Doménech, 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

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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.

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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)

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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)

http://pubs.acs.org/action/showImage?doi=10.1021/ac7024333&iName=master.img-003.png&w=166&h=407


7/13

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)



8/13

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.



9/13

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.



10/13

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.



11/13

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



12/13

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.



13/13

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


Documents

Doménech, A. et al. Study cobalt and copper pigments in damaged frescoes. 2008