Abstract
As one of the most desired and expensive artists’ materials throughout history, there has long been interest in studying natural lapis lazuli. The traditional method of extracting the blue component, lazurite, from lapis lazuli, as outlined in Cennini’s Il Libro dell’Arte, involves a lengthy purification process: (1) finely grind the rock; (2) mix with pine rosin, gum mastic, and beeswax; (3) massage in water to collect the lazurite. Repeating the process produces several grades of the pigment, typically referred to as ultramarine blue. Here, we investigate the sulfur environment within the aluminosilicate framework of lazurite during its extraction from lapis lazuli. The sulfur XANES fingerprint from samples taken at the different stages in Cennini’s extraction method were examined. All spectra contain a strong absorption peak at 2483 eV, attributable to sulfate present in the lazurite structure. However, intensity variations appear in the broad envelope of peaks between 2470 and 2475 eV and the pre-peak at 2469.1 eV, indicating a variation in the content of trisulfur (S3−˙) radicals. By studying the effect of each step of Cennini’s process, this study elucidates the changes occurring during the extraction and the variability within different grades of the precious coloring material. The increasing application of XANES to the study of artist’s materials and works of art motivated extending the research to assess the possibility of X-ray induced damage. Direct comparison of micro-focused and unfocused beam experiments suggests an increase of the S3−˙ radicals with prolonged exposure. Analysis indicates that induced damage follows first-order kinetics, providing a first assessment on the acceptable amount of radiation exposure to define the optimal acquisition parameters to allow safe analyses of lapis lazuli and ultramarine pigments.
Introduction
Lapis lazuli is a complex rock whose composition is defined by the presence of the mineral lazurite (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2 [1, 2], which is responsible for its overall blue hue. Inclusions of several other minerals are also common, including pyrite (FeS2), calcite (CaCO3), diopside (CaMgSi2O6), forsterite (Mg2SiO4), and wollastonite (CaSiO3), in varying amounts [1, 3, 4, 5, 6]. Lazurite itself is a member of the sodalite group [7, 8] – which includes sodalite (Na8(Al6Si6O24)(Cl2)), nosean (Na8(Al6Si6O24)(SO4)·H2O) and hauyne ((Ca,Na)4−8(Al6Si6O24)(SO4,S,Cl)1−2) [9, 10] – and is typically considered a sulfur-rich hauyne [3, 11, 12]. The sodalite minerals contain frameworks of alternating silica and alumina tetrahedra creating large cubo-octahedral cages, known as β-cages; extra-framework cations (Ca2+, K+, or Na+), anions (Cl−, OH−, SO42−, or Sn−), and neutral species (H2O) are entrapped [4, 13, 14] within these cages. Lazurite’s blue color is attributed to sulfur polyanion radicals trapped in the β-cage structure [14, 15]. Variations in color appear to be related to ratios between various sulfur species: the trisulfur radical (S3−˙) is mainly responsible for the blue color, but contributions from disulfur (S2−˙) and tetrasulfur (S4−˙) radicals can shift the color towards yellow or red, respectively [14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28]. While in nature most lapis lazuli is blue due to the predominance of S3−˙ radicals, smaller concentrations of the yellow chromophore S2−˙ have also been identified, and hence varieties of green lapis lazuli can also be found [14, 29, 30].
Lapis lazuli has been exploited and prized for its deep blue color since at least the 5th millennium BC [31] when it was first used to fashion jewels, amulets, seals, and inlays. The semi-precious stone quickly became one of the most desirable and expensive art-making materials in all of history. Lapis was first used as a pigment (known as ultramarine) in the 7th century CE, and has been colorfully described as “illustrious, beautiful, and most perfect, beyond all other colors” in painting treatises [5, 31, 32, 33].
Beginning in the 13th century, a number of experimental/alchemical processes were developed with the intent of augmenting the intensity of the pigment’s blue hue after grinding the raw lapis lazuli [32, 34, 35, 36]. One traditional method, as reported in Cennino Cennini’s Il Libro dell’Arte [32], is a lengthy extraction process that serves to separate lazurite from the accessory minerals. As shown in Fig. 1, in Cennini’s process (a protocol still used by some modern artisans), deep blue lapis lazuli of the highest quality (i.e. dominated by lazurite) is ground to a fine powder. This powder is intimately blended with a mixture of melted pine rosin (melting temperature, Tm=100–120°C), gum mastic (Tm=95°C), and beeswax (Tm=64°C) producing a doughy mixture known as a pastello. The pastello is stored for a week at room temperature and kneaded once per day, then massaged in a warm solution of ash (dilute potassium hydroxide, KOH) which preferentially separates the blue pigment particles. Repeated several times, this process produces several grades of pigment, each less saturated in color than the one before. The first grade possesses a deep blue hue; the last and crudest grade has a pale-blue hue and is typically called ultramarine ash [1, 5]. For centuries, it has been shown that Cennini’s extraction process selected the most intense blue particles and concurrently removed most of the accessory minerals from lapis lazuli, ultimately resulting in a pigment with the characteristic deep blue hue.
Cennino Cennini’s process for the production of ultramarine blue from lapis lazuli.
Whether a physical or chemical process is responsible for this preferential selection of the bluest lazurite is not yet known. Selection of the lazurite phase has been, in recent years, attributed to its higher affinity toward water when compared to the other phases present in the pastello [37]; however, scientific evidence in support of this hypothesis has yet to be provided. Favaro et al. [36] investigated the hypothesis that the extraction process is driven by sedimentation, but observed that similarly sized particles are, in fact, not separated according to their densities (e.g. lazurite is more efficiently separated than denser particles such as diopside). Instead, Favaro proposed two complementary processes. First, the retention of impurities in the pastello can be explained by a process similar to saponification. The alkali modifiers (Ca, Na, Mg, K, etc.) of the silicate/aluminosilicates minerals are replaced/exchanged by protons from the free carboxylic acids formed during the hydrolysis reaction of the esters present in the organic mixture with the alkaline environment of the KOH solution [38, 39, 40]. This reaction takes place preferentially in silicates (diopside, wollastonite) over aluminosilicates (feldspars and feldspathoids such as lazurite, sodalite, nepheline), given that the anionic tetrahedral sites, such as AlO4− in aluminosilicate minerals, are orders of magnitude more resistant to ion exchange than non-bridging oxygens in a pure silicate structure [41, 42]. The results of Favaro et al. [36] provide the only evidence that a purely physical explanation is insufficient to describe why the Cennini process works, and that the chemistry of the process needs to be better understood.
In this work, we characterize ultramarine at each stage of Cennini’s extraction process, using sulfur K-edge X-ray absorption near edge structure (XANES) to probe the sulfur environment within the aluminosilicate framework of lazurite in order to further elucidate the chemical changes occurring during the production of this important artists’ material.
XANES is particularly well-suited to characterize the local bonding environment of sulfur, although interpretation of sulfur K-edge XANES spectra for the lazurite structure is complicated by the lack of reference compounds to account for this specific environment [27]. Characteristic XANES peaks associated with lazurite and ultramarine pigments have been discussed in the literature [27, 28, 43], revealing the presence of (i) a distinctive absorption feature centered at 2469.1 eV associated with the S3−˙ polysulfide radical anion chromophore [27], (ii) a series of reduced sulfur radicals (S2−˙, S3−˙, S4−˙) resulting in a broad absorption envelope between 2470 and 2475 eV, (iii) a sulfite (SO32−) absorption at 2478.5 eV, and (iv) a peak at 2482.5 eV attributed to S6+ ions from sulfate-based species present in the lapis lazurite accessory materials and/or the lazurite structure [2, 28, 44]. These XANES features, together with complementary X-ray diffraction (XRD) studies to identify mineral phases in the raw rock and in the blue pigment products, were used to track chemical speciation within lazurite at varying stages of purification.
The increasing application of micro-focused XANES to the study of artist’s materials and works of art for speciation, characterization, provenance and degradation studies [15, 28, 45, 46, 47, 48, 49, 50] motivated extending the research to also assess the possibility of X-ray induced damage to the ultramarine pigment. If radiation-induced damage occurs, through redox reactions associated with ionizing radiation, it is possible it could induce the formation of new sulfur species in-situ, affecting the ratios of S-based compounds and thereby compromising our understanding of the extraction process, and also possibly resulting in color changes, either permanent or time reversible. These types of X-ray experimental effects are often considered in the spectroscopic studies of other polysulfide systems, most notably LiS battery systems [51, 52, 53]. The studies presented here were all performed in removed samples, but it is possible that in future similar studies may be performed directly on works of art. Therefore, before such studies are performed, a thorough understanding of the effects of radiation exposure is critical to define optimal acquisition parameters to allow safe analyses.
Experimental section
Samples
Lapis lazuli rock, powdered lapis lazuli rock, and three grades of processed ultramarine pigment (1st grade, 2nd grade, and 3rd grade) prepared following Cennini’s traditional recipe were purchased from Master Pigments (Lake Forest, CA, USA). For synchrotron experiments, a standard procedure consisting of spreading powder (powdered lapis lazuli rock and pigments) on Saint Gobain M60 sulfur-free polyester tape was followed [54]. While direct measurement of thickness was not performed for these samples, uniformity was assured by following this approach. The unpolished lapis lazuli rock fragment was mounted on modeling clay for analysis. X-ray diffraction measurements were conducted on the free powder.
Synchrotron experiments
XANES spectra and X-ray fluorescence (XRF) maps were collected on BL 14-3 at the Stanford Synchrotron Radiation Lightsource (SSRL), Stanford, USA. A Si(111) double crystal monochromator selected the energy of the incoming beam, and was calibrated such that the pre-edge thiol peak of sodium thiosulfate (Na2S2O3) was defined as 2472.02 eV. This calibration results in a sulfate peak for gypsum at 2482.5 eV. The monochromatic beam was focused using Kirkpatrick-Baez mirrors resulting in a focal point of 5×5 μm2 and a flux of 2×1010 photons/s at the sample surface. The sample was mounted at a 45° angle to the incoming beam in a helium-purged chamber. A Vortex Silicon Drift Detector (SDD, Hitachi USA High Technologies) using Xspress3 high-throughput pulse-processing electronics (Quantum Detectors), mounted at 90° relative to the incident beam, monitored the fluorescence signal for all measurements. A 2500 eV incident beam was used to acquire XRF maps. The raster scanning was performed with a pixel size of 10×10 μm2 and a 50 ms dwell time to map Al, Si, and S in each sample. Al and Si are characteristic elements of lazurite, and areas rich in both were targeted for XANES analyses. Spectra were collected in fluorescence mode at the sulfur K-edge from 2460 to 2540, with a spectral resolution of 0.2 eV from 2460 to 2540 eV, and of 2 eV from 2500 to 2540 eV. A micro-focused beam was used to overcome possible limitations related to the sample heterogeneity at the micrometer scale. On average, 10 XANES spectra were acquired on different lazurite particles in order to minimize the possible influence of radiation damage (see below). The spectra from each sample were averaged to improve the signal to noise ratio.
Radiation damage on the extracted pigments was assessed by acquiring a series of 30 S K-edge XANES spectra at a fixed position on each sample, using both micro-focused (10×10 μm2) and unfocused (500 (v.)×1000 (h.) μm2) beams. Dose for each acquisition was calculated from the time of exposure of each XANES scan (ca. 8 min) to the incident X-ray energies using the RADDOSE-3D calculation program [55], at approximately 10 MGy per XANES scan in bulk beam configuration and 50 MGy per XANES scan in micro-focus mode.
General data processing was performed with the SIXPACK [56] and Athena [57] software packages. The energy range presented in this study is centered at 2465–2482 eV. The high energy absorption attributed to sulfate-based materials that do not present specific changes along the extraction process are not presented. Moreover, the effect of self-absorption that often affects the intensity of the sulfate peak in fluorescence mode complicates the discussion about the intensity of this feature.
X-ray diffraction
X-ray diffraction (XRD) analyses were conducted in the Jerome B. Cohen X-ray Diffraction Facility at Northwestern University using a Rigaku SmartLab diffractometer with Cu Kα radiation on a zero background quartz holder. The diffractometer was operated at 40 kV and 20 mA with a 1 mm beam size, from 10°≤2Θ≤90°, with a step size of 0.1° and a dwell time of 1 s.
Radiation damage assessment
In order to define the optimal acquisition parameters to allow safe analyses of the ultramarine pigments without inducing in-situ radicalization or scission of the sulfur polyanions, the effect of possible radiation damage on the S-based species was assessed.
Time resolved XANES spectra of the 1st grade of pigment under an unfocused beam are presented in Fig. 2a (trace color darkening with increasing number of exposures) and are characterized by a relatively intense absorption at 2469.1 eV, together with a characteristic envelope feature with a maximum absorption at 2472.7 eV with two flanking shoulders at 2471.4 and 2473.3 eV. The red PCA trace in Fig. 2a summarizes the effect of the radiation damage on the S K-edge XANES: increasing exposure appears to decrease the absorption intensity of the maxima of absorption at 2472.7 eV, while increasing the intensity at 2469.1 eV, suggesting a decrease in the amount of S–S bonds of polysulfides and elemental sulfur, which leads to a strong increase of the S3−˙ radical blue center [27, 28, 43]. The visible image (Fig. 2a, inset) was taken after prolonged beam exposure and shows the resulting darkening of the pigment, confirming the correlation between the sulfide radical and an increase in blue coloration. Figure 2b shows a principal component analysis (PCA) of the time series data from unfocused X-rays. The first two major components reflect the differences in the pigment grade as well as the beam-induced radiation damage. The pigment grades can be described by the vector in the component space from the lower left to the upper right (green arrow, showing an increasing amount of S3−˙ in e.g. the 1st grade vs. the other grades or the lapis lazuli rock), while the reaction from the beam goes from the upper left to the lower right (red arrow, showing the progression of the XANES time series). Note that all grades of pigment extraction, as well as the powdered lapis lazuli rock present similar radiation damage response with continued exposure to the X-rays, suggesting that all ultramarine-based materials are subject to damage from prolonged X-ray exposure (~1 h) at these energies.
Effect of radiation damage on the S-based species. (a) Time-resolved XANES spectra during the first 30 min of beam exposure (~8 min each) of the 1st grade pigment under unfocused beam showing a decrease of the absorption intensity of the peak at 2472.7 eV, together with an increase of the intensity at 2469.1 eV. The visible image on the lower right corner was taken after prolonged beam exposure, and shows darkening of the pigment (inside the circle); (b) Principal component analysis (PCA) of the time resolved XANES during the first 60 min of beam exposure, showing that all grades of pigment extraction, as well as the powdered lapis lazuli rock, present similar radiation damage response with continued exposure to the X-rays.
Figure 3a shows the kinetics of the beam-induced radiation damage from the unfocused beam (solid markers): the extent of reaction is reported as PC distance, defined as the Euclidean distance of the eigenvalues between the initial XANES measurement and the later reaction time steps in the PCA reaction space of component 1 vs. component 2. The kinetics of the reaction were fit using a pseudo first-order rate law with the same first-order rate constant for both the powdered lapis lazuli rock and the different pigment grades, with the only variable being the effective concentration of the S–S linkages. From these fits, the half-life, in units of dose, is calculated to be 94 MGy, and the 10% level of damage is 7.7 MGy. Similar kinetics analysis (Fig. 3a, open markers) using the micro-focused beam data (again expressed as PC distance), as in the unfocused beam case, shows that this dataset also fits within the kinetic/dose parameters given above, with the major parameter difference being that the smaller beam size allowed the measurement of a higher concentration of pigment.
Kinetics of the beam induced radiation damage. (a) First-order rate kinetics, calculated using a pseudo first-order rate law, of unfocused beam S K-edge XANES scan for all types of lapis lazuli samples. Similar kinetics controls the micro-focused beam experiments. (b) Comparison of the initial scan (t0) on the 1st grade pigment in micro-focused (solid line) and unfocused (broken line) bulk beam indicates an increase in the absorption at 2469.1 eV by the micro-focused beam.
A corresponding micro-focused beam study gives an exposure dose at completion of the scan of 50 MGy, indicating the possibility of significant beam damage within a single XANES scan. Comparison of the initial scan (t0) on the 1st grade pigment in micro-focused (solid line) and unfocused (broken line) beam configuration (Fig. 3b) indicates a more intense absorption at 2469.1 eV by the micro-focused beam, indicating that a significant difference in the amount of damage can occur during the first scan.
For rapid XRF imaging experiments under these conditions (e.g. 2×1010 photons/s, 5×5 μm2 beam spot), a micro-focused exposure at a reasonable dwell time (100 ms) results in a dose to the sample of ~175 kGy. If one allots a maximum safe limit at the 10% level, then a single scan provides a margin of safety of nearly 45-fold. This also implies that using a chemical imaging strategy, using multiple energy image maps where one reduces the dose on the sample by performing images at several key, significant energies (on the order of 1 MGy per pixel total), would also be well within the safe and allowable dose limit, as well as providing speciation information. However, during a single micro-focused XANES scan, the pigment is exposed to a dose of 50 MGy, above the 10% limit, potentially exposing the materials to damage even at t0. It is therefore recommended that comparisons with a measurement performed with unfocused beam are performed to monitor the eventual damages due to X-ray interaction at the beginning of the experiment. In this study, the data obtained using a micro-focused beam were validated by the results obtained using an unfocused beam.
Pigment processing analysis
The sulfur K-edge XANES of the lapis lazuli rock was investigated to elucidate the chemistry of the sulfur in the lazurite phase of the rock prior to processing. In Fig. 4, the average XANES spectrum of lazurite from the unprocessed rock (solid blue trace) is characterized by a relatively intense absorption at 2469.1 eV, together with a characteristic envelope feature with a maximum absorption at 2472.7 eV and a shoulder at 2471.4 eV. By comparing these spectral features with XANES lazurite fingerprints from various geographic regions (groups 1–5, see figure caption for description of regions) [28], it is possible to tentatively provenance the raw material provided by Master Pigments: the spectral features are consistent with those observed primarily in samples from Afghanistan confirming the primary origin information given by the manufacturer.
![Fig. 4:
XANES spectrum of the lazurite in the raw material (solid blue trace) compared to five characteristic clusters of lazurite rich-rocks from different geographical origins (adapted from [28]) (dashed traces; group 1: mostly Afghanistan; group 2: mostly Afghanistan; group 3: Chile; group 4: Russia; group 5: North America). The lazurite is characterized by a relatively intense feature at 2469.1 eV, together with a characteristic envelope feature with a maximum of absorption at 2472.7 eV and a shoulder at 2471.4 eV.](/document/doi/10.1515/pac-2017-0502/asset/graphic/j_pac-2017-0502_fig_011.jpg)
XANES spectrum of the lazurite in the raw material (solid blue trace) compared to five characteristic clusters of lazurite rich-rocks from different geographical origins (adapted from [28]) (dashed traces; group 1: mostly Afghanistan; group 2: mostly Afghanistan; group 3: Chile; group 4: Russia; group 5: North America). The lazurite is characterized by a relatively intense feature at 2469.1 eV, together with a characteristic envelope feature with a maximum of absorption at 2472.7 eV and a shoulder at 2471.4 eV.
The first step of Cennini’s recipe involves grinding lapis lazuli rock to a fine powder. As shown in Fig. 5a, the XANES spectrum of the powdered material differs from the rock signature discussed above: the powdered form has (i) lower intensity in the 2469.1 eV absorption peak, (ii) a loss of the shoulder at 2471.4 eV, (iii) a shift towards lower energy (2472.2 eV) of the maximum of absorption of the envelope feature located between 2470 and 2475 eV, and (iv) the appearance of a shoulder at 2477.4 eV. These spectral differences, however, may be attributed to the homogenization of the overall composition of the rock due to grinding rather than a chemical change induced by the mechanical action of grinding. XRD data (Fig. 6) confirm that pyrite (FeS2) is an additional source of sulfide-based minerals within the powder (black trace; other phases identified do not contain S, i.e. diopside (CaMgSi2O6), calcite (CaCO3), sodalite (Na4Cl(Al3Si3O12)), marialite (Na4Al3Si9O24Cl) and quartz (SiO2)). As illustrated in Fig. 5b, the sulfur absorption spectrum of the powdered rock can be described as a linear combination of contributions from both the absorption of the lazurite and pyrite, since the micronized powder has particles sizes smaller than the 5×5 μm2 beam. Least square linear combination fitting (LSLC) results suggest that pyrite may contribute ~30% of the final spectral fingerprint of the powdered rock, explaining the slight variations in the powdered rock when compared to the lazurite in the unground material. Consequently, the change in the sulfur speciation observed in the powdered rock cannot be ascribed to lazurite.
(a) XANES spectra of the lazurite in the lapis lazuli rock (blue trace) and the powdered lapis lazuli (black trace). The powdered rock differs from the lazurite by (i) a lower intensity in the absorption peak at 2469.1 eV, (ii) a shift towards lower energy (2472.2 eV) of the maximum of absorption of the envelope feature located between 2470 and 2475 eV, and (iii) the appearance of a shoulder at 2477.4 eV. These differences are attributed to the homogenization of the rock due to grinding; (b) Least Square Linear Combination fit (green trace) of the powdered rock XANES spectrum (black trace), using as references the XANES spectra of lazurite (blue trace) and pyrite (yellow trace), suggests a contribution of up to 30 wt% of pyrite.
X-ray powder diffraction of the powdered lapis lazuli rock (black trace), 1st grade pigment (dark blue trace), and 3rd grade pigment (light blue trace). The spectra are characterized by a mixture of different ratios of calcite, diopside, lazurite, marialite, pyrite, quartz, and sodalite.
The S K-edge XANES spectrum collected at the surface of the rock itself was used as a reference material for the rest of the study to ascertain whether pigment processing affects the chemical environment in lazurite. By contrast with the powdered rock, XRD showed the purest grade of pigment (1st grade) contains phases associated with lazurite, smaller quantities of calcite and diopside, and only negligible amounts of pyrite (Fig. 6, dark blue trace). These data confirm that the first separation step extracts mainly lazurite as the dominant phase. Subsequent washes lead to a decrease in lazurite and concomitant increase in accessory minerals. Direct comparison of the powdered lapis lazuli rock, 1st grade and 3rd grade pigment (Fig. 6) shows that, for equal levels of diopside, the peak intensity of lazurite is about 30% smaller in the 3rd grade pigment than in the powdered rock, while quartz shows an opposite behavior, with an increase in the 3rd grade pigment. These results confirm prior XRD quantitative measurements on similar samples [58].
The almost complete removal of pyrite during the first extraction eliminates the contribution of this species from the XANES measurements allowing for a direct comparison to the lazurite from the lapis lazuli rock, as shown in Fig. 7a. Interestingly, the 1st grade pigment presents a higher absorption intensity at 2469.1 eV than the unprocessed rock (though this does not change further between e.g. 2nd/3rd grade). The characteristic envelope features, with a maximum of absorption at 2472.7 eV and a shoulder at 2471.4 eV, are relatively stable throughout the processing. However, the pigments present a distinct absorption peak at 2473.3 eV, together with an increase of intensity of the 2471.4 eV shoulder. Of the three pigment grades, the 1st grade presents the strongest overall intensities compared to the 2nd and 3rd grades; no significant differences are observed between 2nd and 3rd grade. Similarly, a decrease in the intensity of the absorption peak at 2469.1 eV, going from 1st grade to 2nd and 3rd grade pigments, together with a less pronounced shoulder at 2471.4 eV and 2473.4 eV, is observed for the experiments done with unfocused beam (Fig. 7b), confirming that these changes are a result of pigment processing and chemical speciation, and are not induced by the beam itself.
(a) XANES spectrum of the 1st grade pigment (dark blue trace) and subsequent pigment grades (light blue traces) compared with the lazurite phase (black trace) in the unprocessed lapis lazuli rock. The three pigment grades present a higher absorption intensity at 2469.1 eV than the unprocessed rock (insert). The maximum of absorption at 2472.7 eV is relatively stable, but the pigments present a distinct absorption peak at 2473.3 eV, together with an increase of intensity of the 2471.4 eV shoulder. The 1st grade pigment presents the strongest overall intensities compared to the ultramarine ashes; no significant differences are observed between 2nd and 3rd grade; (b) The same pigment grades powders analyzed using unfocused beam. Note a decrease in the intensity of the absorption peak at 2469.1 eV in subsequent washes.
In comparison to the in-situ measurements made on the rock, the increase in intensity of the absorption peaks at 2469.1 eV, and 2471.4 eV in the 1st grade may be explained by a higher contribution of the polysulfide radical anion S3−˙ [27, 43]. There is ambiguity in the assignment of S K-edge XANES features in the 2471–2472 eV spectral range mostly related to the fact that the edge peaks for S2− bonded to Na and native sulfur overlap [59]. The decrease of the peak presenting a maximum of absorption at 2472.6 in the 1st grade pigment seems to correspond to a decrease in elemental sulfur (2472.8 eV, here defined as S0, also referred to as neutral sulfur) – as a S8 native sulfur K-edge XANES reference contains a similar absorption feature centered at 2472.8 [28, 60]. Lastly, the increase in absorption at 2473.3 eV can be assigned to trisulfur and tetrasulfur radical anions [43] that may form from the oxidation of elemental sulfur [27]. Table 1 summarizes the species contributions for the spectral features observed in natural lazurite, as discussed in the literature. Data from Fleet and Liu [27] have been recalibrated to fit the monochromator calibration used during this set of experiments.
Possible contributing species to the XANES spectra of lazurite.
| Peak position (eV) | Possible contributing species |
|---|---|
| 2469.1 | S3−˙ (2469.1 eV) [27, 43] |
| S2−˙ (2469.0 eV) [43] | |
| 2471.4 | S3−˙ (2471.4 eV) [27] |
| S2−˙ (2471.4 eV) [43] | |
| S2− (2471.5eV) [59] | |
| 2472.7 | S4−˙ (2472.9 eV) [43] |
| S2−˙ (2472.6 eV) [43] | |
| S0 (2472.8 eV) [28, 60] | |
| 2473.3 | S4−˙ (2473.5 eV) [43] |
| S3−˙ (2473.5 eV) [43] | |
| S2−˙ (2473.8 eV) [43] | |
| S0 (2473.4 eV) [27] |
-
Data from Fleet and Liu [27], have been recalibrated to fit the monochromator calibration used during this set of experiments.
Based on these results, the increased absorption maxima at 2469.1 eV and 2471.4 eV in the processed pigments appear to suggest a relative increase of trisulfur anions that can be attributed either to (i) an increase of trisulfur radicals within the zeolite framework, (ii) a specific selection of zeolites presenting the highest content of S3−˙ radicals during the first extraction, or (iii) a combination of both.
As recently discussed in the study of the origin of the blue/green color in blast furnace slag-based materials [60], sulfide (S2−) and elemental sulfur (S0) may be converted to S3−˙ radicals upon contact with oxygen and alkaline solutions, such as that which occurs during the pastello-washing (in KOH) stage of the pigment preparation [60]. This phenomenon may therefore explain the local increase of S3−˙ radicals in the processed pigments. In highly colored synthetic blue ultramarine pigments, it has been demonstrated that less than half of the lazurite β-cages are occupied by S3−˙ and moderate heating (150–500°C) results in a significant increase in the concentration of S3−˙ and S2−˙ [22]. This implies that sulfur species [likely S6 and S8 formed from the disproportionation of polysulfides (Eq. 2) during synthesis], can be transformed into chromophores (specifically S3−˙) via the application of heat (Eq. 3).
It might be suggested that a similar chemical process governs the speciation in naturally derived lazurite such as the materials studied here. Some proportion of the β-cages in the lapis lazuli’s lazurite phase contains S3−˙ (clearly evident in the black trace in Fig. 7a), the remaining sites could either be occupied by S2−˙, neutral sulfur species (Eq. 1), or be vacant [22].
One possible explanation for the observed changes in the sulfur chemistry is that during the preparation of the pastello, lapis lazuli powder is exposed to elevated temperatures in the range of 150–200°C. At these temperatures, radical anions can form from either the neutral sulfur and/or their dimers in an entropy-driven process [14]. Thus, the increased S3−˙ observed in the processed pigments may stem from these species, consistent with previous work [21], which suggests that sulfur radicals must be generated simultaneously with the formation of the zeolite cages in order to be stable, because the β-cages are impermeable for the color centers following formation.
Interestingly, the radiation damage portion of this work provides additional support for this hypothesis: the changes in the XANES spectra (Fig. 2a) confirm that beam exposure promotes a redox mechanism similar to the one proposed in Eq. 1–3, either related to thermal or photochemical effects of the exposure to X-rays.
However, if the thermal or photochemical effect explain the increase in S3−˙ observed in the 1st grade pigment, it is not clear how the Cennini’s process promotes the selection of the β-cages richer in lazurite. Variable aluminum framework/non-framework ratios characterize the lazurite minerals within a given lapis lazuli rock [25]. Removal of aluminum atoms from the lazurite framework results in a transformation of the four-coordinate aluminum compounds into six-coordinate aluminum compounds, causing a permanent opening of the cage framework, and enabling the release of the chromophore radical anions and subsequent discoloration [15]. A preferential interaction with the pastello might therefore be suggested for the zeolite structure characterized by a “weaker” aluminum framework, poorer in S3−˙ radicals, and thus less blue. This hypothesis might suggest that the interaction of the pastello promotes the selection of lazurite with higher framework structure, richer in S3−˙ anions, and therefore more blue, during the first washout. Further work on the pastello and its organic components is needed in order to elucidate the possible role played during the separation process.
Implications for the analysis of cultural heritage objects
S K-edge XANES was applied to characterize the effect of Cennini’s extraction process on the lazurite chemistry. Overall, the XANES spectral fingerprint from the rock to the pigment is altered, both in terms of peak intensity and absorption positions, complicating the correlation between the rock origin and the end pigment product. This trend needs to be verified for lapis lazuli from regions other than Afghanistan in order to better quantify the changes in fingerprint introduced during processing of lapis lazuli rock to the ultramarine blue pigments. Nonetheless, the extraction process allows for the selection of the more blue particles, and analysis of the XANES spectra allows a mechanism for this to be proposed. The pre-peak at 2469.1 eV is particularly diagnostic of the contents of S3−˙, chromophore responsible for the intense blue coloration. Its concentration increases in the 1st grade, while decreases with subsequent washes, having its minimum concentration in the lapis lazuli rock itself. The phenomenon could be explained by a thermal reaction that promotes the formation of new trisulfur radical species during the extraction process. A preferential affinity with the pastello, and subsequent retention of the zeolite structures poorer in S3−˙, might be the basis of the Cennini’s separation process. This hypothesis is not yet verified; further work focused on the pastello and its organic component, before and after the separation steps, will help clarify the mechanism behind the effectiveness of the historical method.
As synchrotron techniques continue to gain traction, radiation damage assessment is critical. Direct comparison of micro-focused (10×10 μm2) and unfocused (500 (v.)×1000 (h.) μm2) beam experiments has demonstrated the tendency of ultramarine pigments to react to prolonged exposure to low-energy X-rays. Using a micro-focused beam, a thermal or photochemical process is promoted already after the first irradiation (t0). However, the behavior of the pigments to the micro-focused and unfocused beams is comparable, suggesting that the increase in S3−˙ is not beam dependent, but rather is a result of chemical processing during pigment preparation. The definition of optimal acquisition parameters is key: monitoring the possible damage induced by the exposure to micro-focused beam – ideal to overcome the heterogeneity at micrometer scale – with data acquired using unfocused beam has shown to be an effective way to evaluate the most appropriate experimental conditions to allow for safe analysis of real works of art.
Article note
A special issue containing invited papers on Chemistry and Cultural Heritage (M.J. Melo, A. Nevin and P. Baglioni,editors).
Acknowledgments
Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Use of the J.B. Cohen X-Ray Diffraction Facility is supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205). This project was supported by the Getty Conservation Institute (GCI) and by the Northwestern University/Art Institute of Chicago Center for Scientific Studies in the Arts (NU-ACCESS). NU-ACCESS is funded through a generous grant from the Andrew W. Mellon Foundation. The authors are grateful to Karen Trentelman for productive discussions.
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