2 3 New highlights on degradation process of verdigris from easel paintings

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Applied Physics AMaterials Science & Processing ISSN 0947-8396 Appl. Phys. ADOI 10.1007/s00339-014-8253-2

New highlights on degradation process ofverdigris from easel paintings

Carlotta Santoro, Karim Zarkout, Anne-Solenn Le Hô, François Mirambet,Didier Gourier, Laurent Binet, SandrinePagès-Camagna, et al.

1 23

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INVITED PAPER

New highlights on degradation process of verdigris from easelpaintings

Carlotta Santoro • Karim Zarkout • Anne-Solenn Le Ho • Francois Mirambet • Didier Gourier •

Laurent Binet • Sandrine Pages-Camagna • Solenn Reguer • Sigrid Mirabaud • Yann Le Du •

Pascal Griesmar • Nadege Lubin-Germain • Michel Menu

Received: 19 December 2013 / Accepted: 13 January 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Verdigris is a green copper organometallic

pigment, widely used in paintings during the fifteenth and

sixteenth centuries. With ageing, chromatic modifications

like browning or darkening can be observed on those green

painted layers. An original but crucial approach has been

developed based on the characterization of a reference

neutral verdigris pigment—anhydrous copper acetate—and

model samples, made of verdigris and linseed oil. Samples

have undergone artificial ageing (temperature, light) to

reproduce the color change effect. They were analysed

before and after accelerated ageing tests by a comple-

mentary set of classical techniques: colorimetry, electron

paramagnetic resonance, X-ray absorption spectroscopy,

and UV–visible absorption. Our experiments revealed that

the incorporation of the verdigris pigment in linseed oil

induces a transformation of the copper acetate bimetallic

structure, with the formation of monomeric species. These

monomers, however, are not directly responsible for the

darkening. The chromatic alteration seems instead linked

to the transient formation of Cu(I) in the copper complexes

of the pigment/oil system. This formation could be initiated

by ambient light absorption through ligand-to-metal charge

transfer, which favors the decarboxylation of the copper

complexes leading to the reduction of Cu(II) into Cu(I).

Moreover, dioxygen can react with partially decarboxyl-

ated dimers to form peroxy-Cu dimer complexes that can

be responsible for the darkening.

1 Introduction

Verdigris is a green copper organometallic pigment widely

used in paintings during the fifteenth and sixteenth centu-

ries for its transparency and brilliance. Verdigris is a

mixture of blue–green copper acetates with different

degrees of basicity and hydration [1, 2], obtained by the

reaction of pure copper, or copper alloys¸ with the acetic

acid contained in vinegar.

With ageing, chromatic alterations of this pigment,

namely browning, are often observed, modifying the visual

appearance of the paintings in a non-esthetic and irre-

versible way [3, 4].

Browning is, however, not systematic. As it has been

observed in some older paintings, like some from the

Middle-Ages [5], or in the presence of tempera as binder,

the pigment can keep its original color. Moreover, altered

and not altered areas can coexist in the same painting.

C. Santoro � K. Zarkout � A.-S. Le Ho (&) � F. Mirambet �D. Gourier � L. Binet � S. Pages-Camagna � Y. Le Du �M. Menu

C2RMF, Centre de Recherche et Restauration des Musees de

France, Palais du Louvre, Paris, France

e-mail: [email protected]

C. Santoro � N. Lubin-Germain

SOSCO, Laboratoire de Synthese Organique Selective et de

Chimie bioOrganique, EA 4505 CNRS-Universite de

Cergy-Pontoise, Cergy-Pontoise Cedex, France

K. Zarkout � A.-S. Le Ho � F. Mirambet � D. Gourier �L. Binet � S. Pages-Camagna � Y. Le Du � M. Menu

Chimie-ParisTech, Institut de Recherche de Chimie-Paris-IRCP,

UMR8247 CNRS, Paris Cedex 05, France

S. Reguer

Ligne DiffAbs, Synchrotron SOLEIL, Saint-Aubin, France

S. Mirabaud

INP, Institut national du patrimoine, Saint-Denis, France

P. Griesmar

SATIE, laboratoire Systeme de l’Application des Technologies

de l’Information et de l’Energie UMR 8029 CNRS, ENS,

Universite de Cergy-Pontoise, Cergy-Pontoise Cedex, France

123

Appl. Phys. A

DOI 10.1007/s00339-014-8253-2

Author's personal copy

Due to the complexity of this issue and despite several

studies [6–8] the mechanism involved in the color change

is still not fully understood.

Several hypotheses have been formulated. Gunn et al.

[6] correlates the color changes with reactions between the

oil binder, composed of terpenoid and/or poly-unsaturated

fatty acids, and the pigment, leading to the formation of

copper soaps. The formation of these copper organome-

tallic complexes could be responsible for a local molecular

rearrangement of the metal ion coordination structure (due

to hydration or a change in symmetry), which could induce

changes of the chromatic properties, as also observed by

Cartechini et al. [7].

Verdigris is very reactive with fatty and resinic acids

and can catalyze the oxidative degradation of linseed oil

films [8]. This reaction involves copper in Cu(I) and Cu(II)

oxidation states during redox cycles [8, 9]. Recent studies

seem to exclude the presence of copper(I) at the end of the

drying process [7, 10].

Other complementary and sometimes contradictory

hypotheses were formulated, such as poor stability with

other pigments, formation of inorganic copper oxides [11]

and effects of outer SO2 and NO2 pollutants [3, 4, 6, 12].

It is fundamental to unravel the different hypotheses

regarding the degradation process. Understanding the

reactivity of verdigris is in fact the key factor to evaluate

potential evolution of the damages through time and also to

propose new adapted conservation strategies, especially

when dealing with the question of the removal of those

altered layers.

In the present work an original but crucial approach was

developed, based on systems of increasing complexity. The

evolution of a reference neutral verdigris, anhydrous cop-

per acetate, has been studied during the different steps of a

painting’s life.

The analytical methodology is based on the combination

of different complementary techniques. XANES (X-ray

absorption near edge structure), EPR (electron paramagnetic

resonance), UV–Vis (ultraviolet–visible) absorption spec-

troscopy and colorimetric measurements have been per-

formed on pigment before and after mixing with the binder,

and on the samples before and after accelerated ageing.

These analytical steps allowed to relate chromatic variations

of the pigment with changes in the molecular structure.

2 Experimental

2.1 Sample description

The anhydrous acetate used to simulate verdigris (neutral

acetate) was supplied by Strem Chemicals [2]. Linseed oil

was provided by Laverdure (Paris, France).

The pure initial pigment has been mixed with oil (2 %

w/w, 100 mg of pigment in 4.9 g of binder) and the mixture

was spread on pre-polished glass slides using a Spin coater

(SPIN-150) to obtain 100–300 lm thickness samples. The oil

binder and pigment mixture was spread directly on the glass

support. The preparation layer was not simulated. This

choice was motivated by the detailed observation and anal-

yses by scanning electron microscopy with energy-dispersive

(SEM–EDS) of easel paintings from the twelfth to seven-

teenth century museum collections containing altered or not

altered acetate copper pigments. The alteration is localized

only near the surface of the cross sections obtained from

paintings, suggesting that the chemical composition of the

preparation layer does not directly influence the process. The

glass provides a solid and inert surface to work as a support.

However, to check the effect of the support, colorimetric and

UV–Vis measurements have also been performed on sample

spread on canvas (preparation layer made of glue and plas-

ter), naturally aged for 15 years. This sample will be named

‘‘15-year-old model sample’’.

The model samples spread over the glass slide have been

dried in air for 2 weeks before starting the ageing process.

Freshly prepared samples, before drying, will be referred to

as ‘‘Freshly prepared samples’’, 2 weeks dried samples will

be referred to as ‘‘Dried samples’’.

The thermal ageing was carried out for 21 days in an

oven (Bioblock Scientific 45001) at 60 �C and 1 atm, to

prevent or at least greatly reduce the presence of oxygen

and moisture. As the color changed after 3 days of artificial

ageing, XAS and EPR analysis were performed after this

time. The ‘‘Thermally aged’’ samples were dried for

14 days and then thermally aged for only 3 days. Never-

theless, the color measurements were collected at various

times (0, 2, 4, 9, 13 and 21 days) during the artificial

ageing to follow the evolution of the hue angle.

A light-induced ageing was also performed. The sam-

ples were exposed for 600 h in a chamber ageing test XLS

Sun ? Atlas, using four xenon lamps without filter to

irradiate between 300 and 800 nm with a total power of

765 W/m2. Despite the cooling system of the apparatus, the

temperature reached 35 �C, adding a potential partial effect

of thermal degradation.

As for thermal ageing, colorimetric measurements were

performed at different times (0, 3, 11, 19 and 25 days). The

sample called ‘‘Light aged’’ corresponds to a sample dried

for 14 days and then aged under light for 25 days.

2.2 Analytical techniques

2.2.1 UV–Vis absorption and colorimetry

The samples were analyzed with an Ocean Optics USB-

4000 spectrometer, using a quartz optical fiber and a xenon

C. Santoro et al.

123


lamp HPX 2000. Colorimetric measurements were per-

formed at room temperature with illuminant D65 and CIE

2� observer, using a 5-mm-diameter spot, with a 45�geometry (angle between light and sample) and recording

of 10 acquisitions of 50 ms (the measure is the average of

the 10 acquisitions). The distance between the light source

and the sample is approximately 1.5 cm. The analyses were

performed directly on the glass slides. For each sample, at

least four different points were analyzed. The color coor-

dinates L* a* b*, were collected with the Spectra Suite

software. To quantify the color change, the variation of hue

angle was calculated on a graph reporting the a* as

abscissa and the b* as ordinate. We consider browning as

soon as the a* parameter is positive. The hue corresponds

to the angle between the axes of abscissas and the line that

connects the measured points to the origin. The distance

from the origin corresponds to the saturation. A color

change becomes meaningful if the difference between the

angles before and after ageing is at least of 10–15�.

2.2.2 Electron paramagnetic resonance (EPR)

EPR spectra were measured using an EPR Bruker Elexsys

E500 spectrometer in the continuous wave mode at X-band

(&9.4 GHz) and at Q-band (&34.1 GHz) on a Bruker

Elexsys E500 EPR spectrometer. The acquisitions were

performed at room temperature, using a microwave power

of 20 mW. The EPR spectra were lock-in detected using a

modulation of the magnetic field at 100 kHz so that the

spectra represent the derivative of the EPR absorption.

Fresh samples were liquid, while dried and thermally aged

ones were scratched from glass slide and put in the quartz

sampler.

2.2.3 XANES (X-ray absorption near edge structure)

X-ray absorption spectroscopy experiments were per-

formed on the DiffAbs beamline at SOLEIL synchrotron

using a beam size of about 340 lm2. XANES spectra were

collected at the copper K-edge from 8.95 to 9.6 keV with

0.5 eV steps in the edge region, using an Si(111) double

crystal monochromator. A reference metallic copper foil

was measured to calibrate absolute energy of the spectra

(position of the first peak of the spectrum derivative at

8,979 eV). The samples were analyzed both in fluores-

cence and transmission modes using a 4-element SDD and

an AXUV diode, respectively. The intensity of the incident

beam onto the sample was monitored using a thin Si

photodiode. At least two energy scans were collected and

averaged for each sample. The spectra were background-

corrected and edge-normalized using the software Horae

package (Athena, [13]). Normalized XANES and EXAFS

spectra were extracted by subtracting a first-order

polynomial fit to the pre-edge region and using a spline for

the post-edge region.

3 Results

3.1 Colorimetric measurements

Colorimetric measurements clearly show evidence of

browning and allow us to compare the color change at

different evolution stages. A significant color change, but

not a darkening, is observed between the freshly prepared

copper acetate in linseed oil and the same sample at the

beginning of the ageing process (2 weeks of drying in the

air) (Fig. 1). For the thermally aged sample a shift of a*

from negative to positive values was observed, corre-

sponding to darkening (Table 1). A color change is visible

also for light-aged sample, but in our condition a* did not

reach positive values. The hue angle variation for copper

acetate in linseed oil light-aged samples is 75�, passing

from 171� for the fresh sample to 96� (Table 1). This

variation is about half of the one measured on the thermally

aged sample, that shows a hue angle variation of 144�,

ranging from 171� to 27� (Table 1).

Differences are visible also in the kinetics of alteration

between light and thermally aged samples. If we consider

the hue angle variation as a function of time, we can notice

that the browning process is extremely rapid in the case of

thermal ageing (Fig. 2). The main changes are already

visible after 2 days, as shown by an abrupt change of slope.

The color change for light-aged sample is instead less

pronounced and more gradual than the one encountered in

the thermally aged samples. So light ageing seems to

induce an intermediate color variation between the varia-

tion observed during natural and thermal ageing. For this

reason the results in the next part of this article are mainly

presented on the thermally aged samples, where the

mechanism of color change is more advanced and stronger,

so easier to point out.

It is important to notice that there is no browning in the

absence of copper. The linseed oil alone is substantially

stable. Only a slight yellowing is visible with a hue angle

variation of 14� (Fig. 1; Table 1).

3.2 Electron paramagnetic resonance

The Q-Band spectrum of copper acetate after incorporation

in the linseed oil binder is presented in Fig. 3. The EPR

spectrum shows three transitions at 500, 1,000 and

1,350 mT. They are typical of the transitions DMs = ±1

(1,000 and 1,350 mT) and DMs = ±2 (500 mT) of an

S = 1 system formed by two interacting S = 1/2 systems.

These three lines thus correspond to the Cu(II)–Cu(II)

New highlights on degradation process of verdigris from easel paintings

123


dimers of copper acetate (Fig. 4). It should be noticed that

the exchange interaction between the two Cu(II) of the

dimers is antiferromagnetic allowing the splitting of the

ground state configuration into two spin states S = 0 (the

ground state) and S = 1. The antiferromagnetic exchange

interaction is equal to 2 J = -325 cm-1, thus sufficiently

small for the magnetic S = 1 state to be populated at room

temperature.

After lying on the glass slide and drying, the Q-Band

spectrum reveals another signal in the 1,100–1,200 mT

field range consistent with the presence of mononuclear

Cu(II) complexes (S = 1/2) in the copper acetate/linseed

oil system [14]. Figure 5 shows experimental and simu-

lated X-band spectra of the mononuclear complexes, that

allows to determine the principal values of the g-factor and

the hyperfine interaction. The X-band spectrum can be

interpreted with an axial symmetry for the mononuclear

complex. The gk value (corresponding to complexes with

their principal symmetry axis along the magnetic field) is

obtained from the low-field part of the spectrum around

295 mT and the g\ value (corresponding to complexes

with their principal symmetry axis perpendicular to the

magnetic field) from the high-field transition at 330 mT.

The spectrum shows four well-separated hyperfine lines in

the gk component resulting from the interaction of the

unpaired electron of Cu(II) with 63Cu or 69Cu nuclei (both

with spin I = 3/2), while the hyperfine splitting is much

less resolved in the perpendicular component. The

Fig. 1 Hue angle variation for

linseed oil and linseed oil mixed

with copper acetate (2 % w/w).

On the right bottom: optical

images of samples

C. Santoro et al.

123


principal values of the g-factor and of the hyperfine inter-

action for the mononuclear Cu(II) complexes obtained

from the simulation of the X-band EPR spectrum are

reported in Table 2.

The Q-band spectrum of the dimer can be simulated

with the same g-factor and hyperfine interaction as for the

mononuclear complex, but with an additional parameter

corresponding to the zero-field splitting D in the triplet

state [15, 16] given in Table 2. It is possible to find the

relative concentrations of dimer and mononuclear species

using the spin Hamiltonian parameters (g, A, D) to simu-

late the Q-band spectra of each species and calculating

their respective intensities by double integration. The

details of the procedure will be given in a separate paper.

The results are reported in Table 2.

The two spectra of a mixture of copper acetate and

linseed oil recorded before and after thermal ageing did not

reveal any pronounced modifications (Fig. 3). The relative

intensities of the monomer and dimer components are only

slightly affected by thermal ageing, showing a slight

decrease of monomers (Table 2).

The zero-field splitting D parameter, which is the sum of

the (through space) dipolar interaction (Ddip) and the

anisotropic (through bond) exchange term (Dexch) reflects

the interaction between the two Cu(II) of dimmers [15, 16].

It is thus very sensitive to any structural change of the

pigment.

The D values obtained before and after ageing of the

pigment/linseed oil mixture are reported in Table 2. These

values are in good agreement with previous data collected

for Cu(II) dimers with oxygen atoms as donors [17]. The

fact that D values are not modified by ageing shows that

the dimer interaction is not modified during the ageing

process.

Coordination of Cu(II) in the monomeric component

was investigated on the basis of the EPR measurements.

The g and A values are very close to those reported for

copper acetates monomer complexes formed in water [18].

In the same way, no great variations of the spin Hamilto-

nian parameters for the copper monomer complex could be

observed during the ageing process. This result demon-

strates that there are no structural and physical changes of

the copper complexes during the browning phenomena.

3.3 X-ray absorption spectroscopy

X-ray absorption spectroscopy was carried out to charac-

terize the chemical bonding of copper in the samples.

The XANES spectra of copper acetate were collected

before and after the incorporation in linseed oil. The

spectrum acquired before the incorporation is reported in

Fig. 6. Copper acetate spectrum exhibits a very weak fea-

ture (noted A) at 8,979 eV, the white line consists of a high

amplitude peak (noted D) at 8,999 eV and two main

shoulders (noted B and C) along the rising edge around

8,986 and 8,994 eV. At last, after the edge, two small

features are observed in the first oscillation at 9,045 eV

(E) and 9,065 eV (F), respectively.

The signal A is correlated to a 1s ? 3d transition [19],

the C feature is attributed to a 1s ? 4p transition and the

feature B in the rising edge is linked to multi-electron

excitations including a 1s ? 4p transition with simulta-

neous ligand-to-metal charge transfer (LMCT) ‘‘shake-

down’’ transition [20, 21]. The intensity and the position of

the edge including features B and C are generally assigned

to Cu(II) complexes and can be used to predict differences

in local geometry and to get information on the presence of

Cu–ligand interactions [19].

Table 1 Hue angle at different time of sample evolution

Fresh

sample

Dried

sample

Thermally

aged sample

Light-

aged

sample

Color of the samples

Copper acetate

in linseed oil

Green Green Brown Greenish

brown

Linseed oil Colorless Yellow Yellow Yellow

Hue angle

Copper acetate

in linseed oil

171� 123� 27� 96�

Linseed oil – 107� 93� 106�Colorimetric parameters (L*, a*, b*)

Copper acetate

in linseed oil

47.5;

-53.7;

8.2

60.6;

-27.7;

40.2

18.7; 22.3;

11.4

61.6;

-4.6;

30.8

Linseed oil – 76.7;

-3.6;

11.4

71.2; -2.2;

37

95.2;

-7;5;

7;5

Fig. 2 Variation of the hue angle versus time of linseed oil

containing copper acetate (2 % w/w) during thermal and light ageing.

The measurements start from freshly prepared sample. The acceler-

ated ageing process was initiated after 2 weeks of drying


123


After the incorporation in the binding media, the spec-

trum is compared to the previous one (Fig. 6). A decrease

of the intensity of B and C features is clearly observed and

a change in the shape of the first oscillation can also be

observed.

Figure 6 shows also the XANES spectra of samples of

copper acetate mixed with linseed oil after thermal ageing.

A shift towards lower energies of the rising edge with

ageing time is observed at 8,982 eV. In addition, the first

oscillation after the edge has a less pronounced amplitude.

From a previous study, it is accepted that a well-defined

peak in the 8,983–8,984 eV range is the fingerprint of

Cu(I) species corresponding to a 1s ? 4p allowed transi-

tion [19]. Even if the XANES spectrum of the thermal

ageing sample did not show any well-defined and intense

Fig. 3 Q-band spectra of

acetate–linseed oil mixture

freshly prepared, after drying

and 2 weeks of thermal ageing.

Asterisk large signal due to

ferromagnetic species present in

air

RO

Cu

OR

O

O

RO

O R

O

OCu

Fig. 4 Dimeric structure of copper carboxylate

Fig. 5 X-band spectrum of

copper acetate–linseed oil

mixture after 2 weeks of drying

Table 2 Summary of the EPR parameters

Ratio monomer/dimer gk Ak (cm-1) g\ A\ (cm-1) D (cm-1)

Copper acetate in linseed oil

Freshly prepared 0.6 – – – – 0.31 (3)

Dried sample 9.7 2.29 1.76E-02 2.056 1.29E-03 0.33 (5)

Thermally aged 7.6 2.29 1.66E-02 2.05 (6) 1.44E-03 0.33 (5)

C. Santoro et al.

123


peak in this energy range, the visible feature suggests the

presence of Cu(I) atoms in the copper complexes.

3.4 UV–Vis absorption

The UV–Vis absorption spectrum of copper acetate

(Fig. 7a) exhibits three distinct features: two bands around

25,000 and 30,000 cm-1 and one band of high intensity at

15,000 cm-1. The two bands around 25,000–30,000 cm-1

correspond to ligand-to-metal charge transfer transitions

(LMCT). The characteristic splitting is due to the binuclear

structure of copper acetate [7]. The 15,000 cm-1 band

corresponds to the d–d transition of the Cu(II).

After the incorporation of copper acetate in the linseed

oil, the spectrum displays the same absorption bands with a

slight shift of the d–d band towards the lower energies

(Fig. 7a).

Once the pigment and oil binder mixture is laid on the

surface of a glass slide and dried, a major decrease of the

d–d band intensity is observed and the LMCT transitions

are shifted to the visible part of the spectrum (Fig. 7b).

The effect of the thermal ageing which induces the

browning is expressed by the presence in the UV–Vis

spectra of a new large band at 19,000 cm-1 (Fig. 7c)

responsible for the brown coloration.

4 Discussion

A partial transformation of copper(II)-acetate dimer is

induced by the incorporation of the pigment in linseed oil.

EPR experiments allowed us to demonstrate that mixing

copper acetate with oil induces an increase of the mono-

meric component revealing the transformation of the cop-

per environment in the binder.

The increase of the amount of monomer during the

drying influences the UV–Vis spectra.

The decreasing intensity of the d–d transition is thus the

results of two combined phenomena. Monomers generate a

less intense signal than dimers for constant copper con-

centration. This is because d–d transitions are parity for-

bidden for monomers. There is no parity change in d–

d transitions of monomeric complexes [22, 23]. On the

contrary, interactions between Cu of dimers generate

symmetric (g) and antisymmetric (u) combinations of

d orbitals, both in the ground state and in excited states

resulting into a more intense signal due to change in parity

during the transition.

The second possible reason is related to the intensity of

the d–d bands which is also dependent on the orientation of

the electric field with respect to the copper acetate mole-

cules [23]. The drying process could induce specific

molecular orientations of the copper dimer. Thus, the d–

d band intensity will be affected by the polarization

direction of the incident radiation with respect to the

molecular axes of the copper complexes. A similar flat

signal for the d–d transition bands is visible also for the

‘‘15-year-old model sample’’, suggesting that the chemical

composition of the preparation layer does not influence the

phenomena.

The partial transformation from a dimer structure to a

monomeric structure can be due both to hydration of the

copper-complex [18] and to its interaction with free fatty

acids issued from triglycerides initially present in oil [6, 7].

Fig. 6 Comparison of XANES

spectra for copper acetate,

copper acetate–linseed oil

mixture after 2 weeks of drying

and after 3 days thermally aged.

Label signal: A:

1s ? 3d transition at 8,979 eV,

B: 1s ? 4p transition and

LMCT at 8,986 eV, C:

1s ? 4p transition at 8,994 eV,

D: white line at 8,999 eV, E:

9,045 eV, F: 9,065 eV


123


So the increase of monomeric component can be related to

the lability of Cu-carboxylate ligand bonds, through

exchange reactions. The present experiments have clearly

revealed that the apparition of monomeric species in the

model samples is not responsible for the browning since

only a slight modification of the chromatic properties was

highlighted by colorimetric measurements, unlike what

was proposed in a previous study [6]. Besides during the

thermal ageing, when the browning appears, a slow

decrease of monomer is observed (Table 2).

The dimer/monomer transformation could be initiated

by reaction with fatty acids or by exposure to direct

sunlight. It has been previously reported that Cu(II) car-

boxylate complexes such as acetate can undergo a light-

induced process loss of ligand when irradiated by UV-

radiation so that this hypothesis has been brought up to

explain the degradation of many pigments used in painting

[24]. The excitation of LMCT occurs under light irradia-

tion. The excited state of the LMCT corresponds to a

Cu(I) with an electron vacancy on a carboxylate ligand L.

Then in a second step, the decay of the LMCT excited state

can induce the breaking of the Cu(I)–L bond according to

the following reaction:

During the ageing of painting samples, ambient light

could induce LMCT between the acetate bridging ligand

and one of the Cu(II)of the dimers, leading to a loss of a

carboxylic ligand. As the loss of ligand mechanism con-

tinues, Cu(I)–Cu(I) dimers can be generated in the mixture.

This hypothesis is supported by the fact that the spin

Hamiltonian parameters (g, A and D) of the Cu(II)–Cu(II)

dimer and of the mononuclear Cu(II) complexes do not

change during ageing. This means that any change in the

dimer structure is probably accompanied by formation of

EPR-silent Cu(I). Besides from the data collected by

XANES experiments on the aged samples, the existence of

Cu(I) species cannot be excluded.

The photodecarboxylation process allows the formation

of copper complexes containing a binuclear Cu(I) active

site that could activate dioxygen. It is well known that

Cu(I) species produced by photo-reduction can undergo a

fast re-oxidation in an oxidizing environment [24].

UV–Vis spectra revealed that the browning process is

associated with the presence of a new absorption band at

19,000 cm-1. This band was also observed for copper

organometallic protein such as oxyhemocyanin [25]. In this

binuclear complex, Cu(I)–Cu(I) site could react with O2

leading to the formation of Cu(II)–O2–Cu(II) bridges. The

LMCT between the peroxide bridge and the Cu(II) in

peroxy-Cu(II) dimer complexes gives an intense transition

at low energy, between 17,000 and 21,000 cm-1 [25].

Peroxy-Cu(II) dimer is strongly antiferromagnetic

(-2J [ 1,000 cm-1) and the excited state S = 1 cannot be

populated at room temperature in this case. Therefore, this

complex is inactive in EPR and no changes are observed in

the spectrum of Cu(II) dimers.

Thus during ageing, the pigment/oil system in paintings

interacts with environment by redox reactions, and dioxy-

gen molecules can react which the binuclear Cu(I) com-

plexes formed by photodecarboxylation. Considering this

hypothesis, even if a complete understanding of the

browning mechanism has not yet been reached, the color

Fig. 7 UV–visible absorption spectra of model samples for a copper

acetate and linseed oil–copper acetate freshly prepared, b after

2 weeks of drying and c after 3 days of thermal ageing

C. Santoro et al.

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change observed in the model samples could be related to

the LMCT between peroxo bridge ligands and Cu(II)

induced by the reaction of dioxygen with partially decar-

boxylated dimers. The double effect of light and oxygen

can explain why the same pigment shows different alter-

ation depending if it is protected from ambient air or if it is

exposed.

5 Conclusion

The combination of optical absorption, XAS and EPR

measurements allowed us to propose new hypothesis con-

cerning the mechanism of degradation of verdigris pig-

ments. This work revealed that the incorporation of the

pigment in oil induces the formation of monomeric species.

The latter are, however, not directly responsible for the

browning mechanism.

Our results suggest that the browning mechanism seems

to be linked to the transient formation of Cu(I) in the copper

complexes of the pigment/oil system. This formation could

be initiated by ambient light absorption through LMCT,

which favors the release of a carboxylate leading to the

reduction of Cu(II) into Cu(I). Even if the amount of

Cu(I) is weak, a drastic change in the optical properties is

generated. The partially decarboxylated Cu(I)–Cu(I) pairs

are thought to react with oxygen, promoting the formation

of copper bridged by peroxide bonds (Fig. 8). This could be

an explanation of the main features observed on the UV–Vis

spectrum of a darkened model sample. In this hypothesis,

the browning would be due to an additional LMCT near

19,000 cm-1, involving a peroxide bridge ligand.

Nevertheless, at this stage of the study, the complete

understanding of the browning phenomenon remains par-

tially unsolved. To provide additional support to this

hypothesis, further experiments by pulsed EPR have to be

implemented. These experiments would bring crucial

improvement of the EPR resolution and selectivity and

then would allow to get more information about the

structure of the copper complexes involved in the browning

of the aged samples.

Moreover, experiments on real samples collected from

paintings are under progress to detect the presence of

Cu(I) species with peroxide bridges in order to confirm the

evidence highlighted by this preliminary study. It is plan-

ned to determine the relative proportions of dimeric and

monomeric structures in real paints samples and whether

these proportions differ between altered and unaltered

areas.

Acknowledgments This work was supported by the Patrima LabEx

and the Foundation for Cultural Heritage Sciences.

References

1. M. San Andres, J.M. De la Roja, V.G. Baonza, N. Sancho, J.

Raman Spectrosc. 41, 1468 (2010)

2. H. Kuhn, Stud. Conserv. 15, 12 (1970)

3. C. Cennini, The Craftsman’s Handbook, »Il libro dell’Arte»,

Translated by Daniel V. Thompson Jr. (Dover, New York, 1954)

4. L. Da Vinci, Trattato della Pittura (c. 1550), vol. 2, no 214.

Translated by McMahon (1956)

5. R. Wouhuysen-Keller, Hist. Paint. Tech. Mater. Studio Pract.

26–29, 65–69 (1995)

6. M. Gunn, G. Chottard, E. Riviere, J.-J. Girerd, J.-C. Chottard,

Stud. Conserv. 47(1), 12 (2002)

7. L. Cartechini, C. Miliani, B.G. Brunetti, A. Sgamellotti, C. Al-

tavilla, E. Ciliberto, F. D’Acapito, Appl. Phys. A 92(1), 243

(2008)

8. E. Ioakimoglou, S. Boyatzis, P. Argitis, A. Fostiridou, K. Papa-

panagiotou, N. Yannovits, Chem. Mater. 11(8), 2013 (1999)

9. S. Boyatzis, E. Ioakimoglou, P. Argitis, J. Appl. Polym. Sci.

84(5), 936 (2002)

10. C. Altavilla, E. Ciliberto, Appl. Phys. A 83(4), 699 (2006)

11. D.A. Scott, Copper and Bronze in Art: Corrosion, Colorants,

Conservation, vol. 270 (Getty Publications, 2002)

12. M. VanEikema-Hommes, Discoloration in Renaissance and

Baroque Oil Paintings, Thesis, Amsterdam, University, under the

direction of Van de Wetering E. (2002)

13. B. Ravel, M. Newville, J. Synchrotron Rad. 12, 537 (2005)

14. J. Svorec, M. Valko, J. Moncol, M. Mazur, M. Melnık, J. Telser,

Transit. Met. Chem. 34(2), 129 (2009)

15. M.V. Fedin, Dalton Trans. 42, 4513 (2013)

16. C.D. Delfs, R. Bramley, Chem. Phys. Lett. 264, 333 (1997)

17. P. Sharrock, M. Melnik, Can. J. Chem. 63(1), 52 (1985)

18. H. Grasdalen, J. Magn. Reson. 9(1), 166 (1973)

19. L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson,

E.I. Solomon, J. Am. Chem. Soc. 109(21), 6433 (1987)

20. J.L. DuBois, P. Mukherjee, T.D.P. Stack, B. Hedman, E.I. Sol-

omon, K.O. Hodgson, J. Am. Chem. Soc. 122(24), 5775 (2000)

21. T.A. Smith, J.E. Penner-Hahn, M.A. Berding, S. Doniach, K.O.

Hodgson, J. Am. Chem. Soc. 107(21), 5945 (1985)

22. F. Tuczek, E.I. Solomon, Coord. Chem. Rev. 219–221, 1075

(2001)

23. P.K. Ross, M.D. Allendorf, E.I. Solomon, J. Am. Chem. Soc.

111(11), 4009 (1989)

24. P. Ciesla, P. Kocot, P. Mytych, Z. Stasicka, J. Mol. Catal.

A Chem. 224, 17 (2004)

25. E.I. Solomon, F. Tuczek, D.E. Root, C.A. Brown, Chem. Rev.

94(3), 827 (1994)

Fig. 8 Hypothesis on reaction

of oxygen complexation.

L represents a carboxylate

function


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