CHEMICAL REACTIONS BETWEEN COPPER PIGMENTS AND OLEORESINOUS MEDIA
Michele Gunn, Genevieve Chottard, Eric Riviere, Jean-Jacques Girerd and Jean-Claude Chottard
Summary— This study demonstrates that resin and fatty acids are able to extract copper (II) ions
from verdigris (copper acetate) and verditer (basic copper carbonate). Ligand exchange reactions
of basic copper carbonate with fatty acids and resin acids are much slower than is the case with
copper acetate. The browning of paint layers is closely correlated with the relative ease of copper
extraction; the copper diffuses in the form of fatty acid or resin carboxylic acid complexes. These
complexes are formed in the painting layer during grinding of the pigment with binding media
containing oleoresin acids, as well as being formed at the interface of this layer and organic upper
layers, such as varnishes.
Introduction
Copper ions appear to play an important role in the degradation of the organic constituents in various layers of paintings, as well as of a number of materials such as wood, canvas, paper or leather.
An example of damage caused by green copper pigments has been reported, where copper ions are
presumed to bind to cellulosic material through hydroxyl and carboxylate groups resulting in the
oxidation of the cellulose [1].
Some alterations of painting layers have been reported for copper pigments such as verdigris,
azurite or malachite. Azurite-containing paint films darken on aging [2]. Although verdigris seems
to be the most reactive of the three pigments in giving a brown coloration, in certain cases verdigris
has retained its original intense colour [3]. Moreover, previous generations of painters were aware
at an early stage of the reactivity of verdigris [4]. Browning has been identified in fifteenth-century
paintings, such as the 14 portraits of famous men of the Studiolo in Urbino painted by Juste de
Gand and Pedro Berruguete [5].
When considering such alterations, curators often question whether the dark layer is a brown glaze,
a green glaze turned brown by aging, or only a non-original superficial brown upper layer. In this
last case there is also the question of whether to clean the upper layer away to reveal the bright
green pigment of the underlying layer. This type of alteration has previously been observed by
Groen. She studied cross-sections of brown areas from a number of paintings, using scanning
electron microscopy coupled with an X-ray microanalysis system (SEM-EDX). The brown layers
were found to be predominantly organic materials, in some cases containing a small amount of
copper. It was concluded that in these samples the brown layers were neither brown glazes nor
alteration products of copper resinates [6].
The absence of copper in a brown layer does not, of itself, permit us to determine the nature of the
layer, and nor does the presence of this element constitute proof of the existence of an original
green glaze. The question is, therefore, where does the copper come from?
The presence of copper can be attributed to a diffusion of copper ions from an underlying coppercontaining pigment, especially verdigris. This implies that extraction of the metal ion from the
pigment has taken place. Such an extraction may result from competitive binding of the metal ions
by the ligands present in the pigment and those present in the mixture of oils and resins in the
various upper layers.
This study examines the ability of resinic and fatty acids, especially abietic and Cl8 fatty acids
(stearic, oleic, linoleic, linolenic acids), either in their carboxylic or carboxylate forms and under
various conditions, to extract copper(II) ions from verdigris (copper acetate) and basic copper
carbonate pigments such as malachite and azurite. Verditer was chosen as an example of a basic
copper carbonate. Abietic acid, a resinic acid with the conjugated 1,3-butadienyl unit, and linoleic
acid, a fatty acid with the peroxidizable l,4~cis,cis-pentadi-enyl unit, were chosen as representatives
of the two families of compounds present as major constituents in oleoresinous layers. Linseed oil,
a common major component of paint and varnish layers, contains more linolenic acid, but linoleic
acid was found more appropriate for this study because it is less easily oxidized than linolenic acid.
Abietic acid
Portrait of a Man in Black
Three Saints
Rupp retable
Figure 1 Back-scattered electron images for the three samples examined and copper depth
profiles for the paint layers. The extent of the layers examined is indicated alongside each image.
was used with the aim of studying the mechanism of transformation of green glazes of copper
resinate into brown materials.
Results
SEM-EDX analysis of paintings
Three paintings in which this phenomenon is implicated were selected for SEM-EDX analysis.
These were the "Rupp" Retable (fifteenth-century Swiss School. Musee des Beaux-Arts. Dijon).
Three Saints (fifteenth-century German School. Musee Dechelette. Roanne) and Portrait of man
dressed in black (sixteenth-century Italian School. Musee Nationale du Chateau de Fontainebleau).
SEM-EDX microanalysis was performed on samples from brown areas of these three paintings. The
cross-sections were first coated with carbon and semi-quantitative analysis was performed for each
layer after calibration using a cobalt reference sample. The acceleration voltage was 20kV and the
diameter of the beam was OO|xm. The thicknesses of the three brown layers were 6, 11 and 15|j.m.
The results obtained are shown in Figure 1. The copper concentration profiles obtained in these
three cases clearly show the presence of copper in the brown layers.
Analysis of the brown layer in the 'Three Saints'
With brown layers which are relatively thin, X-rays may be emitted by copper atoms located in the
underlying green layer; this phenomenon is a consequence of scattering of the primary electron
beam. To be sure that copper was indeed present in the brown superficial layers, the brown layer
from the Roanne painting was analysed separately by scraping the layer off to give a powder. The
analysis was carried out using the same method as for the cross-sections above. The brown powder
was laid on a carbon support and analysed directly without the addition of a metal coating. The
spectrum obtained demonstrates the presence of copper in the powder: the quantity of copper varies
between different grains (Figure 2). The brown layers are superficial and can be distinguished from
the green paint layer. The low concentration of copper in these layers confirms that they are neither
brown glazes nor alteration products of previous green glazes. Research on fourteenth- and
fifteenth-century paintings has enabled a distinction to be made between an original brown glaze
and a brown colour resulting from the alteration of a copper resinate [7, 8].
Figure 2 SEM-EDX microanalysis of the powder obtained by scraping off the superficial brown
layer of the Roanne painting.
Reactivity of verdigris towards carboxylic acids
For copper pigments that are in contact with fatty acids and resin acids in the painting layers, it is
important to investigate the replacement of the acetato, carbonate and hydroxo ligands that surround
the copper(II) ion by carboxylate groups from the free acids in the medium. Based on the relative
lability of acetato, carbonate and hydroxo ligands, basic copper carbonate is expected to exhibit less
mobility than copper acetate.
Copper complexes of carboxylic acids can be prepared from pure acids in the liquid state with copper carbonate or from the sodium or potassium salts of the acids with a copper(II) salt such as copper chloride, CuCl2. Abietic acid (AbH) dissolved in benzene has been found to extract copper(II)
ions from aqueous solutions [9. 10]. The compounds obtained were described as copper abietates,
with formulae Cu(Ab)2-2AbH and Cu(Ab)2-4AbH. To the best of our knowledge, these compounds
were not fully characterized.
We studied copper complexation reactions by infrared (IR) spectroscopy in the 1800 to 1300cm-1
range, to follow the transformation of the carboxylic group (—COOH) or carboxylate ion (—COO-)
into a copper(II) carboxylato ligand (Table 1). The formation of carboxylato-Cu(II) complexes was
indicated by the appearance of a
Table 1 IR spectral bands for each type of carboxylato (RCOO~) ligand in the carbonyl fvCOj
region
characteristic band close to 1600cm~', together with absorptions in the visible spectrum at 633nm
for copper(II) abietate and at 670nm for copper(II) linoleate. Proton nuclear magnetic resonance
(1H-NMR) and mass spectrometry (MS) were used to identify the ligands after they had complexed
to the copper(II) ion. Electron paramagnetic resonance (EPR) spectroscopy and Raman
spectroscopy also revealed characteristic features of the structures of
Figure 3 Extraction of copper from copper acetate by (a) abietic acid and (b) linoleic acid. The
concentration of both acids was 0-04M and the reactions took place in dichloromethane. The initial
rate for abietic acid was 15mg.ml~'h~' and for linoleic acid 9-5mg.ml -1h -1.
the resin and fatty acid copper(II) complexes formed. For the moment, no crystals have been isolated for crystallographic studies. Copper abietate was obtained as an amorphous powder and
copper linoleate as an oil.
Copper extraction by abietic and fatty acids
The extraction was conducted in the solid/liquid phase with dichloromethane under an inert atmosphere. Abietic and linoleic acids react easily with copper(II) acetate monohydrate to give copper(II)
complexes that are soluble in the organic phase. Aliquots of this phase of the reaction mixture were
withdrawn and analysed by IR and ultraviolet-visible (UV-vis) spectroscopy until no further evolution of the complex could be observed. The graphs obtained show that, in both cases, a limiting concentration of copper(II) carboxylate was reached in about 10 hours (Figures 3 and 4). The
extractions were carried out using different initial concentrations of the acid to determine the
influence of the latter on the extraction of copper; Table 2 reports the data for linoleic acid.
It was observed that the initial rate and the final concentration of the copper linoleate formed both
Figure 4 Extraction of copper from copper acetate by linoleic acid as a function of time, monitored
by (a) visible light at 670nm, and (b) infrared radiation at 1610cm-'.
Table 2
Variation in reaction parameters with the concentration of linoleic acid
increased in line with the initial concentration of linoleic acid. But in all cases, the time at which the
final concentrations were reached was 10 hours. The reactions in which these copper carboxylates
are formed from copper acetate seem to be chemical equilibria. In effect, when an aliquot of the
organic phase of the reaction mixture is washed with water, a drop in the concentration of copper
linoleate is observed, due to the reaction of acetic acid with copper linoleate to regenerate copper
acetate which is water soluble. Moreover, addition of a large quantity of acetic acid to the reaction
solution gave a precipitate of copper acetate. Starting from the free acids, it proved difficult to
isolate the copper(II) complexes from the reaction mixture. Standard chromatographic methods
were unsuccessful because of the similar polarities of the acids and their copper derivatives.
Copper(II) complexes were prepared from the alkali and ammonium salts of the acids in alcohol,
preparation (a), or in water, preparation (b). Sodium or ammonium salts of abietic and fatty-acids
reacted easily with copper(II) acetate in an organic solvent, giving copper(II) complexes. Copper
acetate and other salts could easily be separated from the hydrophobic copper(II) complexes by
washing the organic phase with water. The complexes were isolated by evaporating the solvent under reduced pressure and were stored under
argon. In the case of copper(II) abietate, further purification was achieved by quick flash chromatography. This procedure failed for copper(H) linoleate, due to copper decomplexation induced by
adsorption onto the stationary (silica) phase.
Identification of copper(1I) complexes of abietic and fatty acids
Copper (II) abietate
The copper(II) complexes obtained were blue-green solids and exhibited some differences in their
IR and Raman spectra. An inspection of the IR spectra of abietic acid, sodium or ammonium
abietate and copper(II) abietate showed that each type of carboxylic and carboxylate group could be
clearly distinguished from the others (Table 1). Preparation (a) yielded a product exhibiting only the
intense IR band at 1600cm-1, characteristic of a carboxylate bound to a copper(II) ion. In the
products from preparation (b), using the abietate anion in water, two extra IR bands were present as
well as extra Raman bands in the 1350 to 1650cm~' region, which reflect the presence of a mixture
containing free acid (Table 3). In spite of the use of a salt, the reaction product was a mixture of
abietic acid and copper carboxylate (the presence of acid is due to the use of an aqueous solution):
Besides the observed shift of the vasymmetrical COO
asymmetrical
vibration in the IR spectrum, complexation of the abietate moiety leads to the appearance of a low
frequency Raman band at 500cm-1, in a region of the spectrum associated with metal-oxygen
interactions. The type of binding of the carboxylate group can be deduced from the difference in the
frequencies of the asymmetrical (vas) and symmetrical (v) COO stretching frequencies (∆vas - vs)
[11]. From the IR spectrum it is easy to determine v s due
Table 3 Vibrational frequencies for the carboxylate depending on the method of preparation of
copper (II) abietate
(s) = strong; (m) = moderate; (w) = weak.
to its strong intensity, while v; is available from the Raman spectrum. However, the latter peak
occurs in a region where deformation modes of the —CH,— and —CH, groups are also present,
making precise assignment difficult. For sodium abietate, vas was 1550cm"1 and vs was assigned
either to the peak at 1469cm~' or that at 1433cm"1. leading to a Av value of 81 or 117cm~'. The
slightly-higher values observed for copper abietate allow a unidentate coordination, characterized
by Av > 200cm"1, to be excluded.
The room temperature powder EPR spectra of the copper abietate obtained from preparations (a)
and (b), both showed signals at around 4500G. as is also the case for copper acetate. This signal can
be attributed to a spin of S = 1 and. therefore, to a binuclear copper(II) complex. As the temperature
was reduced to 4K the spectra exhibited signals at 3000G. corresponding to a spin state of S = Vz,
indicating a mononuclear copper(II) complex [12]. A concurrent attenuation of the 4500G signal
was observed. A study of the magnetic properties of copper(II) abietate was carried out to
understand this phenomenon, and to determine if the compound is a mononuclear or binuclear
complex.
Magnetic susceptibility measurements have revealed particular features which correspond neither to
a mononuclear nor to a binuclear complex. A plot of magnetic susceptibility (XT) as a function of
temperature between 0 and 300K gave a two-phase plot, obeying the Curie law up to 60K. and
characteristic of a dimeric species above 60K (Figure 5). A mathematical treatment of the second
part of the plot of x versus temperature enabled us to fit the experimental data with the presence of
20% of a monomeric Cu(II) complex, 40% of a dimeric Cu(II) complex with antiferromagnetic coupling (J = -317cm"1), and with the remainder of the copper present in the form of Cu(I) species.
This composition also explains the decrease in the absorption band at around 600 to 700nm. due to
d-d transitions in the Cu(II) complexes, that is discussed in a later section. The experimental plot
can be considered as the sum of two contributions, one from a mononuclear and the other from a
binuclear species. The values of x obtained from 50 to 300K are too low7 for a pure binuclear
complex and result from the presence of Cu(I). which is diamagnetic (Figure 6).
Comparison of the 'H-NMR spectrum of the cop-per(II) abietate complex from preparation (a) with
that of free abietic acid indicated a perturbation of only one of the two conjugated double bonds.
The chemical shifts for the protons attached to the other double bond remained unchanged; the peak
at 5-34ppm in the free acid became two broad peaks at 5-01 and 5-66ppm. A broadening of the
peaks was
Figure 5 Temperature dependence of x for copper abietate, assuming a molecular weight of 1619
g.mor1 and a binuclear complex.
expected, due to the influence of the paramagnetic Cu(II) ion bound to the carboxylate group.
CopperIIIj linoleate and other C18 fatty acid-copper complexes
Copper(II) linoleate was prepared in organic solvents using preparation (a). The appearance of a
narrow IR band at 1610cm-1 indicated binding of linoleate to copper(II) ions, and this band was
used to monitor complex formation.
As in the case of copper(II) abietate. 'H-NMR revealed a paramagnetic effect on the ethylenic protons of the linoleate copper complex. It is of note that the chemical shifts remained in the
diamagnetic range and that only peak broadening was observed; no extra peaks were present. This
difference with respect to the case of copper(II) abietate probably results from the longer distance
between the paramagnetic centre and the 9,11-pentadienyl system compared to that present in the
more rigid copper abietate system.
The Clg fatty acid copper(II) complexes seem to be binuclear or polynuclear complexes (Table 4).
Figure 6 Temperature dependence of the magnetic susceptibility. Open circle = experimental curve,
solid line = fit, solid square = theoretical dimeric Cu" curve (J = 317cm-1), dotted line =
monomeric Cu" contribution, dot and dash line = dimeric Cu11 contribution.
Their EPR spectra showed the same features as copper acetate and copper abietate, except copper
stearate. which exhibited a signal around 3000G (corresponding to a g-value of 2), characteristic of
mononuclear copper(II), although hyperfine coupling with the copper nucleus was not well
resolved.
Reaction of linseed oil with copper (II) acetate
A mixture of linseed oil and copper(II) acetate was allowed to stand at room temperature under an
inert atmosphere to avoid oxidation reactions. Dichloromethane was added to extract the copper(II)
fatty acid complexes formed. After filtration of insoluble copper(II) acetate, the solution obtained
was washed with water, dried with magnesium sulphate and investigated by IR spectroscopy.
Comparison of the bands at 1700 and 1610cm"1 in the IR spectra with those for abietic and linoleic
acids showed that the product extracted was a mixture of linseed oil and copper carboxylates. The
latter result from ligand exchange reactions between free acids present in the oil and copper(II)
acetate.
Reactivity of verditer towards carboxylic acids
When abietic or linoleic acid or their alkali salts were allowed to stand at room temperature with
verditer (basic copper carbonate), no band at 1610cm-1 was detected after several days. In contrast,
copper acetate formed immediately in the same conditions upon addition of acetic acid. Copper
carboxylate was only detected by Fourier transform IR after several months. The explanation for
this may lie in hydrophobic interactions.
Transformation under atmospheric conditions
When dichloromethane solutions of copper abietate or linoleate were allowed to stand under
oxygen, the solution, which was initially blue-green, turned yellow-green and the d-d transition
band in the visible spectrum disappeared, signalling the transformation of Cu(II) to Cu(I); this was
confirmed by the magnetic susceptibility study of copper abietate. The following mechanism for
this reaction is proposed, which involves decarboxylation of the cop-per(II) complex:
The alkyl radical generated in this first step may in turn react as follows:
Table 4
Vibrational data for fatty acid copper(II) complexes
Another possibility for the reaction mechanism is a Cu(II)/Cu(I) redox reaction initiated by the
abstraction of a hydrogen atom from the ligand:
Discussion
The aim of this work was to test the hypothesis that the copper(II) ion is extracted from copper pigments by the fatty and resin acids present in paint layers. Once liberated, the copper(II) ions could
contribute to redox processes involving oxygen and lead to peroxidation of the unsaturated
carboxylic acids. The first objective was to isolate the complexes formed from abietic or linoleic
acid and copper acetate in a pure state and characterize them. The best results were obtained with a
two-phase system of solid copper acetate and the sodium or ammonium salts of abietic or linoleic
acid in non-aqueous solvents, preparation (a). In these conditions the use of a carboxylate avoids the
need to deprotonate the acid, which w:ould be energetically very difficult in aprotic solvents and.
furthermore, favours the carboxylato-acetato ligand exchange. Moreover, during the work-up of this
reaction mixture, the unreacted copper acetate and the sodium acetate formed were easily removed
by aqueous washing, giving pure copper complexes of abietic and linoleic acids. When free
carboxylic acids were used—more akin to the situation present in the paintings, since oils and resins
contain these species—the reaction did not go to completion and a mixture of acid and copper
carboxylate was detected.
The 1600cm-1 band in the IR spectrum, characteristic of the copper(II)-carboxylato bond, revealed
the formation of copper carboxylates when fatty or resinous material were in contact with copper(II)
acetate or copper alloys. In both cases the carboxy-lato ligand was bound to copper(II) either
already present in verdigris (or other copper pigments) or formed by oxidation of the metallic
copper in alloys. The ligands were detected by MS. either with electron impact (El) or chemical
ionization (CI) systems, or by gas-phase chromatography after decomplexation and methyl ester
formation with boron trifluoride/methanol or diazomethane. The copper(II) complexes could not be
identified by mass spectrometry. but the appearance of a new-band in the Raman spectrum is a
distinctive feature
of copper complexation. Elemental analyses showed that two carboxylate ligands were present per
copper atom, which is compatible with either mono-nuclear or binuclear structures.
The IR and Raman studies gave A(vas - v_) <200cm~', which is characteristic of bidentate
carboxylates, as in copper(II) acetate. This agreed with the room temperature solid-phase EPR
spectrum, which did not exhibit the signal at around 3000G (g = 2) normally expected for a
copper(II) ion and suggested a binuclear or an infinite polymeric structure for these complexes.
Only copper(II) stearate was found to be a mononuclear compound. The Raman analysis was
difficult to perform on this copper(II) complex because of fluorescence, although a low frequency
vibration was observed at 427cm"1, which might correspond to a Cu-O stretching vibration. The
absence of paramagnetic shift in the 'H-NMR signals suggests the presence of a diamagnetic
complex with a metal-metal bond between the copper(II) ions (electronic configuration d9 with an
unpaired electron).
Ligand exchange reactions of basic copper carbonate with fatty acids and resinic acids are much
slower than with verdigris, which can be understood in terms of lower lability of hydroxo and carbonato ligands compared to carboxylato ligands. The reactions described in this paper were run in
Schlenk flasks, which are closed systems, in contrast with paintings which can be considered as
open systems. In paintings, the acetic acid formed may be partially evaporated, or could migrate to
another layer. Because of their low polarity, abietic and fatty acid copper(II) complexes could be
extracted by oleoresinous layers, where they could participate in further reactions in the presence of
oxygen or other components of air such as SO,, CO, CO, or NO, (Figure 7).
Conclusion
This work has demonstrated the capacity of fatty and resin acids, exemplified by linoleic and abietic
acids, either in the free acid or carboxylate forms, to extract copper(II) ions from verdigris (copper
acetate). A kinetic study based on infrared and UV-vis spectroscopy showed that these two acids
have similar reactivities and that extraction of copper ions begins immediately the starting materials
are mixed. Verdigris ground with oil or with a mixture of oil and resin is thus partly transformed by
exchange of the acetato ligands with the carboxylic groups of the acids present in the system. Such
an extraction is also observed when an oleoresinous layer is placed on a paint layer containing
verdigris.
Figure 7 Schematic representation of the possible reactions which may occur in the paint and
varnish layers.
When the same reaction is carried out with verditer a basic copper carbonate with carbonate and
hydroxo ligands (as in malachite and azurite), copper extraction is not immediately visible. This
relative lack of reactivity could explain the fact that whilst browning is observed with verdigris, to
the best of our knowledge no important examples exist of a similar change in painting layers
containing malachite. The browning of the paint layers is thus closely correlated with the relative
ease of copper extraction.
From our results, we predict that copper extraction from pigments by fatty and resinic acids will be
a slow process, since they are present in oils and resins both as the free acids and as esters. The fatty
acids are mainly present as triesters of glycerol and hydrolysis is required to liberate the free acids:
this hydrolysis could be induced by copper ions, as has been shown previously [13]. We believe that
copper extraction could occur not only with resins or oily media but in the presence of other
substances that are capable of coordinating copper ions, such as waxes and proteins.
The question of the brown oleoresinous layers must also be carefully considered, since these alterations may be due to changes in the original paint layer or may equally be a result of changes in layers added during restoration. The chemical reactions involved in copper extraction are balanced
equilibria, so that the removal of these brown layers carries with it the risk of disturbing a balance
reached over centuries. The application of a new protective layer containing compounds liable to
form complexes with copper ions could begin again the process of copper extraction, and thereby
further degrade the painting.
Acknowledgements
This work was supported by the CNRS (Centre National de la Recherche Scientifique) and
the
DMF (Direction des Musees de France). We thank Myriam Eveno and Alain Duval for help with
electron microscopy studies and Marie-Noelle Lagier for NMR spectroscopy. We thank Elisabeth
Martin, Jean-Paul Rioux and Jason Hart-Davis for advice. We also thank Dr Jean-Pierre Mohen.
Director of the C2RMF (Centre de Recherche et de Restauration des Musees de France).
Appendix: experimental details
General procedures
All syntheses were performed under a dry argon atmosphere using standard Schlenk techniques.
Reagents were purchased either from Prolabo or from Aldrich Chemical Co. Before use linoleic
acid was purified by chromatography on a silica gel column using a 5% ethoxyethane
(ether)/hexane eluent; abietic acid was purified by crystallization from ethanol/water. The solvents
used were of synthesis grade and were kept under argon. IR spectra were recorded on a PerkinElmer 783 spectrophotometer. UV-vis spectra on Uvikon-Kontron 810 and 941 spectrophotometers
and 'H-NMR spectra on a Bruker AM-250 spectrometer operating at 250MHz. EPR spectra were
recorded at the Laboratoire de chimie inorganique. Universite de Paris-Sud (Orsay). Raman spectra
were obtained from solutions in CHC1, or on solids, using a JYU 1000 double monochromator with
496-5nm excitation. Elemental analyses were performed by the CNRS service at Vernaison.
Preparation of sodium abietate
Abietic acid (0-70g, 2-3mmol) was dissolved in a minimal quantity of dry ethanol under argon. A
solution of NaOH (0-lg. 2-5mmol) in 6ml of ethanol was added to this solution. The mixture was
stirred at room temperature for 15 minutes then the solvent was concentrated under reduced
pressure. The white solid obtained was washed several times with small quantities of dry7 ethanol,
then with ether. The sodium abietate was dried under vacuum. Infrared (13mm KBr pellet): vcoo
1550cm-' (symmetrical, strong, broad), 1400cm"1 (asymmetrical, moderate).
Preparation of ammonium abietate
A concentrated aqueous solution of ammonia was added to a solution of abietic acid until a white
solid appeared. The solvent was evaporated to dry-ness under vacuum. The salt obtained was used
for copper abietate preparation without any further purification.
Preparation of copper (II) abietate
Svnthesis (a)
An ethanolic solution of abietic acid (0-5g. l-65mmol) was reacted with NaOH (0-07g, l-75mmol)
dissolved in ethanol. A fine powder of sodium abietate precipitated from the solution. A solution of
copper acetate (0-165g. 0-82mmol) in ethanol was added to this mixture by syringe. The mixture
was allowed to react at room temperature until the fine powder disappeared and a greenish-blue
solution was obtained. The alcohol was then removed at reduced pressure. The crude greenish-blue
solid was dissolved in ether and washed several times with water to eliminate unreacted copper
acetate or sodium abietate. After evaporating the solvent, a green-blue solid was obtained. Infrared
(13mm Kbr pellet): vCOOCu 1600cm-' (strong). 'H-NMR (in CDCL, chemical shift. 8): olefin protons.
5-68ppm (singlet), 5-08ppm (singlet, broad). For comparison, olefin protons in abietic acid: 574ppm (singlet) and 5-3ppm (doublet). Elemental analysis, calculated for Cu(C,0H,9O,)2: C 72-12%,
H 8-71%; found: C 72-3%, H 9-39%'.
Synthesis (b)
Sodium abietate (l-7g. 3-3mmol) was dissolved in 20ml water. Copper acetate (0-66g, 3-3mmol)
was added to this solution to give a green precipitate which was filtered off. Further purification can
be achieved using flash chromatography on a short silica gel column with a 1:2 etherpentane eluent.
Infrared (13mm KBr pellet): vco 1700cm-' (moderate). vCOOCu 1600cm-' (strong). Visible-ultraviolet
(in CH,C1,): 244nm (strong). 270nm (shoulder), 633nm (broad, weak). Elemental analysis,
calculated for Cu(C,0H,9O,),: C 72-12%, H 8-71%; found: C 72-65%, H 9-55%.
Synthesis of copper(II) linoleate from ammonium linoleate and copper acetate
A solution of linoleic acid (443mg, l-58mmol) in ethanol (0-4ml) was treated with a few drops of
concentrated NH4OH. The mixture was evaporated to dryness under vacuum and was dissolved in
10ml of ethanol. Copper(II) acetate (0-16g, 0-8mmol) was added to this solution, which first turned
blue, then blue-green. The alcohol was evaporated and the crude product was dissolved in 10ml of
ether and washed with water. The aqueous phase w7as coloured blue by unreacted copper acetate.
After removal of the ether a blue-green oil was obtained. Infrared (in CH,C1, or CHCL): vCOOCu
1610cm-' (strong). Visible:ultraviolet (in CH,C1,): 300nm (strong). 400nm (shoulder). 670nm
(broad, weak). Elemental analysis, calculated for Cu(Cl8H,]O,)1: C 69-50%, H 9-97%; found: C 6947%, H 9-94%.
Copper extraction by abietic and linoleic acids
Copper acetate (0-95g. 0-97mmol) was placed in dichloromethane (CH,C1,. 50ml). To this suspension abietic acid (0-602g. l-99mmol) was added. Aliquots of 0-1 ml of the resulting blue solution
were taken and quenched with liquid ammonia during the course of the reaction. These aliquots
were analysed by infrared (using the band at 1590cm"1) and visible-ultraviolet spectroscopy (using
the band at 633nm). After several days there remained a suspension of unreacted copper acetate. In
a similar way the extraction of copper by linoleic acid was followed by observing the infrared band
at 1600cm"1 and the visible band at 670nm.
References
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2 BOMFORD, D., and ROY, A.. The techniques of two paintings by Dieric Bouts'. National Gallery
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3 WOUDHUYSEN-KELLER. R.. "Aspects of painting technique in the use of verdigris and copper
resinate' in Historical Painting Techniques, Materials, and Studio Practice, ed. A. WALLERT, E.
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Authors
MICHELE GUNN obtained
a degree in physical sciences from the Cheikh Anta Diop University of
Dakar, Senegal (West Africa). Subsequently she obtained a doctorate in applied chemistry from the
University of Paris (Paris VI), followed by postdoctoral work in bioinorganic coordination
chemistry. She also taught chemistry to painting restorers. From 1996 to 2001 she worked in the
Centre de Recherche et de Restauration des Musees de France (C2RMF) in Paris. Address: Musee
du quai Branly, Et. public, 5 rue Auguste Vacquerie, 75116 Paris, France.
GENEVIEVE CHOTTARD graduated
from the Ecole Nationale Superieure de Chimie de Paris in 1963.
She obtained her PhD from the University of Paris in 1968. The same year she became a research
fellow7 at the Centre National de la Recherche Scientifique (CNRS). where she now has the position of Research Director. Her research field is resonance Raman spectroscopy, mainly focused on
metalloproteins and their model complexes. Address: Laboratoire de Chimie Inorganique et
Materiaux Moleculaires, Universite Paris 6 Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris,
France.
ERIC RIVIERE obtained
his PhD at the University of Toulouse in 1991. Between 1992 and 1996 he
was in charge of physical measurements at the Laboratoire de Chimie de Coordination du CNRS at
Toulouse. In 1996 he obtained a CNRS Engineer position at the Inorganic Chemistry Laboratory at
the University of Paris South. Address: Laboratoire de Chimie Inorganique, UMR 8613, Universite
Paris Sud, ICMO-Bdtiment 420, 91405 Or say, France.
JEAN-JACQUES GIRERD obtained
his PhD at the University of Paris South in 1982. As a postdoctoral
fellow he worked in 1983 with Professor R.H. Holm at Harvard University. He was a researcher at
CNRS unitl 1988. when he became professor of inorganic chemistry at the University of Paris
South. In 1993 he was appointed director of the
Inorganic Chemistry Laboratory at the University, responsibility for chemistry teaching in the
medical
Address: as for Riviere. sciences. Co-founder in 1974 of the Laboratoire de
chimie et biochimie pharmacologiques et toxiJEAN-CLAUDE CHOTTARD graduated
from the Ecole cologiques. he is currently director of
the
Nationale Superieure de Chimie de Paris in 1963. Biomedical Department of Paris 5 University and
He obtained his PhD at the Ecole Normale administrator of the Saints-Peres University
Centre.
Superieure (ENS) and subsequently became a Address: Laboratoire de Chimie et
Biochimie
research scientist with the CNRS. Since 1972 he Pharmacologiques et Toxicologiques, UMR
8601,
has been professor of organic chemistry at the Universite Rene Descartes, 45 Rue des Saints
Peres,
Universite Rene Descartes (Paris 5), with special 75270 Paris 06, France.
Resume—Cette etude demontre que les resines et les acides gras sont susceptibles d'extraire les
ions cuivriques (cuivre II) du vert-de-gris (acetate de cuivre) et de la cendre bleue (carbonate de
cuivre basique). Les reactions d'echange de ligand entre ce dernier produit et les resines et acides
gras sont beaucoup plus lentes que dans le cas de I'acetate de cuivre. Le brunissement des couches
picturales est etroitement relie a la facilite relative avec laquelle a lieu I'extraction; le cuivre
diffuse sous forme de complexes carboxyliques de resines ou d"acides gras. Ces complexes se
forment dans la couche de peinture durant le broyage du pigment avec un liant contenant des
acides oleo-resineux et peuvent aussi se former a I'interface entre la couche picturale et une couche
superieure de nature organique comme un vernis.
Zusammenfassung—Die Studie :eigt, daft Harze und Fettsduren Kupfer-(IIj-Ionen aus Griinspan
(Kupferacetat) oder basischem Kupferacetat herauslosen konnen. Bei den basischen
Kupferacetaten ist die Geschwindigkeit des Ligandenaustausches mil Fettsduren oder Harzen weit
langsamer als bei Kupferacetat. Die Verbraunung der Malschichten kann direkt mil der relativen
Leichtigkeit der Extraktion des Kupfers kor-reliert werden. Kupfer diffundiert als Komplex mil
Fettsauren und Harzsduren. Diese Komplexe werden einer-seits bereits wdhrend des Reibens des
Pigmentes mit Harz- und Olsduren enthaltenden Bindemitteln gebildet, entstehen andererseits aber
auch an der Grenze zwischen Malschicht und daruberliegenden Schichten, nie beispielsweise dem
Firnis.
Resumen—Este estudio demuestra que los dcidos resinicos y grasos son capaces de extraer iones
de cobre (II) del verdigris (acetato de cobre) y del verditer (carbonate bdsico de cobre). Las
reacciones de inter cambio de ligandos del carbonato bdsico de cobre con los dcidos grasos y con
los dcidos resinicos son mucho mas lentas que en el caso de las del acetato de cobre. La
decoloration marron de las capas de pintura estd intimamente relacionada con la facilidad en la
extraction del cobre; el cobre se difunde en forma de complejos de dcidos grasos o de dcidos
resinicos carboxilicos. Estos complejos se forman en la capa de pintura durante el proceso de
molido del pigmento con un aglutinante que contiene dcidos oleoresinicos, aunque tambien se
forman en la zona de contacto entre esta capa y las superiores de naturaleza orgdnica, como por
ejemplo barnices.