Analytica Chimica Acta 407 (2000) 275–289

Identification of inorganic pigments from paintings and polychromed

sculptures immobilized into polymer film electrodes by stripping

differential pulse voltammetry

A. Doménech-Carbó

a

,∗

, M.T. Doménech-Carbó

b

, M. Moya-Moreno

b

,

J.V. Gimeno-Adelantado

a

, F. Bosch-Reig

a

a

Departament de Qu´ımica Anal´ıtica, Universitat de València, Dr. Moliner, 50, 46100 Burjassot, València, Spain

b

Departament de Conservació i Restauració de Bens Culturals, Universitat Politécnica de València, Cam´ı de Vera 14, 46022, València, Spain

Received 14 May 1999; received in revised form 24 September 1999; accepted 1 October 1999

Abstract

Inorganic pigments in paintings and polychromed sculptures are studied by cyclic voltammetry and differential pulse

stripping voltammetry using micro-sample coatings in Paraloid B72-film modified electrodes. Characteristic cathodic and
anodic differential pulse profiles were obtained in the

+0.4 to −1.0 V vs. SCE potential range for different cadmium, copper,

lead, mercury and zinc pigments used in traditional colour palettes. Under optimized conditions, excellent reproducibility
was obtained. Microsamples extracted from polychromed sculptures, wall paintings, canvas paintings, panel paintings and
altarpieces from Spain, Ethiopia and Italy from the 12th to the 20th centuries have been analyzed and cadmium, copper, lead,
mercury and zinc pigments such as cadmium red, cadmium yellow, azurite, copper resinate, malachite, verdigris, lead–tin
yellow, lead white, minium, Naples yellow, vermilion, chrome orange, chrome yellow and white zinc have been identified
by this technique in agreement with PLM, SEM/EDX, XRD and FT-IR analysis. ©2000 Elsevier Science B.V. All rights
reserved.

Keywords: Inorganic pigments; Polymer film electrodes; Stripping differential pulse voltammetry

1. Introduction

Chemical analysis and technical examination of

works of art play an essential role in the field of con-
servation and restoration of historic and cultural her-
itage. The analytical studies previous to the restora-
tion of Michael Angelo’s frescoes in the dome of the
Sistine Chapel [1] are an outstanding example of the
importance of the intervention of analytical chemists
in the conservation and restoration of works of art. In

∗

Corresponding author.

addition to this paradigmatic case study, a number of
studies have evidenced the increasing attention given
to analytical problems in the work of conservation
[2,3]. In particular, the identification of pigments and
the study of their alteration processes that are vital to
providing a scientific diagnosis of the conservation
conditions of works of art.

Several instrumental techniques have been tradi-

tionally used in the analysis of pigments present in
works of art. Microchemical tests and polarized light
microscopy (PLM) were early techniques and are still
used nowadays. SEM/EDX is an important tool in

0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 7 8 1 - 3

276

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

the characterization of strata distribution of protective
films, paint layers, priming layers and grounds and
it provides the elemental composition of areas of the
sample as small as a pigment grain [4]. Together with
microscopy techniques, FT-IR spectroscopy and XRD
allow the analyst to obtain the chemical composition
of the sample [4–6]. Conventional FT-IR and XRD
provide satisfactory results for the majority of sam-
ples; however, for multi-component samples, IR spec-
tra and diffractograms can become complicated, mak-
ing the detection of minor and trace components dif-
ficult. IR-microscopy coupled with high-performance
MCT-A detector [7] and FT-IR transmission or re-
flectance microscopy provides IR spectra of very small
areas of materials, being highly useful for pictorial
samples [8–10]. When analysis of very small areas is
required, complementary XRD techniques are avail-
able [11].

To complement the existing techniques and facili-

tate routine analysis of work of art samples, an elec-
trochemical procedure for identifying inorganic pig-
ments, based upon the immobilization of solid probes
in polymer film electrodes, has been developed in this
work.

The area of chemically modified solid electrodes

(CMSEs) is a rapidly growing field giving rise to the
development of new electroanalytical methods with
increased selectivity and sensitivity for the determina-
tion of a wide variety of analytes [12]. CMEs are typ-
ically used to pre-concentrate the electroactive target
analyte(s) from the solution. The most popular meth-
ods for collecting analytes are based on complexa-
tion reactions [13] and electrostatic interactions [14]
at surfaces carrying an appropriate ligand or ion ex-
changer. Such pre-concentration has proven extremely
useful in conventional stripping voltammetry for anal-
ysis of metal traces in solution phase [15]. In this case,
the pre-concentration/deposition step may be the re-
duction of metal ions to metal, oxidation of the elec-
trode material and formation of insoluble salts with
the analyte, oxidative precipitation of the analyte, or
adsorption of the depolarizer [13,15]. In particular, the
use of film electrodes with insoluble compounds is
well known in electroanalytical chemistry [16]. Exten-
sive reviews covering stripping analysis are available
[15,17].

Alternatively, the analyte itself can be used as

a modifier and its electrochemical response can be

modulated by using an appropriate electrolyte. Elec-
trochemical identification of solid materials has been
reported by several research groups [18–21], includ-
ing the electrochemical phase analysis developed by
Brainina and co-workers [18,22,23]. Modified car-
bon paste electrodes have been used for analyzing a
wide variety of solid compounds [24–26], including
non-stoichiometric copper sulfides [27] to natural and
synthetic manganese oxides [28]. Analysis of metals,
alloys, pigments and minerals by abrasive stripping
voltammetry in paraffin-impregnated graphite elec-
trodes have been described by Scholz et al. [29–32].
The main drawbacks of this approach are the exis-
tence of strong background currents and the difficul-
ties in sample recuperation. The latter is a problem in
the field of conservation and restoration where only a
limited number of very small samples from the works
of art is available.

As alternative approaches, the preparation of films

of solid materials on the electrode surface [33], and
the immobilization of solid particles in polymer films
[34–38] have been described. More recently, pulse
methods have been applied to the analysis of a variety
of solid samples [39–42].

The use of these approaches is a potentially valuable

tool for analyzing pictorial samples in the field of con-
servation and restoration of works of art. The purpose
of this work is to describe the use of pigment-modified
polymer film electrodes for the microchemical identi-
fication of inorganic pigments used in different types
of works of art. Traces of the solid sample, if neces-
sary less than 0.01 mg, are transferred to the electrode
surface from a dispersion of the powdered solid into a
solution of an acrylic polymer in acetone and allowing
the resulting coating to dry in air.

Determination of different pigments has been

achieved by cyclic voltammetry (CV) and differen-
tial pulse voltammetry (DPV) and also by anodic
stripping differential pulse voltammetry (ASDPV)
preceded by an electrogeneration step at

−1.0 V vs.

SCE in different media. The electrochemical response
is conditioned by the supporting electrolyte, complex-
ing agents, pre-electrolysis time and electrochemical
parameters such as scan rate, amplitude pulse, etc. A
series of cadmium, copper, lead, mercury, and zinc
pigments has been studied. These include the most
used pigments in traditional colour palettes (azurite,
copper resinate, malachite, verdigris, lead–tin yellow,

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

277

lead white, litharge, minium, Naples yellow and ver-
milion) and those extensively used in the last 150
years (cadmium red, cadmium yellow, chrome or-
ange, chrome yellow, and zinc oxide). An excellent
reproducibility has been obtained permitting the anal-
ysis of a variety of pictorial samples. Application to
samples from the most representative types of works
of art: canvas paintings, wall paintings, polychromed
sculptures and altarpieces, is described. The results
obtained for a series of pictorial samples from the 12th
to the 20th centuries are reported. In all cases identifi-
cation of pigments is in agreement with that obtained
from PLM, SEM/EDX, XRD and FT-IR analysis.

2. Experimental

2.1. Instrumentation

Cyclic voltammograms were obtained with a

Newtronics 200P triangular wave generator, an
HQ 101 potentiostat and a Riken–Denshi F35 x–y
recorder. Differential pulse voltammograms were ob-
tained using a Metrohm E506 polarecord stand. The
potential scan rate varied from 10 to 500 mV/s. Pulse
amplitude values ranging from 10 to 100 mV were
used. A standard three-electrode arrangement was
used with a platinum counter electrode and a saturated
calomel reference electrode (SCE).

Examination of Paraloid B72-film modified elec-

trodes was carried out using a Jeol JSM 5410 Scan-
ning Electron Microscope-Cryostation CT1500C Ox-
ford Instruments. Accelerating voltage was 10 kV.

Energy-dispersive X-ray spectra of pigments from

work of art samples were obtained using a Jeol JSM
6300 Scanning Electron Microscope operating with
a Link–Oxford–Isis microanalysis system. The ana-
lytical conditions were: 20 kV accelerating voltage,
2

× 10

−9

A beam current and 15 mm as working

distance.

IR spectra of pigments from work of art samples

were obtained using a Perkin–Elmer 7700 Model
1750 Fourier transform infrared spectrometer with
a FR-DTGS (fast recovery deuterated triglycine sul-
phate) temperature-stabilized coated detector. Number
of co-added scans: 10; Resolution 2 cm

−1

.

Diffractograms of pictorial samples were obtained

using a Siemens D-500 diffractometer operating at

40 kV and 20 mA employing Cu K

␣

(

λ = 1.5418 Å)

and using graphite monochromator, scintillation
counter, step-scan range 1.2

◦

2

θ, angular increment

0.05

◦

/2

θ and count time 5 s.

2.2. Materials

Commercially available pigments were used for

blank probes. The standards of pigments were lead
(II) oxide (Probus p.a.), and minium (Probus p.a.).
Lead white (Charbonnel, 13 quai Montebello, 75005
Paris), cadmium yellow (Charbonnel) and cadmium
red (Charbonnel) were supplied by RCM Productos
de Conservación, Barcelona, Spain. Naples yellow
(Kremer, Farbmühle, D-88317 Aichstten/Allgäu, Ger-
many), verdigris, synthetic copper acetate (Kremer),
copper resinate (Kremer), lead–tin yellow (Kremer),
vermilion from the historic mine at Monte Amiata
in Southern Tuscany (Kremer), and natural azurite
extra fine ground (Kremer) were supplied by AP
Fitzpatrick, London. Zinc white (Winsor & Newton,
Ltd., Harrow, Wealdstone, Middlesex HA3 5RH, UK)
and chrome orange (Winsor & Newton) was supplied
by Viguer S.L. Productos de Arte y Conservación,
Valencia, Spain. Malachite (Zecchi-Colori-Belle Arti,
Via dello Studio 19r, 50122, Florence, Italy) was sup-
plied by Zecchi. Chrome yellow from the collection
of artists’ pigments of the Department of Conserva-
tion and Restoration of the Faculty of Fine Arts of
Valencia was used.

Paraloid B72; Rohm and Haas Co., Philadelphia, PA

was supplied by Dumi-Restauro Productos de Conser-
vación, Valencia, Spain.

2.3. Polymer film electrode preparation

Commercially available Paraloid B72 acrylic resin,

an ethyl methacrylate (70%) methyl acrylate (30%)
co-polymer (P[EMA/MA]) [43] was selected for poly-
mer film electrode preparation because of its ability
to form uniform thin films and its good adhesion on
all kinds of substrates. In addition, Paraloid B72 is
highly soluble in organic solvents such as acetone,
toluene or xylene and is insoluble in water. Although
oxidation and other changes such as cross-link on ul-
traviolet exposure of acrylates and chain-scissioning
reactions of PEMA have been reported, these changes
take place significantly near the glass transition tem-

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A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

perature (T

g

= 40

◦

C for Paraloid B72) and they occur

slowly at room temperature. For this reason, Paraloid
B72 has been considered a standard of stability in the
restoration field [43]. Interactions between pigments
and acrylic polymers have not been reported. The use
of poly(methyl methacrylate)-modified electrodes for
stripping analysis has been reported by other research
groups [44].

Paraloid B72 films obtained from solutions in ace-

tone were examined by SEM [45]. Specimens of poly-
mer films were fractured under cryogenic conditions
(

−170

◦

C) using liquid nitrogen in order to obtain

cross-sections. When the film was not fractured in a
dry state, the sample was dipped with a water drop be-
fore freeze-fracturing. This process takes place in the
cryostation after a previous cryofixation on slush ni-
trogen. The specimens were allowed to sublimate for
10 min at

−90

◦

C in the SEM chamber. Then, they went

back to the cryostation to be gold-coated at

−170

◦

C

using a classical sputtering process. After this, the
specimens were placed again in the SEM chamber to
be observed and images were taken. Temperature of
observation was

−130

◦

C.

The porous aspect of the top surface (the surface

of the film in contact with the solution) formed with
Paraloid B72 diluted to 0.5% in acetone (environment
conditions were 23

◦

C and 39% R.H.) is shown in

Fig. 1. Cross-sections from the films processed on the
cryostation attached to the SEM were measured and
the thickness was estimated to be 2–3

␮

m.

Fig. 1. Cryo-SEM secondary electron image of the top surface
of the coating formed from a 0.5% solution of Paraloid B72 in
acetone.

2.4. Pigment standards preparation

Pigment-coating electrodes were prepared on the

basis of the methods devised by Bard and co-workers
[34], Liu and Anson [35], Li and Calzaferri [37], and
Bessel and Rolison [32]. 0.1 mg of the pigment was
mixed with 0.1 ml of 0.5% acetone solution of Paraloid
B72 acrylic polymer in a small glass vial. Then, the re-
sulting dispersion was placed for 5 min in an ultrasonic
bath. The modified electrode was prepared by trans-
ferring a few microlitres of the dispersion onto the sur-
face of a freshly polished glassy carbon electrode and
allowing the solvent to evaporate. The glassy carbon
electrode surface was previously polished with 0.1

␮

m

alumina aqueous suspension on a polishing cloth. The
coatings examined contained 0.1–0.2 mg cm

−2

of the

dry pigment.

2.5. Sample preparation

The pigments analysed were from different paint

layers of the work of art which is commonly tens of mi-
crometers thick. In some cases the different paint lay-
ers were intended to separate. The sample was taken
with a sharp tip of a microscalpel and placed on a con-
cavity slide. Then the paint layers were mechanically
separated by using a scalpel with diamond lancets, dia-
mond dissecting knife and tungsten needles. A stereo-
scopic light microscope with a wide separation be-
tween the stage and the objective lens was used for
this stage. After this, the sample was grounded and ho-
mogenized in a small agate mortar. Finally, 0.2–0.4 mg
of the sample was weighed and the same procedure
used for pigments was followed.

2.6. Electrochemical measurements

All electrochemical experiments were performed

at 25

◦

C after the immersion of the modified elec-

trodes in well de-aerated solutions. Acetic acid/sodium
acetate (0.50 M), NaNO

3

(0.25 M), NaCl (1 M) and

NaNO

3

(0.25 M)

+ Na

2

EDTA (from 10

−3

to 0.10 M)

were used as electrolytic solutions. To avoid contami-
nation effects the solution was renewed for each sam-
ple after electrochemical measurements.

Anodic stripping experiments were carried out by

applying a given reduction potential for 1–15 min. The

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

279

electrogeneration/deposition potential was

−1.0 V in

all experiments unless otherwise stated. Anodic strip-
ping was initiated from the electrogeneration poten-
tial without a rest time. Linear potential scan or dif-
ferential pulse voltammetry was employed during the
subsequent oxidation step.

3. Results and discussion

3.1. Pigment analysis

In a first step, the redox response of a series of

pigments immobilized in polymer film electrodes
was examined. Fig. 2 shows cyclic voltammograms
for three PFEs containing different lead pigments,
namely, (a) chrome yellow, (b) minium and (c)
lead–tin yellow in NaCl 1 M. These are representative
of the different kinds of electrode processes that are
recorded in pigment-modified PFEs. For chrome yel-
low a well-defined reduction peak appears. As judged
by the linear dependence of the cathodic peak cur-
rents on the square root of the sweep rate, reduction
processes can be considered predominantly controlled
by diffusion of lead species in a solution generated
by the partial dissolution of the lead salt attached at
the electrode surface:

Fig. 2. Cyclic voltammograms for different pigment-modified PFEs
in NaCl 1 M. Scan rate 100 mV/s. (a) chrome yellow; (b) minium;
(c) lead–tin yellow.

MX

(surf)

= M

2

+

(sol)

+ X

2

−

(sol)

(1)

M

2

+

(sol)

+ 2e

−

= M

(surf)

(2)

In some cases, such as minium, the stripping reduc-

tion of the solid product attached to the electrode sur-
face occurs, as suggested by prominent tall cathodic
peaks recorded during the first and successive scans.

MX

(surf)

+ 2e

−

= M

(surf)

+ X

2

−

(sol)

(3)

These processes can be mediated by species existing

in the solution phase; for instance, for litharge, it can
be expected that the electrode process

PbO

(surf)

+ 2H

+

(sol)

+ 2e

−

= Pb

(surf)

+ H

2

O

(4)

occurs. These results reproduce those described
by Bond and Scholz [46] for the voltammetry of
lead–dithiocarbamate complexes attached at graphite
electrodes.

Finally, for lead–tin yellow, the cyclic voltam-

mogram exhibits an ill-defined cathodic wave fol-
lowed, in the reverse scan, by a prominent anodic
stripping peak. This voltammetric response can be
interpreted in terms of the model developed by
Roizenblat et al. [47] on the electrochemistry of
insoluble non conducting oxides immobilized into
CPEs. The reduction process involves a preceding
chemical reaction with formation of protonated sur-
face complexes localized at the three-phase boundary
of graphite/oxide/supporting electrolyte. Mass trans-
fer accompanying the electron transfer step is then
accomplished in the diffusion-limited region of the
colloidal surface layer of the oxide particles.

In these cases, differential pulse voltammetry at low

scan rates provides well-defined cathodic peaks. As
can be seen in Fig. 3, different voltammetric profiles
are obtained for the different pigments. The anodic
portion of cyclic voltammograms correspond, in all
cases, to the stripping oxidation of metallic layers re-
tained at the electrode surface. As can be seen in Fig.
2, the anodic profiles differ remarkably from one pig-
ment to another. These results support the idea that
both the cathodic and anodic regions of the voltam-
mograms are, in principle, usable for analytical pur-
poses. In fact, the morphology of DPV curves is in-
sensitive to changes in the potential scan rate in the
1–60 mV/s range, whereas amplitude pulse variations
promote only small morphologycal changes in the

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A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

Fig. 3. Cathodic scan for DPVs at PFEs modified by. (a) malachite;
(b) copper resinate; (c) azurite; (d) verdigris; (e) zinc white; (f)
litharge. Acetic acid/sodium acetate 0.50 M. v

= 20 mV/s. 1U = 25

mV.

Fig. 4. Cathodic scan for DPVs at PFEs modified by (a) lead
white, (b) Naples yellow, and (c) mixture of lead white

+ Naples

yellow (1 : 1 w/w). Na

2

EDTA 0.01 M

+ NaNO

3

0.25 M solution.

v

= 20 mV/s. 1U: 25 mV.

voltammograms. Interestingly, the voltammetric pat-
terns of individual pigments remains unchanged in
pigment mixtures. This can be seen in Fig. 4 in which
differential pulse voltammograms for (a) lead white
and (b) Naples yellow-modified electrodes in EDTA
0.01 M

+ NaNO

3

0.25 M are plotted. Stripping peaks

at

−575 and −510 mV vs. SCE are obtained, respec-

tively. These peaks are usable for analytical purposes
as evidenced by Curve (c) in the same figure, corre-
sponding to a PFE from a mixture of both pigments. In
this case, two overlapped peaks at

−570 and −510 mV

appear. Table 1 summarizes the DPP data relevant for
the identification of selected metallic pigments from
cathodic peaks.

In some cases the distinction between two pigments

from the cathodic portion of CVs and DPVs becomes
uncertain. Then, the anodic portion of CVs and DPVs
can be used to elucidate the composition of sam-
ples. As occurring in conventional stripping analysis
[15,18], the process of metal electrodeposition on a
foreign substrate is preceded by the appearance of a
single-layer of adsorbed adatoms, which in suitable

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

281

Table 1
Pigment-modified Paraloid B72 film electrodes. Reduction peak potentials vs. SCE for the differential pulse voltammograms (v

= 20 mV/s;

1V = 25 mV) in (a) HAc 0.50 M + NaAc 0.50 M, (b) NaCl 1 M

Pigment

Ep(a) (mV)

Ep(b) (mV)

Azurite

−80

−184

−552

t

+160

b

−265

b

−655

t

Cadmium red

−24

−636

−836

−305

b

Cadmium yellow

+16

−544

−656

−872

−912

−890

t

Chrome orange

−370

b

Chrome yellow

+312

t

+128

−608

t

−615

t

Copper resinate

−56

−136

−512

b

−712

−920

−380

b

Lead–tin yellow

+200

b

−240

−384

−865

b

Lead white

+32

b

−672

−780

−590

t

Litharge

−550

−780

−280

b

Malachite

+24

−120

−320

b

Minium

−225

t

−580

t

−330

b

Naples yellow

+336

b

−312

−600

t

−752

−784

−510

t

Verdigris

−64

t

−164

−504

−616

−760

−350

b

Vermilion

+104

+24

−584

−728

−304

b

−665

t

Zinc white

−64

t

−176

−512

−640

−808

−280

b

−560

b

b

Broad peak.

t

Tall peak.

electrode potential conditions are capable of forming
stable crystallization nuclei. Surface concentration
of adatoms depends on values of electrode potential,
chemical and phase composition, texture and surface
morphology, crystallographic orientation and reac-
tions with substrate compounds. Two-dimensional
adsorption of metal atoms preceding the process of nu-
cleation onto the electrode surface occurs. Nucleation
and growth take place with participation of surface
diffusion adatoms and direct incorporation of atoms.
When the nucleating phase does not block completely
the electrode surface by a two-dimensional adsorption
layer, the whole surface becomes thermodynamically
accessible for the formation of stable nuclei of new
phases [48–50]. As a result, different metallic layers
can be formed undergoing different re-oxidation pro-
cesses and thus the anodic portion of CVs becomes
strongly substrate-dependent.

Accordingly, the technique of controlled potential

cathodic electrogeneration followed by anodic strip-
ping with a linear potential step has been systemati-
cally employed. Focusing attention on the exploitation
of the above-described behaviour for the identification
of pigments in work of art samples, the influence of
typical experimental parameters was examined.

Preliminary experiments on the influence of the

amount of polymer deposited on the electrode surface
indicate that good results are obtained by using

0.8 mg cm

−2

, corresponding to a film thickness of

2.5

␮

m. On lowering the amount of the modifier below

this value, the adherence of the coatings decreased
progressively. Increasing the amount of coating above
4 mg cm

−2

causes a distortion of voltammetric peaks,

probably owing to an excessive increase in the ohmic
resistance of the coating.

The amount of pigment varied from 1 to 0.01

mg cm

−2

. As expected, the intensity of voltammet-

ric peaks increased as the amount of sample in-
creased, but no significant differences were obtained
in the peak potentials and morphology of both the
cathodic and anodic voltammetric profiles. Routine
experiments were performed with coatings contain-
ing 0.2–0.4 mg cm

−2

of the sample on the electrode

surface.

Voltammetric curves are, however, sensitive to

the electrogeneration potential. As can be seen in
Fig. 5, anodic DPV curves changed significantly from
electrogeneration potentials up to

−1.4 V, but there

are no remarkable alterations in the number and peak
potential of the recorded peaks for electrogeneration
potentials ranging from

−0.9 to −1.2 V. As expected,

peak currents increased as the scan rate increased, but
no significant alterations of the potential peaks were
obtained in the range from 1 to 100 mV/s.

The morphology and height of the voltammetric

peaks were independent of the electrogeneration/

282

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

Fig. 5. Effect of electrogeneration potential variations on an-
odic differential pulse stripping voltammograms for Naples
yellow-modified PFEs in Na

2

EDTA 0.01 M

+ NaNO

3

0.25 M.

Sweep rate 20 mV/s.

1U: 20 mV. Electrogeneration potential (a)

−1.4 V; (b) −1.2 V; (c) −1.0 V; (d) −0.8 V.

deposition time for more than 2–5 min. Accordingly,
quantitative measurements can be carried out even us-
ing short electrogeneration times. In our experiments,
a 5 min electrogeneration time was routinely used.

Under optimized conditions, excellent reproducibil-

ity was obtained for measurements on (a) different
freshly coated electrodes, and (b) for repeated
cathodic/anodic measurement cycles on the same
modified electrode.

The electrochemical behavior of surface-confined

electroactive species is conditioned by limitations in
the overall rate of the redox process by mass transfer
within the film, charge transfer at the substrate/film in-
terface, associated chemical reactions of the electroac-
tive species, and non-idealities in the film structure

[51,52]. Accordingly, it can be expected that the elec-
trochemical response may be modulated by several
factors and, in particular, by the addition of complex-
ing agents to the electrolytic solution. This observation
enhances the scope of the proposed method to a suit-
able identification of pigments on the basis of the
use of different electrolytic media. Fig. 6 presents the
anodic differential pulse voltammograms obtained in
NaCl 1 M media under optimized conditions for a se-
ries of pigments included in this study. Table 2 sum-
marizes the peak potential data in different electrolytic
media.

3.2. Analysis of pictorial samples

To examine the possibility of using modified

PFEs to identify inorganic pigments in real sam-
ples, differential pulse voltammograms for a series
of sample-modified electrodes have been obtained.
Application of modified electrodes to pictorial sam-
ples is non-trivial; apart from the need to use traces
of the sample, other problems arise from possible
interference caused by the presence of (a) different
electroactive pigments in the sample and (b) other
substances such as binding media and varnishes, that
can produce unexpected perturbations of the electro-
chemical response. In the first case, cadmium, copper,
lead, mercury and zinc can be co-plated during the
Faradaic pre-electrolysis step, producing a variety of
intermetalic compounds. This is a typical problem in
stripping analysis of trace transition metals in solution
which requires the use of more selective modified
electrodes [15,18]. However, under our experimental
conditions, it appears that this problem has minimal
influence because the various components exist as
separate microcrystallites in the sample.

The studied samples from canvas paintings, wall

paintings, polychromed carvings and altarpieces dated
from the 12th to the 20th century are described in de-
tail in Appendix A. Results of analysis carried out by
PLM, SEM/EDX, FT-IR and XRD in these samples
are given in Table 3. The peak potential values ob-
tained under optimized conditions are shown in Table
4. Identification of pigments established by electro-
chemical methods in the studied samples is in good
agreement with that from the other instrumental tech-
niques usually used.

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

283

Fig. 6. Anodic differential pulse stripping voltammograms in 1 M NaCl for (a) malachite, (b) azurite, (c) minium, (d) chrome yellow,
(e) litharge, (f) lead white, (g) lead–tin yellow, (h) chrome orange, (i) cadmium red, (j) cadmium yellow, (k) vermilion, (l) zinc white.
Electrogeneration potential

−1.0 V. Sweep rate 20 mV/s. 1U: 20 mV.

Table 2
Pigment-modified Paraloid B72 film electrodes. Anodic peak potentials (mV vs. SCE) for the differential pulse voltammograms with
their morphology. Electrogeneration time 5 min.; v

= 20 mV/s; 1V = 20 mV. Electrolytes: (a) HAc 0.50 M + NaAc 0.5 M; (b) NaNO

3

0.25 M

+ Na

2

EDTA 0.01 M; (c) NaCl 1M

Pigment

Ep(a)

Ep(b)

Ep(c)

Azurite

+40

t

+104

−70

b

+55

t

+170

b

−250

t

+150

b

Cadmium red

−896

t

−624

t

−328

−144

−72

b

−730

b

−850

b

−745

b

Cadmium yellow

−848

−608

t

−264

−168

−128

−640

b

−105

b

−905

s

−705

b

−425

s

Chrome orange

−680

b

−730

b

+105

b

−720

b

Chrome yellow

−720

b

−520

b

−415

t

−615

b

−730

b

−570

b

Copper resinate

+64

t

−690

b

−50

t

−50

o

−300

o

Lead–tin yellow

+200

b

−240

−384

−750

b

−110

b

−720

b

Lead white

−572

t

−36

+104

b

−340

t

−640

b

−560

t

Litharge

−680

b

−720

t

−415

t

−750

b

−450

t

Malachite

−8

t

−720

s

−470

t

+65

o

−710

b

−215

t

+15

o

Minium

−650

b

+335

b

−625

b

−455

t

+50

s

−660

b

−480

b

Naples yellow

−544

t

−380

−48

b

−475

t

−30

t

−670

b

Verdigris

−104

t

−8

+96

−585

b

−390

t

+160

o

+320

o

−490

b

Vermilion

−592

−136

−56

t

+4

−590

b

−370

t

−50

s

+320

t

+20

o

+200

o

Zinc white

−592

b

−88

−720

b

−950

s

−950

s

b

Broad signal.

o

Overlapped peaks.

s

Shoulder.

t

Tall signal.

284

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

Table 3
Sample-modified Paraloid B72 film electrodes. Peak potentials vs. SCE for the differential pulse voltammograms. t: tall signal; b: broad
signal; s: shoulder; o: overlapped peaks. Electrogeneration time 5 min; v

= 20 mV/s; 1V = 20 mV. Cathodic peaks; electrolyte: HAc

0.50 M

+ NaAc 0.50 M. Anodic peaks; electrolyte: NaNO

3

0.25 M

+ Na

2

EDTA 0.01 M. Characteristic X-ray emission lines obtained in the

paint layer of the sample using SEM/EDX. Major IR absorption bands obtained using FT-IR. Major characteristic x-ray lines obtained
using XRD. The values in brackets correspond to the relative intensities from the highest intensity of each pigment that have been found
in the diffractogram of the paint layer analysed

Sample

Remarks

SEM/EDX Main characteristic X-ray
emission lines identified (keV)

FT-IR Main IR absorption bands
identified (cm

−1

)

XRD Main characteristic
x-ray lines identified, d
(Å)

1

Reddish-brown layer,
35

␮

m thickness, tem-

perature

Minium: 2.3 M

␣

(Pb), 10.6 L

␣

(Pb),

12.6 L

␤

(Pb), Iron oxide red, red

earth: 1.3 K

␣

(Mg), 1.5 K

␣

(Al), 1.7

K

␣

(Si), 3.3 K

␣

(K), 3.7 K

␣

(Ca), 4.0

K

␤

(Ca), 6.4 K

␣

(Fe), 7.1 K

␤

(Fe),

Carbon black: 3.7 K

␣

(Ca), 4.0

K

␤

(Ca)

–

Minium: 3.38 (100);
2.905 (48), 2.784 (42),
2.629(28), 1.750(25),
Iron oxide red: 2.69
(100), 2.51 (30), 1.69
(21), Quartz: 3.34 (100),
4.262 (21), 2.455 (10),
2.452 (14), 2.287 (20),
2.237 (13), 2.128 (18),
1.976 (13), 1.817 (33),
1.671 (12), 1.605 (11),
1.540 (20), 1.452 (11)

2

Light blue layer,
123

␮

m thickness,

Fresco

Azurite: 8.1 K

␣

(Cu), 8.9 K

␤

(Cu),

Lead

white:

2.3

M

␣

(Pb),

10.6

L

␣

(Pb), 12.6 L

␤

(Pb)

Azurite:

ν OH (3421), δ OH (950),

ν

3

CO

3

2

−

(1450, cm

−1

) strong

and broad,

ν

2

CO

3

2

−

(837 cm

−1

)

weak,

Lead

white:

ν

3

CO

3

2

−

(1423 cm

−1

) strong and broad,

ν

2

CO

3

2

−

(876 cm

−1

) weak,

ν

4

CO

3

2

−

(683 cm

−1

) sharp

–

3

Red layer, 24

␮

m

thickness, oil painting

Vermilion: 2.2 M

␣

(Hg), 10.0

L

␣

(Hg), 11.8 L

␤

(Hg), 2.3 K

␣

(S)

–

–

4

Green layer, 94

␮

m

thickness, oil painting

Malachite: 8.1 K

␣

(Cu), 8.9 K

␤

(Cu)

Malachite:

ν OH (3441), δ OH

(1045),

ν

3

CO

3

2

−

(1500, 1400 cm

−1

)

strong

and

broad,

ν

1

CO

3

2

−

(1113 cm

−1

) weak,

ν

2

CO

3

2

−

(819,

743 cm

−1

) weak

–

5

Deep green glazed
layer, 34

␮

m thick-

ness, oil painting

Copper resinate: 8.1 K

␣

(Cu), 8.9

K

␤

(Cu)

–

–

6

Green layer, 67

␮

m

thickness, oil painting

Malachite: 8.1 K

␣

(Cu), 8.9 K

␤

(Cu)

Malachite:

ν OH (3441), δ OH

(1045),

ν

3

CO

3

2

−

(1500, 1400 cm

−1

)

strong

and

broad,

ν

1

CO

3

2

−

(1113 cm

−1

) weak,

ν

2

CO

3

2

−

(819,

743 cm

−1

) weak

–

7

Red–orange layer,
48

␮

m thickness, oil

painting

Minium: 2.3 M

␣

(Pb), 10.6 L

␣

(Pb),

12.6 L

␤

(Pb)

–

–

8

Red–orange layer,
40

␮

m thickness, egg

tempera/oil painting

Minium: 2.3 M

␣

(Pb), 10.6 L

␣

(Pb),

12.6 L

␤

(Pb)

–

–

9

Light yellow layer,
69

␮

m thickness, egg

tempera/oil painting

Lead–tin yellow: 2.3 M

␣

(Pb), 10.6

L

␣

(Pb), 12.6 L

␤

(Pb), 3.4 L

␣

(Sn),

3.7 L

␤

(Sn)

–

–

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

285

Table 3 (Continued)

Sample

Remarks

SEM/EDX Main characteristic X-ray
emission lines identified (keV)

FT-IR Main IR absorption bands
identified (cm

−1

)

XRD Main characteristic
x-ray lines identified, d
(Å)

10

Deep green glazed
layer, 85

␮

m thick-

ness, egg tempera/oil
painting

Copper resinate: 8.1 K

␣

(Cu), 8.9

K

␤

(Cu)

–

–

11

Yellow–green layer,
115

␮

m thickness, oil

medium

Naples

yellow:

2.3 M

␣

(Pb),

10.6 L

␣

(Pb), 12.6 L

␤

(Pb), 3.6

L

␣

(Sb), 3.8 L

␤

(Sb), Lead white:

2.3 M

␣

(Pb),

10.6

L

␣

(Pb),

12.6

L

␤

(Pb), Green earth: 1.3 K

␣

(Mg),

1.5 K

␣

(Al), 1.7 K

␣

(Si), 3.3 K

␣

(K),

3.7

K

␣

(Ca),

4.0

K

␤

(Ca),

6.4

K

␣

(Fe), 7.1 K

␤

(Fe)

–

–

12

Blue–green layer,
30

␮

m thickness, oil

medium

Verdigris: 8.1 K

␣

(Cu), 8.8 K

␤

(Cu)

–

–

13

Red layer (repaint),
10

␮

m thickness, glue

temperature

Vermilion:

2.2 M

␣

(Hg),

10.0

L

␣

(Hg), 11.8 L

␤

(Hg), 2.3 K

␣

(S)

–

–

14

Flesh tone, 106

␮

m

thickness, oil painting

Lead

white:

2.3 M

␣

(Pb),

10.6

L

␣

(Pb), 12.6 L

␤

(Pb)

Lead white:

ν

3

CO

3

2

−

(1423 cm

−1

)

strong

and

broad,

ν

2

CO

3

2

−

(876 cm

−1

)

weak,

ν

4

CO

3

2

−

(683 cm

−1

) sharp

–

15

Red layer, 56

␮

m

thickness, oil painting

Vermilion:

2.2 M

␣

(Hg),

10.0

L

␣

(Hg), 11.8 L

␤

(Hg), 2.3 K

␣

(S),

Minium: 2.3 M

␣

(Pb), 10.6 L

␣

(Pb),

12.6 L

␤

(Pb), Azurite: 8.1 K

␣

(Cu),

8.9 K

␤

(Cu)

–

–

16

Yellow layer,
15

␮

m thick-

ness, glue temperature

Chrome yellow: 2.3 M

␣

(Pb), 10.6

L

␣

(Pb), 12.6 L

␤

(Pb), 5.4 K

␣

(Cr),

6.0 K

␤

(Cr)

Chrome

yellow:

ν

1

CrO

4

2

−

(845 cm

−1

) strong and broad,

ν

3

CrO

4

2

−

(883 cm

−1

) sharp

–

17

Yellow layer, 10

␮

m

thickness, Fresco

Cadmium yellow: 3.1 K

␣

(Cd), 3.3

K

␤

(Cd), 2.3 K

␣

(S)

–

–

18

Red layer, 18

␮

m

thickness, Fresco

Cadmium red: 3.1 K

␣

(Cd), 3.3

K

␤

(Cd), 2.3 K

␣

(S), 11.2 K

␣

(Se),

12.5 K

␤

(Se)

–

–

19

White ground, 61

␮

m

thickness, glue tem-
pera

Zinc white: 8.6 K

␣

(Zn), 9.6 K

␤

(Zn),

–

–

Many pigments considered in this work are usually

found in traditional colour palettes. The mineral azu-
rite, a basic copper carbonate (2CuCO

3

·Cu(OH)

2

);

verdigris, a copper basic acetate (Cu(C

2

H

3

O

2

)

2

·2Cu

(OH)

2

), vermilion, mercuric sulphide (HgS); red

lead or minium, lead tetroxide (Pb

3

O

4

) and lead

white, a basic lead carbonate (2PbCO

3

·Pb(OH)

2

) are

mentioned in almost all of the source materials and

catalogue lists of pigments from ancient times to the
present [53–56]. Malachite, basic copper carbonate
(CuCO

3

.Cu(OH)

2

), does not appear to have been used

in European painting as extensively as azurite [55].

Azurite was found in Sample 2 (polychromed sculp-

ture, 13th century) and Sample 15 (canvas painting,
19th century, Vicente López). Malachite pigment was
found in Samples 4 and 6 (wall painting in San Juan

286

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

Table 4
Sample-modified Paraloid B72 film electrodes. Peak potentials vs. SCE for the differential pulse voltammograms. Electrogeneration
time 5 min; v

= 20 mV/s; 1V = 20 mV. Cathodic peaks; electrolyte: HAc 0.50 M + NaAc 0.50 M. Anodic peaks; electrolyte: NaNO

3

0.25 M

+ Na

2

EDTA 0.01 M

Sample

Ep(a) (mV)

Ep(b (mV)

Assignment of pigments from ASDPV

1

−220

t

−575

t

−620

b

−445

t

−100

s

Minium

2

+37

b

−75

s

−560

t

−665

s

−345

b

−70

b

+60

b

+180

s

Azurite

+ lead white

3

+100

s

+28

s

−575

s

−732

−595

b

−375

t

−70

s

Vermilion

4

+29

t

−125

s

−725

s

−470

t

+75

s

Malachite

5

−62

−141

s

−515

b

−712 −915

−685

b

−55

t

Copper resinate

6

+21

t

−122

s

−715

s

−460

t

+60

s

Malachite

7

−225

t

−588

t

−635

b

−465

b

−100

b

Minium

8

−230

t

−574

t

−635

b

−475

b

−130

b

Minium

9

+204

b

−245 −390

s

−740

b

−110

b

Lead–tin yellow

10

−47

−124

s

−505

b

−698 −910

−685

b

−55

t

Copper resinate

11

+328

b

−303

s

−592

t

−743

s

−776

s

−470

t

−350

t

−30

t

Naples yellow

+ lead white

12

−60

t

−158

s

−496

s

−625 −751

580

b

−390

t

−150

s

+330

o

Verdigris

13

+98

s

+14

s

−575

s

−720

−565

b

−355

t

−90

s

Vermilion

14

+26

b

−664

s

−771

−345

t

Lead white

15

+104

s

+20

s

−231

t

−561

t

−582

t

−760

b

−625

b

−465

t

−410

t

−120

b

Vermilion

+ minium + azurite

16

+321

t

+115 −617

t

−620

b

Chrome yellow

17

+28

−536

s

−643

s

−860

s

−900

−650

b

−110

b

Cadmium yellow

18

−21

−625

s

−847

−735

b

Cadmium red

19

−56

t

−188

s

−523

s

−652

s

−811

s

−960

s

Zinc white

b

Broad signal.

o

Overlapped peaks.

s

Shoulder.

t

Tall signal.

del Hospital Church, 13th–14th centuries). Verdigris
was found in Sample 12 (canvas painting, 16th cen-
tury attributed to Tintoretto). Lead white is a ubiqui-
tous pigment, it was found in Sample 2 (polychromed
sculpture, 13th century), Sample 11 (Altarpiece, 16th
century, anonymous) and Sample 14 (polychromed
carving in the Cathedral of Málaga, 16th century).

As comparison between Tables 1, 2 and 4 reveals,

vermilion was found in Samples 3 (proceeding from
a polychromed sculpture, 13th century), 13 (from
a panel of the frescoes in the Cathedral of Málaga
(Spain), end of the 16th century), and 15 (canvas
painting, 19th century, Vicente López). Similarly, red
lead (minium) was identified in the wall paintings
of the rock-hewn churches in Lalibela (Ethiopia),
dated 12th century (Sample 1); the wall paintings
from San Juan del Hospital church in Valencia, dated
13th–14th centuries, (Sample 7); the Cinctorres Al-
tarpiece, dated 14th century (Sample 8). This pigment
was also found in Sample 15 (canvas painting, 19th
century, Vicente López).

Lead monoxide yellow pigments (PbO) massicot

and litharge were used in pre-dynastic times in Egypt,
in Roman monuments and it has only very rarely been
found in any painting made between the 13th and 20th
centuries. Litharge is the fused and crystalline oxide
formed from the direct oxidation of molten metallic
lead [55,57].

Use of Naples yellow, a lead antimoniate (Pb

3

(SbO

4

)

2

), as a colorant in glass dates back at least

to the 18th Egyptian dynasty and its further use is
reported in both fresco and oil painting in the 16th
century [53]. Lead white and Naples yellow in Sam-
ple 11, proceeding from a 16th century altarpiece,
have simultaneously been identified by comparing
the cathodic DPVs in Fig. 3 for (a) lead white, (b)
Naples yellow, (c) Sample 11, and (d) a mixture of
lead white and Naples yellow (1 : 1).

There is little certain evidence of the use of lead–tin

yellow (Pb

2

SnO

4

) from the older literature on paint-

ing techniques; nevertheless this pigment has been
identified in a number of paintings and polychromed

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

287

sculptures from the 14th to the 18th century [55]. In
general, there has been considerable confusion regard-
ing the occurrence of this pigment. Use of Naples yel-
low, the lead antimoniate pigment, was attributed to the
Old Masters from this period but this idea has been re-
jected. Similarly, use of Naples yellow was attributed
to Rubens but it is lead–tin yellow that has been iden-
tified on several of his paintings by XRD and emission
spectroscopy [54]. These examples show the impor-
tance of establishing reliable methods of analysis to
distinguish these two yellow pigments. The lead–tin
yellow pigment was clearly identified for a sample (b)
proceeding from the Saint Michael Altarpiece (Sam-
ple 9) painted by Vicente Macip (father of Joan de
Joanes) in 1527, confirming the use of such pigment
in Valencian workshops during the 16th century.

Copper resinate or transparent copper green is a

transparent green glaze obtained from copper salts of
resin acids. The occurrence of this glazing pigment
in illuminated manuscripts from the 8th to the 16th
century was established on the basis of the brittle-
ness of these paint layers, microscopic observations
and positive test of copper. The use of this pigment
has also been reported in paintings from northern Eu-
rope in the 15th and 16th centuries and paintings from
Italy in the 16th century. After this period, the use
of this pigment declined considerably [55]. Copper
resinate has been found in Sample 5 (wall painting in
San Juan del Hospital Church, 13th–14th century) and
Sample 10 (Saint Michael Altarpiece, 1527, Vicente
Macip).

In addition to the above, several pigments have been

included in this study, whose importance in terms of
artistic use over the last 150 years has been recognized
by those involved in the making and conservation of
works of art. Chrome yellow and chrome orange, lead
chromates (PbCrO

4

) began to be used widely as pig-

ments in art in the second quarter of the 19th century
[54,57]. Chrome yellow is now unavailable and with-
drawn from general use due to its carcinogenic na-
ture. This pigment has been found in Sample 16 (wall
paintings in the Palace of the Marquis of Montortal,
19th century).

Cadmium yellow (CdS) and cadmium red, a

cadmium sulfo-selenide (CdS[Se]), did not become
commercially available as pigments until about the
second half of the 19th century and the first quarter of
the 20th century, respectively [54,57]. These pigments

have been found in Samples 17 and 18 (frescoes in
the Virgen del Rosario Church, 1940, José Ros).

Zinc white is zinc oxide (ZnO), used since 1780

[53] and considered as one of the three white pig-
ments of good hiding together with lead white and ti-
tanium white. Its excellent suspension properties and
absorbance of ultraviolet radiation have made it highly
appreciated in the world of artistic and decorative
paints. This pigment has been identified in Sample 19
(canvas painting, 1942–43, Ramón Stolz).

Finally, it is interesting to note the ability of the

current procedure to identify mixtures of pigments.
This is the case of Samples 2, 11, and 15, as can be
deduced from data in Tables 1, 2 and 4.

4. Conclusions

Under optimized conditions, a judicious combina-

tion of cathodic differential pulse voltammetry—and
eventually anodic differential pulse voltammetry—in
different electrolyte media permits an unambiguous
identification of pigments in a variety of pictorial sam-
ples. The use of PFEs avoids some problems associ-
ated with carbon electrodes such as strong background
currents and facilitates sample recuperation after elec-
trochemical measurements. In particular, it is possible
to identify the components of pigment mixtures in the
paint layers of paintings and polychromed sculptures.
In this study, the results obtained in the analysis of pic-
torial samples using differential pulse voltammetry are
in good agreement with those obtained using other in-
strumental techniques such as PLM, SEM/EDX, XRD
and FT-IR spectroscopy. The scope of the proposed
procedure is limited by the requirement of samples
at the microgram level. In spite of the fact that sam-
ples for restoration are in the nanogram-to-microgram
level, the reported procedure can be considered as an
useful tool in the field of conservation and restoration
of works of art allowing a rapid identification of pig-
ments in a wide variety of pictorial samples.

Acknowledgements

Financial support by the Valencian regional

Government Project GV97-RN-1411 is gratefully
acknowledged. The authors would like to thank Mr.

288

A. Dom´enech-Carb´o et al. / Analytica Chimica Acta 407 (2000) 275–289

Manuel Planes, technical supervisor responsible for
the Electron Microscopy Service of the Polytechnic
University of Valencia.

Appendix A. Samples studied in this work

1. LA-9-7/98: sample from a red zone in the ‘Biet

Maryan’ wall paintings of the rock-hewn churches
in Lalibela (Ethiopia). The site has been regis-
tered on the World heritage List. Anonymous, 12th
century. This study is included in the Project of
Preservation of the rock-hewn churches in Lal-
ibela conducted by UNESCO and financed by
the Commission Directorate General for Develop-
ment, 7th European Development Fund , 1994.

2. SP-1.15: sample from the blue tunic of Saint Peter

polychromed sculpture (limestone support) of the
façade of the archpriestal church of San Mateo
(Spain). Anonymous, 13th century.

3. SP-1.16: sample from the red mantle of Saint Peter

polychromed sculpture (limestone support) of the
façade of the archpriestal church of San Mateo
(Spain). Anonymous, 13th century.

4. SJH 1.10: sample from the green background

in the wall paintings found in the San Juan del
Hospital church in Valencia (Spain). Anonymous,
13th–14th century.

5. SJH 4.4: sample from the green background in

the wall paintings found in the San Juan del Hos-
pital church in Valencia (Spain). Anonymous,
13th–14th century.

6. SJH 4.7: sample from the green draperies of the

principal figures found in the San Juan del Hos-
pital church in Valencia (Spain). Anonymous,
13th–14th century.

7. SJH-50: sample from the red background in the

wall paintings found in the San Juan del Hos-
pital church in Valencia (Spain). Anonymous,
13th–14th century.

8. RA-CN: sample from the red robes of the princi-

pal figures in the upper panel of the Saint Anthony
and Saint Blaise Altarpiece (Cinctorres, Spain)
painted by an anonymous artist in workshops of
Morella or San Mateo (Maestrazgo School located
in the Maestrazgo region in the north of the Va-
lencian Community), 14th century.

9. 4-SM/93: sample from a light yellow zone in

the Saint Michael Altarpiece painted by Vicente
Macip (father of Joan de Joanes) in 1527.

10. 2.A-SM/90: sample from the green mantle of the

Virgin in the Saint Michael Altarpiece painted by
Vicente Macip (father of Joan de Joanes) in 1527.

11. V.2-9/97: sample from a green zone in an Altar-

piece, (16th century, anonymous) from Valencia
(Spain).

12. C.7-15/97: sample from the green mantle of the

Virgin in the canvas painting ‘The Crucifixion’,
16th century. Anonymous painting attributed to
Jacopo Tintoretto.

13. 1.B-4/98: sample from the red mantle of Judas in

‘La Flagelación’, panel of the frescoes in the High
Altar of the Cathedral of Malaga (Spain). Cesar
Arbassia (1550–1607).

14. SG-4/98: sample from the flesh tone in ‘Saint Gre-

gory’ polychromed carving in the High Altar of
the Cathedral of Malaga (Spain). Cesar Arbassia
(1550–1607).

15. VL.6-16/97: sample from a red brown zone in

the canvas painting ‘Virgen de Gracia’ painted by
Vicente López (1772–1850).

16. CX.5-3/92 : sample from a yellow zone in the wall

painting of the Palace of the Marquis of Montortal
(Carcaixent, Spain), anonymous, 19th century.

17. RO.22–2/98: sample from the yellow robes of a

main character in the frescoes of the Virgen del
Rosario church in Valencia, painted by José Ros
in 1940.

18. RO.16-2/98: sample from the red robes of a main

character in the frescoes of the Virgen del Rosario
church in Valencia, painted by José Ros in 1940.

19. PC.22-1/94: sample from the ground of the canvas

painting ‘Saint Philip the Apostle’ in the Basilica
de la Virgen de los Desamparados in Valencia,
painted by Ramón Stolz in 1942–43.

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