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8/3/2019 Domnech, A. et al. Study cobalt and copper pigments in damaged frescoes. 2008
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Quantitation from Tafel Analysis in Solid-StateVoltammetry. Application to the Study of Cobaltand Copper Pigments in Severely DamagedFrescoes
Antonio Domenech,*, Mara Teresa Domenech-Carbo, and Howell G. M. Edwards
Departament de Qumica Analtica. Universitat de Vale`ncia. Dr. Moliner, 50, 46100 Burjassot, Vale`ncia, Spain, Institut deRestauracio del Patrimoni, Universitat Polite`cnica de Vale`ncia. Camde Vera s/n. 46022 Vale`ncia, Spain, and UniversityAnalytical Centre, Chemical & Forensic Sciences, School of Life Sciences, University of Bradford, Bradford, BD7 1DP, UK
A novel method, using Tafel plots, for quantifying elec-
troactive species in solid materials when their voltammet-
ric signals are strongly overlapped is described. This is
applied to the analysis of submicrosamples from the
highly damaged frescoes painted by Palomino (1707) in
the ceiling vault of the Sant Joan del Mercat church in Valencia, Spain. These paintings, which were fired in
1936, contained cobalt smalt plus azurite mixtures, this
last being altered to tenorite (CuO). The reported method
provides a quantitation of the cobalt smalt/azurite, teno-
rite/(azurite + smalt) relationships in samples, thus
providing direct information on pigment dosage (smalt/
azurite ratio) in pristine paintings, extent of alteration, and
temperature experienced by the frescoes during the
gunfire episode. Distinction between Palomino paintings
and other painters was clearly obtained due to the pres-
ence of malachite in these last.
Quantitation of components in samples is a general aim foranalytical purposes. In the fields of archaeometry, conservation,
and restoration, quantitation of species in solid microsamples is
of interest for characterizing materials and techniques, thus
obtaining information for authentication, geographical location,
etc.
In the last years, the scope of available techniques for analyzing
solid materials has been increased by the voltammetry of micro-
particles (VMP), a general methodology developed by Scholz et
al.1,2 This approach, which extends classical studies on carbon
paste electrodes,3-6 can be used for identification, speciation and
quantitation of electroactive components in sparingly soluble
solids, as described in recent extensive reviews.7,8
In this methodology, relative quantitation can be obtained from
coulometric data9,10 or via measurement of peak areas in
voltammograms5,9-18 and peak potential shifts.7,12,13Absolute quan-
titation can be obtained, also using the above parameters, by
means of addition of internal standards.19-22All these procedures
require that analytes (and eventually standards) yield separatedvoltammetric peaks, a requirement that does not hold in a number
of cases. In the current report, a method is proposed for the
relative quantitation of components in solid samples using solid-
state voltammetry when such components produce highly overlap-
ping signals, based on the Tafel analysis of the rising portion of
the common voltammetric curve. The use of this kind of analysis
for identifying individual components in mixtures has been
previously described.23
The proposed method is applied to the determination of the
composition of a series of 18 microsamples containing cobalt and
copper pigments from the frescoes on the vaulted ceiling of the
* To w hom correspon dence shoul d b e a dd ressed. E-m ail :
[email protected]. Universit at de Valencia. Universit at Politecnica d e Valencia. University of Bradford.
(1) Scholz, F.; Nitschke, L.; Henrion, G. Naturwissenschaften 1989, 76, 71-
72.
(2) Scholz, F.; Nitschke, L.; Henrion, G.; Damaschun, F. Naturwissenschaften
1991, 76, 167-168.
(3) Schultz, F. A.; Kuwana, T. J. Electroanal. Chem. 1965, 10, 95-103.
(4) Kuwana, T.; French, W. G. Anal. Chem. 1964, 36, 241-242.
(5) Lamache, M.; Bauer, D. Anal. Chem. 1979, 51, 1320-1322.
(6) Brainina, K. Zh.; Vidrevich, M. B. J. Electroanal. Chem. 1981, 121, 1-28.
(7) Scholz, F.; Meyer, B. In Electroanalytical Chemistry, A Series of Advances;Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 20,
pp 1-87.
(8) Grygar, T.; Marken, F.; Schroder, U.; Scholz, F. Collect. Czech. Chem.
Commun. 2002, 67, 163-208.
(9) Scholz, F.; Nitschke, L.; Henrion, G. Electroanalysis 1990, 2, 85-87.
(10) Scholz, F.; Lange, B. Fresenius J. Anal. Chem. 1990, 338, 293-294.
(11) Scholz, F.; Rabi, F.; Muller, W.-D. Electroanalysis 1992, 4, 339-346.
(12) Zhang, S.; Meyer, B.; Moh, G. H.; Scholz, F. Electroanalysis 1995, 7, 319-
328.
(13) Meyer, B.; Zhang, S.; Scholz, F. Fresenius J. Anal. Chem. 1996, 356, 267-
270.
(14) Grygar, T.; van Oorschot, I. H. M. Electroanalysis 2002, 14, 339-344.
(15) Cepr ia, G.; Garca-Gareta , E.; Perez-Arant egui, J. Electroanalysis 2005, 17,
1078-1084.
(16) Domenech, A.; Domenech, M. T. ; Osete, L.; Gimeno, J. V.; Bosch, F.; Mateo,
R. Talanta 2002, 56, 161-174.
(17) Domenech, A.; Domenech, M. T.; Osete, L.; Gimeno, J. V.; Sanchez, S.;Bosch, F. Electroanalysis 2003, 15, 1465-1475.
(18) Domenech, A. ; San chez, S. ; Yusa, D. J.; Moya, M.; Gimeno, J. V.; Bosch, F.
Electroanalysis 2004, 16, 1814-1822.
(19) Domenech, A. ; San chez, S. ; Yusa, D. J.; Moya, M.; Gimeno, J. V.; Bosch, F.
Anal. Chim. Acta 2004, 501, 103-111.
(20) Domenech, A.; Moya, M. ; Dom enech, M. T. Anal. Bioanal. Chem. 2004,
380, 146-156.
(21) Domenech, A.; Domenech, M. T.; Gimeno, J. V.; Bosch, F. Anal. Bioanal.
Chem. 2006, 385, 1552-1561.
(22) Bosch, F.; Domenech, A; Domenech, M T; Gimeno, J V. Electroanalysis
2007, 19, 1575-1584.
(23) Domenech, A.; Domenech, M. T.; Gimeno, J. V.; Bosch, F.; Saur, M. C.;
Casas, M. J. Fresenius J. Anal. Chem. 2001, 369, 576-581.
Anal. Chem. 2008, 80, 2704-2716
2704 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008 10.1021/ac7024333 CCC: $40.75 2008 American Chemical SocietyPublished on Web 03/07/2008
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church of Sant Joan del Mercat in Valencia, Spain. The frescoes,
painted by Antonio Palomino (1655-1726) in 1707, were severely
damaged by fire in the Spanish Civil War in 1936. As a result, an
important fraction of the wall paint was destroyed and the
surviving areas underwent severe deterioration, including chro-
matic changes among other dramatic damage. The current
process of restoration, initiated in 2001, required the development
of new analytical tools for facing the problem of identifying
pigments and their alteration products.24Two additional samples
from that vault, whose attribution to Palomino was uncertain, werealso studied.
Apart from the need to use as minimal amount of sample as
possible, this analytical objective is made difficult, even for well-
conserved paints, by the presence of interfering pigments and the
coexistence of additives (binders, varnishes, compounds in ground
layers). In the case of damaged paints, the appearance of
efflorescences, debries, poultice and deposits, and alteration
products complicates seriously the identification of pigments in
the sample.
Accordingly, a synergic collection of several techniques,
namely, optical end electron microscopies, atomic force micros-
copy, Fourier transform infrared and Raman spectroscopies, and
solid-state electrochemistry was used for obtaining informationabout the original pigments, binders, and substrate treatments
employed by Palomino.24 Thermal alteration of earth pigments
was studied by applying multivariate chemometric techniques to
VMP data.25
Quantitation via VMP was tested using synthetic specimens
of pigment or mineral mixtures and samples from Palominos
frescoes. The copper pigments mainly used in this period were
azurite and malachite, two basic copper carbonate minerals
(2CuCO3Cu(OH)2 and CuCO3Cu(OH)2, respectively), and ver-
digris, a basic copper acetate (Cu(CH3COO).Cu(OH)2).26 The
synthetic analogues of azurite and malachite, respectively, blue
and green verditer, were in production since the earlier 19th
century. Smalt, a cobalt-containing glass-type pigment, was used
since the 17th century. In contrast, cobalt blue (Co 3O4) was only
used since 1774.26
Alteration of copper pigments leads to copper trihydroxychlo-
rides, Cu(OH)3Cl, (different polymorphs, generically, minerals of
the atacamite group) but, as occurs for bronze disease, nantokite
(CuCl) and cuprite (Cu2O) may be formed.27,28 As reviewed by
Scott,27 the atacamite group comprises atacamite, clinoatacamite,
and botallackite, but as pointed out by Antonio and Tennent,28
even under laboratory conditions, the mode of production of
copper trihydroxychlorides is critical. Apart from classical spec-
troscopy and microscopy techniques, identification of copper
pigments and their alteration products by VMP23 and Raman
spectroscopy29-35 have been recently reported.
In the current report, the VMP approach was used for
identifying and quantifying cobalt and copper species existing in
microsamples from the Palominos frescoes. Since the majority
of involved cobalt and copper compounds produce almost coin-
cident voltammetric responses, conventional methods, based on
separated peak record, cannot be used. In particular, three
problems arise: (i) the distinction between different pigments,
(ii) determination of dosages in pigment mixtures, and (iii)
identification and eventually quantitation of alteration products.
The two first problems deal with the characterization of materialsand techniques used by the artist whereas the later provides
information on the extent of the alteration in paint layers.
Linear potential scan, cyclic and square wave voltammetries
(LSV, CV, and SQWV, respectively) have been used, this last
technique being of particular interest because of its inherently
high sensitivity and immunity to capacitive effects.36 It should be
noted that application of VMP for quantitation suffers from the
difficulty in controlling the amount of sample transferred to the
electrode, thus causing problems of reproducibility. In the ap-
proach for data treatment presented here, quantitation is derived
from shape-dependent parameters, which are independent of
sample loadings, thus avoiding the main source of repeatability
problems.Voltammetric data were crossed with Raman spectros-copy and scanning electron microscopy coupled with X-ray energy
dispersive analysis (SEM/EDX) for obtaining information for
conservation/restoration purposes.
EXPERIMENTAL SECTION
Materials and Chemicals. Reference materials were CoO
(Aldrich), CuO (Baker), CuCl (De Haen), and Cu2O (Carlo Erba)
reagents, and copper trihydroxichlorides prepared by means of
recommended procedures.28,29
Clinoatacamite was prepared by immersion of a sheet of copper
(1 5 cm) into a slurry of CuCl in water (0.1 g/L). After 24 h, a
crystalline green precipitate was developed in contact with the
copper sheet. The crystals were separated and rinsed with water
and ethanol. Atacamite was prepared following a similar procedure
but using a CaCO3 suspension (0.1 g/L) in a 0.1 g/L solution of
CuCl22H2O (Merck) in water and stirring the solution magneti-
cally for 24 h in contact with the copper sheet. Botallackite was
prepared by an identical procedure, but the suspension was left
unstirred. To prevent recrystallization into atacamite, the resulting
green crystalline precipitate was merely separated from the
aqueous suspension and desiccated.28 Paratacamite, a similar
compound where Ni, Co, or Zn replaces some of the Cu, 28,29was
not considered here. Reference pigments were azurite natural
(standard, K10200), azurite natural (fine, K10210), azurite natural
(dark standard, K10250), azurite natural (dark fine, K10260),
(24) Edwards, H. G. M.; Domenech, M. T.; Hargraves, M. D.; Domenech, A. J.
Raman Spectrosc. In press.
(25) Domenech, A.; Domenech, M. T.; Edwards, H. G. M. Electroanalysis2007,
19, 1890-1900.
(26) Gettens, R. J.; FitzHugh, E. W. In Artists Pigments. A Handbook of their
History and Characteristics; Roy, A. Ed.; National Gallery of Art; Washington
and Oxford University Press: Oxford, UK, 1993; Vol. 2, pp. 23-36.
(27) Eastaugh, N.; Walsh, V.; Chaplin, T. D.; Siddall, R. The Pigment Compendium;
Elsevier: New York, 2004.
(28) Scott, D. A. Stud. Conserv. 2000, 45, 39-53.
(29) Tennent, N. H.; Antonio, K. M. ICOM Committee for Conservation 6th
Triennial Meeting, Ottawa, 1981.
(30) Bell, I. M.; Clark, R. J. H.; Gibbs, P. Spectrochim. Acta, Part A 1997, 53,
2159-2179.
(31) David, A. R.; Edwards, H. G. M.; Farwell, D. W.; De Faria, D. L. A.
Archaeometry2001, 43, 461-473.
(32) Gilbert, B.; Denoel, S.; Weber, G.; Allart, D. Analyst 2003, 128, 1213-
1217.
(33) Frost, R. L.; Martens, W.; Kloprogge, J. T.; Wiliams, P. A. J. Raman Spectrosc.
2002, 33, 801-806.
(34) Frost, R. L. Spectrochim. Acta, Part A 2003, 59, 1195-1204.
(35) Hayez, V.; Costa, V.; Guillaume, J.; Terryn, H.; Hubin, A. Analyst2005,
130, 550-556.
(36) Lovric, M. In Electroanalytical Methods; Scholz, F., Ed.; Springer: Berlin,
2002; p 111.
Analytical Chemistry, Vol. 80, No. 8, April 15, 2008 2705
8/3/2019 Domnech, A. et al. Study cobalt and copper pigments in damaged frescoes. 2008
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azurite natural (greenish gray-blue, K10280), azurite MP reddish
deep (63-120 m, K10201), azurite MP, deep (K10203), azurite
MP (cobalt blue-type, K10204), azurite MP (cerulean blue,
K10206), azurite MP (greenish light (K10207), azurite MP
(exclusive, K10208), malachite natural (standard grind, K10300),
malachite natural (very fine, K10310), malachite MP (coarse,
K10341), malachite MP (medium, 80-100 m, K10343), malachite
MP (fine, 63-80 m, K10344), malachite MP (very fine, 0-63
m, K10345), malachite arabian (K10370), verdigris (synthetic,
K44450), smalt (standard grind, K10000), smalt (very fine grind,K10010), and dark cobalt blue (K45700), all supported by Kremer.
Heated specimens of azurite and smalt were prepared by heating
in furnace at 200, 400, and 600 C. A second series of specimens
consisting of azurite + malachite, azurite + atacamite, and azurite
+ smalt mixtures were prepared from K10200, K10300, and
K10000 materials. Compositions were 70:30, 50:50, and 30:70 w/w.
A third series was prepared from the above adding CaCO3 (50%
w/w). These mixtures were accurately powdered and homo-
genized in mortar and pestle before electrochemical measure-
ments.
Electrode Modification and Conditioning. Paraffin-impreg-
nated graphite electrodes were prepared as described in the
literature1,2,7,8 and consisted of cylindrical rods of diameter, 5 mm.
Prior to the series of runs for each material or sample, a
conditioning protocol was used for increased repeatability. The
electrode surface was polished with alumina, rinsed with water,
and submitted to potential cycles between +0.85 and -0.85 V
during 10 min in contact with phosphate buffer. An amount of
10-20 g of reference materials and 1.0 g of samples was
powdered in an agate mortar and pestle and further extended on
the agate mortar forming a spot of finely distributed material. Then
the lower end of the graphite electrode was gently rubbed over
that spot of sample and finally rinsed with water to remove ill-
adhered particles.
Instrumentation and Procedures. Electrochemical experi-ments were performed at 298 K in a three-electrode cell under
argon atmosphere. SQWVs and complementary CVs were ob-
tained with CH 420I equipment. Paraffin-impregnated graphite
working electrodes were dipped into the electrochemical cell so
that only the lower end of the electrode was in contact with the
electrolyte solution. This procedure provides an almost constant
electrode area and reproducible background currents.7 A AgCl
(3 M NaCl)/Ag reference electrode and a platinum wire auxiliary
electrode completed the conventional three-electrode arrange-
ment. A 0.50 M phosphate buffer (Panreac) was used as the
electrolyte solution. Hierarchical cluster analysis was performed
using the Minitab14 software package.
Raman spectra were acquired using a Renishaw InVia confocalRaman microscope, operating with diode and gas laser excitation
at 785, 633, 514.5, and 488 nm wavelengths and CCD detection.
Minimal laser powers of the order of microwatts were used to
prevent damage to sensitive pigments with lens objectives of 20
and 50, which provided spectral footprints between 2 and 5 m.
A spectral resolution of 2 cm-1 was used over the wavenumber
range 1800-200 cm-1, with the accumulation of between 10 and
20 scans to improve the signal-to-noise ratios. Calibration was
effected using a silicon wafer and wavenumbers of sharp bands
are accurate to 1 cm-1.
Morphology of the surface of paintings was characterized using
a Jeol JSM 6300 scanning electron microscope operating with a
Link-Oxford-Isis X-ray microanalysis system. The analytical condi-
tions were accelerating voltage 20 kV, beam current 2 10-9 ,
and working distance 1.5 mm. In parallel to the morphological
examination of microsamples, elemental analysis was performed
by means of SEM/EDX. Samples were carbon-coated to eliminate
charging effects. Qualoitative analysis was performed in punctual
mode. Quantitative microanalysis was carried out using the ZAF
method for correcting interelemental effects. The counting timewas 100 s for major and minor elements. Concentrations were
calculated by stoichiometry from element percentages generated
by ZAF software on the Oxford-Link-Isis EDX.
Samples. As previously noted, the Palominos paintings in the
ceiling vault of the Sant Joan del Mercat church in Valencia, dating
from 1707, were gunfired during the Spanish Civil War in 1936.
As a result, only some 20% of the original frescoes remain and
they are in a serious condition. Over an extensive part of the
paintings, the outer ground layer (intonaco) has been destroyed,
exposing the intermediate ground layer (arricio), which itself has
been removed in several parts along with the inner ground layer
(arenato) to reveal the underlying brickwork. Figure 1 shows an
image illustrative of the damage suffered by the paintings.Sampling was exercised from a representative selection of remain-
ing fresco fragments prior to their consolidation during the
conservation tasks. Samples were undertaken with a scalpel using
minimal intervention but including, wherever possible, pigment
particles that were adhered to the substrate. Each sample was
divided in three aliquots for analysis using SEM/EDX, Raman
spectroscopy, and VMP.
Samples were taken during 2002 and 2005 from different areas
of the hemicylindrical-shaped vault and were initially classified
into two groups: blackened samples (PVB7, PVB8, PVB9),
exhibiting a gross black surface layer, all excised from the central
axis of the vault (highest part), and dark samples (PV1, PV2,
PV3, PV3b, PV4, PV5, PV7, PV8, PV3a, PA4b, PA5b, PA7, PV8b,
PV10, PV11) taken in different locations external to the central
axis of the vault. Two additional samples, U7 and U11, were taken
from the lunettes placed at the lowest part of the vault. The
attribution of these samples to Antonio Palomino was uncertain
because it is documented that, at this level of the vault, the painter
Vicente Guillo Barcelo (1645-1698) started to execute a prior
frescoe, which was, partially, maintained despite Antonio Palomino
finally being the painter in charge for the decoration of the
complete vault.
RESULTS AND DISCUSSION Analysis of Voltammetric Responses. Figure 2 shows the
CV responses of (a) azurite, (b) malachite, and (c) smalt, attached
to PIGEs and immersed into 0.50 M phosphate buffer (pH 7.4).
In the initial cathodic scan voltammograms of copper pigments,
two overlapping cathodic waves appear at-0.10 and -0.20 V
versus AgCl (3M NaCl)/Ag, followed, in the subsequent anodic
scan, by a stripping peak at+0.02 V eventually exhibiting certain
peak splitting. In the second and following cathodic scans, a more
intense reduction peak -0.05 V was recorded. If the potential
scan is switched at-0.15, the stripping peak vanishes. For smalt,
the CV presents a main cathodic peak at -0.18 V, accompanied
by a stripping oxidation peak at -0,08 V.
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In Figure 3, the SQWV responses of the following are
compared: (a) azurite, (b) cuprite, (c) verdigris, and (d) atacamite,
all immersed into 0.50 M phosphate buffer. On initiating the
potential scan at+0.45 V in the negative direction, reduction peaks
at -0.10 and -0.25 V appear. SQWVs for all other azurite
specimens as well as malachite ones were similar. In contrast,verdigris and cuprite exhibit a unique peak at-0.15 V looking
like two strongly overlapped signals, preceding a weak signal at
-0.55 V. Atacamite and botallackite exhibit a similar profile, with
peaks at -0,15 and -0,25 V, while clinoatacamite produces a
unique peak at-0,16 V. The voltammetric response of all azurite
and malachite pigments (see Supporting Information) exhibited
a close similarity, with variations lower than 10-15 mV in the
peak potential and peak width from one specimen to another. The
voltammetry of CuO, however, was clearly different (vide infra),
consisting of a prominent cathodic peak at-0.60 V, also differing
from that of CuCl, for which a unique reduction peak at -0.35 V
was recorded in phosphate buffers.
Figure 4 shows the response of (a) smalt, (b) cobalt blue, and
(c) a smalt specimen heated at 600 C during 24 h. Smalt yields
a main reduction peak at -0,14 V, whereas cobalt blue yields
waves at+0.20 and -0.50 V. The heated smalt specimen produces
the reduction peak at-0.14 V followed by a broad wave at -0.50
V.
The voltammetry of cobalt and copper pigmenting species can
be described in terms of the overall reduction of the parent
compounds to the corresponding metal, followed by the oxidative
dissolution of the metal deposit to metal ions (Co2+, Cu2+ ) in
solution.
The reduction of copper pigments proceeds apparently via two
successive one-electron steps. Interestingly, upon addition of NaCl
(in concentrations between 0.05 and 0.10 M) to the electrolyte,
the voltammetric pattern of the different copper pigments remains
essentially unchanged. Since in the presence of chloride ions, Cu-
(I)-chloride complexes in solution should be formed, thusproviding a marked two-peak response,37 the above feature clearly
suggests that the reduction of copper pigments involves a solid-
state Cu(II) to Cu(I) transformation followed by epitactic reduction
to copper metal lightly accompanied by a dissolution-metal
deposition mechanism involving intermediate species in solution
phase. The reduction process is then governed by proton insertion
and the advance of a hydrated layer along the grains of pigment,
similarly to the electrochemical reduction of lead oxide to lead
metal described by Hasse and Scholz.38 Consistently, on increasing
the potential scan rate, the second reduction peak for azurite and
malachite decreases with respect to the first one while both peaks
are lightly shifted in the negative direction. The overall reaction
of reduction for azurite can be described as
(s) denoting solid phases. In the subsequent anodic scan, the
deposit of Cu metal is oxidized to Cu2+ (aq) ions, which in turn
(37) Vazquez, J.; La zaro, I.; Cruz, R. Electrochim. Acta 2006, 52, 6106-6117.
(38) Hasse, U.; Scholz, F. Electrochem. Commun. 2001, 3, 429-434.
Figure 1. Image of a portion (area 1 m2) of the damaged Palominos frescoes in the vault of the Sant Joan del Mercat church in Valencia,
Spain.
2CuCO3Cu(OH)2 (s) + 6H+ (aq) + 6e-f
3Cu (s) + 2CO2 + 4H2O (1)
Analytical Chemistry, Vol. 80, No. 8, April 15, 2008 2707
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are reduced to Cu metal in the second and successive potential
scans.
In the case of cobalt pigments, the response appears to depend
on the structural environment of cobalt ions in the material, and,
in particular, on the presence of both octahedral and tetrahedral
Co2+ ions, as described for cobalt cordierites.39 Thus, smalt
produces a reduction peak at -0.14 V (Figure 4a). Upon heating
there is certain tetrahedral/octahedral interconversion, as de-
scribed in the literature,26 so that an additional signal at -0.50 V
appears (Figure 4c). For cobalt blue, where both tetrahedral and
octahedral Co2+ ions coexist, but in a spinel-type structure, far
from the glass smalt environment, two reduction waves at +0.20
and -0.50 V are recorded (Figure 4b).
These electrochemical processes can be described on the basis
of the model developed by Lovric, Oldham, Scholz et al. for the
electrochemistry of nonconducting solids attached to inert
electrodes.40-43 Here, the redox reaction is initiated at the particle/
electrolyte/electrode three-phase junction and propagates throughthe solid particle via electron hopping and proton insertion into
the solid lattice. It should be noted that, for the studied systems,
the overall reduction process can be controlled not only by the
kinetics of the proton insertion or electron-transfer process but
also by the kinetics of the nucleation and nuclii growth involved
in the formation of the metal.
(39) Dom enech, A. ; Torres, F. J.; Alarcon, J. J. Solid State Electrochem. 2004, 8,
127-137.
(40) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1997, 1, 108-113.
(41) Lovric, M.; Scholz, F. J. Solid State Electrochem. 1999, 3, 172-175.
(42) Oldham, K. B. J. Solid State Electrochem. 1998, 2, 367-377.
(43) Schroder, U.; Oldham, K. B.; Myland, J. C.; Mahon, P. J.; Scholz, F. J. Solid
State Electrochem. 2000, 4, 314-324.
Figure 2. CVs of PIGEs modified with (a) azurite (K10200), (b)
malachite (K10300), and (c) smalt (K10000), immersed into 0.50 Mphosphate buffer, pH 7.4. Potential scan rate 50 mV/s. Figure 3. SQWVs for (a) azurite (K10200), (b) cuprite, (c) verdigris
(K44450), and (d) atacamite, in contact with 0.50 M phosphate buffer,pH 7.4. Potential scan initiated at +0.45 mV in the negative direction.
Potential step increment 4 mV; square wave amplitude 15 mV;frequency 2 Hz.
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For our purposes, the relevant point to emphasize is that the
electrochemical response is phase-dependent, allowing for the
characterization of solid compounds. In view of the close vicinity
between the voltammetric curves for the different copper and
cobalt species, multiparametric fitting, and multivariate regression
procedures were tested. For these purposes, a series of shape-
dependent parameters, which can be easily measured for the main
reduction peak, were taken: (i) peak potential, Ep, (ii) onset
potential obtained from the intersection of the almost linear portion
of the peak with the baseline for current measurement, Eon, and
(iii) peak-to-half peak potential separation, Ep(I)-Ep/2, were used.
Pertinent data are summarized in Table 1. Hierarchical cluster
analysis, however, indicated that although such parameters should
provide a distinction between the studied species, the percentages
of difference were small (See Supporting Information).
Tafel Analysis. In view of the close vicinity between the
voltammetric curves for azurite, malachite, verdigris, smalt, and
the specimens of the atacamite group, Tafel analysis of voltam-
metric curves was used in order to obtain more discriminating
parameters and quantitative data for pigments.
As originally studied by Reinmuth for irreversible electron-
transfer processes involving species in solution phase,44,45 the
rising portion of voltammetric curves can be approached, in several
cases, to a exponential variation of the current with the applied
potential. In particular, this assumption applies for linear scan
voltammograms of reversible and irreversible electron-transfer
processes involving species attached to the electrode surface.46
In this last case, the current satisfies
where o represents the surface concentration of the electroactive
species, Rna the product of the coefficient of electron transfer by
the number of electrons involved in the rate-determining step, kothe electrochemical rate constant at the zero potential, and the
other symbols have their usual meaning. Extension of this
treatment to SQWV is complicated by the recognized influence
of potential step increment and square wave amplitude in theshape of voltammetric curves obtained by this technique, so
that numerical solutions of diffusion equations rather than analyti-
cal ones are in general used. In the case of reversible electron
transfer between species in solution, as long as the square wave
amplitude, ESW, is lower than 0.5RT/nF, a condition easily
accomplished under the usual experimental conditions, the net
current flowing during the anodic and cathodic half-cycles can
be represented, following Ramaley and Krause by an expression
of the type:47,48
fbeing the square wave frequency, C a numerical constant, and
the other symbols having their customary meaning. For a
reduction process, both eqs 2 and 3 can be reduced to a linear
variation of lni on E when the applied potential is clearly larger
than the formal electrode potential, Eo; i.e., at the foot of the
voltammetric peak. Using reported numerical solutions for the
diffusion equations,49-53 a similar Tafel-type relationship can be
approximated, under favorable conditions, in SQWVs for oxida-
tive/reductive dissolution of species immobilized on the electrode
surface,49-51quasi reversible surface processes,52 and surface-
confined electrochemical reactions.53
(44) Reinmuth, W. H. Anal. Chem. 1960, 32, 1891-1892.(45) Buck, R. P. Anal. Chem. 1964, 36, 947-949.
(46) Bard, A. J.; Faulkner, L. R. Electrochemical methods; John Wiley & Sons:
New York, 1980; pp 521-525.
(47) Ramaley, L.; Krause, M. S.; Jr. Anal. Chem. 1969, 41, 1362-1365.
(48) Krause, M. S. Jr.; Ramaley, L. Anal. Chem. 1969, 41, 1365-1369.
(49) Lovric, M.; Komorsky-Lovric, S. J. Electroanal. Chem. 1988, 248, 239-
253.
(50) Lovric, M.; Komorsky-Lovric, S.; Bond, A. M. J. Electroanal. Chem. 1991,
319, 1-18.
(51) Komorsky-Lovric, S.; Lovric, M.; Bond, A. M. Anal. Chim. Acta 1992, 258,
299-305.
(52) ODea, J. J.; Osteryoung, J. G. Anal. Chem. 1993, 65, 3090-3097.
(53) Komorsky-Lovric, S.; Lovric, M. Anal. Chim. Acta 1995, 305, 248-
255.
Figure 4. SQWVs for (a) smalt (K10000), (b) cobalt blue (K45700),and (c) a smalt specimen heated at 600 C for 24 h in contact with
0.50 M phosphate buffer, pH 7.4. Potential scan initiated at +0.45mV in the negative direction. Potential step increment 4 mV; square
wave amplitude 15 mV; frequency 2 Hz.
i ) nFAkooe exp(-RnaF(E- Eo)/RT)
exp[RTko
RnaFvexp(-RnaF(E- E
o)/RT)] (2)
idif) Cn2F2AD1/2cESWf
1/2
RT1/2exp(nF(E- Eo)/RT)
[1 + exp(nF(E- Eo)/RT)]2(3)
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Although there is no disposal of a detailed model for describing
reduction processes such as represented by eq 1, the Grygar
model54 for reductive dissolution of solids provides a possible
approach. Assuming that both linear scan and square wave
voltammograms behave similarly, the current at the beginning of
the voltammetric peak can tentatively be represented as
where qo represents the total charge involved in the complete
reaction of the electroactive solid. Equation 4 predicts a linear
dependence of lni on E whose slope depends on the phase-
characteristic coefficient Rna, while the ordinate at the origin
depends on the electrochemical rate constant and the net amount
of depolarizer deposited on the electrode regardless of the
granulometry of the solid.55,56 In order to eliminate the contribution
of this last quantity, it is convenient to use normalized currents.This is possible because in both linear scan46,54 and square wave
voltammetries49-51 the peak current for the reduction of surface-
immobilized species can be approached by an expression of the
type
Hbeing an electrochemical coefficient of response characteristic
of the electrochemical process and the electrode area and the
potential scan rate (LSV) or the square wave frequency (SQWV).
Combining eqs 4 and 5, one obtains
Here, both the generalized Tafel slope (SL ) RnaF/RT) and the
ordinate at the origin (OO ) ln(koRT/HRnaF)) become charac-
teristic of the solid analyte regardless of the amount of sample
deposited on the electrode.
For a two-component system, one can write
If RjnajFE/RT , 1 (j ) X,Y), one can use the approximation e-z
1- z, so that the above equation reduces to
If voltammetric peaks for X and Y are strongly overlapped, a
unique peak will be recorded, the peak potential being approached
by
Thus, the i/ip ratio will be given by the approximate expression:
This equation fits to a linear dependence of ln(i/ip) on E so that
the slope and the ordinate at the origin will be intermediate
between those obtained for the X and Y components separately
via eq 7.
For quantitation of a mixture of X plus Y, one can combine
the Tafel dependence predicted by eq 10 for that mixture, with
the Tafel dependence described by eq 5, applied separately for
(54) Grygar, T. J. Electroanal. Chem. 1996, 405, 117-125.
(55) Grygar, T. J. Solid State Electrochem. 1998, 2, 127-136.
(56) Bakardjieva, S.; Bezdicka, P.; Grygar, T.; Vorm, P. J. Solid State Electrochem.
2000, 4, 306-333.
Table 1. Electrochemical Data for Reference Pigmenting Materiala
specimenEon
(mV)Ep
(mV)Ep-Ep/2
(mV) Tafel SL (mV -1 ) Tafel OO r2
azuriteb +30 ( 5 -110 ( 5 90 ( 5 -0.0115 ( 0.0004 -1.00 ( 0.02 0.9996malachiteb +35 ( 5 -105 ( 5 70 ( 5 -0.0160 ( 0.0005 -1.30 ( 0.02 0.9997atacamitec +15 ( 5 -155 ( 5 90 ( 5 -0.0154 ( 0.0005 -1.66 ( 0.03 0.9995botallackitec +20 ( 5 -160 ( 5 90 ( 5 -0.0196 ( 0.0005 -1.82 ( 0.04 0.9994clinoatacamitec -65 ( 5 -165 ( 5 60 ( 5 -0.0203 ( 0.0005 -2.62 ( 0.04 0.9996verdigrisc +30 ( 5 -150 ( 5 120 ( 5 -0.0195 ( 0.0005 -1.26 ( 0.02 0.99998smaltb +5 ( 5 -155 ( 5 95 ( 5 -0.0089 ( 0.0004 -1.57 ( 0.02 0.9996
cobalt bluec -
60(
5-
250(
10 120(
5-
0.0140(
0.0005-
2.98(
0.08 0.9993Azurite (200 C) +35 ( 5 -105 ( 5 90 ( 5 -0.0112 ( 0.0004 -1.00 ( 0.02 0.9994smalt (600 C) +15 ( 5 -145 ( 5 90 ( 5 -0.0137 ( 0.0005 -1.53 ( 0.02 0.9995
a From SQWVs at specimen-modified PIGEs immersed into 0.50 M phosphate buffer, pH 7.4. Initiated at +0.65 V in the negative direction.Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz. bMean value for specimens listed in the Experimental Section,c Mean values for five independent measurements on the same material.
i qoko exp(-RnaF
RTE) (4)
ip ) H(RnaF
RT )qo (5)
ln(i/ip) ) ln(koRT
HRnaF) -
RnaF
RTE (6)
i qoX
koX
exp
(-RXnaXFE
RT )+ q
oYk
oYexp
(-RYnaYFE
RT )(7)
i (qoXkoX+ qoYkoY)
exp[-(qoXkoXRXnaX+ qoYkoYRYnaY)(FE/RT)
qoXkoX+ qoYkoY ] (8)
ip HX(RXnaXF
RT )vqoX+ HY(RYnaYF
RT )vqoY (9)
i
ip
(qoXkoX+ qoYkoY)RT
(HXRXnaXqoX+ HYRYnaYqoY)nFv
exp[-(qoXkoXRXnaX+ qoYkoYRYnaY)(FE/RT)
qoXkoX+ qoYkoY ] (10)
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the individual components. As a result, the X to Y molar ratio, g
()qoX/qoY) can be expressed as
In this equation, SLM represents the Tafel slope for the mixture
of X plus Y, and SLX, SLY, the Tafel slopes for the individual
components. This equation enables the a determination ofgfrom
Tafel representations providing that the quotients between the
individual electrochemical rate constants, koX and koY, and the
electron-transfer coefficients, RXnaX, RYnaY, are known.
Considering eq 7, these ratios can be directly obtained from
the normalized Tafel ordinates at the origin, OOX, OOY, and the
Tafel slopes for the individual components, so that, finally
In view of the close similarity between the predictions for SQWV
and LSV concerning the Tafel-type behavior to be expected in
the initial portion of voltammetric peaks, it will be assumed that
eq 10 also applies for SQWVs of sparingly soluble electroactive
solids mechanically attached to inert electrodes. On the basis of
that assumption, the above treatment can be taken as a semiem-
pirical approach whose application should be confirmed by
experimental data.
Analysis of Reference Materials. Figure 5 shows generalized
Tafel plots of ln(i/ip ) versus E for azurite, malachite, atacamite,
and verdigris. In all cases, an excellent linearity was obtained (see
Supporting Information) for potentials between 200 and 100 mV
before the corresponding voltammetric peak. The values of SL
and OO determined for the reference materials are listed in Table
1. Confirming the suitability of the Tafel analysis previously
described, current-potential curves in the rising portion of SQWV
peaks for all the studied pigments fitted well to linear ln(i/ip) on
Edependences, with correlation coefficients larger than 0.999 in
all cases (see Table 1 and Supporting Information).
Figure 6 presents a two-dimensional diagram in which SL and
OO were used as variables. As can be seen in this figure, data
points representative for the different species fall in localized and
well-separated regions of the diagram.
In order to test the validity of the proposed methodology for
analysis of mixtures, different specimens consisting of azurite +
malachite, azurite + atacamite, and azurite + smalt mixtures were
prepared. In order to approach the conditions of paint samples, a
second series was prepared incorporating CaCO3 as diluent (50%
w/w). In all cases, the voltammetric responses of the specimens
were similar to those of the reference materials. Tafel analysis of
the rising portion of the main reduction peak provided linear ln(i/
ip ) versus E plots (correlation coefficients larger than 0.999; see
Supporting Information), the values of SL and OO being inter-mediate between those determined for the parent materials
separately. The corresponding data points are also depicted in
Figure 6. Interestingly, no significant differences were obtained
between pigment mixtures and pigment + CaCO3 ones.
For these systems, quantitation using Tafel parameters pro-
vided results in satisfactory agreement with the nominal composi-
tion of the azurite + malachite mixtures, with standard deviations
lower than 5% for all compositions. For azurite + atacamite and
azurite + smalt mixtures, however, some major deviations (10-
15%) were obtained from nominal compositions. A reason for this
can be obtained on considering data in Table 1. Thus, while for
azurite and malachite the main reduction peak possesses identical
peak potential, the peak potentials for azurite and atacamite (and
for azurite and smalt) differ in 50-100 mV; i.e., one of the
conditions for quantitation using Tafel analysis does not apply
strictly.
This situation can be summarized on considering that the peak
current in these mixtures will be lower than the sum of the peak
currents for the separated components (eq 7), thus distorting the
i/ip values with respect to those for exactly coincident voltam-
metric peaks. Apart from this, eventual interactions between the
components during electrochemical turnovers may distort volta-
mmetric responses, as reported for iron and manganese oxide
Figure 5. Generalized Tafel plots for azurite (rhombs), malachite
(solid squares), atacamite (triangles), and verdigris (open squares)
from SQWV data in phosphate buffer. Potential step increment, 4mV; square wave amplitude, 25 mV; frequency, 5 Hz.
g)
(
SLY- SLM
SLM-
SLX)(
koY
koX)(
RYnaY
RXnaX)
(11)
g)
(SLY- SLM
SLM - SLX)(exp(OOY)
exp(OOX))(SLY
SLX)(12)
Figure 6. Two-dimensional Tafel slope vs Tafel ordinate at the origindiagram for pigmenting materials studied here (solid rhombs) and
azurite plus malachite (squares) and azurite plus smalt (triangles)mixtures. From SQWVs at specimen-modified PIGEs immersed into
0.50 M phosphate buffer, pH 7.4. initiated at +0.65 V in the negativedirection. Potential step increment 4 mV; square wave amplitude 20
mV; frequency 5 Hz.
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materials.14To account for this effect, theoretical working current-
potential curves for azurite + atacamite and azurite + smalt
mixtures were obtained from experimental voltammograms of
azurite, atacamite, and smalt. The resulting Tafel parameters were
close to those experimentally determined for the corresponding
mixtures. A representation of theoretical curves and experimental
data points for azurite plus smalt mixtures is presented in Figure
7. Here, experimental data agree with theoretical ones taking apeak potential separation of 50 mV.
Analysis of Real Samples. Chemical and morphological
analysis by SEM/EDX of the studied samples informed on the
pigment distribution in the different paint strata as well as on the
elemental composition of the different grains and crystalline
aggregates identified on the secondary and backscattered electron
images of the cross-section of the studied samples. It should be
noted that samples, in general, consisted of mixtures of several
pigments, which often appeared applied in different strata. Most
of the studied samples exhibited X-ray emission lines characteristic
of smaltKR(Si), KR(K), KR(As), K(As), KR(Co), and K (Co) and
copper pigments KR(Cu) and K(Cu). Interestingly, black color
was observed in the cross section of the samples when they were
observed with the light microscope in some grains and crystalline
aggregates Cu-rich suggesting the probable transformation of the
original pigment in tenorite, a black CuO. Red earths, Naples
yellow, green earth, and iron oxide red were others of the
pigments appearing in the set of samples studied corresponding
to the brownish-green and blue areas of the vault (See Supporting
Information.).
SQWVs of samples from the Sant Joan del Mercat church can
be divided into three morphological groups, respectively repre-
sented in Figure 8 by samples: (a) PV8B, (b) PV7, and (c) PV1.
For blackened samples PVB7, PVB8, and PVB9 (Figure 8a), a
prominent reduction peak at-0.60 V appears, preceded by a less
intense peak at-0.10 V. For samples PV3b, PV7, PV8, PA3, PA4b,
PA5b, PA7, PV10, and PV11; a main reduction peak located
between -0.10 and -0.16 V is accompanied by broad signal at
-0.60 V, as shown in Figure 8b. Finally, samples PV1, PV2, PV3,
PV4, PV5, and PV8b show (see Figure 8c) a main reduction peak
at -0.12 V, followed by weak signals at -0.20 and -0.60 V. A
similar response was obtained for samples U7 and U11. In several
samples, an additional reduction peak at -0.55 V, accompanied
by a stripping anodic peak at -0.48 V, representative of Naples
yellow,57 was also recorded (see Supporting Information) in
agreement with SEM/EDX data.
SQWVs performed on scanning the potential from-
0.85 V inthe positive direction also provide relevant information for analyti-
cal purposes. This can be seen in Figure 9, where the voltammetric
responses for (a) azurite, (b) sample PV8b, (c) smalt, and (d)
sample PA5b are shown. Copper pigments yield a unique stripping
peak at-0.05 V whereas cobalt pigments produce a main anodic
peak at+0.02 V accompanied by overlapping peaks at -0.02 and
+0.22 V. SQWV in Figure 9b is representative of the response
obtained for samples PV8b, PA7, U7, and U11, consisting of only
one single stripping peak near to 0.0 V, characteristic of copper.
(57) Domenech, A. ; Domenech, M. T.; Mas, X. Talanta 2007, 71, 1569-1579.
Figure 7. Theoretical variation of the Tafel ordinate at the origin
for azurite + smalt mixtures taking peak potential separations (from
upper to below) of 0, 50, 75, and 100 mV. Data points correspond tosynthetic specimens containing pure azurite; pure smalt; and 70:30,50:50, and 30:70 (%, w/w) azurite-smalt mixtures. From SQWVs at
specimen-modified PIGEs immersed into 0.50 M phosphate buffer,pH 7.4 initiated at +0.65 V in the negative direction. Potential step
increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.
Figure 8. SQWVs for samples: (a) sample PVB9, (b) PV7, and
(c) PV1 immersed into 0.50 M phosphate buffer, pH 7.4. Potentialscan initiated at +0.45 or +0.65 mV in the negative direction. Potential
step increment 4 mV; square wave amplitude 15 mV; frequency 2Hz.
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Samples PV3b and PA4b should be composed by smalt while all
other samples showed a voltammetric profile that can be described
in terms of the cobalt stripping or as a superposition of the
stripping processes for cobalt and copper, as can be seen in Figure
9d for sample PA5b. In this voltammogram, an additional stripping
peak appears at-0.48 V, due to the presence of Naples yellow in
the sample. The foregoing set of data indicates that copper pig-
ment and copper+ cobalt pigment mixtures exist in the samples.
The prominent signal at-0.60 V in blackened samples can
unambiguously be attributed to tenorite (CuO), as can be assumed
from a comparison between SQWVs in Figure 8a with those for
tenorite and tenorite plus azurite mixtures (see Supporting
Information). Formation of tenorite from copper pigments should
occur during the gunfire episode suffered by the paintings, as
clearly suggested by thermochemical data. Thus, upon heating,
azurite and malachite undergo loss of CO2 and water at 345 C to
give CuO. Further heating yields Cu2O at 840 C.58-60 In turn,
copper acetate dehydrates at 190 C with partial decomposition
at 220 C forming CuO accompanied by small amounts of Cu2O
and Cu3O4, further oxidized in air at 400 C.
Tafel analysis of the rising portion of the reduction peak at
-0.10 V produced linear log(i/ip ) versus E plots for all thestudied samples, as indicated by statistical parameters (correlation
coefficients larger than 0.999; see Supporting Information). The
corresponding SL and OO values are listed in Table 2. Insertion
of such parameters into a two-dimensional diagram is illustrated
in Figure 10. Here, one can observe that (i) data points for samples
U7 and U11 fall in the malachite region, (ii) data points for
blackened PVB7, PVB8, and PVB9 samples are located in a central
position in the diagram, distanced from smalt and copper pig-
ments, and (iii) all other samples are located in a region between
azurite and smalt.
These results suggest that samples PV3b, PV7, PV8, PA3,
PA4b, PA5b, PA7, PV1, PV2, PV3, PV4, PV5, PV8b, PV10, and
PV11 are constituted by azurite, smalt, and azurite + smaltmixtures, while samples U7 and U11 are composed of malachite.
These results were confirmed by Raman spectroscopy. Azurite
and smalt were identified on the basis of their characteristic
vibrations at 402, 1430/1459, and 1577 cm-1for azurite and 1086,
475, 430, 377, 358, and 1370 cm-1 for smalt, whereas malachite
displays characteristic signal at 433 cm-1, all in agreement with
the literature.30-35Additionally, the majority of the studied samples
showed carbon signatures, whether arising from the fire or from
addition as a darkening agent to other pigments, all being
assignable to vegetable- or plant-based origin.24
In order to test the possible influence of thermal stress in the
voltammetric response of the pigments, two additional series of
specimens were prepared upon heating azurite (K10200) and smalt
(K10010) in furnace during 24 h at 200, 400, and 600 C. As
expected, up to 400 C, azurite was converted into tenorite, as
denoted by blackening of the sample. For the sample treated at
200 C, the voltammogram was essentially indistinguishable from
that of the parent azurite pigment, with coincident Tafel param-
eters. Pertinent data are summarized in Table 1. For smalt, only
a light change in the hue of the sample was obtained after thermal
treatments. Remarkably, although the general profile of the
voltammogram remained unchanged, Tafel parameters for the
reduction peak at-0.15 V changed significantly with the temper-
ature. Insertion of the corresponding data points into the SL versus
OO diagram (see Figure 10) reveals that data points for blackened
samples become now intermediate between the regions of azurite
and smalt heated at 600 C.
The smalt/azurite ratio was determined from Tafel parameters
using the proposed procedure. Pertinent data are summarized in
Table 3. Remarkably, data points for samples PVB7, PVB8, PVB9,
PA3, PV3b, PV7, PV8, PA4b, PA5b, PV1, PV2, PV3, PV4, and PV5
(58) Frost, R. L.; Ding, Z.; Kloprogge, J. T.; Martens, W. V. Thermochim. Acta
2002, 390, 133-144.
(59) Kiseleva, I. A.; Ogorodova, L. P.; Melchakova, L. V.; Bisengalieva, M. R.;
Becturganov, N. S. Phys. Chem. Miner. 1992, 19, 322-333.
(60) Mansour, S. A. A. J. Therm. Anal. 1996, 46, 263-274.
Figure 9. SQWVs for (a) azurite (K10200), (b) sample PV8b, (c)
smalt (K10010), and (d) sample PA5b, in contact with 0.50 Mphosphate buffer, pH 7.4. Potential scan initiated at -0.85 mV in the
positive direction. Potential step increment 4 mV; square waveamplitude 15 mV; frequency 2 Hz.
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cover a relatively wider region between azurite and smalt as
depicted in Figure 11. Here, samples PV1-PV5, PV7, and PV8
can be assigned to azurite plus smalt mixtures because data points
fall in the Tafel diagram close to the theoretical working SL versus
OO curve for a peak potential separation of 50 mV. Quantitation
using Tafel parameters (eq 10) provides smalt molar percentages
relative to the azurite + smalt mixture grouped in few dosages:
pure azurite, pure smalt, and azurite plus smalt mixtures concen-
trated in smalt molar percentages of 55, 72, and 85%. Consistently,
application of this method to blackened samples using Tafel
parameters for azurite and smalt heated at 600 C (see data in
Table 1) provide smalt percentages just in the aforementioned
dosages (see Table 3).
These results suggest that the painter used several fixed
azurite + smalt dosages in order to obtain the desired chromatic
effect in different areas of the frescoes. In view of the consistent
use of azurite + smalt mixtures by Palomino, one can conclude
that samples U7 and U11, where malachite is the copper pigment,
should be attributed to Guillo.
Interestingly, samples PV3b, PA4b, and PA5 fall in a region of
the SL versus OO diagrams in Figures 10 and 11 clearly separated
from the theoretical curve for a peak potential separation of 50
mV. This can mainly be attributed to the following: i) the use of
different pigment sources by the painter and/or their alteration
Table 2. Electrochemical Data for Samples from the Sant Joan del Mercat Church, from SQWVs at
Specimen-Modified PIGEs Immersed into 0.50 M Phosphate Buffer, pH 7.4a
sampleEon
(mV)Ep
(mV)Ep-Ep/2
(mV) Tafel SL(mV -1 ) Tafel OO
PVB7 +50 ( 5 -160 ( 5 110 ( 5 -0.0126 ( 0.0005 -1.30 ( 0.03PVB8 +40 ( 5 -155 ( 5 100 ( 5 -0.0128 ( 0.0005 -1.24 ( 0.03PVB9 +45 ( 5 -150 ( 5 105 ( 5 -0.0133 ( 0.0005 -1.27 ( 0.04PA3 +45 ( 5 -155 ( 5 110 ( 5 -0.0098 ( 0.0005 -1.03 ( 0.03PA4b +35 ( 5 -160 ( 5 110 ( 5 -0.0088 ( 0.0005 -1.02 ( 0.03PA5 +65 ( 5 -155 ( 5 120 ( 5 -0.0095 ( 0.0005 -0.87 ( 0.03
PA7 +40 ( 5 -115 ( 5 100 ( 5 -0.0111 ( 0.0005 -1.11 ( 0.03PV3b +35 ( 5 -165 ( 5 115 ( 5 -0.0089 ( 0.0005 -1.12 ( 0.04PV7 +20 ( 5 -155 ( 5 105 ( 5 -0.0096 ( 0.0005 -1.41 ( 0.06PV8 +20 ( 5 -155 ( 5 95 ( 5 -0.0098 ( 0.0005 -1.24 ( 0.04PV1 +30 ( 5 -145 ( 5 95 ( 5 -0.0105 ( 0.0005 -1.03 ( 0.03PV2 +40 ( 5 -150 ( 5 105 ( 5 -0.0100 ( 0.0005 -1.33 ( 0.03PV3 +35 ( 5 -145 ( 5 100 ( 5 -0.0106 ( 0.0005 -1.08 ( 0.03PV4 +30 ( 5 -150 ( 5 110 ( 5 -0.0101 ( 0.0005 -1.26 ( 0.04PV5 +40 ( 5 -155 ( 5 95 ( 5 -0.0103 ( 0.0005 -1.16 ( 0.04PV8b +35 ( 5 -110 ( 5 90 ( 5 -0.0115 ( 0.0005 -1.01 ( 0.03PV10 +40 ( 5 -150 ( 5 100 ( 5 -0.0095 ( 0.0005 -1.06 ( 0.03PV11 +40 ( 5 -150 ( 5 100 ( 5 -0.0105 ( 0.0005 -1.18 ( 0.03U7 +50 ( 5 -105 ( 5 80 ( 5 -0.0164 ( 0.0005 -1.32 ( 0.03U11 +45 ( 5 -110 ( 5 70 ( 5 -0.0170 ( 0.0005 -1.34 ( 0.03
a Initiated at+0.65 V in the negative direction. Potential step increment 4 mV; square wave amplitude 20 mV; frequency 5 Hz.
Figure 10. Two-dimensional Tafel slope vs Tafel ordinate at the
origin diagram for samples from the Sant Joan del Mercat church.From SQWVs at specimen-modified PIGEs immersed into 0.50 M
phosphate buffer, pH 7.4 initiated at +0.65 V in the negative direction.
Potential step increment 4 mV; square wave amplitude 20 mV;frequency 5 Hz. Squares, dark samples; triangles, strongly blackened
samples; rhombs, samples whose attribution to Palomino wasuncertain.
Table 3. Quantitative Data for Samples from the Sant
Joan del Mercat Church Derived from Electrochemical
Data
sample
tenorite/(azurite+ smalt)
(w/w) ratio
% of smalt(mol/mol)from Tafel
analysis
% of smalt(w/w) from
peakpotentials
PVB7 0.27 ( 0.04 59 ( 4 78 ( 11PVB8 0.37 ( 0.04 67 ( 4 78 ( 11PVB9 0.71 ( 0.06 86 ( 2 67 ( 9PA3 0.10 ( 0.02 81 ( 3 78 ( 11PA4b 0.08 ( 0.02 100 ( 1 100 ( 12PA5b 0.10 ( 0.02 88 ( 2 78 ( 11PA7 0.09 ( 0.02 0 ( 1 6 ( 6
PV3b 0.14 ( 0.03 100 ( 1 100 ( 12PV7 0.08 ( 0.02 86 ( 2 78 ( 11PV8 0.12 ( 0.03 81 ( 3 78 ( 11PV1 - 58 ( 4 50 ( 8PV2 - 76 ( 3 67 ( 9PV3 - 54 ( 4 50 ( 8PV4 - 73 ( 2 67 ( 9PV5 - 75 ( 3 78 ( 11PV8b - 0 ( 1 0 ( 4PV10 0.24 ( 0.04 72 ( 3 78 ( 11PV11 0.12 ( 0.03 53 ( 4 50 ( 8
2714 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008
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by effect of thermal stress, and (ii) the presence of a disturbing
matrix. The issue i appears to be in contravention with the above
data, because points for samples PV3b, PA4b, and PA5 separate
not only from the azurite + smalt region but also from the azurite
+ heated smalt one. With regard to the issue ii, it should be noted
that experiments with CaCO3 plus pigment for both azurite and
smalt produced Tafel responses essentially identical to those
displayed by pure pigments. In view of this, a possible option
should be the presence of any remaining binding media in suchsamples, just obtained from zones of the frescoes far from the
central axis of the vault. Since in these zones the thermal stress
during the gunfire was relatively smooth (vide infra), one can
conjecture that the rest of binding media remain, thus modifying
the voltammetric response of the pigments. This is consistent with
prior observations on lead pigments.57 Analyses carried out by
means of gas chromatography/mass spectrometry have evidenced
the presence of amino acids in samples containing copper
pigments in a few Palomino samples.24 This result suggests that
Antonio Palomino could bind pigments with some protein-
aceous medium in order to prevent their alteration from the
strongly alkaline medium provided by the Ca(OH)2 formed in the
fresco technique. These results are in agreement with therecommendations published by the artist in his treatise on pic-
torial techniques El Museo Pictorico y Escala Optica, published in
1769.61
Quantitation can be completed with the determination of the
amount of tenorite relative to the azurite + smalt mixture in
samples. This was estimated from the peak areas for the azurite
+ smalt peak at -0.10 V and the tenorite peak at -0.60 V. Since
the specific response of all materials was not identical, a calibration
graph, constructed from electrochemical data for azurite + smalt
+ tenorite mixtures was used. The resulting graph is shown in
Figure 12, while the calculated percentages of tenorite are shown
in Table 3.
Crossing all these data with the position of the samples in the
nave provides a scene for the gunfire attack suffered by the
paintings. Thus, blackened samples PVB7, PVB8, and PVB9,
having high tenorite content, were placed along the central axis
of the vault. Crossing the foregoing set of data with those derived
from the analysis of earth pigments,25 one can conclude that the
central part of the vault reached temperatures between 600 and
650 C during the gunfire. Samples with minor amounts of tenorite
provided from the lateral zones of the nave probably experiencedtemperatures in the 350-460 C range. Samples from paintings
near the lunettes, for which no significant tenorite signals were
recorded, reached probably temperatures of260 C.
CONCLUSIONSTafel analysis of voltammetric curves can be used for quantify-
ing components in solid micro- and submicrosamples, where
strongly overlapping peaks for two electroactive components are
recorded, taking a semiempirical approach based on the assump-
tion that the involved electrochemical processes approach this
kind of current-potential dependence in a reasonably wide range
of conditions. Experimental SQWV data for the reduction of
copper and cobalt pigments and samples from the Sant Joan del
Mercat church in Valencia satisfied Tafel-type equations. Two-
dimensional diagrams, using Tafel slope and ordinate at the origin,
calculated from the rising portion in current/potential curves,
enable the identification of individual components in
such samples. This methodology permits the following: (i) char-
acterization of Palomino paintings, with distinction between
azurite, smalt, or azurite + smalt compositions; (ii) a satisfactory
discrimination between the paintings executed by Antonio Palo-
mino from those others from Vicente Guillo-Barcelo, where,
in contrast to that found for Palomino paintings, malachite was(61) Palomino, A. El museo pictorico y escala optica; Translation from the original
published in 1759. Aguilar: Madrid, 1947; p 745.
Figure 11. Detail of the SL vs OO diagram in the azurite + smalt
region for samples from the Sant Joan del Mercat church. FromSQWVs at specimen-modified PIGEs immersed into 0.50 M phos-
phate buffer, pH 7.4 initiated at +0.65 V in the negative direction.Potential step increment 4 mV; square wave amplitude 20 mV;
frequency 5 Hz.
Figure 12. Calibration graph for estimating the tenorite/(azurite +
smalt) ratio in thermally altered samples from the Sant Joan del
Mercat church using the quotient between the peak currents forvoltammetric signals at -0.10 and -0.60 V.
Analytical Chemistry, Vol. 80, No. 8, April 15, 2008 2715
http://pubs.acs.org/action/showImage?doi=10.1021/ac7024333&iName=master.img-011.png&w=239&h=205http://pubs.acs.org/action/showImage?doi=10.1021/ac7024333&iName=master.img-010.png&w=225&h=2268/3/2019 Domnech, A. et al. Study cobalt and copper pigments in damaged frescoes. 2008
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the only copper pigment used; (iii) quantification of pigment
mixtures and determination of the extent of alterations in paint
specimens in highly damaged frescoes. This methodology is
limited, however, by the confidence level of the aforementioned
Tafel approximation.
Voltammetric data, confirmed by SEM/EDX and Raman
spectroscopy data, indicated that azurite, very frequently ac-
companied by smalt, was used by Palomino in the frescoes of the
Sant Joan del Mercat church. The composition of azurite-smalt
mixtures was relatively homogeneous, including applications ofessentially pure azurite to mixtures containing smalt propor-
tions 60% (w/w) of smalt until pure smalt. This result informs
on the technique used by the artist: azurite and azurite + smalt
mixtures were used preferentially in some dosages by the painter
in order to obtain the desired chromatic effect. As a result of the
gunfire attack suffered by the frescoes in the past, tenorite was
formed, thus producing considerable chromatic changes in the
paint.
This study illustrates the capabilities of the voltammetry of
microparticles for obtaining information potentially interesting for
archaeometry, conservation, and restoration of cultural goods from
solid samples in relatively complicated systems.
ACKNOWLEDGMENT
Financial support is gratefully acknowledged from the Gener-
alitat Valenciana GVAE07/140 and ACOMP/2007/138 Projects
and the MEC Projects CTQ2005-09339-C03-01, 02 and CTQ2006-
15672-C05-05/BQU, which are also supported with ERDEF funds.
The authors thank Dr. Pilar Roig Picazo and Dr. Ignacio Bosch
Reig art conservator and architect in charge of the conservation
project of the San Joan del Mercat church. Financial support of
this conservation project is kindly acknowledged from Lubasa and
Fundacion Aguas de Valencia. The authors thank Mr. Manuel
Planes Insausti and Dr. Jose Luis Moya Lopez for technical
assistance.
SUPPORTING INFORMATION AVAILABLE
Additional information as noted in text. This material is
available free of charge via the Internet at http://pubs.acs.org.
Received for review November 28, 2007. AcceptedJanuary 28, 2008.
AC7024333
2716 Analytical Chemistry, Vol. 80, No. 8, April 15, 2008