- Research article
- Open Access
Combined photoluminescence and Raman microscopy for the identification of modern pigments: explanatory examples on cross-sections from Russian avant-garde paintings
© The Author(s) 2019
- Received: 4 November 2018
- Accepted: 2 March 2019
- Published: 19 March 2019
In conservation science, the identification of painting materials is fundamental for the study of artists’ palettes, for dating and for understanding on-going degradation phenomena. For these purposes, the study of stratigraphic micro-samples provides unique information on the complex heterogeneity of the pictorial artworks. In this context, we propose a combined-microscopy approach based on the application of time-resolved photoluminescence (TRPL) micro-imaging and micro-Raman spectroscopy. The TRPL device is based on pulsed laser excitation (excitation wavelength = 355 nm, 1 ns pulse width) and time-gated detection, and it is suitable for the detection of photoluminescent emissions with lifetime from few nanoseconds to hundreds of microseconds. In this work, the technique is beneficially applied for identifying different luminescent semiconductor and mineral pigments, on the basis of their spectral and decay kinetic emission properties. The spatial heterogeneities, detected in the micro-sample, are investigated with Raman spectroscopy (785-nm in CW mode) for a further identification of the paint composition on basis of the molecular vibrations associated with the crystal structure. The effectiveness and limits of the proposed combined method is discussed through analysis of a corpus of stratigraphic micro-samples from Russian Avant-garde modern paintings. In the selected samples, the method allows the identification of modern inorganic pigments such as cadmium-based pigments, zinc white, titanium white, chrome yellow, ultramarine and cinnabar.
- Photoluminescence microscopy
- Time-resolved photoluminescence
- Raman microscopy
- Painting materials
- Pigment identification
In conservation science, the identification of painting materials is fundamental for the study of artists’ palettes, for dating and for understanding on-going degradation phenomena. The in-depth knowledge of artist materials plays a crucial role in the fine-tuning of restoration and conservation protocols. For these purposes, the study of stratigraphic micro-samples provides unique information on the complex heterogeneity of pictorial artworks.
In this contest, Raman spectroscopy is retained as standard non-destructive technique for pigment identification in polychrome artworks, as paintings, sculptures and ancient manuscripts [1–6]. The recent research has clearly demonstrated how the method can be highly effective both for the direct in situ analysis of artwork—with the aid of portable and compact devices —and for the in-depth study of micro-samples in the laboratory [4–6]. The identification of materials is made even easier by online Raman spectra databases, which are accessible without any restrictions and usable also by non-expert users [7, 8]. The main limit of the method remains the intrinsic weakness of the Raman scattering phenomenon, whose scattered signal can be hindered by other competitive processes, first of all the broadband fluorescence from some samples. One effective way to improve the efficiency of Raman spectroscopy is to combine several excitation wavelengths as reported in  in order to optimized the tradeoff between the signal to noise ratio and the competitive fluorescence baseline arising from different materials. Another possibility is the use of a single near-infrared laser, for reducing the fluorescent contribution, and applying specific acquisition protocols, such as the Subtracted Shifted Raman spectroscopy (SSRS) or the Shifted Excitation Raman Difference Spectroscopy (SERDS) [9–11]. All these strategies aim at diminishing the limitations imposed by fluorescent samples.
Similarly, UV-induced fluorescence microscopy, based on the use of conventional epi-fluorescence microscope coupled with filtered mercury lamp for sample excitation and with a color digital camera for image registration, is a widely employed method in restoration laboratories. However, the fluorescence phenomenon is considered only for qualitative inspection, allowing one to simply evaluate the distribution of heterogeneities on surfaces or on micro-samples . This is due to the limited chemical specificity of the fluorescent processes, since—when considering organic compounds typically encountered in artworks as binders and varnishes—the emission occurs from a variety of fluorophores , giving rise to a broadband and often poorly specific signal.
Nonetheless, the investigation of the fluorescent or more generally photo-luminescent (PL) emission can be advantageous when investigating specific categories of highly luminescent pictorial materials. This is the case of luminescent semiconductor and mineral pigments (see Additional file 1: Table S1). In fact, in direct semiconductor pigments, the free pair electron–hole radiative recombination is related to the energy of the band gap and hence highly specific of the semiconductor type. In addition in both direct and indirect semiconductors a radiative recombination from trap states can occur, associated to the defects and impurities within the semiconductor crystal structure. This latter emission can be informative of the semiconductor material and in particular of its synthesis process [14–16]. Finally, in mineral pigments, the presence of substitutional ions can give rise to a specific PL emission. An illustrative example is the presence of Cu2+ ions in the cuprorivaite mineral, which give rise to the exceptional infrared emission of the Egyptian Blue pigment . By employing a spectrally-resolved and time-resolved PL approach, the characteristics of the photoluminescence emissions, i.e. the emission spectrum and lifetime, can be retrieved providing key-parameters for the identification of luminescent pigments.
In this work, we investigate the combination of time-resolved photoluminescence (TRPL) microscopy and micro-Raman spectroscopy for pigment identification, taking the advantages of the different sensitivity of the two methods to the phenomena described before and of the elevated flexibility of both systems that are custom-built. The effectiveness and limits of the proposed combined-analytical method are discussed through analysis of a corpus of stratigraphic micro-samples from precious Russian modern paintings, with a particular focus on semiconductor pigments—such as cadmium yellow/orange, zinc white and titanium white—widely diffuse in the modern age. We considered here selected micro-samples from oil paintings by Mikjail Larionov (1881–1964) and Natalia Goncharova (1881–1962), leaders of the Russian avant-garde (also known as Rayionism), mainly active in the period between the 1912 and 1915 [18, 19].
Three micro-samples from Russian modern paintings are considered in this work, coming from Archivio Gallone hosted at the Physics Department at the Politecnico of Milan . Samples were taken from two paintings belonging to a private collection and attributed one to Mikjail Larionov (following labelled as sample L5 and L6) and the other to Natalia Gocharova (sample G3). The Goncharova’s painting belong to the ‘Rayonist Forest’ series, while Larionov’s painting is untitled. Both paintings are not dated, but they supposedly painted after the 1914. Samples from these two paintings were prepared as stratigraphic cross-sections by embedding them in epoxy resin.
The PL properties of micro-samples are probed with a TRPL microscope. A scheme and a detailed description of the setup is provided elsewhere  and here is briefly summarized. The system is based on a Q-switching laser source (FTSS 355-50, Crylas GmbH, Berlin, Germany, λ = 355 nm, pulse energy = 70 μJ, pulse duration = 1 ns, repetition rate = 100 Hz) and a fast time-gated intensified CCD camera, coupled together with an epi-fluorescence microscope. The microscope mounts a 15× and a 50× objectives, which allows analysis of a field of view of 900 μm and 300 μm in diameter, respectively, with a spatial resolution down to 1 μm and 0.6 μm, respectively. The microscope is also equipped with 12 band-pass transmission filters (FKB-VIS-40, Thorlabs Inc, spectral range covered: 370–870 nm) in the detection path. The time-gated camera allows the detection of a two multi-spectral imaging datasets of the PL emission occurring at nanosecond and microsecond timescales, as described in details in the “Protocol” section.
The Raman device is a flexible homemade system, described elsewhere . The system is based on a solid state laser emitting at 785-nm in CW mode and on a spectrometer coupled to a cooled Si-based CCD camera. The detection of Raman peaks is made in the spectral range 130–3000 cm−1 with a spectral resolution close to 10 cm−1. The excitation and detection units are connected to a micro-probe that allows the detection of Raman spectra at high signal-to-noise-ratio on selected spots of 15 μm in diameter at a working distance of around 2 mm.
The cross-section is then analyzed with the TRPL microscope system. A sequence of PL time-gated images at a fixed delay is recorded in different spectral bands. In the present case study, analysis of the emissions occurring at the nanosecond and microsecond timescales are achieved by employing, for the former, a gate with a temporal width of w = 10 ns synchronous with laser pulse (delay D = 0 ns), whereas, for the latter, a gate with a temporal width of w = 10 μs set at a delay D = 0.2 μs after the pulsed excitation. Following correction for the spectral efficiency of the detector, this procedure gives rise to the creation of two time-gated multispectral imaging datasets, related to the spectral emission behavior of the cross-section in a certain temporal regime (nanosecond or microsecond) of the emission decay. Each dataset is composed of a sequence of grey-colour images related to the emission intensity of the sample in different spectral bands.
Following this, a subset of images of the multispectral dataset are merged, achieving in this way a false color image useful for material discrimination. The PL spectrum is further reconstructed selecting a region of interest (ROIs). In this reconstruction procedure, for the sake of simplicity, each bandpass filter is modelled as a Dirac delta function centered at the filter central wavelength and the spectral transmission of filters are accounted for in the overall spectral detection efficiency. The PL spectrum in each ROI is shown as the mean of intensity values within the ROI with error bars reporting the ROI standard deviation. Data obtained are compared with literature and standard samples purchased from Kremer Pigmente and Sigma Aldrich.
After micro-TRPL analysis, the cross-section is then investigated with Raman spectroscopy on analysis points selected on the basis of previous measurements and observations. Raman measurements are typically carried out with an acquisition time between 5 and 15 s and an irradiance on sample between 700 and 3500 W cm−2. On the basis of the collected and post-processed Raman spectra (baseline subtraction and SSRS), material identification is achieved through comparison with reference Raman data from a free online published database [7, 8] or with Raman spectra of standard samples purchased from Kremer Pigmente and Sigma Aldrich.
With a conventional microscope, it is possible to recognize three layers in the stratigraphic sample L5 (Fig. 1). From the top to the bottom, a first layer (Layer 1) is constituted by two shades of yellow paints (light one and dark one), mixed together with a greenish colour, that appears as stripes through the layer. Beneath, a white layer (Layer 2) mixed with brown is superimposed to a tiny blue layer (Layer 3), where blue pigments grains are coarsely mixed with a white paint. UV photography (Fig. 1) highlights the complex and heterogeneous morphology of the yellow layer, with the appearance of reddish and greenish emitting region and a relevant presence of luminescent heterogeneities in all the paint layers. The brown colour does not show any visible emission, and similarly the blue layer. The UV image does not underline any other specific detail.
The spectral profile of the luminescent pigments in the stratigraphy are reconstructing achieving the identification of three main luminescent pigments in sample L5. In details, the yellow layer (Layer 1) shows a short emission (lifetime of order of few ns) peaked in the spectral region between 450 and 500 nm and a microsecond emission in the NIR region, peaked around 700 nm (Fig. 3c). These features can be associated to the presence of cadmium yellow (Cd1−xZnxS, 0 < x < 0.25) paint, as largely discussed in [22, 23]. The white layer (Layer 2) shows an intense blue short emission (380–400 nm) combined with a green microsecond emission (500–550 nm), properties that correspond to zinc white (ZnO) paint [15, 24]. As already quoted, PL micro-imaging reveals the presence of luminescent heterogeneities in the blue layer (Layer 3), characterized by microsecond NIR emission beyond 800 nm, features that can be associated to trap state emission of cadmium based pigment, possibly cadmium red (CdS1−xSex, 0 < x < 0.50) . However, the reconstruction of the spectrum on nanosecond timescale (associated to the band edge emission of the semiconductor) is critical due to the very intense emission of ZnO, diffusing within the stratigraphy of the micro-sample.
Pigments identified on sample L5 on the basis of Raman spectroscopy
354(s), 375(m), 397(w), 839(vs)
256(m), 679(s), 746(s), 951(m) and in SM: 1143(m), 1337(s), 1448(m), 1523(vs)
253(s), 280(sh-w), 343(w)
Pigments identified on sample L6 on the basis of Raman spectroscopy
Raman modes (cm−1)
253(s), 280(sh-w), 343(w)
Calcium sulphate dihydrate
419(m), 490(w), 619(w), 669(w), 1008(vs)
331(w), 383(w), 438(s)
Goncharova cross-section G3 is not made by a clear stratigraphy. Colours are one into the others in accordance with the painting technique of the artist. White, yellow, orange-yellow and dark green are recognizable as main colours. Specifically, Layer 1 and Layer 2 are mainly composed of dark yellow and orange pigments; the latter layer includes also a dark green stripe. Layer 3 and Layer 5 are white, the first one with blue inclusion coming from a thin layer between Layer 2 and 3. A bright yellow pigment (Layer 4) divides the two white layers. Epi-fluorescence microscopy does not add any details with respect to the visible microscopic image.
Pigments identified on sample G3 on the basis of Raman spectroscopy
Raman modes (cm−1)
331(w), 383(w) 438(s)
147(vs), 198(vw), 396(m), 515(m) 640(m)
354(s), 375(m), 397(w), 839(vs)
The results of the pigments identification are summarised in Table 1. PL approach recognizes ZnO as the main white pigment in all samples, while the presence of TiO2 is found in G3 sample. The yellow and orange pigments have been associated to cadmium-based pigments. Raman measurements complete the list and in Larionov samples allowed the detection of Cinnabar (HgS) as red colour (which instead has any luminescent emission when excited with UV). In the yellow layer of L4 sample, the use chrome yellow (or lead chromate) can be recognized, while the greenish stripes within the yellow layer is due to the presence of the blue Phthalocyanine dye. Two white pigments required a further discussion. The first one is lead white in sample L6. This white pigment has two crystalline structures (cerussite and hydrocerussite) and it highly absorbs above 5 eV . It has been shown that when excited below this threshold (as in our case, around 3.5 eV), both crystal structure show an emission in the green due to trap states. In case of sample L6, this emission is superimposed to the green and more intense band of the zinc white pigment. Thus, its presence is hardly detectable with the use of PL techniques when it is mixed with other strongly luminescent pigments. On reverse, Raman spectroscopy has clearly inferred its presence. A second observation is related to titanium white in G3 sample. In this case, Raman spectroscopy allowed to detect the presence of the Anatase form of TiO2, whereas TRPL analysis it is possible to suppose the presence of the Rutile form of TiO2. Indeed, the two crystal forms are usually present in titanium white pigments, especially in the early production of the pigment . From an historical point of view, this result indicates that the particular micro-sample analyzed might belong to a series of Goncharova of her Paris period, since titanium white is expected in oil painting after the 1920, when a French firm started an intense production [28, 29]. As final observation, the use of these pigments has been detected in other works belonging to the Russian avant-gard, as reported in already published literature [30–32].
The results of the case studies presented in this work put in evidence the sensitivity of TRPL and Raman spectroscopies to different materials. We have illustrated how in case of artist materials, the complementarity of these two methods can be highly advantageous: the time-resolved PL microscopy is highly sensitive to the presence of luminescent semiconductor pigments. Further, it gives an insight on the distribution of the luminescent compounds and on the material heterogeneity, whereas Raman spectroscopy can confirm and/or complement the information through a vibrational pattern specific of the chemical composition. The use of micro-Raman spectroscopy alone does not allow the identification of all the pigments present in a complex stratigraphy: indeed, as illustrated in the present case, modern semiconductor pigments are not or are hardly detectable with Raman microscopy, even when employing a proper post-processing data (like SSRS). Similarly and complementary, TRPL microscopy has allowed only the identification of luminescent pigments and cannot add any information on materials that do not have a specific luminescent fingerprint. Instead, when considered together, the two methods have provided a clearer view of the complexity of the paints and pigment mixture employed by the two Russian artists, as deeply discussed above.
List of pigments found in the Russian avant-garde painting cross-sections 
On the market
Identification provided by Raman
Identification provided by TRPL
L5, L6, G3
Principal conventional techniques applied to the analysis of paint stratigraphy
Elements (Z > 10)
~ 100 nm
Organic and Inorganic compounds
Organic and Inorganic compounds
In this context, our approach—based on Raman and TRPL studies—add further information to the ones achieved with the conventional approach based on SEM–EDX and ATR-FTIR, providing clues on the presence of materials with specific Raman or PL fingerprints. Moreover, being non-contact and non-invasive the proposed approach can be considered as mandatory in presence of high-valuable samples, for which any surface pre-treatment is allowed, or for the rapid and non-invasive screening of a large number of samples. In a future research, it will be of interest to study—on the same paint stratigraphy—how many pigments can be identified with the combination of TRPL and Raman measurements with respect to the combination of SEM–EDX and ATR-FTIR or ATR-FTIR and Raman.
In this work, we have applied two techniques, based on TRPL micro-imaging and on micro-Raman spectroscopy, that have been rarely used together as microscopy methods. We propose here this novel approach for the investigation of pigments and artist materials. The method instead of being alternative to other conventional analysis protocol (as the previously quoted SEM–EDX and ATR-FTIR analyses) provides complementary data for further understanding the complex and heterogeneous material composition of paint layers in paintings. In the future, taking into account the high sensitivity of the two methods to the detection of minerals, the proposed approach could be extended to other case studies, as stratigraphic micro-samples taken from ancient sculpture, wall painting and heritage stones.
All authors contributed to research. AA and MG were responsible for photoluminescence measurements, analyses. SM carried out Raman spectroscopy measurements and analyses. DC coordinated the publication. All authors were responsible for interpretation of results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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