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Composition of Renaissance Paint Layers: Simultaneous Particle Induced X-ray Emission and Backscattering Spectrometry L. de Viguerie,*,† L. Beck,†,‡ J. Salomon,† L. Pichon,† and Ph. Walter† Centre de Recherche et de Restauration des Muse´es de France (C2RMF, CNRS UMR 171), Palais du Louvre-Porte des Lions, 14 Quai Franc¸ois Mitterrand, 75001 Paris, France, and Institut National des Sciences et Techniques Nucle´aires (INSTN), UESMS-CEA Saclay, 91191 Gif-sur-Yvette Cedex, France Particle induced X-ray emission spectroscopy (PIXE) is now routinely used in the field of cultural heritage. Various setups have been developed to investigate the elemental composition of wood/canvas paintings or of cross-section samples. However, it is not possible to obtain information concerning the quantity of organic binder. Backscattering spectrometry (BS) can be a useful complementary method to overcome this limitation. In the case of paint layers, PIXE brings the elemental composition (major elements to traces) and the BS spectrum can give access to the proportion of pigment and binder. With the use of 3 MeV protons for PIXE and BS simultaneously, it was possible to perform quantitative analysis including C and O for which the non-Rutherford cross sections are intense. Furthermore, with the use of the same conditions for PIXE and BS, the experiment time and the potential damage by the ion beam were reduced. The results obtained with the external beam of the Acce´le´rateur Grand Louvre pour l’Analyse Ele´mentaire (AGLAE) facility on various test painting samples and on cross sections from Italian Renaissance masterpieces are shown. Simultaneous combination of PIXE and BS leads to a complete characterization of the paint layers: elemental composition and proportion of the organic binder have been determined and thus provide useful information about ancient oil painting recipes. In the study of paintings, the determination of the original composition can be very useful to date and even authentify them. To study the working method and the materials used by the artist is also important for art historical research and conservation purpose. To know the current chemical and physical state of the painting is crucial for responsible decisions on conservation and restoration treatments. Various analytical techniques have been used for this purpose. Particle induced X-ray emission (PIXE) has been used for the past decade for painting analysis.1-5 This technique is well adapted to the detection of the elements heavier than sodium up to * To whom correspondence should be addressed. † Palais du Louvre-Porte des Lions. ‡ Institut National des Sciences et Techniques Nucle´aires (INSTN). (1) Mando`, P. A.; Fedi, M. E.; Grassi, N.; Migliori, A. Nucl. Instrum. Methods 2005, 239, 71–76.
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uranium, and as a consequence, it has given successful results for the characterization of pigments based on mineral compounds.6,7 Particle induced γ-ray emission (PIGE), complementary to PIXE, has been used in canvas and wood painting for the detection of sodium and thus the identification of lapis lazuli (3Na2O, 3Al2O3, 6SiO2, 2Na2S). Indeed low-energy X-rays are strongly absorbed in the protective varnish whereas γ-rays are easily detected.8 Depth profiling was tested by changing the energy of the beam or the ion beam incident angle.9-11 X-ray diffraction and X-ray fluorescence are also used to identify the mineral fraction: the XRF spectra give similar information as PIXE, and XRD allows the determination of the mineralogical nature of the pigments.12-15 Used in a mapping mode, these techniques provide new information on the distribution of the pigments. The mapping mode in µ-XRD permitted identification, location, and quantification of the different mineral phases of lead whites in the stratigraphy of the Grunewald painting cross sections,16 and the XRF scan of a Van Gogh panel revealed a previous hidden composition.17 (2) Andalo`, C.; Bicchieri, M.; Bocchini, P.; Casu, G.; Galletti, G. C.; Mando`, P. A.; Nardone, M.; Sodo, A.; Plossi Zappala`, M. Anal. Chim. Acta 2001, 429, 279–286. (3) Neelmeijer, C.; Wagner, W.; Schramm, H. P. Nucl. Instrum. Methods 1996, 118, 338–345. (4) Neelmeijer, C.; Ma¨der, M. Nucl. Instrum. Methods 2002, 189, 293–302. (5) Denker, A.; Opitz-Coutureau, J. Nucl. Instrum. Methods 2004, 118, 677– 682. (6) Rajta, I.; Ontalba, M. A.; Koltay, E.; Kiss, A. Z. Nucl. Instrum. Methods 1997, 130, 315–319. (7) Mando`, P. A. Nucl. Phys. A 2005, 751, 393c–408c. (8) Grassi, N.; Migliori, A.; Mando, P. A.; Calvo del Castillo, H. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 219-220, 48–52. (9) Brissaud, I.; Lagarde, G.; Midy, P. Nucl. Instrum. Methods 1996, 117, 179– 185. (10) Lagarde, G.; Midy, P.; Brissaud, I. Nucl. Instrum. Methods 1997, 132, 521– 527. (11) Weber, G.; Martinot, L.; Strivay, D.; Garnir, H. P.; George, P. X-Ray Spectrom. 2005, 34, 297–300. (12) Mantler, M.; Schreiner, M. X-Ray Spectrom. 2000, 29, 3–17. (13) Hocquet, F. P.; Garnir, H. P.; Marchal, A.; Clar, M.; Oger, C.; Strivay, D. X-Ray Spectrom. 2008, 37, 304–308. (14) Bontempi, E.; Benedetti, D.; Massardi, A.; Zacco, A.; Borgese, L.; Depero, L. E. Appl. Phys. Mater. Sci. Process. 2008, 92, 155–159. (15) Gianoncelli, A.; Castaing, J.; Ortega, L.; Dooryhe, L.; Salomon, J.; Walter, P.; Hodeau, J. L.; Bordet, P. X-Ray Spectrom. 2008, 37, 418–423. (16) Welcomme, E.; Walter, P.; Bleuet, P.; Hodeau, J. L.; Dooryhee, E.; Martinetto, P.; Menu, M. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 825– 832. (17) Dik, J.; Janssens, K.; Van Der Snickt, G.; van der Loeff, L.; Rickers, K.; Cotte, M. Anal. Chem. 2008, 80, 6436–6442. 10.1021/ac901141v CCC: $40.75 2009 American Chemical Society Published on Web 09/04/2009
Figure 1. Analyzed cross sections. Table 1. Advantages and Limitations of 3 MeV Protons for Painting Analysis routine conditions PIXE
2-3 MeV protons
RBS
2-3 MeV helium ions
PIGE
3 MeV protons
selected conditions for paintings 3 MeV protons
Organic pigments as well as binders or varnishes are not seen by all these X-ray based techniques. The same is true for SEM which is routinely used to know the elemental composition of painting cross sections but does not give accurate C and O content. In order to analyze the organic components of the paintings, other independent techniques, such as Fourier transform-infrared spectrometry (FT-IR),18 gas chromatography (GC)19 and related methods (mass spectrometry GC/MS, pyrolysis PyGC, and highperformance liquid chromatography HPLC20) have to be carried out. Up until now and for specific cases, only infrared and Raman spectroscopies are able to qualitatively analyze organic and mineral matters.21,22 In this article, we propose to associate two ion beam techniques in order to simultaneously collect information on mineral compounds by PIXE and on organic compounds by backscattering spectrometry (BS).23,24 (18) van den Berg, K. J.; Boon, J. J.; Pastorova, I.; Spetter, L. F. M. J. Mass Spectrom. 2000, 35, 512–533. (19) Rioux, J. P. Techne` 1995, 2, 80–85. (20) van der Doelen, G. A.; van den Berg, K. J.; Boon, J. J.; Shibayama, N.; de la Rie, E. R.; Genuit, W. J. L. J. Chromatogr., A 1998, 809, 21–37. (21) Lo´pez-Gil, A.; Ruiz-Moreno, S.; Miralles, J. J. Raman Spectrom. 2006, 37, 966–973. (22) Cotte, M.; Susini, J.; Sole´, V. A.; Taniguchi, Y.; Chillida, J.; Checroun, E.; Walter, P. J. Anal. At. Spectrom. 2008, 23, 820–828.
advantages high cross sections of X-ray production non-Rutherford cross sections for light elements (improvement of C and O detection) large analyzed depth high cross section for γ-ray production
limits low-Z elements (Z < 11) not detected. no information on organic compounds low selectivity for high-Z elements
low cross sections for high-Z elements
This method was tested with the AGLAE facility, (AGLAE for “Acce´le´rateur Grand Louvre pour l’Analyse Ele´mentaire”) of the C2RMF laboratory (Centre de Recherche et de Restauration des Muse´es de France), located in the Louvre palace in Paris. AGLAE is a tandem pelletron particle accelerator (6SDH-2, National Electronic Corporation) entirely devoted to the study of cultural heritage artifacts.25 The possible ion beam analyses (IBA) are noninvasive. This work is part of a research project about components and recipes for oil painting in the Italian Renaissance and particularly about the realization of carnations. At that time, pigments were traditionally mixed with drying oils (linseed or nut oil). This liquid binding medium holds the pigment in suspension, allows it to be applied with a brush, and then dries to bind it to a canvas support. For centuries painters have been experimenting with paint formulas and adjusting compositions according to the effects they (23) Maxwell, J. A.; Campbell, J. L.; Teesdale, W. J. Nucl. Instrum. Methods 1989, B43, 218–230. (24) Beck, L.; Gutie´rrez, P. C.; Salomon, J.; Walter, P.; Menu, M. Proceedings of the XI International Conference on PIXE and its Analytical Applications, Puebla, Mexico, May 25-29, 2007. (25) Dran, J. C.; Salomon, J.; Calligaro, T.; Walter, Ph. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 219-220, 7–15.
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expected. The rheological and mechanical properties of a paint, and thus its behavior during and after application and the resulting visual aspect obtained, are linked to the proportion of the various ingredients and to their preparation (mainly mixing, heating, and addition of driers). Knowing these proportions is crucial to study the physical and chemical properties of Renaissance paint layers and then predict their conservation. The old recipes provide details about the way to make the colors or to prepare the oils but little information can be found about the exact proportion pigment/ oil. The historical sources are generally difficult to interpret in terms of quantities and even the nature of the materials is currently missing. As an example, the most precise indication that could be found is in the important manuscript Secreti diversi, preserved in Venice in the Marciana library and dated from the beginning of the 16th century: “Grind up the colour with linseed or nut oil as stiff as you can, that is, with as little oil as possible”.26 In order to improve our knowledge about Renaissance artworks and painting recipes, the analysis of painting cross sections can provide precious information. The recent setup developed by the AGLAE facility was used to obtain new information by analyzing painting cross sections from Italian Renaissance masterpieces. EXPERIMENTAL CONDITIONS Samples. In the first step, experiments were performed with samples of known composition in order to confirm the efficiency of the setup and to define the analytical procedure and its accuracy. Various samples of paint were prepared. Every paint is constituted by a mixture of microscopic pigment particles mixed in linseed oil. It was possible to compare the results on pure pigments on one hand with the same pigments in oil on the other hand. As an example, the results concerning zinc oxide, a modern white pigment, are reported here. Then, experiments were carried out on cross sections taken from a painting prepared by a restorer according to the traditional recipes of the 15th-16th centuries.27 The painting is composed of a gesso layer, a mixture of gypsum and animal glue, laid on a wood support as a preparation layer. Various layers of lead white mixed with linseed oil were applied on the gesso substrate. In some parts of the painting, a red pigment was added, vermilion or iron oxide. Finally, four painting cross sections taken from Italian masterpieces of the 15th-16th centuries were analyzed. Indeed the C2RMF laboratory owns a large collection of painting cross sections from many Italian Renaissance painters such as Giotto, Botticelli, Raphael, Boltraffio, and Leonardo da Vinci. The description of the analyzed painting cross sections is summarized in Figure 1. Measurement Setup. Samples were analyzed with the external proton beam of AGLAE under He flux (4.5 L/min). The external beam setup28 allows us to perform simultaneously PIXE, PIGE, and BS with two Si(Li) X-ray detectors, one HPGe γ-ray detector, and one charged particle detector. A 3 MeV-proton beam has been selected for the experiments.
Despite its poor discrimination for heavy elements, BS with protons has some interesting characteristics, especially a large analyzed depth in the material and intense non-Rutherford cross sections for low-Z elements (Table 1). The 3 MeV protons allow us to increase the sensitivity for the detection of C and O, which can partially be attributed to the organic ingredients of the paint. Furthermore, the simultaneous operation of PIXE and BS presents other advantages, such as the same area is analyzed by one experiment reducing experimental time and potential discoloration of the pigments.29-31 Moreover, for the analysis of the painting cross section, the beam size was reduced to less than 20 µm and it was measured before each set of experiments. In order to lower the damage risks, particular care was also devoted to monitoring the beam current intensity which was decreased as much as possible (around 150 pA in runs lasting typically from 100 to 200 s) but keeping the capacity to measure the irradiating particle dose.
(26) Merrifield, M. P. Marciana manuscript. In Original Treatises on the Arts of Painting; Dover Publications: New York, 1967; pp 302-370. (27) Dunkerton, J.; Gordon, S. F.; Penny, N. Durer to Veronese, Sixteenth-Century Painting in the National Gallery; National Gallery Publications Limited, Yale University Press: London, 1999. (28) Calligaro, T.; Dran, J.-C.; Salomon, J.; Walter, Ph. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 226, 29–37.
(29) Gutie´rrez, P. C.; Beck, L. Proceedings of the XI International Conference on PIXE and its Analytical Applications, Puebla, Mexico, May 25-29, 2007. (30) Enguita, O.; Caldero´n, T.; Ferna´ndez-Jime´nez, M. T.; Beneitez, P.; Millan, A.; Garcı´a, G. Nucl. Instrum. Methods Phys. Res., Sect. B 2004, 53, 219– 220. (31) Absil, J.; Garnir, H.-P.; Strivay, D.; Oger, C.; Weber, G. Nucl. Instrum. Methods Phys. Res., Sect. B 2002, 198, 90–97.
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Figure 2. BS spectra of the white pigment ZnO, with and without oil. (a) Comparison between the BS spectra of the pure pigment (pink line) and a mixture of pigment (52%) and linseed oil (48%) (black line). The simulated spectra obtained by SIMNRA of the pure pigment (pink 9) and the pigment mixed with oil (dark blue b) are also indicated. (b) Detail of the mixture spectra simulation: Zn (purple b) comes from the pigment, C (blue b) from the oil, and O (red 2) from both. The simulation of the contribution of the O content of the pigment is indicated by an orange dashed line. Experimental conditions: H+, 3 MeV, θ ) 150°, I ≈ 1 nA, Q ) 0.2 µC, beam diameter ≈ 100 µm.
Table 2. Results Obtained on the Painting Model Samplesa 78 wt % pigment (PbSO4, 3PbO), 22 wt % binder area B
area C measured
Pb wt % S wt % O wt % C wt % pigment wt % binder wt % ratio pigment/binder
measured
expected
average
standard deviation
expected
average
standard deviation
66.4 2.3
67.3 3.1 12.0 13.6 83 17 4.9
1.0 0.4 0.6 0.5 0.9
66.4 2.3
59.5 3.0 12.8 14.5 75 25 3
1.7 0.5 1.4 1.7 1.6
78 22 3.5
78 22 3.5
a Comparison between the expected values and the experimental values (element wt %, pigment and binder wt %, and their ratio) for the lead white layers. The composition chosen by the restorer leads to the expected values. For the calculated values, the average of all the results obtained on the lead white layer is indicated with their standard deviation: around 30 measurements for sample B and 10 for sample C were considered. The carbon and oxygen wt% were determined by the SIMNRA fit; the lead and sulfur wt% were obtained after normalization of the GUPIX calculations.
Figure 3. Results obtained on the model samples prepared by a restorer: elemental profile of concentrations with steps of 5 µm.
A Peltier-cooled X-ray detector is usually used for monitoring the beam dose using the Si-K peak emitted by the Si3N4 exit window. With such a beam size and a low current, the intensity of the Si-K peak is very low and cannot be measured accurately for the BS analyses. Thus, we used a recently developed microbeam setup, extensively described in ref 32, which permits simultaneous or sequential PIXE/BS combination with an improved dose monitoring. The exit nozzle houses an annular surface barrier detector. This detector collects the BS signal from the sample and is also used for dose monitoring via a second BS signal from a gold foil that is hit by the beam a fraction of the time through a beam-deflection system. This setup permitted us to combine very low damage risks with (32) Salomon, J.; Dran, J.-C.; Guillou, T.; Moignard, B.; Pichon, L.; Walter, P.; Mathis, F. Appl. Phys. Mater. Sci. Process. 2008, A92, 43–50.
Figure 4. BS and PIXE spectra on the cross section from Salvator Mundi (all the points in the lead white). (a) PIXE spectra and (b) BS spectra recorded at the same time as the PIXE spectra (black line), with the contribution of each element: C (b), O (2), and Pb (9). Pb comes from the pigment (PbCO3, Pb(OH)2); C and O are present in both pigment and binder. The global simulation obtained by SIMNRA, not visible here, is the sum of these contributions. Experimental conditions: H+, 3 MeV, θ ) 170°, I ≈ 0.14 nA, Q ) 0.015 µC, beam diameter ≈ 15 µm.
accurate beam dose monitoring which is essential for data treatment. Elemental concentration profiles from the surface were performed on the painting cross sections. Samples were translated in front of the external microbeam with steps of 20 µm (samples 1 and 2) or 5 µm (sample B and C, 3 and 4) in order to obtain Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
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Table 3. Results Obtained on the Lead White Based Layers from the Painting Cross Sectionsa cross section (1) Salvator Mundi School of Vinci (hair) (2) Young Lady with a dog School of Vinci, (neck) (3) Virgin of the Casio’s Boltraffio (child, carnation) (4) Pieta, Solario (wrist) a
layer
binder wt %
lead white wt %
colored pigment
red layer white layer white layer reddish layer white layer white layer
35 15 5 15 10 22
48 85 95 80 90 78
HgS (8%) + Ca, Si, Al, Sn (≈ 9%) none none Si, Fe, K (≈ 5%) none none
Calculated values of the pigment and binder content. The reported value is an average of all the measurements made on the same layer.
Then BS spectra are simulated with SIMNRA35 for both Rutherford and non-Rutherford parts of the spectra. This step provides the concentration of the main elements, light elements such as C and O, as well as heavier elements such as Ca and Pb. It is not possible to know the exact amount of hydrogen. However it has sometimes to be introduced and the amount is adjusted so that the BS spectrum is well simulated. When the nature of the binder is known, estimations can be made: for oils, according to the fatty acids formulas, it can be considered approximately that H is twice more abundant than C. The C and O content of the binder is calculated by subtracting their pigment contribution. The binder proportion is then obtained by summing the binder contribution of C, O, and H. As a final step, since organic elements are not seen in PIXE, the concentrations obtained by GUPIXWIN treatment (matrix and traces) are normalized with the proportion of inorganic elements, calculated by SIMNRA. In summary, the combination of PIXE and RBS provides absolute element concentrations in light and heavy elements, leading to the determination of pigment to binder proportion.
continuous profiles representing elemental distributions as a function of depth. Microdiffraction experiments were also carried out on the painting cross sections. An equipment recently developed at C2RMF for the analysis of artworks and archeological materials allows the nondestructive analysis of samples and objects belonging to cultural heritage and, in our case, to perform µXRD analysis directly on the painting or sample cross sections. The system is built with a Rigaku MSC Micromax-002 X-ray tube with copper anode (wavelength of 1.5418 Å) and a microfocused electron beam of 20 µm in diameter and a power of 30 W. A quasi parallel beam is formed by reflection on a Kirkpatrick-Baez mirror to provide a small beam with a high flux. Its diameter is below 200 µm, and its residual beam divergence is 3.2 mrad. The flux on the sample is 5 × 107 photons per second using a collimator of 100 µm and 2 × 108 photons per second for 200 µm. Two laser pointers and a CCD video-microscope intersect at the analyzing position where the X-ray beam impinges the surface of the sample. The data were collected with a R-Axis IV scanner and imaging plates as a 2D detector.15 The time of measurements is 5-20 min. A corundum sample (a-Al2O3) is used for the calibration of the geometrical parameters of the system. Complementary SEM observations were performed on a Philipps XL 30 CP, operating at a current intensity of about 100 µA and an excitation potential of 20 kV. Data Treatment: General Procedure. The main difficulty in estimating the binder/pigment ratio is that C and O are often present in both pigment and binder. In order to overcome this problem, a three-step methodology has been developed: (i) the nature of the pigments is identified by µ-XRD and elemental concentrations measured by PIXE; (ii) the global elemental composition of the matrix, containing pigment and binder, is determined by BS; and (iii) the C and O contribution of the pigment is subtracted from the global composition, thus leading to the C and O binder contribution. Indeed, the first step is the identification of the main pigments by µ-X-ray diffraction. The free software FIT2D33 allows the transformation of 2D-images into standard XRD diagrams, and the Bruker-AXS EVA software is used to select the crystalline phases from the diagrams. Second, the PIXE data are processed in order to measure the quantity of each element heavier than Z ) 11 that will have to be considered for BS data treatment. PIXE spectra have been fitted by GUPIXWIN,34 which extract elemental concentrations. In our analytical procedure, the mineral contribution of the matrix is calculated from these concentrations.
RESULTS Thick Targets of Painting. Figure 2 shows the measured and simulated BS spectra of a pure ZnO pigment and a thick paint layer of the same pigment mixed with linseed oil. The pigment spectrum is characterized by two edges corresponding to the backscattered ions on zinc and oxygen. The intensity of the plateau leads to the 1:1 stoichiometry of Zn and O. The paint spectrum shows the same features with, in addition, the C signal from the organic binder. By extraction of the elemental concentrations by SIMNRA and assuming that the C signal mainly originates from the linseed oil, it is possible to calculate the pigment-binder proportion. In this example, the weight ratio of ZnO pigment and linseed oil was found to be 1.3 ± 0.2 (57 wt % of pigment, 43 wt % of binder) for an expected ratio of 1.1 ± 0.1 (52 wt % of pigment, 48 wt % of binder). The uncertainty on the experimental results is due to the uncertainty on experimental parameters as well as on the SIMNRA fit. Concerning the expected ratio, the uncertainty is due to the painting preparation (weighing and mixing). Such analyses were undertaken for another set of pigments and commercial paintings containing various pigments and extenders. The PIXE/BS experimental results were in agreement with the expected values.36 Painting Cross Sections. Model Samples. For these samples, the pigments used by the restorer have been analyzed by X-ray diffraction before mixing with oil: CaSO4, 2H2O (gypsum) and
(33) Hammersley, A. P.; Riekel, C. Synch. Rad. News 1989, 2, 24–26. (34) Maxwell, J. A.; Campbell, J. L.; Teesdale, W. J. Nucl. Instrum. Methods Phys. Res., Sect. B 1989, 43, 218–230.
(35) Mayer, M. Nucl. Instrum. Methods Phys. Res., Sect. B 2002, 194, 177. (36) Beck, L.; Gutierrez, P. C.; Salomon, J.; Walter, P.; Menu, M. Les Cahiers de la Musique 2008, 9, 56–62.
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Figure 5. Elemental profile of concentrations with steps of 5 µm obtained on the lead white based layers from (a) the painting cross section 3 taken from the “Pieta” and (b) the painting cross sections 4 taken from the “Salavator Mundi”.
Figure 6. SEM observations of the painting cross section 4 (from the “Salvator Mundi”).
PbSO4, and 3PbO (sold as a lead white) were identified as the main pigments. For each measurement on the cross section, the matrix composition and the trace elements are determined, respectively, from the PIXE low-energy and high-energy X-ray spectrum. Then, the elemental concentrations in carbon and oxygen and the main inorganic elements have been calculated from the BS spectra. The proportion of pigment and oil were deduced after subtracting the O contribution of the pigment. Once the organic content is known, the absolute concentrations of each element in the painting layers were calculated. Table 2 indicates the results obtained on the lead white layer from the two model samples: these results are in close agreement with the expected values chosen by the restorer. In order to estimate the heterogeneity of the paint layer composition, the
standard deviation on all the measurements performed on the lead white layer is also indicated. The absolute elemental concentrations profiles are presented in Figure 3, providing an accurate representation of the distribution of the pigments in the differents layers. Painting Cross Sections of the School of Leonardo da Vinci. The same procedure was applied on a set of paintings of the Italian Renaissance (15th-16th centuries) presented in Figure 1. The PIXE low-energy spectrum and the BS spectrum obtained on the lead white layer of the cross section (4) from the “Salvator Mundi” are presented in Figure 4. The proportion pigment/binder could be obtained for each sample and are presented in Table 3 for the lead white layers. These results are quite similar from one painting cross section to another: when the pigment is not mixed with other colored pigments, the proportion of lead white is from 85 to 95 wt % for a binder content of 5-15 wt % (corresponding to a pigment/binder ratio from 6 to 19). In the cross section (3) (taken from the “Pieta” by Solario), this percentage is lower (78%). However, this result must be taken cautiously: the beamsize was reduced to approximately 15 µm whereas the layer thickness is around 20 µm. Lower ratios (from 2 to 6) can be observed if color pigments are added. Indeed in the cross section (4) taken from the painting “Salvator Mundi”, the binder content of the colored layer is 35%, whereas it is around 15 wt % in the layer containing only lead white. This can be explained if we consider the absorption value of each pigment. The oil absorption value is the minimum amount of linseed oil which must be added to a pigment to transform it Analytical Chemistry, Vol. 81, No. 19, October 1, 2009
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from a powder to a cohesive plastic mass. For lead white, the oil absorption value is around 8% in weight.12 The results around 10% obtained on the paintings of the Italian Renaissance are clearly in agreement with this value and with the advice given in the Marciana manuscript of the 16th century: “Grind up the colour (. . .) with as little oil as possible”.24 This text, which was the only one found in several books of recipes about the proportion pigment/binder, refers clearly to a very small amount of oil, which corresponds to the modern definition of the oil absorption value. For umber and vermillion, the oil absorption values can vary a lot: 25-70% and 12-30%, respectively.37 Higher oil contents are also observed in the studied Renaissance paintings when lead white is mixed with these colored pigments. It has to be noted that the binder proportion also depends on the transparency required: a varnish layer which has to be transparent is completely organic. Glaze layers will appear translucent with low pigment content whereas layers with high pigment content are totally opaque, as lead white layers. Varnish and glazes are not visible in these cross sections; however, in the second cross section (taken from “Young lady with a dog”) a layer with higher binder content is also present over the lead white layer. This layer, which appears white in the picture in Figure 1, has around 90 wt % binder content. The documentation about this painting (“Young lady with a dog”) does not permit us to conclude about the nature of this layer which may be due to a later repaint. The complete elemental profiles of concentrations are presented for two painting cross sections (from the “Salvator Mundi” and the “Pieta”) in Figure 5. Variations in the pigment proportions and concentrations profile in one layer are clearly visible and have to be linked to the intrinsic heterogeneity of a painting layer (Figure 6). Moreover in the painting cross section (4) taken from the “Salvator Mundi”, the proportions profile of the lead white based layer (Figure 5a, from 60 µm) indicates pigment proportions between 80 and 100%. For a pigment content of 100%, no binder has been detected, which indicates that only pigment has interacted with the beam. This has also to be linked to the SEM picture (Figure 6) of this painting cross section in which the presence of variable size lead white grains is visible. In our experiment, the analyzed area can have been one pigment grain with a diameter higher than 15 µm (which corresponds to the beam size). In that case, and if no binder has penetrated inside the grain, the pigment content is 100%. We focused our study on the lead white layers; however, the proportion of pigment/binder has been also estimated for the preparation layers, present in the painting cross sections 1 and 3 (respectively, based on calcium sulfate and calcium carbonate compounds). One should note however that the binder used in the preparation layers are commonly animal glue and water and (37) Mantler, M.; Schreiner, M. X-Ray Spectrom. 2000, 29, 3–17.
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not oil as in the others layers. PIXE and RBS do not permit the identification of the nature of the binder but only the proportion. In sample 3, the calcium carbonate concentration is around 90%; the calcium sulfate content is around 70% in sample 1. It would be necessary to study more preparation layers to be able to conclude on common calcium compound concentrations. CONCLUSIONS Most of the available techniques for the painting cross sections studies permit the identification and the (semi-) quantification of the mineral part. To access the organic part of painting cross sections remains one of the challenges open today in painting studies. However, the determination of the organic proportion is fundamental for the study of the paint properties which partly determine its conservation. Thanks to the combination of XRD, PIXE, and BS, we achieved the quantification of the organic matter in painting cross sections. PIXE and XRD give access to the elemental composition (from major elements to traces) of the mineral compounds, pigments, and extender. BS brings the organic compound estimation and, in some cases, the thickness information. The simultaneous PIXE and BS experiments have the advantages of analyzing the same area in one experiment. The data treatment is handy by using routinely available simulation softwares. The proportion binder/pigment and profiles of the concentrations were calculated for various painting cross sections of the Italian Renaissance. Similarities in composition were found for the lead white layers. These results provide information about the ancient oil painting recipes: the binder proportion is kept the same for each pigment and corresponds to the oil absorption value. As this study focused on the lead white based layers it may be interesting to analyze more cross sections to get information on the colored or the organic layers. This kind of study could also be enriched with other techniques, such as time-of-flight-secondary ion mass spectrometry (TOF-SIMS) or FT-IR microscopy, in order to access information on the nature of the binder.38,39 ACKNOWLEDGMENT This issue is dedicated to the memory of Joseph Salomon, our colleague and friend. The authors wish to thank Brice Moignard from the AGLAE team for its help during work at AGLAE. We also express our gratitude to Carolina Gutie´rrez and Aurelia Chevalier for the paint samples preparation. Received for review May 25, 2009. Accepted August 19, 2009. AC901141V (38) Keune, K.; Boon, J.-J. Anal. Chem. 2004, 76 (5), 1374–1385. (39) Mazzeo, R.; Joseph, E.; Prati, S.; Millemaggi, A. Anal. Chim. Acta 2007, 599 (1), 107–117.
