Journal of
MOLECULAR
STRUCTURE
ELSEVIER
Journal of Molecular Structure 440 (1998) 105-111
The identification by Raman microscopy and X-ray diffraction of
iron-oxide pigments and of the red pigments found on Italian pottery
fragments
Robin J.H. Clark*, M. Lucia Curri
Christopher Ingold Laboratories, University College London, 20 Gordon Street, London, WC1H OAJ, UK
Received 17 April 1997; accepted 12 May 1997
Abstract
The technique of Raman microscopy has been used to identify and characterise the pigments used in red shards of medieval
and earlier items of pottery which have been found in various archaeological sites in the South of Italy. The research has led to
the identification, on the basis of their characteristic Raman/resonance Raman spectra, of the red pigments as iron(Ill) oxide
(e.g. Indian Red, Red Ochre or Venetian Red) and the yellow pigments as hydrated iron(Ill) oxyhydroxide (e.g. Yellow Ochre
and Mars Yellow). X-ray powder diffraction experiments confirm the conclusions drawn above. © 1998 Elsevier Science B.V.
Keywords: Raman microscopy; Inorganic pigments; Iron oxides; Medieval pottery
1. Introduction
Red pigments were used widely from early antiquity until the late Middle Ages for surface decorations of stylistically different classes of pottery [1].
Early potters were clearly attracted to the rich variety
of colours and shades that iron oxides could afford;
their range is wide, their variations subtle, and they do
not disappoint when fixed. Moreover, red iron-oxide
pigments are highly variable in colour, texture, lustre,
and hardness. Different types of painted pottery, both
glazed and unglazed, have been manufactured in the
South of Italy for a long time, e.g. RMR, where R
denotes 'ramina', a copper-based compound, M
denotes manganese, and R denotes a red decoration
* Corresponding author. Tel.: +44 171 3807457; fax: +44 171
3807463; e-mail: [email protected]
(glazed ware only) [2]. The remains (shards) of this
pottery are characteristic of particular sites and
periods of time, but the traditions have been preserved
from the Middle Ages to the present time. The typological method of identifying the components of pottery, in particular the decorative features of similar
fragments, is unreliable for shards found in multilayered sites, especially when excavations have been
carried out without a stratigraphic study. Archaeologists and historians of art thus require accurate
methods of identification and data-handling in order
to define and recognise technologies in pottery
production and decoration [3-5].
The aim of this work was to identify the red pigments on painted ceramic ware, both glazed and
unglazed, from different medieval archaeological
sites and different periods of time (fifth century B.C.
to the late Middle Ages). The archaeometric study of
0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved
PII S0022-2860(97)00239- 1
106
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
Table 1
List of the shards analysed
Code a
Provenance site
Description
Bak 1
CF 1
CF 2
CF 3
C 14
C6
Class. 1
Class. 2
Class. 3
RMR 1
RMR 2
UC 9
Bail, Cathedral
Castel Fiorentino, Foggia
Castel Fiorentino, Foggia
Castel Fiorentino, Foggia
Torre di Mare, Metaponto, Matera
Torre di Mare, Metaponto Matera
Grotta Pacelli, Bail
S.Francesco della Scarpa, Bari
S.Francesco della Scarpa, Bail
Bail, Cathedral
Bail, Cathedral
Gallana, Brindisi
Green/red painted 'sgraffiato' fragment. Medieval
Shard with band decoration in red. Medieval
Shard with band decoration in red. Medieval
Red decorated fragment. Medieval
Red/green brown decorated fragment. Medieval
Red/brown decorated fragment. Medieval
Red decorated fragment.
Red/brown decorated fragment. VI-V century B.C.
Red/brown decorated fragment. VI-V century B.C.
Green/brown/red decorated glazed fragment. Medieval
Green/brown/red decorated glazed fragment. Medieval
Red band decorated shard. Late Middle ages
aClass., classical, i.e. pre-medieval.
medieval pottery artefacts from the South of Italy has
only begun recently [3-5]. It is difficult to obtain
samples without contamination from the accompanying ceramic material and, moreover, decorative
materials have been used in such small quantities
that their study requires the use of very sensitive techniques. Few such studies o f pigments and colourants
in pottery have been made [4-9], the most recent
technique to have been applied being Raman microscopy [8,9].
The shards which have been investigated (Table l)
are representative of stylistically different classes of
ceramic from the following areas:
(a) Northern Apulia, from the ancient site of Castel
Fiorentino (Torremaggiore, Foggia), and mainly from
the church discovered during the excavation carried
out by researchers o f the Institute of Medieval History
of Art (University of Bari) in collaboration with Ecole
Francaise of Rome and the Ecole des Hautes Etudes of
Paris (France) [ 10-12[.
(b) Central Apulia, from the Cathedral of Bari,
where the excavations and restorations have been
undertaken by the local 'Soprintendenza ai Beni
A . A . A . S . ' in Puglia [13,14].
(c) Southern Apulia, from the archaeological site of
Gallana (Oria, Brindisi), from a collection of shards
which were discovered at the surface and not by
excavation.
(d) Basilicata, from the excavation carried out by
the local 'Soprintendenza Archeologica', at Torre di
Mare (Metaponto, Matera) [15].
(e) Central Apulia, from different sites excavated
near Bari.
2. Experimental section
Raman microscopy [16,17] is a highly specific and
non-destructive analytical technique with excellent
spatial resolution and is uniquely suited to the analysis
of grains of pigment located on or in works of art,
especially where the use o f destructive or sampling
techniques is precluded [18-23]. Raman microscopy
is also a suitable technique for the identification o f
microscopic particles of pigment 'in situ' and an
appropriate one for the study of the shards selected.
Raman spectra of the samples have been obtained
using an Olympus BH-2 microscope coupled to a
Dilor XY triple grating spectrometer with an intensified photodiode array detector. The samples were
excited using a Coherent Radiation Model 52
krypton-ion laser (647 nm), filtered through a specific
narrow band-pass filter. The pigments on the unglazed
shards were studied directly. However, the glaze on
shards constitutes an impediment to analysis by
Raman microscopy; such samples were placed for
analysis on the XY translation stage of the microscope
using a holder which permitted adjustment by rotation
about an axis perpendicular to the light beam. In this
way the cross-sections of the glazed shards, i.e. the
broken edges, could be studied by looking at the
coloured layer coating the ceramic material.
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
X-ray powder diffractometry experiments were
performed to identify the commercial pigments from
Winsor and Newton and from Aldrich used as reference materials. The data were collected in the transmission mode on a D 5000 Siemens powder X-ray
diffractometer using Ge-monochromatized Cu Kct
radiation (X = 1.5406 ,~) and a scintillation counter
detector. Samples of the pure pigment were ground
and placed between transparent tape. The data were
measured in steps of 0.040 ° in 28 with an integration
time of 3.0 s, and collected over 5 - 9 5 ° in 20. The
powder diffraction data file was displayed and treated
by the EVA program which permits a search and
match directly into the JCPDS database.
(d)
(o)
I
400
r-
I
400
300
I
I
300
200
I
I
200
I
The Raman spectra of various iron-based pigment
powders obtained from Winsor and Newton were
obtained for reference purposes (Fig. 1, Table 2).
Natural iron oxides are processed from several different ores, including haematite, limonite, siderite,
and magnetite, providing a wide range of reds,
yellows, purples, browns and blacks [24-28]. Synthetic iron oxides are prepared in a variety of grades
from light reds to dark reds, sold under several names
such as Indian Red, Red Ochre, or Venetian Red (CI
77491) [24,29] and prepared from ground classified
haematite [24]; the formula for these materials is
essentially Fe203, but has not been closely defined.
Mars Yellow (as in Yellow Ochre) is a commercial
iron oxyhydroxide which is usually referred to as
400
300
200
400
300
20O
I
400
I
I
300
I
I
I
200 600
400
200
Wawnumbcr / cm -I
Fig. 1. Raman spectra of red decorated pottery shards (a) UC9, (b)
C6, (c) Class 1; and of pure pigment samples provided by Winsor
and Newton, (d) Indian Red, (e) Red ochre, (f) Iron(III) hydroxide.
X0= 647.1 nm, 4-s integration time (100 accumulations).
having the formula FeO(OH)x.H20 (CI 77492)
[24,29]; however, since it is an iron(III) compound,
the formula is probably better represented as
FeOl+x(OH)l_2x.H20, 0 -< x -< 0.5. Such pigments
are formed from hydrated iron oxides and dehydrate
Table 2
R a m a n spectra o f c o m m e r c i a l iron oxide p i g m e n t s
Pigment
Band w a v e n u m b e r " ( c m -I)
Venetian Red h
220 s
Indian Red c
220 s
238 w
Red Ochre c
220 s
Red O x i d e c
Mars Orange c
220 s
219 s
285 s
405 m
405 m
238 w
286 s
290 vs
240 w
239 w
286 s
285 s
406 m
392 m
406 m
412 m
487 w
Iron H y d r o x i d e c
244 w
296 m
383 s
415 w
478 w
M a r s Yellow c
244 w
299 m
384 s
412 w
476 w
295 m
383 m
403 m
Yellow Ochre c
aVS, very strong; s, strong; m, m e d i u m ; w, weak; vw, very weak.
bWinsor and Newton.
CAldrich.
i
(f)
(c)
3. Results and discussion
107
546 w
543 w
490 w
108
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
Table 3
X R D data for the p i g m e n t s studied a
Yellow ochre ( W & N ) b
C a S O 4 anhydrite
F O M = 1.21
4.169
3.4973
3.359
2.8644
2.685
2.3278
2.450
2.2145
2.189
-3.4988
-2.8494
-2.3282
-2.2090
--
F e 2 0 3 (Aldrich)
F e 2 0 3 haematite
F e O ( O H ) goethite
IAdl ( x 10 3)
1.5
15
0.4
5.5
29-0713 F O M = 2.85
IAdl ( × 103)
4.1830
-3.383
-2.693
-2.450
-2.190
14
24
8
0
1
F e 2 0 3 haematite
2 4 - 0 0 7 2 F O M = 0.39
IAdl ( x 103)
33-0664 F O M = 0.54
IAdl
2.699
2.515
2.205
1.838
1.693
1.475
1.455
2.7030
2.5190
2.208
1.8428
1.6966
1.4873
1.4543
4
4
3
4.8
3.6
12.3?
0.7
2.7000
-2.2070
1.8406
1.6941
1.4859
1.4538
1
Red O c h r e ( W & N )
F e 2 0 3 haematite
2
2.6
1.1
?
1.2
F e 2 0 3 haematite
33-0664 F O M = 0.20
Imdl ( x 103)
2 4 - 0 0 7 2 F O M = 0.28
IAdl
2.699
2.519
1.842
1.694
1.599
1.486
1.452
1.258
2.700
2.519
1.841
1.694
1.599
1.486
1.454
1.259
1
0
1
0
0
0
2
I
2.703
2.519
1.843
1.697
-1.487
1.454
--
4
0
1
3
-1
2
--
Indian Red ( W & N )
F e 2 0 3 haematite
3.682
2.697
2.516
2.204
1.840
1.695
1.486
1.454
x 103)
x 103)
F e 2 0 3 haematite
3 3 - 0 6 6 4 F O M = 0.33
I&dl ( × 103)
2 4 - 0 0 7 2 F O M = 0.39
IAdl
3.684
2.700
2.519
2.207
1.841
1.694
1.486
1.453
2
3
3
3
1
1
0
1
3.686
2.703
2.519
2.208
1.843
1.697
1.487
1.453
4
6
3
4
3
2
1
1
x 103)
109
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
Table 3 (continued)
Mars Orange (W&N)
Fe203 haematite
Fe203 haematite
24-0072 FOM = 0.49
IAdl ( × 103)
33-0664 FOM = 0.56
IAdl ( × 103)
3.682
2.693
2.516
2.204
1.839
1.692
1.601
1.485
3.686
2.703
2.519
2.208
1.843
1.692
1.601
1.487
4
10
3
4
4
0
0
2
3.684
2.700
2.519
2.207
1.841
1.694
1.603
1.486
2
3
3
3
2
2
2
1
Mars Yellow (W&N)
FeO(OH) goethite
FeO(OH) goethite
29-0713 FOM = 0.26
IAdl ( × 103)
17-0536 FOM = 0.56
IAdl ( × 103)
4.180
2.693
2.583
2.452
2.191
1.721
4.183
2.693
2.583
2.450
2.190
1.720
3
0
0
2
1
1
4.180
2.690
2.580
2.452
2.192
1.721
3
3
3
2
1
1
Red Ochre (W&N)
Fe203 haematite
2.699
2.519
1.842
1.694
1.599
1.486
1.452
1.258
Fe203 haematite
33-0664 FOM = 0.20
I~1 ( x 103)
24-0072 FOM = 0.28
lAd[ ( X 103)
2.700
2.519
1.841
1.694
1.599
1.486
1.454
1.259
1
0
1
0
0
0
2
1
2.703
2.519
1.843
1.697
-1.487
1.454
--
4
0
1
3
-1
2
--
aThe two columns relating to the data set with the lower FOM are listed first.
eWinsor and Newton.
at 177°C [27]; t h e i r h e a t r e s i s t a n c e is less t h a n t h a t o f
the r e d oxides. M a r s O r a n g e is also a m a n u f a c t u r e d
pigment, probably a mixture of Red Ochre and Mars
Yellow.
The results show that different patterns of Raman
b a n d s arise for the d i f f e r e n t g r o u p s o f i r o n - b a s e d pigm e n t s . In p a r t i c u l a r F e 2 0 3-based c o m p o u n d s g i v e rise
to c h a r a c t e r i s t i c s t r o n g R a m a n b a n d s at ca. 220, 285,
a n d 4 0 5 c m -l, w h e r e a s the m o s t c h a r a c t e r i s t i c b a n d o f
iron h y d r o x i d e a n d h y d r a t e d c o m p o u n d s is a s t r o n g
o n e at ca. 383 c m -l t o g e t h e r w i t h o n e or t w o w e a k
o n e s at or a b o v e 4 7 6 c m -1 w h i c h m a y b e r e l a t e d to the
p r e s e n c e o f F e - O - F e a n d / o r - O H g r o u p s in the crystal s t r u c t u r e [30]. It s h o u l d b e n o t e d t h a t the p i g m e n t s
Red Ochre, Red Oxide and Yellow Ochre show some
b a n d s b e l o n g i n g to b o t h o f t h e b a n d g r o u p s specified
a b o v e . T h e results s u g g e s t t h a t t h e s e p i g m e n t s m a y
consist of various mixtures of hydroxylated, hydrated
or o x y b r i d g e d c o m p o u n d s a n d iron oxide.
XRD experiments have been carried out on a range
o f c o m m e r c i a l p i g m e n t s in o r d e r to test the c o m p o sition a n d r e l i a b i l i t y o f the latter as r e f e r e n c e s a m p l e s
for R a m a n i n v e s t i g a t i o n s . T h e m o s t i n t e n s e lines in
the d i f f r a c t o g r a m s w e r e a s s i g n e d to k n o w n c o m pounds by means of a procedure for searching and
matching with lines of known crystalline compounds
in t h e d a t a b a s e . T h e X R D p a t t e r n s w h i c h w e r e
o b t a i n e d ( T a b l e 3) w e r e u s e d to d e t e r m i n e the c o m p o s i t i o n o f e a c h r e f e r e n c e s a m p l e . H o w e v e r it w a s
p o s s i b l e to i d e n t i f y o n l y t h e m a j o r c o m p o n e n t s in
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
110
Table 4
R a m a n spectra o f the red shards e x a m i n e d
Sample a
Band
Wavenumber/cm
BAK 1
CF 1
97 w
140 s
CF 2
CF 3
C 14
185 w
96 w
C 6
184 v w
Class. 1
Class. 2
Class. 3
185 w
88 w
137 m
UC 9
221 w
220 s
245 v w
288 m
286 s
404 w
220 s
240 w
286 s
402 w
219 s
286 s
222 w
288 s
402 w
404 w
220 w
402 w
288 s
219 m
240 w
283 s
219 s
252 w
285 s
396 m
401 w
221 m
288 s
404 w
404 w
220 vs
286 s
405 w
aFor the identification o f the codes, see Table 1.
this way, due to the detection limits of the technique.
It is important to note the usage of C a S O 4 a s a filler in
some pigments such as Yellow Ochre and Venetian
Red. In particular Venetian Red cannot be considered
to be a reliable XRD reference material due to the fact
that the filler is present in high percentage.
The wavenumbers of the main Raman bands of all
the shards examined are listed in Table 4. Bands
present in the shards of pottery fragments reveal interesting information on the pigment used for red
decoration (Fig. 1). In the Raman spectra of almost
all of the shards, bands at ca. 220, 285, and 405 cm -1
were observed and so it is reasonable to conclude that
red Fe203 is the main component of the pigments
used. The added presence of iron hydroxides and/or
hydrated or oxy-bridged compounds is also possible
in samples CF 1, CF 2, Class. 1, Class. 2, whose
Raman spectra show evidence for an additional band
at ca. 396 cm -1. These results could be explained by
recognising the fact that hydrated iron compounds can
be converted to iron(Ill) oxide at relatively low temperatures [27]; the additional band may be indicative
of residual traces of the compounds originally applied
onto the surface of the ware and almost, but not completely, converted to" the red oxide, Fe203. Alternatively it is possible to hypothesise a post-firing
application process; however it is difficult to say by
visual observation alone whether decorations have or
have not been effected in this way [31]. In fact it is
important to recognise that the fired ferric oxide paints
are sometimes granular and powdery, showing no
apparent effects of firing.
It should also be noted that bands at 95, 140,
185 cm -1 have been detected in only a few spectra
'of the pottery fragments studied and not in those of
the commercial pigments, and also that several bands
in the spectra of pigments on some pottery shards are
unusually broad (BAK 1, Class. 1, Class. 2, Class. 3, C
6, C 14). These observations may be related to the
conversions Fe203.H20 ---* "y-Fe203 (maghaemite,
red brown) and "y-Fe203 -'-* a-Fe:O3 (haematite,
bright red), at 259 and 700°C respectively [32].
When glazed shards (RMR 1, RMR 2) were examined in cross-section, no signal was detected. This
might be due to the presence of the glazed layer, in
which very fine grains of pigment are almost completely dispersed. In fact no red grains were visible under
a microscope, only a large red area of uniform glass.
4. Conclusions
This study of artefacts from the South of Italy is one
of the first applications of Raman microscopy to the
analysis of pigments in archaeological pottery, both
on the surface and in cross-sections of fractured
shards. The data sets obtained in this work have
allowed the separation of the pigments into two
groups, Fe203 and hydrated or oxybridged iron(Ill)
oxide, by virtue of the characteristic Raman bands
detected. XRD experiments provided a further check
on the identification of the pigments. Raman spectra
of the red pigments in the shards show bands which
are also observed in commercial pigments based on
iron oxides and hydroxides. It has thus been possible
to confirm that the use of red iron oxide in pottery
R.J.H. Clark, M.L. Curri/Journal of Molecular Structure 440 (1998) 105-111
decoration is both widespread and long-standing
[1,33-36].
Acknowledgements
The authors thank the ULIRS for financial support
and Drs D.A. Ciomartan and C. Laganara and
Professor R. Striccoli for valuable discussions.
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