spectrochimica Acta Part A 53 (1997) 2159-2179

Raman spectroscopic library of natural and synthetic pigments (P re- N 1850 AD)

Ian M. Bell, Robin J.H. Clark *, Peter J. Gibbs Christopher Ingold Laboratories, UniversitJj College London, 20 Gordon Street, London WClH OAJ, UK

Received 24 April 1997; accepted 24 May 1997

Abstract

To assist in the greatly increasing number of applications of Raman microscopy as a tool for non-intrusive, in situ archaeometric analysis, the Raman spectra of over 60 pigments, both natural and synthetic, known to have been in use before - 1850 AD, have been studied by Raman microscopy. Fifty-six pigments have yielded high quality spectra which have been arranged, by colour, into a spectroscopic library for reference purposes. The spectroscopic files may be downloaded from http://www.ucl.ac.uk/chem/resources/raman/s~clib.html 0 1997 Elsevier Science B.V.

Keywords: Raman microscopy; Pigment; Dye; Archaeometry; Conservation science

1. Introduction

Raman microscopy is now established as the analytical technique which is the most specific, sensitive, spatially refined and immune to interfer- ence for the in situ, non-intrusive analysis of historical artefacts [l]. Interest in its use for archaeometric analysis has increased very significantly over the last 5 years. With the recent development of robust, compact Raman microscopes, such as the Renishaw Ramascope and the Dilor Labram systems, involving notch filters, air-cooled lasers and charge coupled device (CCD) detectors, the accessibility of the technique to conservation scientists has increased greatly.

Although it is possible to envisage many areas

* Corresponding author.

of conservation science where the technique of Raman microscopy may be of use (for example, the study of paint binders and extenders or of degradation and corrosion products) research to date has concentrated on the identification and study of pigments used on illuminated manu- scripts [ l-101 and ceramics [ 1 1 - 131. The examina- tion of pigments will continue to be an important application of this technique, and it is for this reason that a spectroscopic library has been compiled. The library, which consists of the spectra obtained by Raman microscopy of pigments arranged by colour, will allow easy identification of any pigment on a historical sample. It contains as many common pigments [14-181, both mineral and synthetic, as it was possible to obtain, although the authors welcome suggestions for possible future additions. The library has been restricted to pigments in use before the isolation,

1386-1425/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIIS1386-1425(97)00140-6

by Perkin (1856 AD), of the first synthetic organic dye, mauveine [19,20]; since then, many hundreds of pigments and dyes, principally organic, have been synthesized. By limiting the library to pigments in use before b 1850 AD, a database of the Raman spectra of < 70 pigments is sufficient as a reference for thousands of years of pigment use.

2. Experimental

The Raman microscopes used in this study are two modern instruments of the type that will be of most use to conservation scientists. The first was a Dilor Labram, supplied generously by In- struments SA (UK) and configured with an inter- nal Olympus BX-40 confocal microscope, an air-cooled Instruments SA 20 mW helium-neon laser light source (632.8 nm), and an air-cooled CCD detector operating at - 70 C. The second was a Renishaw Raman System 1000, supplied generously by Renishaw PLC, configured with an external Olympus BH-2 confocal microscope, an air-cooled Spectraphysics 21 mW argon-ion laser light source (514.5 nm), and an air-cooled CCD detector operating at - 70 C. With both instruments, neutral density filters were used to set the laser power at the sample surface to a value between 0.5 and 6.0 mW.

Each instrument is fitted with a notch filter assembly to remove light at the excitation fre- quency with the consequence that little spectroscopic information could be obtained from either within 100 cm- of the laser line. The notch filters also affect significantly the spectral response of each instrument between 100-200 cm - from the excitation line; as a consequence, the Raman bands in this region appear less intense than they

At wavenumbers greater than 200 cm - i from the laser line the spectral response functions of these instruments are domi- nated by the grating and CCD efficiencies, In the region 200--3000 cm - the spectral response of the Renishaw Ra- mascope (514.5 nm excitation) was found to increase, almost linearly. by a factor of approximately two. Over the same range, the measured spectral response of the Dilor Labram (632.8 nm excitation) varies by approximately 25%, showing maxima at 1000 and 2700 cm - and a minimum at 2100 cm-

1800 1600 1400 1200 1000 800 Wavenumber I cm

Fig. 1. Ivory black, 1, = 632.8 nm, 6 mW.

1800 1600 1400 1200 1000 800

Wavenumber I cm

Fig. 2. Lamp black, 1, = 632.8 nm, 6 mW.

1600 1350 1100 850 600 350 100

Wavonumber / cm-

Fig. 3. Azurite, A0 = 514.5 nm, 2 mW.

I.M. Bell et al. /Spectrochimica Acta Part A 53 (1997) 2159-2179 2161

800 700 600 500 400 Wavenumber I cm

300 200

Fig. 4. Cerulean blue, R, = 514.5 mu, 4 mW. Fig. 7. Lazurite, A,, = 514.5 nm, 4 mW

600 500 400 300 200 100 Wavenumber I cm-

Fig. 5. Cobalt blue, 1, = 514.5 nm, 4 mW.

1100 900 700 500 300 100

Wavenumber I cm-'

1300 1100 900 700 500 300 100 Wavenumber I cm"

1100 900 700 500 Wavenumber I cm-'

300 100

Fig. 8. Posnjakite, 1, = 632.8 mu, 3 mW.

f

2100 1700 1300 900 500 100 Wavenumber I cm"

Fig. 6. Egyptian blue, 1, = 514.5 nm, 4 mW. Fig. 9. Prussian blue, A, = 514.5 nm, 2 mW.

I.M. Bell et ui. : Spertrochin~iea Acta Part A 53 (1997) 2159-2179

900 700 500 300 100 800 700 600 500 400 300 200 Wavenumber / cm- Wavenumber! cm-

Fig. 10. Smalt, /i,, = 514.5 nm, 2 mW. Fig. 13. Cobalt green, %, = 514.5 nm, 4 mW.

c 1 1100 900 700 500 300 100

Wavenumber I cm 3100 2350 1600 850 100

Wavenumber I cm-

Fig. 11. Atacamite*, 1, = 514.5 nm, 4 mW. F ig. 14. Emerald green*, lo = 514.5 nm, 0.5 mW

500 400 300 Wavenumber I cm

1500 1300 1100 900 700 500 300 100

Wavenumber I cm

Fig. 12. Chromium(II1) oxide, R. = 514.5 nm, 4 mW. Fig. 15. Malachite, 1, = 514 nm, 1 mW.


900 700 500 300 100

Wavenumber I cm

3600 3100 2600 2100 1600 1100 600 100

Wavenumber I cm-

Fig. 16. Scheeles green, 1, = 514.5 nm, 2 mW. Fig. 19. Verdigris (l), A,, = 514.5 nm, 1 mW.

1100 900 700 500 300

Wavenumber I cm-

Fig. 17. Terre verte, 1, = 514.5 nm, 1 mW.

3600 3100 2600 2100 1600 1100 600 100 Wavenumber I cm-

3600 3100 2600 2100 1600 1100 600 100 Wavenumber I cm

Fig. 20. Verdigris (2), ,I0 = 514.5 nm. 1 mW.

600 500 400 300 Wavenumber I cm

200 100

Fig. 18. Raw verdigris, A0 = 514.5 nm, 1 mW. Fig. 21. Viridian, 1, = 514.5 nm, 4 mW.

2164 I.M. Bell et rd. !Spectrochimica Acta Parr A 53 (1997) 2159-2179

600 500 400 300 Wavenumber I cm-

200

Fig. 22. Mars orange, & = 632.8 nm, 3 mW


1500 1300 1100

Wavenumber I cm

900

Fig. 23. Litharge, 1, = 632.8 nm, 6 mW.

Fig. 25. Purpurin*, 1, = 632.8 nm, 1.5 mW


100

Fig. 26. Realgar, 1, = 632.8 mn, 0.6 mW.

700 600 500 400 300 200 100 Wavenumber I cm

500 400 300 200 100 Wavenumber / cm-

Fig. 24. Mars red, &, = 632.8 nm, 3 mW. Fig. 27. Red ochre, /2, = 632.8 mn, 3 mW.

I.M. Bell et al. /Spectrochimiea Acta Part A 53 (1997) 2159-2179 2165


200 100

Fig. 28. Red lead, 1, = 632.8 nm, 3 mW. Fig. 31. Bone white, A0 = 514.5 nm, 4 mW

T z .Z r 2 E

400 300 200

Wavenumber I cm-

100 900 700 500

Wavenumber I cm-

Fig. 29. Vermilion, A,, = 632.8 mn, 6 mW Fig. 32. Chalk, 1, = 514.5 nm, 4 mW.


Fig. 30. Barytes, 1, = 514.5 nm, 4 mW. Fig. 33. Gypsum, 1, = 514.5 nm, 4 mW

4 1200 1000 600 600 400

Wavenumber I cm

T 5 .%

f 5


I.&f. Bell et ul. 1 Spectrorhin~ica .4cta Part A 53 (1997) 21.59-2179

T 5

$

f

E

1100 900 700 500 300 100 Wavenumber / cm-

Fig. 34. Lithopone, 1, = 514.5 nm, 4 mW.

1200 1000 800 600 400 Wavenumber I cm-

Fig. 35. Lead white, 1, = 514.5 nm, 4 mW. Fig. 38. Berberine*, 1, = 632.8 nm, 3 mW

600 500 400 300 200 Wavenumber I cm-

II J,k ,

900 700 500 300 100 Wavenumber I cm-

Fig. 37. Barium yellow, 1, = 514.5, 4 mW.

1800 1600 1400 1200 1000 Wavenumber I cm

500 400 300 Wavenumber J cm-

Fig. 36. Zinc white, A0 = 514.5 nm, 4 mW. Fig. 39. Cadmium yellow*, A,, = 514.5 nm, 4 mW

Inte

nsity

-3

Inte

nsity

-3

P

Inte

nsity

-3

in

tens

ity

--f

>

Inte

nsity

--f

Inte

nsity

+

Inte

nsity

9

inte

nsity

--t

In

tens

ity-j

27

?Q

H ,

Inte

nsity

-3


would using, for example, a triple grating spec- trometer. However, the spectra have not been corrected for the spectral response of the instruments as it is anticipated that this spectroscopic library will be of use principally to conservation scientists using this new generation of Raman microscope, all of which use notch filters, and the spectra obtained here are illustrative of the spectra to be expected from such an instrument.

References have been included if the Raman spectrum of a particular pigment has previously been studied in detail and the bands assigned.

Wavenumber I cm-

Fig. 52. Pararealgar, 632.8 nm, 1.5 mW.

Wavenumber calibration was achieved by su- perposition of neon emission lines on the spectrum recorded for each pigment (in the case of excitation at 514.5, the 546.1 nm mercury emis-

2000 1800 1600 1400 1200 1000 Wavenumber I cm

Fig. 53. Saffron*, 1, = 514.5 nm, 1 mW.

900 700 500 300 100 Wavenumber I cm

E Gg. 55. Yellow ochre, A, = 632.8 nm, 1.5 mW.

Wavenumber I cm-

700 500 300 100 Wavenumber I cm-

Fig. 54. Strontium yellow, i., = 514.5 nm, 4 mW. Fig. 56. Zinc yellow, 1, = 632.8 nm, 6 mW.

Table I Black pigments

Name Composition Band wavenumbers (crn~~ ) Excitation and relative intensiti& wavelength

(nm)

Notes and date

Ivory black Carbon 961 m ( = \,,(a,) PO:-); 632.8 Antiquity. Also contains cal- I - 1325 vs(br): - 1580 vs(br) cium phosphate

Lamp black Carbon - 1325 vs(br); - 1580 vs(br) 632.8 Antiquity 2 _.

a Approximate centres of broad bands in the laser fluorescence spectrum. b s, strong; m, medium; v. very; br, broad. The pigment is either specified to be a mineral or the date of its first manufacture is listed.

sion line was also used). Following routine calibration of each instrument, linear interpolation was found to be adequate, and gave a typical root-mean-square deviation of 0.3 cm - for the fitted neon lines. After the corrected Raman wavenumbers were rounded to integers, and al- lowing for regions of the spectrum in which calibration lines are sparse, it is expected that all quoted wavenumbers are accurate to -t 1 cm- r. Exceptions to this level of accuracy occur in the case of bands described as broad (br) or shoulders (sh) in the peak tables.

Data were collected via a PC, and the spectra analyzed and the peak positions picked using the GRAMS/32 software package. The baseline of eight spectra, principally those of the organic pigments, were corrected and these are indicated by an asterisk (*) after the pigment name in the figure caption.

The pigments used were genuine artists pigments from Winsor and Newton, Cornelissen, and Kremer. Most were purchased from the specialist pigment suppliers L. Cornelissen and A.P. Fitz- patrick (both London, UK) and some were do- nated by the National Gallery, London and the British Museum Department of Scientific Re- search.

3. Results and discussion

A total of 64 pigments were studied using 514.5 First, the common name of a pigment may not and 632.8 nm laser excitation lines. High quality be sufficient to specify the composition or struc- Raman spectra were obtained for 56 of the pig- ture. The term chalk, for example, is interpreted

ments, with only eight of those selected failing to give an adequate spectrum with either of the excitation lines. The spectra obtained are illustrated in Figs. l-56 and are arranged by pigment colour: Figs. 1 and 2, black; Figs. 3- 10, blue; Figs. 11-21, green; Fig. 22, orange; Figs. 23-29, red; Figs. 30-36, white; and Figs. 37-56, yellow. The calibrated wavenumbers (Raman shift-cm - ) and relative intensities (uncorrected for spectral response) are listed in Tables l-7. Those pigments that failed to give adequate Raman spectra with the excitation lines and instrumentation described are listed in Table 8; some of these, nota- bly the fluorescent pigments/dyes, are known to give Raman spectra by FT-NIR Raman spectroscopy (2, = 1064 nm). The tables are arranged alphabetically by pigment colour (Table 1, black; Table 2, blue; Table 3, green; Table 4, orange; Table 5, red; Table 6, white; and Table 7, yellow), and they also indicate the laser excitation wavelength used, whether the pigment is a mineral or, if synthetic, the date of its earliest known use, and references to its Raman spectrum if the bands have been assigned in the literature [Zl-431.

The spectroscopic library is intended to allow rapid identification of a pigment; however, there are a number of factors that may result in spectra obtained from a historical artefact, and even from a standard pigment of different origin, differing somewhat from those of the standard samples reported here.

Tabl

e 2

Blue

pi

gmen

ts

Nam

e Co

mpo

sition

Ba

nd

wave

num

bers

(c

m-)

an

d re

lative

in

tens

itiesb

Ex

citat

ion

wave

lengt

h

(nm

)

Note

s, da

te

and

Ram

an

litera

- Sp

ectru

m

illus-

tu

re

refe

renc

es

trate

d in

fig

ure

Azur

ite

Ceru

lean

blue

Co

balt

blue

Egyp

tian

blue

Lazu

rite

Posn

jakit

e

Prus

sian

blue

Smal

t

Basic

co

pper

(H)

carb

onat

e 2C

uCO,

.Cu(

OH),

Coba

lt(H)

stann

ate

COO.

nS

n0,

Coba

lt(II)-

dope

d al

umin

a gl

ass,

Co

0 Al

,O,

Calci

um

copp

er(B

) sil

icate

Ca

CuSi

,O,,

SF

& S;

in

a

sodi

um

alum

ino-

silica

te

mat

rix

Nas[A

l,Si,O

,,]S,

Ba

sic

copp

er(I1

) su

lfate

Cu

SO,.3

Cu(O

H),.H

,O

Iron(

II1)

hexa

cyan

ofer

rate

(I1)

Fe,[F

e(CN

),],

.14-1

6HaO

Co

balt(I

1)

silica

te

COO.

nS

i0,

145

w;

180

w;

250

m;

284

w;

335

w;

403

vs;

545

w;

746

w(sh

); 76

7 m

; 83

9 m

; 94

0 w;

10

98

m;

1432

m

: 14

59

w;

1580

m

; 16

23

vw

495

m(s

h);

532

s; 6

14

vs

203

vs;

512

vs

114

m;

137

m;

200

w;

230

w;

358

m;

377

m;

430

vs;

475

m(s

h);

571

w;

597

VW;

762

w;

789

w;

992

w;

1012

w;

10

40

w;

1086

s

258

w;

548

vs;

822

w;

1096

m

135

VW;

208

VW;

278

VW;

327

vw;

467

w;

612

w;

983

vs;

1092

VW

; 11

39

VW

282

VW;

538

vs;

2102

m

; 21

54

vs

462

vs;

917

m

514.

5 M

inera

l

514.

5 51

4.5

514.

5

1821

17

75

3000

BC

. Al

so

know

n as

cu

pror

ivaite

514.

5

632.

8

Mine

ral

(lapi

s laz

uli).

Synt

hetic

c.

18

28

= ult

ram

arine

[2

6628

] M

inera

l

514.

5

514.

5

1704

Ea

rlies

t m

oder

n sy

nthe

tic

m 1

500

b s,

stro

ng;

m,

med

ium;

w,

weak

; v,

very;

sh

, sh

oulde

r. _.

-

The

nie

men

t is

eith

er

soec

ified

to

be

a m

inera

l or

th

e da

te

of

its

tirst

m

anuf

actu

re

IS h

sted.

3 7 8 9 10

Tabl

e 3

Gree

n pi

gmen

ts

..___

- -

Nam

e

Atac

amite

Chro

mium

ox

ide

Coba

lt gr

een

Emer

ald

gree

n

Mala

chite

Sche

ele

s gr

een

Copp

er(I1

) ar

senit

e Cu

(AsO

,),

Terre

-verte

Verd

igris

raw

Verd

igris

(no.

1)

Com

posit

ion

Band

wa

venu

mbe

rs

(cm

i)

and

relat

ive

inte

nsitie

sb

Excit

atio

n wa

velen

gth

b-4

Basic

co

pper

(H)

chlo

ride,

Cu

Cl,

.3Cu(

OH),

Chro

mitm

r(III)

ox

ide,

Cr,O

,

Coba

lt(H)

zinca

te

COO.

n&

O

Copp

er(I1

) et

hano

ate

tri-co

pper

(I1)

arse

nite

Cu[C

,H,O

,] .3

Cu[A

sO,],

Basic

co

pper

(I1)

carb

onat

e Cu

CO,

Cu(O

H),

Varia

tions

on

K

[(AI

, Fe

)(Fe

, M

g)],

(A%

Si

.J

OdOH

), Co

pper

(I1)

etha

noat

e Cu

(CH,

COO)

,

Basic

hy

drate

d co

pper

(I1)

etha

noat

e [C

u(CH

,COO

)&.

Cu(O

H)>

.5H,O

Verd

igris

Basic

co

pper

et

hano

ate

(no.

2)

Cu

(CH,

COO)

, Cu

(OH)

,

Virid

ian

Chro

mium

(II1)

ox

ide

CrzO

1. 2H

z0

~--_

__

a +

I cm

-.

122

m:

149

m;

360

w;

513

vs;

514.

5 82

1 m

; 84

6 s;

911

s;

974

s

221

vw;

308

w;

349

w;

552

vs;

514.

5 61

1 w

328

m(b

r);

434

vs;

471

m(s

h);

555

s(br)

514.

5

122

w;

154

vs;

175

vs;

217

vs;

514.

5 24

2 vs

; 29

4 m

; 32

5 m

; 37

1 m

; 42

9 m

; 49

2 m

; 53

9 m

; 63

7 vw

; 68

5 w;

76

0 w;

83

5 w;

95

1 m

; 13

55

vw;

1441

m

; 15

58

m;

2926

s

155

s;

178

s;

217

m;

268

m;

354

m;

514.

5 43

3 vs

; 50

9 m

; 55

3 s;

55

8 w;

75

7 VW

; 10

51

m;

1085

m

; 14

92

vs

136

s; 2

01

m(b

r);

236

w;

275

m;

514.

5 37

0 vs

; 44

5 w;

49

5 m

; 53

7 VW

; 65

7 VW

; 78

0 s

145

vs;

399

w;

510

w;

636

m;

514.

5 68

5 m

; 82

0 vw

; 10

07

m;

1084

m

126

m;

180

m;

233

m;

322

vs;

514.

5 70

3 m

; 94

9 s;

13

60

w;

1417

w;

14

41

w;

2943

m

; 29

90

w;

3027

w

139

vw;

181

w;23

1 w;

32

8 w;

51

4.5

392

w;

512

w;

618

w;

680

w;

939

s;

1351

w;

14

17

m;

1441

m

; 15

52

w(br

); 29

37

vs;

2988

m

; 30

26

w 19

3 s;

271

VW

; 32

1 w;

37

1 w;

51

4.5

526

m;

619

VW;

676

w;

939

s;

1351

w;

14

24

m;

1524

w;

29

39

vs;

3192

m

; 34

76

s;

3573

s

266

w;

487

vs;

552

m;

585

VW

514.

5

b s,

stro

ng:

m,

med

ium;

w,

weak

; v,

very;

sh

, sh

oulde

r; br

, br

oad.

The

pigm

ent

is e

ither

sp

ecifie

d to

be

a

mine

ral

or

the

date

of

its

fir

st

man

ufac

ture

is

list

ed.

Note

s, da

te

and

Ram

an

litera

- Sp

ectru

m

illus-

tu

re

refe

renc

es

trate

d in

fig

ure

___

~.

Mine

ral

Early

18

00s

[29]

1780

1814

Mine

ral

[30]

15

1778

16

Mine

ral.

The

Ram

an

spec

tra

of

othe

r gr

een

earth

s m

ay

diffe

r fro

m

that

illu

stra

ted

here

Sy

nthe

tic

(BC)

17

18

Synt

hetic

(B

C)

19

Synt

hetic

(B

C)

20

1838

(?

1850

) I


Table 4 Orange pigment

Name Composition Band wavenumbers (cm-) and relative intensities

Excitation wavelength (nm)

Notes and date Spectrum illustrated in figure

Mars orange Synthetic iron(II1) 224 vs; 291 vs; 407 m; 494 w; 608 632.8 Middle 19th C 22 oxide, FezOX m

as being calcium carbonate; however, there are five known polymorphs of calcium carbonate: aragonite, vaterite and three forms of calcite [21]. In each case the principal band of interest arises from v,(a;) CO:-: for calcite, as studied here, this band is at 1088 cm- (see Fig. 32 and Table 7), but for aragonite it has been reported to occur at 1085 cm- * [21]. There is little chance of such a small variation causing confusion with other car- bonates, such as basic lead(I1) carbonate (lead white-2PbC0,. Pb(OH),) for which vl(aJ CO: - is at approximately 1050 cm- , but careful calibration will be required to determine the exact identity of any polymorph of chalk. For the same reason, the spectra of only three types of verdigris have been illustrated (Figs. 18-20) although this pigment occurs in many possible hy- drated and basic forms, each of which will have a different Raman spectrum.

Second, the degree of hydration of a pigment will also affect the wavenumbers of the Raman bands. For example, the well known mineral gypsum is a dihydrate, CaS0,.2H20, for which vl(al) SO:- is approximately 1007 cm- (see Fig. 33 and Table 7); however, the important rock-form- ing mineral anhydrite (CaSO,) is often found with gypsum, and a study of the Raman spectrum of the former has shown that the band attributed to the vibration v,(a,) SO:- is at approximately 1016 cm- [22].

Third, the earth pigments, such as red earth (Fig. 27 and Table 5) and green earth (terre verte-Fig. 17 and Table 3) encompass pigments from many possible sources, obtained, as their names suggest, from the soil. Red earths have the same chromophore, iron(II1) oxide (Fe,O,), which has a characteristic spectrum (Fig. 27). However, due to the source of the pigment, it is not incon-

ceivable that a particular red earth may contain components that result in the Raman spectrum differing slightly from that illustrated here. This is especially likely for green earths (Fig. 17) since these are complex pigments of general formula K[(Al, Fe)(Fe, Mg)], (AlSi,, Si4)010(OH)2.

Finally, it is often possible to purchase pigments, especially the more modern ones, in vari- ous shades. The colour of a pigment arises as a consequence of absorption (via. ligand-field. charge-transfer, or intervalence charge-transfer bands) and specular reflectance, and it is affected by the absorption coefficient and band width of electronic bands in the visible region [3]. The colour of a pigment can be modified by mixing with another lighter or darker pigment, or by alteration of the particle size, as is well known for CuSO,. 5H,O, since this affects the relative im- portance of diffuse and specular reflectance [23,24]. In certain cases, however, different shades of a pigment have significantly different Raman spectra. To illustrate this, the Raman spectra of three shades of chrome yellow (lead(I1) chromate, PbCrO,) have been included here: chrome yellow (Fig. 40), chrome yellow deep (dark yellow--Fig. 41) and chrome yellow-orange (Fig. 42). The Ra- man spectra of chrome yellow (Fig. 40) and chrome yellow deep (Fig. 41) are very similar, but the wavenumber of the strongest band [$!,(a,) CrOz -1 shifts from 841 cm - for the standard yellow, to 838 cm- for the deeper shade; that of chrome yellow-orange is significantly different from the others, and the wavenumber of the strongest band shifts to 828 cm - . In this case, the depth of colour is dependent on the propor- tion of PbO present with PbCrO, in the lattice, and this affects both the wavenumber and the shape of the Raman bands. Comparison with a

Tabl

e 5

Red

pigm

ents

Nam

e Co

mpo

sition

-___

~

Litha

rge

Mar

s Re

d

Purp

urin

Realg

ar

Tetra

gona

l lea

d(I1

) ox

ide,

PbO

Synt

hetic

iro

n(II1

) ox

ide,

Fe,O

,

1,2,

4-Tr

ihydr

oxy-

anth

raqu

inone

GJhO

,

a-Ar

senic

(I1)

sulfid

e,

As,S

,

Red

earth

s/red

oc

hre

Red

lead

Verm

ilion

Iron(

II1)

oxide

ch

rom

opho

re

(Fe,O

, +

clay+

sil

ica)

Dilea

d(I1

) lea

d(IV

) ox

ide:

Pb,0

4

a-M

ercu

ry(H)

su

lfide.

Hg

S

-

a _+

1 c

m-.

- Ba

nd

wave

num

bers

(c

m-l)

an

d Ex

citat

ion

Note

s, da

tec

and

Ram

an

litera

ture

Sp

ectru

m

illus-

re

lative

in

tens

itiesb

wa

velen

gth

refe

renc

es

trate

d in

fig

ure

(nm

)

145

vs;

285

VW;

336

w 63

2.8

Antiq

uity,

cf.

the

yello

w pig

men

t m

assic

ot

[3 l

] 23

224

vs;

291

vs;

407

m;

494

610

w;

632.

8 M

iddle

19th

C

24

m;

660

w(sh

) 95

3 m

; 10

19

w;

1049

m

; 10

91

w;

632.

8 A

chro

mop

hore

, wi

th

aliza

rin,

in

25

1138

w;

11

60

VW;

1229

13

12

s;

vs;

the

mad

der

dye

(300

0 BC

) 13

34

s(sh

); 13

94

s;

1452

vs

14

2 w;

16

4 w;

17

1 w;

18

2 vs

; 19

2 63

2.8

Mine

ral.

Unde

rgoe

s a

light

in-

26

s;

220

s;

233

m

; 32

1 vw

; 34

2 m

; du

ced

trans

form

ation

to

th

e ye

llow

354

s;

367

w;

375

w co

mpo

und

para

realg

ar

[32-

341

220

vs;

286

vs;

402

m;

491

w;

601

632.

8 M

inera

l [1

2]

17

W 12

2 vs

; 14

9 m

; 22

3 w;

31

3 w;

34

0 63

2.8

Antiq

uity

[3 l

] 28

VW

; 39

0 w;

48

0 VW

; 54

8 vs

25

2 vs

; 28

2 w(

sh);

343

m

632.

8 M

inera

l (c

innab

ar)

and

synt

hetic

79

(8

th

C)

[35,

36].

May

unde

rgo

a lig

ht

indu

ced

trans

form

ation

to

bla

ck

a-Hg

S ~.

___.

~.

b s,

stro

ng;

m,

med

ium,

w,

weak

; v,

very,

sh

, sh

oulde

r.

The

pigm

ent

is e

ither

sp

ecifie

d to

be

a

mine

ral

or

the

date

of

its

fir

st

man

ufac

ture

is

lis

ted.

Table

6

Whi

te

pigm

ents

Nam

e Co

mpo

sitio

n

Bariu

m

white

Bone

whi

te

Chal

k (c

alcit

e)

Gyp

sum

J>ith

opon

e

Lead

wh

ite

Bariu

m

sulfa

te,

BaSO

, 45

3 m

; 461

w(s

h);

616

w; 6

47 w

; 98

8 vs

63

2.8

Mine

ral

(bar

ytes

) [3

9]

Calci

um

phos

phat

e,

Ca,

(PO

,),

431

w;

590

w;

961

vs;

1046

w;

1071

VW

15

7 VW

; 28

2 VW

; 10

88 v

s 18

1 w;

41

4 m

; 49

3 w;

619

vw

; 67

0 VW

; 100

7 vs

; 113

2 m

21

6~;

276

vw;

342

m;4

53

m;4

61

w(sh

); 61

6 w;

647

w;

988

vs

665

VW;

687

VW;

829

VW;

1050

vs

632.

8 An

tiqui

ty

Calci

um

carb

onat

e,

CaC

O,

Calci

um

sulfa

te d

ihyd

rate

Ca

SO,

.2H

,O

Zinc

su

lfide

and

bar

ium

su

lfate

Zn

S an

d Ba

SO,

Basic

lea

d(H)

ca

rbon

ate

2PbC

03.

Pb(O

H),

514.

5 An

tiqui

ty

[21,

40]

514.

5 M

inera

l [4

1]

514.

5 18

74

Zinc

whi

te

zinc

oxid

e,

ZnO

514.

5

514.

5

Rare

m

iner

al

(hyd

roce

russ

ite).

Synt

hesiz

ed

in

antiq

uity

(p

re-5

00

W

J311

18

34

a *

1 cm

-.

h s,

stro

ng;

m,

med

ium

; w,

wea

k;

v, v

ery;

sh,

sho

ulde

r, c T

he

pigm

ent

is e

ither

sp

ecifi

ed t

o be

a m

iner

al

or t

he d

ate

of i

ts f

irst

man

ufac

ture

is

lis

ted.

4 %

--

Band

wa

venu

mbe

rs

(cm

-),

and

Excit

atio

n N

otes

, da

te

and

Ram

an

litera

ture

Sp

ectru

m

illus-

B

rela

tive

inte

nsitie

sb

3 wa

vele

ngth

re

fere

nces

tra

ted

in f

igur

e 2

(nm

) 4 ? \

30

31

32

33

34

35

36

Tabl

e 7

Yello

w pi

gmen

ts

Nam

e

Bariu

m

yello

w

Berb

erine

Cadm

ium

yello

w

Chro

me

yello

w

Chro

me

yello

w de

ep

Chro

me

yello

w-

oran

ge

Coba

lt ye

llow

Gam

boge

India

n ye

llow

Lead

tin

ye

ll. Ty

pe

I Le

ad

tin

yell.

Type

II

Mar

s ye

llow

Mas

sicot

Napl

es

yello

w

Orpim

ent

Para

realg

ar

Com

posit

ion

Bariu

m

chro

mat

e Ba

CrO,

[C&,

HrsN

,O,]+

pl

us

sulfa

te

or

chlo

ride

anion

Cadm

ium

sulfid

e Cd

S

Lead

(II)

chro

mat

e Pb

CrO,

352

m;

355

m(s

h);

403

w:

427

514.

5 VW

; 86

3 vs

; 90

1 m

12

03

m;

1235

w;

12

76

m;

1342

63

2.8

w;

1361

w;

13

97

vs;

1424

w;

14

49

m;1

501

s;

1518

vs

; 15

68

w;

1626

s

304

vs;

609

s 51

4.5

338

w;

360

s; 3

12

m;

403

w;

632.

8 84

1 vs

Le

ad(I1

) ch

rom

ate

PbCr

O,

PbO

336

w;

358

s; 3

74

m;4

01

w;

632.

8 83

8 vs

Le

ad(B

) ch

rom

ate

PbCr

O,

PbO

149

m;

346

w(br

); 82

8 vs

Pota

ssiu

m

coba

lt ni

trite

K,

[Co(

NO&]

nH

,O

LX- a

nd

B-Ga

mbo

gic

acid

s,

G8fLO

s an

d G9

H3&

Mag

nesiu

m

salt

of

euxa

nthic

ac

id

WA%

%~

5&O

179

m;

274

s; 3

04

vs;

821

vs;

549

632.

8 83

6 m

; 12

57

w;

1326

vs

; 13

98

w

w;

1215

w;

12

46

m;

1265

w;

13

30

632.

8 w;

14

33

m;

1592

s;

16

33

m

143

484

w;

610

w;

631

w;

697

w;

vs:

289

s; 3

85

w

632.

8

632.

8

772

VW;

811

w;

877

VW;

1009

VW

; 10

47

w;

1097

w;

11

27

s;

1178

m

; 12

18

m;

1266

vw

; 13

45

s;

1414

w;

14

76

s;

1503

s;

15

99

vs

129

vs;

196

s; 2

75

w(br

); 29

1 w;

51

4.5

303

w;

379

w;

457

m;

525

w 13

8 vs

; 32

4 m

(br)

514.

5

245

w;

299

m;

387

s; 4

80

w;

632.

8

Lead

(I1)

stann

ate

Pb,S

nO,

WOH

),

Silic

on

subs

titut

ed

lead(

H)

stan-

na

te,

PbSn

_

Si

0

Orth

orhr

ombic

lea

d(H)

ox

ide,

1 x

x 3

Synt

hetic

iro

n(lI1

) hy

drox

ide,

PbO

-___

__

Band

wa

venu

mbe

rs

(cm

-),

Excit

atio

n No

tes,

date

an

d Ra

man

lite

ra-

Spec

trum

illu

s-

and

relat

ive

inte

nsitie

sb

wave

lengt

h tu

re

refe

renc

es

trate

d in

fig

ure

(nm

) -

Early

19

th

C 37

Antiq

uity.

Prin

cipal

ch

rom

opho

re

38

of

the

huan

gbo

and

kihad

a dy

es

632.

8

Lead

(I1)

antim

onat

e Pb

,Sb,

O,

140

vs;

329

m(b

r);

448

w(br

) 63

2.8

Arse

mc(

II1)

sulfid

e As

,&

136

w;

154

s;

181

VW;

202

w;

632.

8 22

0 VW

; 23

0 VW

; 29

2 m

; 30

9 s;

35

3 vs

; 38

1 w

Arse

nic(B

) su

lfide

As&

141

w;

152

w;

157

VW;

171

w;

632.

X 17

4 w;

19

0 w;

19

5 w;

20

2w:

222

VW;

229

vs;

235

s; 2

73

w;

319

w;

332

m;

344

m

Mine

ral

(gre

enoc

kite)

an

d sy

n-

39

thet

ic c.

18

45

Rare

m

inera

l cr

ocoi

te.

Synt

hetic

, 40

18

09

Synt

hetic

, 18

09

41

Synt

hetic

, 18

09

42

1861

. Al

so

know

n as

Au

reoli

n 43

Befo

re

1640

. gu

m

resin

44

15th

ce

ntur

y. Ex

tracte

d fro

m

the

45

urine

of

ca

ttle

fed

on

man

go

leave

s

Antiq

uity?

[3

7]

46

Antiq

uity?

Sp

ectru

m

show

n is

of

47

PbSn

o.7.

&.24

03

I371

M

iddle

19th

C

38

Antiq

uity,

cf.

the

red

pigm

ent

49

lithar

ge

[3 1

1 Sy

nthe

tic

(Egy

pt,

1570

- 12

93

BC)

50

[121

M

inera

l [3

3,34

,38]

. 51

Ligh

t in

duce

d tra

nsfo

rmat

ion

prod

uct

of

realg

ar

[33,

343

I.M. Bell et al. /Spectrochimica Acta Part A 53 (2997) 2359-2179

217X

Table 8

I.M. Bell et 01. Spectrochivlicrr Actu Part A 53 (1997) 2159%.?I 79

Pigments with no detectable Raman signal using either 514.5 or 632.8 nm excitation

Colour Name Composition Notes, date and literature references

Black Magnetite Iron(H) di-iron(III) oxide. Fe,O, Mineral. Transforms rapidly to Fe,O, in the laser beam 1371. [431

Mars black Synthetic iron(I1) di-iron(II1) Middle 19h C. Transforms rapidly to Fe,O, in the laser beam oxide, Fe,O, [421, [431

Blue Indigo Indigotin. C,,H,,N,O, Plant leaf (BC) Brown Van Dyck brown Humic acids, allomelanins Lignite containing iron (16th C?) Purple Tyrian purple 6,6-dibromo-indigotin Marine mollusc (1400 BC)

Cd-IdrJW2 Red Carmine Carminic acid, C22HZ,,0,3 Scale insect, cochineal (Aztec)

kermesic acid, Ci6Hi00s Scale insect, kermes (antiquity) Alizarin CJW4 Secondary component (after purpurin) of the madder root dye

(3000 BC) Yellow Quercitron CdboO,~ A flavonoid dye from the inner bark of the Quercus oak

(antiquity)

a The pigment is either specified to be a mineral or the estimated date of its first use is listed.

study of the similar MnO; ion [25], indicates that the presence of PbO increases the lattice size, which reduces the constraints on the CrO$ - ion and results in a lower wavenumber for its v,(a,) mode.

This library is comprehensive, encompassing all common pigments in use pre- - 1850 AD, but it is not exhaustive: as mentioned above there are many types of red and green earths that could have been examined individually, different polymorphs of pigments that could have been included, and many shades of the more modern pigments that could have been studied. However, the extra space required to illustrate the Raman spectra of these many possible inclusions is not justifiable as the spectra would differ only slightly, if at all, from those included in this work.

4. Conclusions

The Raman spectra of 56 common pigments in use before - 1850 AD have been recorded by Raman microscopy and compiled into a spectroscopic library. The library will enable conservation scientists rapidly to identify unknown pigments on historical artefacts.

Acknowledgements

We are indebted to the Leverhulme Trust for the award of a fellowship (PJG), to the EPSRC and ULIRS for financial support, to Instruments SA (UK) Ltd. for the loan of the Dilor Labram, to Renishaw PLC for the loan of the Renishaw Raman System 1000, and to the National Gallery, the British Museum Scientific Research Depart- ment, and A.P. Fitzpatrick for the donation of pigments.

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PI

PI

[31 [41

[51

PI

[71

PI [91

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Raman Spectroscopic Library of Natural and Synthetic Pigments