Revealing the Origin and History of Lead-White Pigments by Their

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Revealing the origin and history of Lead-white pigments by their photoluminescence properties. Victor Gonzalez, Didier Gourier, Thomas Calligaro, Kathleen Toussaint, Gilles Wallez, and Michel Menu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04195 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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Analytical Chemistry

Revealing the origin and history of Lead-white pigments by their photoluminescence properties. Victor Gonzalez‡†#, Didier Gourier*†‡, Thomas Calligaro‡†, Kathleen Toussaint†‡, Gilles Wallez†‡#, and Michel Menu‡†. †

Chimie-ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie-Paris (IRCP), F-75005 Paris, France Centre de Recherche et de Restauration des Musées de France (C2RMF), Palais du Louvre, F-75001 Paris, France. # Sorbonne University, UPMC Université Paris 6, F-75005 Paris, France ‡

Corresponding Author * Didier Gourier: [email protected]

ABSTRACT: The lead white pigment, composed of two main mineral phases cerussite PbCO3 and hydrocerussite 2PbCO3.Pb(OH)2, has been used in paintings since the Antiquity. The study of historical sources revealed that a large variety of lead white qualities were proposed, depending on the degree of sophistication of the pigment synthesis. Investigation of photoluminescence of the two constitutive mineral phases gave insight into the origin of the visible emission of these materials, and emphasized the influence of structural defects on their photoluminescence properties. These effects were observed by combining emission and excitation spectra in two-dimensional representations. For each excitation wavelength, between 250 nm and 400 nm (4.9 – 3.1 eV), luminescence spectra were collected between 400 nm and 800 nm (3.1 – 1.5 eV). Two types of emission-excitation bands were identified: an emission excited in the optical bandgap of the compounds (about 5 eV), which depends on the constitutive phase (2.8 eV in cerussite and 2.1 eV in hydrocerussite), and broad emission bands in the same energy range excited below the optical gap, which are sensitive to the synthesis method and the nature of post-synthesis treatments. It is proposed that this sensitivity of photoluminescence properties of lead-white pigments could be used as fingerprints of their origin and history.

Lead-white pigment is ubiquitous in paint layers from the antiquity to the 20th century, sometimes mixed with other colors.1 Lead-white is composed of a mixture of two crystalline phases,2 namely cerussite PbCO3 (labelled C) and hydrocerussite Pb3(CO3)2(OH)2 (labelled HC) in variable proportions.3,4 The traditional manufacturing of lead-white, based on the corrosion of lead metal, remained almost unchanged since the Antiquity until the 19th century.5 Metal sheets were placed into jars above vinegar (acetic acid solution). The jars were then covered up by horse manure, and left to rest for 20 to 90 days. Several rows of jars were stacked up, explaining the name “stack-process” often associated to the lead white synthesis. While the acetic acid vapors led to the formation of lead acetate, the decomposition of the manure produced heat and carbon dioxide that allowed the formation of cerussite C and hydrocerussite HC. It is known that various post-synthesis treatments were applied to the pigment, among which the washing and grinding of the pigment in water or in acidic solutions (vinegar), the heating in water and the levigation in order to select pigment particles according to their size.6 Another particular post-synthesis treatment mentioned in historical writings consisted in the exposure of the synthesized pigment to sunlight.7 After these post-synthesis operations, painters mixed the mineral powder with an organic binder and other pigments of their choice, before using the paint. Starting from the 19th century, the development of modern chemistry led to the multiplication of synthesis processes.8 Transformation of metallic lead in lead carbonates could take place in dry as well as in aqueous environment and numerous synthesis ways were proposed such as precipitation, electrolysis, etc. Historical sources reveal that numerous lead-white qualities were proposed by the manufacturers, at very different prices.9-11 Questions remain as to what constituted those various qualities, and how they were obtained. A first approach consists in a precise crystallographic analysis of the two phases of interest. A recent Synchrotron HR-XRD study conducted on lead-white micro-samples collected on Masters paintings of the Louvre Museum allowed the identification of crystallites morphologies and phase proportions as potential markers of pigment qualities.12

In many historical treatises, optimal optical features were described as the main indicator of a high quality material.13 Hence, the “whiteness”, “brightness” or the “strength” of lead white were praised, and artists were undoubtedly seeking for particular grades of lead white according to the pictorial results they were looking to achieve. The study of the optical properties of the pigment thus appears as a good way to better understand the nature of those qualities. Photoluminescence (PL) properties of molecules and solids are sensitive to the electronic structure of the optically active species (ie their nature) which determine the wavelength and the intensity of PL bands, and to the electron-phonon coupling (ie their interaction with vibrations of the host) which determine the shape of the PL bands and their shift from the corresponding excitation band (Stoke shift). Consequently, these PL properties are influenced by the nature and structure of the host, its organization and also the presence of impurities and structural disorder. As structural defects and impurities are controlled by thermodynamics, kinetics and chemistry (starting materials, synthesis process), PL properties of a material should reflect all its history, from the synthesis to the evolution under interaction with its environment. For several years, the study of photoluminescence properties plays an increasing role in the analysis of artworks and cultural objects. Analytical techniques based on the luminescence present numerous advantages. They can be used to analyze both the inorganic and organic parts of those complex systems, with a good level of sensitivity and reproducibility. Since the 1980’s, many studies have been devoted to luminescence properties of pigments, organic binders and varnishes.14-20 The resolution of emission spectra is generally higher than that of absorption spectra, however except for rare earth ions, photoluminescence spectra are often difficult to interpret in absence of other information, such as the absorption transitions which produce these emissions (excitation). Thus a systematic combination of photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra should provide a kind of two-dimensional spectral character to optical emission spectroscopy. In the present work, we used laser induced PL-PLE spectroscopy to

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probe lead-white pigments, including historical pigments as well as pure C and HC phases synthesized by lead corrosion and co-precipitation method. We observed that PL properties are sensitive to the synthesis methods as well as post-synthesis treatments, and this work shed some light on the origin of the visible PL emission in lead-white and on the criteria that determined the quality of lead white pigments used by Old Masters. EXPERIMENTAL

Samples Synthesis. Pure cerussite (C) and hydrocerussite (HC) phases were prepared by two methods. The first one consisted in co-precitation of Pb(NO3)2 and Na2CO3 (Merck, powders, 99%) in stoichiometric proportions. The equilibrium between the two phases formation is pH dependent. pH must be maintained to a value < 6 in the case of C, and must be increased between 8 and 10 in the case of HC, which is achieved by adding a solution of NaOH (Normapur, granule, 100 %). Two samples were chosen for each of the two phases: one immediately after synthesis (labelled Cchem and HCchem), and one after 90 days of maturing, with control of the pH (labelled Cchem(Mat90) and HCchem(Mat90). Lead white powders were also synthesized by dry corrosion of metallic lead in order to mimic the historical stack process. Lead foils (Goodfellow, thickness 2.0 mm, purity 99.95) were placed in a reactor equipped with ventilation slits, over a beaker containing acetic acid diluted to 8 w% (analogous to vinegar), and another one containing a 5 w% sugar aqueous solution pre-heated to 50 °C, and mixed with 10 g yeast. This mixture was renewed every 4 days to maintain a constant production of carbon dioxide. The CO2 partial pressure in the reactor during the experiment could not be measured from gas absorption in a potash solution (< 1 %). The thin corrosion film, visible after 1 hour was shown by X ray diffraction (XRD) to be made of lead acetate hydrate (Pb(CH3COO)2.3H2O) then plumbonacrite Pb10(CO3)6O(OH)6 formed after a few hours. The latter phase was firmly fixed on the metal. After 1 day, the corrosion film appeared to be constituted of hydrocerussite Pb3(CO3)2(OH)2, with traces of cerussite PbCO3, whereas the former phases had vanished. Conversely to plumbonacrite, hydrocerussite develops as flakes easily removable from the metal. Further exposition to corrosion leads to a decreasing HC:C ratio, but even after two weeks, the two phases still co-exist, as a possible consequence of a local equilibrium also involving metallic lead. So, in order to obtain purer cerussite, flakes were removed from the substrate before sustaining a 9-day exposition in the same conditions. Samples synthesized by this corrosion method are labelled Ccorr and HCcorr. One commercial sample synthesized by the traditional stack process (slow corrosion of metallic lead in acidic conditions), was manufactured by the Natural Pigments company (Willits, CA 95490, USA), starting from 99.99 % lead sheets of 2-3 mm thickness and 5 % content acetic acid vinegar. Corrosion time was 3 months. XRD shows that this sample is a mixture of HC and C in proportion HC:C=90:10. It is hereafter labeled HC90/C10stack or more simply HC/Cstack. Sample

Post-synthesis treatment HC:C (w%) Cchem 0:100 None Cchem(Mat90) 0 :100 Maturing in H2O for 90 days HCchem 100:0 None HCchem(Mat90) 100 :0 Maturing in H2O for 90 days Ccorr 0:100 None HCcorr 100:0 None HC90/C10stack 90:10 Washing in water + grinding HC30/C70hist 30:70 Not documented HC65/C35hist 65:35 Not documented Cchem(UV) 0:100 UV exposure for t = 7 days HCchem(UV) 100:0 UV exposure for t = 7 days HCchem(AcOH) 86:14 Washing in AcOH (5%) for t = 5 min Cchem(AcOH) 0:100 Washing in AcOH (5%) for t = 5 min HCchem(H2O) 100:0 Heating in H2O at 80°C for t = 3 h HC/Cstack(AcOH_w) 78:22 Washing in AcOH (5%) for t = 5 min HC/Cstack(AcOH_g) 67:33 Grinding in AcOH (5%) for t = 5 min HC/Cstack(H2O) 100:0 Heating in H2O at 80°C for t = 3 h Table 1. Origin and composition of cerussite and hydrocerussite samples. Synthesis methods are given in each sample name : chem = coprecipitation in aqueous phase ; corr = corrosion of metallic lead ; stack = « commercial »: reproduction of the stack process; hist = « Historical »: process not documented. The ratio HC:C were determined by XRD.

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Two historical lead-white samples (references NMS6143 and NMS6144) synthesized in 1867 were provided by the National Museum of Scotland. These samples are also mixtures of HC and C in proportions HC:C=30:70 for NMS6143 and 65:35 for NMS6144, and are hereafter labelled HC30/C70hist and HC65/C35hist, respectively. They belong to a collection illustrating the manufacturing of lead-white pigments, and present a particular interest because NMS6143 was labeled “first quality”, while NMS6144 was labeled “second quality”. This illustrates the diversity of lead-white grades available in the past. All the sample characteristics are summarized in Table 1. Post-synthesis treatments. We made several reconstitutions of postsynthesis treatments used by artists and paint manufacturers in the past. Simulation of sun exposure was achieved using a Xenotest 150S chamber, equipped with a 2.7 kW Xenon lamp. This setup was used in “outdoor solar light” mode (combination of IR filters) and the duration of illumination was 7 days, producing 70 W/m2 in the 300-400 nm range. This corresponds to 2 months of mean sunlight illumination in Europe. The temperature reached inside the chamber was 40 °C. For the acidic treatment, a 5 % acetic acid solution was used to simulate vinegar. Phases of interests were either washed (for 5 min) or ground (for 5 min in a mortar) in this solution. The remaining powder was washed with water and dried. For the heating treatment in water, powders were simply placed into water, at T=80°C, for 3h. The XRD study of the treated materials revealed that the effect of acidic treatment on hydrocerussite was the formation of cerussite. Conversely, the formation of hydrocerussite was observed after heating of cerussite in water. The characteristics of these samples are given in Table 1.

Materials and methods X-ray diffraction. All samples at the exception of historical ones were first characterized by X-ray diffraction (XRD) using a Panalytical X’Pert Pro, 45 kV, 40 mA, λKα1 = 1.5405 Å, combined with Rietveld refinement. Samples synthesized by precipitation (Cchem and HCchem) and lead corrosion (Ccorr and HCcorr) were composed of pure phases, while the “commercial” sample synthesized by lead corrosion (HC/Cstack) was composed of a mixture of cerussite (~10 % ±1 %) and hydrocerussite (~90 % ±1 %). The XRD analysis of the two historical samples (NMS6143 and NMS6144) and the commercial sample was carried out at the ID22 High-Resolution XRD beamline (ESRF, France). XRD patterns are given in Supplementary information (Figure S1). Optical absorption. Absorbance spectra were recorded on powder samples using a Varian CARY 6000i UV-Vis-NIR spectrophotometer, equipped with a InGaAs detector (Peltier cooling) and a diffuse reflectance accessory (integrating sphere of 110 mm diameter, coated in PTFE). Luminescence. Photoluminescence excitation (PLE) was performed using a pulsed laser EKSPLA NT342B/SFG-10-WW, at a frequency of 10 Hz. This laser is based upon an OPO laser pumped using a YAG laser. The excitation range was fixed between 250 nm (4.9 eV) and 400 nm (3.1 eV). The width of the laser beam is less than 5 cm-1 and the pulse duration 5 ns. The photoluminescence (PL) signal was recorded through an Acton Research SP2300 spectrometer, with a focal of 300 mm and equipped with 3 gratings. The grating selected for this study was a 300 /mm, blazed at 500 nm. At the exit of the spectrometer, an ICCD camera Princeton Instrument PIMAX IV (pixel size 26 x 26 µm) equipped with 1024 integrated diodes allowed simultaneously visualizing an entire wavelength range according to the chosen grating position. Furthermore, because this camera is intensified, it is possible to record the signal with a delay (gate delay) ranging from 27 ns to 25 ms, and a diode exposure time (gate width) also tunable from few nanoseconds to several milliseconds. In our study, the selected emission range was 400 – 800 nm (3.1 – 1.8 eV). An emission spectrum was collected at each excitation energy (every 10 nm). For each sample, the PL spectra were plotted versus PLE in twodimensional diagrams, with PL intensities in false colors scaled to the most intense emission of the data set. Samples were shaped under the form of powder pellets of 12 mm diameter and ~0.5 mm thickness. They were placed directly in front of the laser beam. We checked that the laser beam did not induce any damage or creation of color centers, by scanning the PLE from low to high energy and then back to low energy, for HCchem and Cchem. The obtained PL spectra were identical. RESULTS AND DISCUSSION

Pure phases and historical pigments The main features of photoluminescence (PL) properties of lead white depend on excitation energy. Figure 1 shows the PL spectra of a series of


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samples excited at 5 eV (248 nm) (Figs. 1A and 1B) and at 3.44 eV (360 nm) (Figs. 1C and 1D). For excitation at 5 eV, cerussite is characterized by an emission band peaking around 2.7-2.8 eV (~ 458-443 nm), with a full width at half maximum (FWHM) of about 100 nm, and a weaker broad emission at lower energy, peaking around 2.1 eV. The main emission band is shifted to 2.1-2.0 eV (~590-610 nm) in hydrocerussite, with the same FWHM, but with a lower intensity and an additional weaker emission around 2.8 eV. It thus appears that the emission at 2.7-2.8 eV is characteristic of cerussite while the emission at 2.0-2.1 eV is characteristic of hydrocerussite. We may hypothesize that the weak emission bands around 2.8 eV in hydrocerussite and 2.1 eV in cerussite can be ascribed to traces of cerussite in hydrocerussite, and of hydrocerussite in cerussite, respectively, in quantity below the detection limit of XRD (a few %). We

may also conclude that visible emission bands excited at 5 eV are intrinsic to the lead white matrices. On the contrary, PL spectra excited at lower energy, namely 3.44 eV (Figs. 1C and 1D) exhibit more variability. It is no longer possible to distinguish cerussite from hydrocerussite, and the spectra are dependent on post-synthesis treatments as discussed below. The spectra are clearly the superposition of 2 to 3 different emission bands. This variability of photoluminescence properties of lead-white pigments with their chemical history strongly suggests that optical transitions are influenced by disorder in the matrices (atomic or molecular defects) and (or) by impurities.

Figure 1. PL spectra of cerussite (A, C) and hydrocerussite (B, D). Emissions were excited at 5 eV (A, B) and at 3.44 eV (C, D). In order to gain deeper insight into the influence of synthetic route of lead white pigments, the emission spectra were recorded by varying the excitation from 3.0 eV (413 nm) to 5.0 eV (248 nm) by steps of 10 nm. The ensemble of results are plotted in the form of two-dimensional diagrams representing emission energy versus excitation energy. The emission intensities are represented in false colours, and scaled to the highest intensity in each diagram. The results are shown in Fig. 2 for “pure” cerussite and hydrocerussite phases synthesized by precipitation process (Figs. 2A and 2D) and by corrosion of metallic lead (Figs. 2H and 2I), and for two types of cerussite/hydrocerussite mixtures: “historical samples” (Figs. 2J and 2K) and “commercial” sample made by the stack process (Fig. 2L). This synthetic representation of PL and PLE data highlights the fact that PL and PLE bands of lead white pigments are influenced by their synthesis route. Regarding Cchem and HCchem samples, Figs. 2A and 2D confirm and precise the information given in Fig. 1, namely the presence of a matrix emission band excited at 5 eV and peaking at 2.8 eV and 2.1 eV for cerussite and hydrocerussite, respectively. Also Figs. 2A and 2D show the presence of weaker and broad emission bands excited below 4 eV, which

we attribute to the effect of structural defects. Contrary to samples synthesized by precipitation in aqueous phase, which are dominated by the matrix emission, samples synthesized by corrosion of metallic lead are largely dominated by the defect-related emission bands (Figs. 2H and 2I). The fact that matrix emission bands are weak for cerussite (Fig. 2H) or apparently absent for hydrocerussite (Fig. 2I) is due to the intensity scaling adopted for this type of diagram, which minimizes the contribution of apparently weaker emission bands. However we checked that the 2.8 eV and 2.1 eV matrix emissions were present. Examination of Figs. 2H and 2I shows that the defect-related PLE and PL bands are different in cerussite and hydrocerussite samples synthesized by lead corrosion. If we compare PL-PLE diagrams of “historical” samples (Figs. 2J and 2K) with those of synthetic samples (Figs. 2A,D, H, I), a clear similitude exists between the former and samples synthesized by corrosion (Figs 2H and 2I). This indicates that the two “historical” samples have been synthesized by lead corrosion, as anticipated. However the defect-related bands in these “historical” samples (HC/Chist) are weaker than in synthetic samples HCcorr and Ccorr, as shown by the fact that the 2.8 eV matrix emission of cerussite is more apparent in “historical” samples than in synthetic cerus-



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site sample (compare Figs. 2H-I and Figs. 2J-K). This could indicate that the two “historical” samples have less defects than our samples synthesized by corrosion. Also, the higher cerussite content of “historical” sample HC30/C70hist compared to HC65/C35hist, as determined by XRD (Table 1), is clearly shown by the higher intensity of the 2.8 eV matrix emission of cerussite in the former. As anticipated, the PL-PLE diagram of “commercial” sample HC/Cstack, reproducing the classical stack process, is also dominated by defects (Fig. 2L). However the defect-related PL and PLE bands in this sample appear at higher energy than in “historical” samples HC/Chist and the samples HCcorr and Ccorr synthesized by corrosion. This suggests that the

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defects present in our samples synthesized by lead corrosion are more similar to those of “historical” samples than those of the “commercial” sample, which may indicate similar thermodynamic and (or) kinetic conditions of synthesis of Ccorr, HCcorr and HC/Chist samples. It was not possible to identify these defects by their luminescence properties only, however some hypotheses are discussed in the next section (see also Supplementary information). PL and PLE characteristics of the different samples are summarized in Table 2.

. Figure 2. Two-dimensional representation of PL and PLE spectra of: (A) Cerussite Cchem and (D) hydrocerussite HCchem synthesized by precipitation in aqueous phase; (H) and (I) cerussite Ccorr and hydrocerussite HCcorr synthesized by corrosion of metallic lead; (J) and (K) “historical” samples HC65/C55hist and HC30/C70hist; (L) “commercial” sample HC90/C10stack synthesized by the classical stack process; (B) and (E) Effect of 7-days UV exposure in a solar simulator at 40°C on Cchem and HCchem; (C) and (F) Effect of acidic treatment (acetic acid 5 %) for 5 min on Cchem and HCchem synthesized by precipitation; (G) and (M) Effect of heating in water at 80 °C for 3 hours of hydrocerussite HCchem synthesized by precipitation and of the “commercial” lead-white sample HC90/C10stack. Panel (N) represents the intensity scale in false colours. It is instructive to compare the optical absorbance spectra of samples with the corresponding PLE spectra, which are obtained from horizontal slices in diagrams of Fig. 2. These spectra are shown in Fig. 3 for samples Cchem and HCchem synthesized by precipitation method and for the “commercial” sample HC/Cstack, by considering emissions at 2.1 eV and 2.8 eV. Concerning optical absorbance spectra, the main feature around 4.5-5 eV is the band gap absorption of the lead carbonate host.21 It appears that this

transition is steeper in HC/Cstack than in Cchem and HCchem, showing the presence of structural disorder which gives a tail in the absorption band, just below the intrinsic absorption at ~ 4.5-5 eV, in samples synthesized by precipitation. PLE spectra of the pure phases Cchem and HCchem show that emission at 2.8 eV in cerussite and at 2.1 eV in hydrocerussite are excited at photon energy ≥ 4.5 eV, which corresponds to the optical band gap of the host. These two emissions are also excited at lower energy, ~ 3-


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4 eV in cerussite and hydrocerussite, despite the weak absorbance of the matrix in this energy range. This confirms that PLE in the range 3-4 eV corresponds to absorption related to defects in the carbonate structure, which only weakly contribute to the total absorbance. It is striking that the tail in the band gap absorption (in the range 4-4.5 eV) of Cchem and HCchem, does not lead to PL. The opposite behavior is observed for HC/Cstack, which has a steep band-gap absorption, but exhibits defect-related emission excited in the range 3.5-4.5 eV, where there is no apparent absorption due to defects. This contradictory behavior can be tentatively explained by considering that Cchem and HCchem contain a high concentration of defects absorbing in the range 3.5-4.5 eV, which gives the tail in the absorbance spectrum, but the emission is suppressed by the concentration quenching effect. On the contrary, defect concentration is low in HC/Cstack for this energy range, which results in a steeper band gap absorption, but which avoids concentration quenching so that the emission at 2.8 eV is observed. Anyway, Fig. 3 confirms that emission in the visible range can be excited by both host absorption and by defect-related absorption inside the bandgap.

Figure 3. Comparison of absorbance spectra (black, right scale) and PLE spectra (blue and red, left scale) of emission at 2.8 eV and 2.1 eV for cerussite Cchem and hydrocerussite HCchem synthesized by precipitation, and the “commercial” sample HC/Cstack synthesized by the classical stack process.

Origin of PL and PLE bands Emission excited above 4.5 eV. The luminescence of ions with s2 configuration is known to strongly depend on the host compound.21, 22 This is the case of MCO3 hosts (M = Ca2+, Sr2+, Ba2+, Pb2+), where Pb2+ is either a dopant or a constituting M2+ ion.23, 24 Basically, Pb2+ has the 6s2 configuration, corresponding to the 1S0 ground state. The electric-dipole allowed excited configuration 6s6p (Laporte rule ∆l = ± 1) possesses two spin states S=0 and 1 giving two spectroscopic terms 1P and 3P, with 3P at lower energy. Spin-orbit coupling splits the 3P terms into three multiplets 3 P0, 3P1 and 3P2 in order of increasing energy, corresponding to total angular momentum J = 0, 1 and 2, respectively. Only the 1S0→1P1 transition in the far UV is spin and symmetry allowed. However the strong spin-orbit coupling of Pb2+ also produces a second order mixing of 3P1 and 1 P1 states, so that the spin forbidden 1S0→3P1 transition becomes allowed. This transition occurs at 5 eV in cerussite. 21, 23 Excitation in this absorption band in cerussite gives not only the 3P1→1S0 emission at 4.0 eV, observed only below ~100 K, but also the so-called D-band at 2.8 eV which can be observed up to room temperature.21 Without any doubt, the emission at 2.8 eV excited at ~5 eV observed in our cerussite samples (Figs. 1 and 2) corresponds to the D-band reported by other authors for cerussite. To the best of our knowledge, hydrocerussite has not been studied by PL spectroscopy. However it appears that the origin if this visible emission is not well understood yet.21 Various mechanisms were proposed for explaining the very large Stoke shift of ions with 6s2 configuration (Tl+, Pb2+, Bi3+). For example, it has been attributed to a dynamic Jahn-Teller effect in cubic matrices25, to a ligand-to-metal charge transfer,22, 23 or to an exciton trapped by an impurity.26 As the visible emission is observed only when Pb2+ is introduced in a matrix, its interpretation must consider the energy levels of Pb2+ relative to electronic states of the carbonate host. It does not seem that band structure calculation have already been performed for PbCO3. However we may consider the band structure of other carbonate compounds such as calcite CaCO3, magnesite MgCO3 and Smithsonite ZnCO3.27, 28 These calculations show that the top of the valence band (VB) is dominated by oxygen 2p orbitals of CO32-, and the bottom of the conduction band (CB) by carbon 2p orbitals of CO32- . The forbidden gap, determined by the optical absorption threshold, is of the order of 6 eV.24 Consequently we may use molecular orbitals of CO32- anion to represent electronic states of the carbonate host, and consider Pb2+ states with respect to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CO32- anions. Fig. 4A shows the 1 S0 and 3P1 states of Pb2+ (left), and the molecular orbital schemes of CO32(middle) and CO33- (right), the latter corresponding to a CO32- ion of the lattice that has trapped an electron. CO32- ion is planar (D3h symmetry) and its HOMO is built with oxygen 2p orbitals in the molecular plane and perpendicular to the C-O bond, while the LUMO is a pure 2pz orbital of carbon along the C3 axis. As proposed by Folkerts and Blasse,23 the visible emission is present (absent) when the 3P1 state of Pb2+ lies above (below) the LUMO of CO32- ions, which could explain why it is present in PbCO3 and Pb2+-doped BaCO3, but not in Pb2+-doped CaCO3. When the excited 3P1 state of Pb2+ lies above the LUMO of CO32- (the case of cerussite), excited Pb2+ ion can auto-ionize, with the ejected electron lying in the LUMO of a neighboring CO32- ion, forming an electronhole pair (exciton) according to the reaction Pb2+(3P1)-CO32- →Pb3+-CO33. It is important to note that electron trapping by planar CO32- ion (D3h symmetry) induces a bending of the resulting CO33- ion (C3v symmetry), which possesses a singly occupied molecular orbital (SOMO) made of hybridized 2pz-2s carbon orbital.29, 30 This bending of the carbonate ion upon electron capture induces a stabilization (ie a self-trapping) of the electron, so that the SOMO of CO33- lies at lower energy than the LUMO of CO22- (right part of Fig. 4A). Such self-trapped electron close to a Pb3+ ion constitutes a self-trapped exciton. Consequently, the visible emission at 2.8 eV in lead carbonate is thought to result from the recombination of this self-trapped exciton, which can be represented by the following charge transfer transition Pb3+-CO33- → Pb2+(1S0)-CO32- + hν. In order to propose a possible explanation of the difference between visible emissions in cerussite (2.8 eV) and hydrocerussite (2.1 eV), the 1S0, 3P1 and photoionized states of Pb2+ are shown in Fig. 4B in configuration coordinate representation, with respect to the HOMO and LUMO of carbonate host. Considering cerussite first (left part of Fig. 4B), the excited states of Pb2+ are represented by a double-well potential corresponding to the 3P1 state of Pb2+ (left potential minimum) and to the charge transfer (or self-trapped exciton) state Pb3+-CO33- (right potential minimum). At low temperature the excited Pb2+ remains in the 3P1 state and gives only a UV emission at 4


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eV.23 At room temperature the thermal energy is sufficient to overcome the potential barrier between the 3P1 state and the charge-transfer state, the latter giving in turn the visible emission at 2.8 eV. In this model, the very large stoke shift of 2.2 eV in cerussite is thus due to the combination of the electron-phonon coupling of Pb2+ and the self-trapping energy of the electron at neighboring carbonate site. The Stoke shift is even larger in hydrocerussite (2.9 eV), which could be tentatively explained by assuming that 1S0 and 3P1 states of Pb2+ are shifted 0.7 eV higher in energy (with respect to electronic states of CO32-) in hydrocerussite compared to cerussite (right part of Fig. 4B). This energy shift of Pb2+ states does not modify the 1S0→3P1 transition of Pb2+ which still occurs at ~ 5 eV (see absorption spectrum in Fig.3) and the stabilization energy of CO33- with respect to the LUMO of CO32-. However the energy of the charge-transfer emission is now decreased by 0.7 eV, giving an emission peaking at 2.1 eV. Following this model, the visible emission of cerussite and hydrocerussite may be equally considered as a self-trapped exciton emission or a ligand-tometal charge transfer emission. The 0.7 eV shift between 1S0 ground state energies of cerussite and hydrocerussite could be explained by the “lone pair” stereochemical effect.23 In cerussite, like in isotypic aragonite CaCO3, the oxygen environment of the unique two-fold cation is roughly regular, with nine Pb-O bond lengths ranging from 2.59 to 2.76 Å.31 In this compound, the 6s electron pair can be considered as nearly spherical and centered, hence at the lowest energy. Conversely, one of the two Pb2+ in hydrocerussite (namely Pb(1)) exhibits much scattered bond lengths (2.36 ≤ Pb(1)-O ≤ 3.26 Å) that account for a marked stereochemical effect.4 In this compound, the ground state for Pb(1) in hydrocerussite becomes destabilized with respect to that of cerussite, which could explain the 0.7 eV difference.

Figure 4. Energy level diagrams of Pb2+ in cerussite and hydrocerussite. A) energy levels for Pb2+, and molecular ions CO32- and CO33-; B) diagrams in configuration coordinate representation with absorption and emission transitions. Emissions excited below ~4.5 eV. Whereas emission bands at 2.8 eV (cerussite) and 2.1 eV (hydrocerussite) excited at ~5 eV may be consider as good markers of the host, excitation at lower energy gives emission features with photon energy (2 to 3 eV), shape and intensity which significantly depend on the synthesis method (Fig. 2) and, as shown in the next

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section, on the post-synthesis history (see also Fig. 1). As proposed in the preceding section, this variability and the fact that these excitation energies are smaller than the optical band gap energy (Fig. 3) strongly suggest that these PL and PLE bands are related to structural defects. These variable emissions can be interpreted by the same mechanism as that described in Fig. 4, namely a carbonate-to-metal charge transfer Pb3+-CO33- → Pb2+(1S0)-CO32- + hν, but with emission and excitation energies modified by the presence of neighbouring defects. These defects are not identified '' yet, however we may suspect the presence of Pb vacancies (noted VPb in Kröger-Vink (KV) notation, and bearing a 2- charge) or neutral oxygen vacancies (noted V

x

O

in KV notation), forming CO22- ions. We may •

exclude the presence of O- vacancies (noted V in KV notation) correO

sponding to a CO2- ion, because this defect is paramagnetic and should be detectable by EPR spectroscopy. We did not observe such EPR signals attributable to such defect (except a very weak signal of Mn2+ impurities, see supplementary Figure S2). Other types of defects appear also likely, such as cationic impurities (see supplementary information) at Pb site. These defects could perturb the energy levels of neighboring Pb2+ and CO32-, which can be responsible for the various excitation bands below 5 eV and for the variable charge transfer emission energy.

Effect of post-synthesis treatments In the preceding sections we showed that PL and PLE spectra are influenced by the synthesis route of lead white pigments. We study in this part the effect of post-synthesis treatments used in the past by paint manufacturers and painters, with two main purposes: (i) to analyze the possible effects of these post-synthesis treatments on PL and PLE spectra, and (ii) to examine whether weak modifications of PL properties by post-synthesis treatments could have provided indicators of pigment quality in the past. We considered three main post-synthesis treatments, namely UV exposure, acidic treatment (washing or grinding in acetic acid) and heating in water. All three processes were frequently advised in historical treatises to obtain a pigment with presumably better optical properties.7, 32 We have already mentioned that PL bands excited at ~5 eV are representative of the matrix. This can be seen in the supplementary Figure S3, which shows the effect of washing in acetic acid and of heating in water on the PL excited at ~5 eV of the “commercial” sample HC/Cstack. As anticipated, the intensity of the 2.8 eV emission of cerussite increases after treatment in acetic acid, which decreases the HC:C ratio, while the intensity decreases after heating in water, which increases the HC:C ratio (see Table 1). Despite the fact that the matrix PL spectrum is sensitive to postsynthesis treatments, it could not provide a quality criterion for artists because it is excited at an energy (> 4.5 eV) far above what is provided by sunlight. On the contrary, defect-related PL bands are located in the domain of sensitivity of human eye, and their excitation correspond to the high energy tail of solar spectrum at Earth surface. However, the PL of cerussite and hydrocerussite excited at ~5 eV constitutes a distinction criterion that may present an interest in the field of cultural heritage research. Indeed, it is possible to differentiate the two phases solely by monitoring PL spectrum excited at 5 eV. This method is far more efficient and faster than classically used techniques such as XRD. Effect of UV exposure. The De Mayerne manuscript dated from 1620 reports the following advice “If you put it two or three times under the sun covered with water, it will become much more white, as one bleach linen”.7, 32 The effect of simulated sun exposure (7 days, 40 °C) on Cchem and HCchem samples is shown in Fig. 2. As for cerussite (Fig. 2B), it can be seen that the defect-related PL bands excited in the range 3-4 eV vanished after UV exposure, while the weak PL band at 2.8 eV excited around 4.6 eV increased (compare with Fig. 2A). This indicates that prolonged solar exposure induced a bleaching of the defect-related PL bands. Indeed, prolonged UV exposure produces many excitation-relaxation cycles of these defect-related bands in the 3-3.7 eV energy range (maximum energy of solar spectrum on Earth). Since the recombination of self-trapped excitons can be partially radiative (PL) and partially non radiative, the energy liberated by the non-radiative modes could induce atomic migration, as shown in other types of materials.33 Thus a continuous excitation of the defect-related PL bands may induce the migration of defects and their annihilation at grain surfaces or at other locations. Also these defects may transform into other types of defects. This could explain the increase of the 2.8 eV emission excited at ~4.6 eV (Fig. 2B, compare with Fig.2A). A different effect was observed in hydrocerussite (Fig. 2E, compare with


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Fig. 2D). In this case, the broad PL and PLE bands of pristine HCchem are replaced by a strong PL band peaking at 2 eV induced by an excitation band peaking around 3.2 eV. Similar to cerussite, we may conjecture that non-radiative recombination of self-trapped excitons in pristine hydrocerussite could transform a broad distribution of defect configurations into a single type of defect, different from the starting ones. Contrary to the initial PL bands which cover all the visible range in pristine cerussite and hydrocerussite, the decrease of these emissions and their shift to orangered in hydrocerussite after prolonged sun exposure could slightly modify the appearance of the pigment, and might have been noticed by the expert eye of the artist. Effect of acidic treatments. The manuscript of Fortunato de Rovigo (~1700, in Stols-Witlox 2011)7 reported the following advice “To render biacca [=lead white] extraordinarily more white, take lead white in flakes and grind it well on the marble with vinegar. […]”. As it was previously noticed, treatment in acetic acid resulted in a decrease of the HC:C ratio (Table 1) by dissolving part of the hydrocerussite and crystallizing neoformed cerussite. Its effect on PL and PLE is shown in Fig. 2 in the case of Cchem and HCchem synthesized by precipitation. This treatment has a strong impact on the defect –induced PL in both cerussite (Fig. 2C, compare with Fig. 2A) and hydrocerussite (Fig. 2F, compare with Fig. 2D). It must be remembered that the intensities are scaled to the stronger emission, so that the apparent decrease of the matrix PL peaks of cerussite (2.8 eV) and hydrocerussite (2.1 eV) reflects only the relative increase of the defect-induced PL bands. It appears that this defect-induced PL in hydrocerussite is excited in the range 3.2-3.5 eV (Fig. 2F), which is accessible by the UV-A of sun light (up to 3.8 eV), while most of the PL of cerussite, excited at higher energy, is inaccessible (Fig. 2C). This emission of hydrocerussite covers all the visible range, so that under natural excitation by sun light, it could eventually appear as a weak white fluorescence in lead-white pigments with high HC:C ratio. Such a weak fluorescence might have influenced the optical quality of lead-white pigments. Effect of water treatments. Birelli (c. 1601, in Stols-Witlox 2011)7 reported “Then pound it, put it in a pignatto and for every libra of this flour [= lead white], add two ounces of water, and put on the fire while stirring it well […]”. In addition to increasing the HC:C ratio as determined by DRX (Table 1), the effect of heating in water is the increase of the defectinduced PL bands, as shown in Fig. 2G in the case of hydrocerussite HCchem (compare with Fig. 2D), similarly to the case of acidic treatment. Again, the weak sunlight excited PL covering all the visible range may increase the brightness of lead-white pigments. The effect of heating in water is less important in the case of the “commercial” sample HC/Cstack (Fig. 2M, compare with Fig. 2L). In particular the defect-induced PL intensity is not modified, and only the PLE is affected by this treatment, with a decrease of the PLE band peaking at 4.4 eV. This relatively weak modification of the “commercial” sample is likely due to the fact that it has already been treated in water during its synthesis process. However it must be noticed that in this sample, the defect-induced PLE is out of the range accessible by sun light, contrary to the case of samples synthesized by coprecipitation or by corrosion and of the two “historical” samples (see Fig. 2). It must be stressed that the PL properties of the two “historical” samples are more similar to our samples synthesized by lead corrosion than to the “commercial” sample synthesized by the traditional method. The PL and PLE characteristics of all samples are summarized in Table 2. Excitation (eV) ~5 ~5

Emission (eV) 2.8 2.1

Attribution

Samples

Comment

CPCT CPCT

C HC

3.2 to 3.8

2.1 to 2.8

CPCT perturbed by defects

Cchem, Ccorr, HCchem, HCcorr, HC/Chist

Vary with synthesis and postsynthesis treatments

3.2

2.0

?

HCchem

UV treatment of HC

4.0 to 4.4

2.6 to 2.9

CPCT perturbed by defects

HC/Cstack

Present only in the “commercial” sample

Table 2: Summary of PL and PLE results. CPCT = Carbonate-to-Pb Charge Transfer; C = cerussite; HC = hydrocerussite. Previous works have addressed the fact that HC and C crystallites vary in size and morphology.34, 35 More specifically, it has been shown that crystallites size could be connected to the time period at which lead white had been synthesized: lead white used in paintings from the Renaissance exhibited particles of smaller size than the pigment obtained during the 19-20th c. This was linked either to changes in the corrosion conditions in the stack process, or to new synthesis ways.12 We investigated the effect of crystal growth, achieved by the maturing of Cchem in aqueous conditions, on the PL and PLE spectra of cerussite. The effect of this growth is shown in Supplementary Figure S4 in the case of Cchem. While the PL bands excited at ~ 5 eV are not affected, crystallites growth induced a modification of defect-related PL bands. The weak defect-related PL at 2.1 eV, excited between 3 and 4 eV for, is now excited between 3.5 and 4.0 eV after 90 days maturing in water. This influence of crystal size, which determines the surface-to-volume ratio, might be due to the recombination of surface defects during the growth of crystallites. This observation suggests that the optical properties of lead-white are closely connected to the pigment microstructure, itself dependent on the synthesis route or the post-synthesis treatments.

Usefulness and limitation of the method Photoluminescence induced by laser excitation has already been used for the analysis of pigments and their binding media.17-20 However the interpretation of PL spectra is generally not trivial as the optical properties of a given chemical species is determined not only by its electronic structure, but it is more or less strongly influenced by its environment (structural defects and impurities). Combining PL with PLE of lead-white pigments in an intensity-scaled two-dimensional spectral representation allowed us to detect subtle spectral variations determined by their synthesis methods and post-synthesis treatments. Despite the fact that we explored pure cerussite and hydrocerussite, and mixture of the two phases obtained by different processes (including two historical pigments) and submitted to different post-synthesis treatments, these model samples do not reflect the complexity inherent to pictorial layers of artworks, which are chemically and structurally heterogeneous at a scale smaller than the laser beam size. First, pigments are mixed with organic binders, which possess their own PL responses. However, in principle the organic and inorganic component should be easily distinguished by their fluorescence lifetimes. Optical transitions in these organic materials are in general fully symmetry and spin-allowed so that the luminescence lifetimes lie in the nanosecond range. On the contrary optical transitions in lead-white pigments are partially spin forbidden (see above), so that their luminescence lifetime is in the microsecond range or larger.21 Consequently the organic and inorganic components of the emission should be easily separated by using time-resolved photoluminescence (TRPL) spectroscopy.14,16 In situ analysis of real paint layers introduces another level of complexity as each chemical component of the layer may contribute to the PL spectrum, and the details of the spectral features can be influenced by the interaction between these different components. For this reason, an optimal use of PL spectroscopy to derive relevant information on materials and techniques used by the artists in the past necessitates a good understanding of the nature of the emitters and their interaction with their environment, which record all the chemical history of these complex materials. It thus appears that an optimal in situ use of the photoluminescence of paint could consist in a three-dimensional analysis combining PL, PLE and luminescence lifetime. CONCLUSION The PL properties of lead-white pigments in the visible range and their PLE in the UV were investigated. We used an intensity-scaled twodimensional representation mode to reveal weak variations of relative intensities, positions and shapes of emission and excitation bands. This method is rapid as it takes only 8-10 min to record a full set of data for each sample. We ascribed the PL bands in the visible to charge transfer transitions of the type Pb3+-CO33- → Pb2+-CO32- , whereby the excited state consists in an electron self-trapped at a carbonate ion (forming a bent CO33-) adjacent to the Pb site where the hole is trapped (forming a Pb3+). It was found that PL and PLE bands of lead-white pigments are sensitive to the cerussite/hydrocerussite composition, to the synthesis processes and to post-synthesis treatments. The main results can be summarized as follows: (i) Excitation of the 1S0→ 3P1 of Pb2+ of the carbonate host at energy > 4.5 eV gives PL bands peaking at 2.8 eV in cerussite and 2.1 eV in hydro-


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cerussite; (ii) PL bands excited below this absorption threshold are ascribed to the same transitions perturbed by defects in these matrices, and their variability reflects the history of the material; (iii) Cerussite and Hydrocerussite samples synthesized by precipitation in aqueous conditions show less defect-related PL than those synthesized by corrosion of metallic lead, which is confirmed by PL properties of “historical” and “commercial” samples synthesized also by the corrosion method; (iv) The effect of prolonged UV exposure (solar simulator) is presumably attributed to the UV-induced migration of various defects which may annihilate or transform into other types of defects; (v) Heating in water (increase of the HC:C ratio) or washing in acetic acid solution (decrease of the HC:C ratio) increase the contribution of the defect-induced PL, with emission covering the visible range. The weak white PL excited by sun light might have been one of the quality criteria for the selection of lead-white by the painters.

AUTHOR CONTRIBUTIONS. VG, KT and GW synthesized and characterized the samples. Optical studies were performed and interpreted by VG and DG. The ensemble of results was discussed with TC and MM. The paper was written by DG and VG. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge P. Aschehoug for his precious help during the fluorescence measurements at the IRCP. This work was funded by the Ministère de la Culture et de la Communication, by the Institut de Recherche de Chimie-Paris (IRCP) and by ED397 of Université Pierre et Marie Curie, Paris.

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21. Kamenskikh, I. A.; Kirm, M.; Kolobanov, V. N.; Mikhailin, V. V.; Orekhanov, P. A.; Shpinkov, I. N.; Spassky, D. A.; Vasil'ev, A. N.; Zadneprovsky, B. I.; Zimmerer, G.. IEEE Trans. Nucl. Sci. 2001, 48, 2324-2329. 21. Boulon, G. Journal De Physique (France) 1971, 32, 333-347. 22. Ranfagni, A.; Mugnai, D.; Bacci, M.; Viliani, G.; Fontana, M. P. Adv. Phys. 1983, 32, 823-905. 23. Folkerts, H. F.; Blasse, G. J. Phys. Chem. Solids 1996, 57, 303-306. 24. Kraienhemke, R.; Semig, P.; Fischer, F. Phys. Status Solidi A 1990, 119, 327-36. 25. Tabakova, V.; Kunev, K. Z. Naturforsch., A 1980, 35A, 308-11. 26. Srivastava, A. M.; Beers, W. W. J. Lumin. 1999, 81, 293-300. 27. Brik, M. G. Physica B 2011, 406, 1004-1012.

28. Bouibes, A.; Zaoui, A.; Tunega. Solid State Commun. 2013, 166, 7682. 29. Marfunin, A.S. in: Spectroscopy, Luminescence Radiation centers in minerals, Springer 1979, p.250; 30. Biktagirov, T.; Gafurov, M., Mamin, G.; Klimashina, E.; Putlayev, V.; Orlinskii, S. J. Phys. Chem. A 2014, 118, 1519-1526. 31. Chevrier, G.,; Giester, G.; Heger, G. Z. Kristallogr. 1992, 199, 67-74. 32. Mayerne, D., Pictoria, In: sculptoria & quae subalternarum artium. 1620. 33. Itoh, N. Nucl. Instr. Meth. B 1998, 135, 175-183. 34. Franke, W.; Lenk, K. Journal of Crystal Growth 1981, 51, 309-313; 35. Sánchez-Navas, A.; López-Cruz, O.; Velilla, N.; Vidal, I. Journal of Crystal Growth 2013, 376, 1-10.

SUPPORTING INFORMATION X-ray diffraction patterns of samples; impurity content of Cchem and HC/Cstack samples; Effect of treatments with acetic acid and water and effect of grain size on PL and PLE spectra. REFERENCES 1. Gettens, R. J.; Kühn, H.; Chase, W. T. Studies in Conservation 1967, 12 (4), 125-139. 2. Welcomme, E.; Walter, P.; Bleuet, P.; Hodeau, J.-L.; Dooryhee, E.; Martinetto, P.; Menu, M. Applied Physics A 2007, 89, 825-832. 3. Chevrier, C.; Giester, G.; Heger, G.; Jarosch, D.; Wildner, M.; Zemann, J. Zeitschrift für Kristallographie 1992, 199, 67-74. 4. Martinetto, P.; Anne, M.; Dooryhee, E.; Walter, P.; Tsoucaris, G. Acta Crystallographica C 2002, 58, 82-84. 5. Harley, R. D. In: Artists' Pigments c. 1600-1835, A study in English documentary sources, 2nd revised edition. Archetype Publications: 1982. 6. Stols-Witlox, M.; Megens, L.; Carlyle, L. In: The Artist's Process: Proceedings of the fourth symposium of the Art Technological Source Research Working Group. Archetype Publications 2012, p. 112. 7. Stols-Witlox, M. In: Studying Old Master Paintings - Technology and Practice, Archetype Publications 2011, p; 284. 8. Sabin, A. H. in: White Lead - Its use in paint. John Wiley, New York, 1920. 9. Nash, S. In: Trade in artist's materials, J. Kirby, S. N., Cannon, J. Ed. Archetype Publications, 2010, pp 97-142. 10. Kubersky-Piredda, S. In: Trade in artist's materials, J. Kirby, S. N., Cannon J. Ed. Archetype Publications: London, 2010; pp 223-243. 11. Berrie, B.; Matthew, L. In: Studying Old Master Paintings - Technology and Practice. Archetype Publications, 2011, p. 295. 12. Gonzalez, V.; Calligaro, T.; Wallez, G.; Eveno, M.; Toussaint, K.; Menu, M. Microchem. J. 2016, 125, 43-49. 13. Merrifield, M. P. in: Original Treatises on the Arts of Painting. Dover Publications, Inc.: New-York, 1849. 14. Nevin, A.; Cesaratto, A.; Bellei, S.; D'Andrea, C.; Toniolo, L.; Valentini, G.; Comelli, D. Sensors 2014, 14, 6338-6355. 15. Thoury, M.; Echard, J.-P.; Refregiers, M.; Berrie, B.; Nevin, A.; Jamme, F.; Bertrand, L. Anal. Chem. 2011, 83, 1737-1745. 16. Nevin, A.; Spoto, G.; Anglos, D. Appl. Phys. A., 2012, 106, 339-361. 17. Miyoshi, T.; Ikeya, M.; Kinoshita, S.; Takashi, K. Jpn. J. Appl. Phys. 1982, 21, 1032-1036. 18. Anglos, D.; Solomidou, M.; Zergiotti, I.; Zafiropoulos, V.; Papazoglou, T.; Fotakis, C. Appl. Spectrosc. 1996, 50, 1331-1334. 19. Nevin, A.; Cather, S.; Anglos, D.; Fotakis, C. Anal. Chim. Acta 2006, 573-574, 341-346. 20. Miyoshi, T. Jpn. J. Appl. Phys. 1987, 26, 780-781.


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For TOC only


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Revealing the Origin and History of Lead-White Pigments by Their

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