**1. Introduction**

After the accidental discovery of mauveine in 1856, a new class of dyestuffs and pigments was developed and introduced to the market. These were formerly defined as coal-tar dyes and are nowadays called synthetic organic pigments (SOPs). SOPs are "manufactured colourants that have a carbocyclic ring skeleton" [1]. In contrast to natural organic pigments, they are synthesized in a laboratory environment and produced on an industrial scale, so that natural organic pigments like indigo and alizarin can be considered as SOPs when they are produced industrially [1].

After their introduction, paint manufacturers started to use SOPs for their tube paints, which led to new shades in artists' colours and to the imitation of traditional pigments [2–4]. As it was initially not mandatory for manufacturers to indicate the use of SOPs in their paint formulations, artists often lacked knowledge of the exact content of their supplies. Considering the minor fastness characteristics of the early SOPs compared to traditional pigments, their use also led to material problems previously unknown to the artists, who tended to take a critical view on these new pigments [2,5]. In contrast to the traditional artists' pigments, the availability of certain SOPs is linked to an exact date of introduction, which can be traced by means of patents. Therefore, the identification of SOPs can not only provide information about the dating of an artwork, but can also expose forgeries [6]. Furthermore, knowledge of the occurrence of certain SOPs can provide useful information for restoration and conservation approaches or art technological studies.

SOPs can be difficult to detect in paint samples, because, due to high tinting strength, relatively small amounts are generally present [7]. Furthermore, these pigments have a very

**Citation:** Pause, R.; van der Werf, I.D.; van den Berg, K.J. Identification of Pre-1950 Synthetic Organic Pigments in Artists' Paints. A Non-Invasive Approach Using Handheld Raman Spectroscopy. *Heritage* **2021**, *4*, 1348–1365. https:// doi.org/10.3390/heritage4030073

Academic Editor: Diego Tamburini

Received: 30 June 2021 Accepted: 13 July 2021 Published: 17 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

small particle size and are usually part of a complex system of ingredients in artists' tube paints. Therefore, traditional methods for pigment identification, such as polarised light microscopy, can hardly be used. Identification with Fourier transform infrared (FTIR) spectroscopy may be equally complicated, because the strong and broad peaks of paint binders, inorganic substrates, or extenders can interfere with the SOPs' information [8]. Invasive and destructive methods like pyrolysis gas-chromatography/mass-spectrometry (Py-GC/MS) can sometimes provide useful information on the presence of certain SOPs, such as azopigments. Others may not be suitable for this technique due to their thermal and chemical instability [9,10]. High performance liquid chromatography-mass spectrometry (HPLC-MS) can be challenging too, as getting the pigment to dissolve can be difficult and the effects of dissolution on the pigment structure may be problematic for interpretation [11]. MicroRaman spectroscopy (μ-RS) has been demonstrated to be a suitable analytical tool for the identification of SOPs. It is relatively sensitive towards these pigments, requires little sample material, and allows samples to be used for further, complementary analysis [12–14]. Approaches for μ-RS SOPs identification in samples from artworks are under constant development [9,10,15,16]. The main limitation in using μ-RS to identify SOPs in artists' paints, however, is related to the fluorescence associated with organic binders, varnish layers and additives, or sometimes even with the target pigments themselves. In order to obtain an interpretable spectrum, it is important to perform measurements in equilibrium between fluorescence and the intensity of the Raman signal, which makes this approach time-consuming [8,17].

All these analytical techniques, however, require the sampling of small paint fragments, which, besides being destructive, can be challenging, as the probability that the samples are not representative for the materials studied might be quite high [18]. An analytical approach using non-invasive devices that allow for unlimited measurements therefore seems desirable [19]. The use of visible reflectance spectroscopy and visible-excited spectrofluorimetry was successful for the non-invasive identification of contemporary SOPs [20]. Yet, for the non-invasive study of artworks, Longoni et al. [20] recommend the use of further complementary non-invasive analysis. The usage of portable Raman systems seems to be promising [21,22]. A portable device that minimizes fluorescence and can operate at low energy is needed. Damage by strong laser absorption should be avoided to prevent local burns, which may occur especially on opaque or dark materials [8].

Different handheld Raman systems have been reviewed and tested for their application in the field of cultural heritage [19,23–25]. Technological developments have countered the most difficult problems, such as low Raman signal, high fluorescence, and environmental interactions, as well as the positioning and focusing of the laser [26,27]. To date, the Bravo handheld Raman instrument developed by Bruker seems to be the most promising device [19] and its applicability to the study of modern art was evaluated [28]. This device employs a DUO Laser system (785 nm and 853 nm), recording spectra in two separate spectral ranges of 300–2200 and 1200–3200 cm<sup>−</sup>1, respectively. The patented sequential shifted excitation (SSETM) works by temperature-shifting the lasers over a small wavelength range and recording three spectra per laser, finally merging them into a seventh averaged spectrum. In this way, and through the use of near infrared lasers, the system is able to reduce fluorescence and excite colour with low efficiency, also enabling the measurement of, for example, varnished paint samples [19,23,27]. However, Pozzi et al. [19] observed that if the peaks and curves of the individual spectra differ too much from each other, there is a risk of creating artefacts within the averaged spectrum. Another disadvantage of the device is related to the fact that the only two adjustable settings are exposure time and acquisition number, but not the energy of the laser, which can be a risk for certain art-technological studies [19]. These aspects, regarding the spectral quality and applicability, are discussed within this study.

The aim of this research is to study the potential of a handheld Raman device in identifying SOPs in historical varnished paint-outs (1932–1950) with consideration to future applications in the non-invasive study of early SOP-containing paintings.

Ongoing work on the collection of SOPs Raman reference spectra (https://soprano. kikirpa.be, accessed on 30 June 2021 [14,29]) enables and simplifies pigments' identification. These reference spectra were often obtained by studying modern artists' supplies and can give an idea of the actual SOPs that can be expected.

Before 1950, the following SOP groups were already developed and established: β-Naphthol pigment lakes; BON pigment lakes (pigments produced with 2-hydroxy-3 naphthoic acid as coupling component); β-Naphthol; Pyrazolones; Hansa Yellows; Diarylides; Naphthol AS; Iron complex Pigment Green B; Phthalocyanines; Nickel Azo Yellow [30]. However, as already mentioned, the actual distribution and use of this "new" group of pigments within artists' materials is still unclear today. Attempts to provide an overview of the SOPs that were known before 1950 and of their occurrence in artists' paints demonstrated the need for more research [7,30]. A starting point could be the study of the production archives of artists' material suppliers.

Our research in the archives of Royal Talens and its main pigment suppliers before 1950, BASF and Bayer, shows that there are considerably more SOPs used in artists' paints than hitherto known [4,11,31]. Notably, most of them have not been traced in paintings.

This study focuses on SOPs in Royal Talens' artists' oil paints from before 1950. Soon after its foundation in 1899, Talens developed into a major international player on the market, already selling more than 1,500,000 oil paint tubes in 1918 [32]. European artists such as van Dongen, Kirchner, Appel, and Munch used Talens' Rembrandt oil paints [33]. In this position, Talens declared the use of SOPs in fine artists' oil paint publicly in the beginning of the 1930s.

Based on pre-1950 oil paint recipes [4,16], a selected set of paint-outs (1932–1950) from the Royal Talens' archive was investigated. For the assessment and evaluation of the handheld Raman spectra, micro samples of the historic Talens paint-outs were also analysed with a benchtop microRaman spectrometer.

Handheld Raman spectra were evaluated, taking into account their major points of criticism, like background fluorescence, the intensity of the Raman signal, spectral resolution, and the risks of laser energy induced damages.

#### **2. Materials and Methods**

#### *2.1. Samples*

The sample selection consisted of paint-outs from 1932 to 1950. These were included within colour charts specifically dedicated to coal-tar pigments, labelled as unalterable Teerkleurstoffen [coal-tar dyes] (Figure 1), which were published in six editions of the book Kunstschildersmaterialen en Schildertechniek [Painting materials and painting technique] [34–40]. The books deal with Talens' fine Rembrandt artists' oil paints, which were on the market since 1904 [4,33]. In addition, some paint-outs that were listed under inorganic names within the inorganic paint colour charts, but suspected to contain SOPs according to recipe interpretation, were examined as well. To obtain results on a larger data set, it was decided to measure (if available) the same set of 42 potentially SOP-containing paint-outs in each Kerdijk edition, even if, according to the recipes, the same SOPs were to be expected. In total, 193 samples were analysed with handheld Raman spectroscopy and for comparison with benchtop microRaman spectroscopy. Samples were labelled with a "K" (Kerdijk) and the year of publication. All paint-outs were varnished.

#### 2.1.1. Instrumentation

The handheld Raman measurements were conducted with a Bravo Spectrometer (Bruker). The device works with two excitation wavelengths, recording spectra in two separate spectral ranges of 300–2200 and of 1200–3200 cm−<sup>1</sup> with a DUO Laser system (785 nm and 853 nm). The near infrared lasers (NIR) aid in reducing fluorescence, as the photon does not possess enough energy to induce high molecular fluorescence [22,41]. Furthermore, using two lasers enables researchers to obtain better quality data in the overlap region of 1200–2200 cm−<sup>1</sup> and lowers fluorescence interference [19]. The energy

reaching the surface during the measurement is officially stated to be less than 100 mW (Table 1). Since the energy cannot be set manually, this indication is too unspecific to be taken as a guide value when analysing delicate cultural heritage objects. In order to estimate the energy that actually reaches the surface of the sample, measurements were carried out with a "Power and Energy Meter" from Thorlabs, using the same settings of the historic Talens paint-outs analyses. The measured energy value was about 45 mW, with measurements conducted at a distance of about half a millimetre, with a spot size of 1 mm. This value also agrees with that determined by Pozzi et al. of about 50 mW [19].

**Figure 1.** Kerdijk colour chart from 1932, presenting a selection of Talens' Rembrandt artists' oil paints 'à base de goudron' (based on coal tar). Please note that the colours are defined as 'inaltérables' (unalterable).

**Table 1.** Characteristics of the microRaman and handheld Raman devices.


<sup>a</sup> Perkin Elmer, RamanMicro 300, coupled to Raman Station 400F; Available online: https://www.perkinelmer. com/CMSResources/Images/46-74804SPC\_RamanStation400RRamanStation400F.pdf (accessed of 14 July 2020). <sup>b</sup> Bravo Bruker [13,23,32]. <sup>c</sup> Due to the lack of a camera and the size of the laser pinhole of 3 mm, it was not possible to precisely focus on this spot size.

To evaluate the handheld device's performance for the identification of SOPs, the colour charts were also analysed with a benchtop microRaman instrument. These reference spectra were recorded with a RamanMicro 300 in combination with a RamanStation 400F (Perkin Elmer, Waltham, MA, USA) and an Olympus BX51M microscope (Table 1). Micro-Raman spectra were acquired from very small samples of the same paint-outs. After the mechanical removal of the varnish from the sampling area, minute paint fragments were taken at the upper right edge where the paint film is thickest. The samples were placed on an aluminium tape, mounted on a glass slide, and positioned as close to perpendicular to the optical axis as possible to reduce fluorescence [42].

Spectra were recorded in the range of 300–2000 cm<sup>−</sup>1, in order to perform the analysis within the same range as the handheld Raman. The microRaman reference spectra were only baseline corrected when necessary for their readability.
