Identification of artificial orpiment in the interior decorations of the Japanese tower in Laeken, Brussels, Belgium
© Vermeulen et al.; licensee Springer. 2015
Received: 7 October 2014
Accepted: 10 February 2015
Published: 15 April 2015
In this paper, we used a multi-technique approach in order to identify the arsenic sulfide pigment used in the decorative panels of the Japanese tower in Laeken, Belgium. Our attention was drawn to this particular pigment because of its relatively good conservation state, despite its known tendency to fade over time when exposed to light. The pigment was used with different painting techniques, bound with oil and urushi in the lacquers and with an aqueous binder in the mat relief panels. In the latter case it is always applied as an underlayer mixed with ultramarine blue. This quite unusual pigment mixture also shows a good state of preservation.
In this study, the orpiment used for the Japanese tower has been identified as an amorphous arsenic sulfide glass (AsxSx) with the aid of light microscopy, PLM, SEM-EDX and Raman microscopy. The pigment features different degrees of As4S4 monomer units in its structure, also known as realgar-like nano-phases. This most likely indicates different synthesis processes as the formation of these As4S4 monomers is dependent of the quenching temperature (Tq) to which the artificial pigment is exposed during the preparation phase.
Introduction and historical context
In characterization studies of arsenic sulfide pigments, there is a high probability that conclusions regarding the exact nature of the compounds present are drawn prematurely. In many cases, the encountered arsenic sulfide is recognized as being a mineral orpiment while in reality it appears to be pararealgar - the yellow degradation phase of realgar - while in others, distinction between natural and artificial orpiment is not made. Contradictions about the exact nature of arsenic sulfide pigments can also be found in published scientific literature or pigment encyclopedias, sometimes confusing realgar with an arsenic sulfide glass of unknown composition . So-called artificial orpiment can be produced using two different processes, wet and dry, using different starting materials [2,3]. In any case, arsenic sulfide pigments are the result but the structure and properties of the reaction products can be different. To understand its composition and characteristics, a multi-technique approach is required. Each used technique, from SEM-EDX to Raman spectroscopy and polarized light microscopy, can increase the knowledge about the processes behind artificial arsenic sulfide. Although the use of such a pigment in a 20th-century artifact seems rather uncommon, its good conservation state is even more striking when one considers how instable this material is over time.
The construction of this Japanese tower took three years and it was inaugurated in 1905. The main entrance porch was taken from the original Parisian pagoda while other elements such as the sliding doors and decorated panels were shipped from Japan; in some cases, these artifacts were re-used elements from disassembled temples. Finally, some elements were created in Europe, by European craftsmen [10,11].
To understand this unusual arsenic sulfide stability, the characterization of the pigment used in the panels has been undertaken using a multi-technique approach. Since arsenic sulfide is a good Raman scattering material, Raman spectroscopy (MRS) along with polarized light microscopy (PLM) seemed the best suited methods to identify the type of arsenic sulfide present in the tower. Nonetheless, scanning electron microscopy coupled with X-Ray detection (SEM-EDX) also seemed useful to some extent. On the other hand, more sophisticated methods such as microscopic X-Ray diffraction (μ-XRD) or time of flight secondary ion mass spectrometry (ToF-SIMS) did not seem very promising due to the poor ionization of the arsenic in the case of SIMS and the lack of signal - except for the broad band characteristic for amorphous material - in the case of μ-XRD.
Materials and methods
Among 165 samples taken from the tower for a complete technical study, a total of 24 samples containing arsenic sulfide pigment and sampled on different floors were selected. The samples were numbered P201.061 to P210.036 for archiving. They were then mounted as cross-sections (C87.129 to C90.158) in order to be analyzed. The cross-sections were prepared by embedding the samples between acrylic resin cubes. The bulk sample was first fixed to a 1 cm3 poly(methyl methacrylate) cube using a white PVA glue, after which the acrylic copolymer resin (Spofacryl®, Spofa Dental CZ-10031, Prague) was poured on the sample and the second cube placed on top. The cross-sections thus obtained were left to dry, then polished with water until the sample surface was nearly exposed. Then, polishing was continued in set circumstances using Micro Mesh© polishing cloths in a sequence of grits (2400, 4000, 6000 and 8000 mesh) leading to a mirror-like surface. In the case of the reference samples, a JEOL Cross Section Polisher IB-09010CP (JEOL, Tokyo, Japan) was used for 12 hours with a 4.0 kV accelerating voltage, 4.4 argon gas flow and a swinging stage. The cross-sections were photographed with an optical microscope Axio Imager 2 (Carl Zeiss, Oberkochen, Germany) under visible and ultraviolet light (excitation bandpass filter from 390 to 420 nm) with magnifications up to 500x.
Reference samples used in this study were the natural orpiment (Kremer Pigmente GmbH & Co, Aichstetten, Germany) and laboratory-prepared arsenic sulfide glass. The latter was synthesized by mixing arsenic trioxide with sulfur (1:1) in a glass tube and heating it for a few minutes over a Bunsen burner as described by Rötter . After grinding, a fraction of the material was mixed with Arabic gum (2/3 in water) until a good consistency was reached. It was then applied on a calcium carbonate coated board before being mounted as a cross-section; other material was kept loose for PLM analysis.
Methods and instrumentation
SEM analyses (backscattered electron images, elemental mapping and point analyses) were carried out on gold-coated (SPI-MODULE™ Sputter Coater, SPI, West Chester, PA, USA) cross-sections using a JEOL JSM6300 scanning electron microscope (JEOL, Tokyo, Japan) equipped with Pentafet Si(Li) and BSE (Tetra Link) X-ray detectors, both from Oxford Instruments. EDX-analyses were run at an acceleration voltage of 15 kV and a working distance of 15 mm. Data was collected using the INCA software system, v. 4.06 (Oxford Instruments).
Micro-Raman spectrometry (MRS)
Micro-Raman spectra were acquired with a Renishaw inVia multiple laser dispersive Raman spectrometer with a Peltier-cooled (203 K), near-infrared enhanced, deep-depletion CCD detector (576 × 384 pixels) using a high-power diode laser (Toptica Photonics XTRA, Graefelfing (Munich), Germany) operating at 785 nm in combination with a 1200 l/mm grating. Based on the particle size, samples were analyzed using either the 50x or 100x objectives in a direct-coupled Leica DMLM microscope with enclosure. To avoid degradation or heat induced physical changes, the power on the samples was reduced to 1 mW with neutral density filters. Integration times of 30 seconds and 5 accumulations were employed; this resulted in an adequate signal-to-noise ratio. Spectra were acquired using the Wire 2 Raman software and were subsequently baseline corrected when necessary.
PLM was carried out on a Zeiss Axio Imager 2 microscope (Carl Zeiss, Oberkochen, Germany) using a 50x or 100x objective. Samples were dispersed and mounted in Canada balsam (n = 1.55) and observed in slightly polarized or cross-polarized light.
Results and discussion
Due to the complexity of the sampling and despite the many samples that were taken/studied, only three samples will be discussed here. They are representative of the arsenic sulfide found and the different techniques in which it was used: samples C90.058 and P210.011 (loose material) are representative for arsenic sulfide mixed with ultramarine blue, bound with protein-based medium and used as underlayer; sample C90.094 (P209.066) corresponds to arsenic sulfide mixed with Prussian blue, used in finishing layers in an urushi-containing oil mediumb; finally, sample C90.101 (P209.074) features unmixed arsenic sulfide used in urushi-containing finishing layers.
For each sample, the same analytical procedure was followed: after embedding into cross-sections, the arsenic and sulfur distribution was recorded via SEM-EDX, the yellow particles were analyzed by means of Raman spectroscopy. Then, a fraction of the remaining loose sample was dispersed in Canada balsam for PLM analysis. The same procedure was used for the natural orpiment and laboratory-made arsenic sulfide used as reference samples.
FitzHugh described the amorphous synthetic arsenic sulfide particles prepared through wet chemistry from thioacetamide solution round in morphology with a particle size of about 1 μm . Grundmann and Rötter differentiated the extremely homogeneous, fine (1–2 μm in diameter) bright yellow particles when obtained from a thioacetamide solution described by FitzHugh from the even smaller (0.1-0.4 μm) particles obtained when prepared from hydrogen sulfide . On the other hand, synthetic arsenic sulfide pigment produced by the dry process is described as being likely to be composed of fine to medium particles  with an average size of 4 μm depending on the grinding . The dry processes give rise to glassy arsenic sulfide cakes (Figure 4b) that contain irregularly shaped and sized particles with conchoidal fractures after grinding as described in the literature [3,19] and as observed in the cross-sections of the laboratory-made g-AsxSx (Figure 4c). Because the pigment investigated in the frame of this study is an artificial arsenic sulfide pigment, the size, and shape of the particles observed in Figure 3 tend to indicate a pigment obtained from a dry rather than from a wet process.
Recapitulative table for the two reference materials and the 3 representative types of arsenic sulfide/techniques found in the decorative panels of the Japanese tower in Laeken, Belgium
Natural orpiment Kremer
g-As x S x laboratory-made
- Thin and elongated forms
Various shapes from spherical to sharp-edged
- Round to elliptical-shaped particles
- Laminated masses or granular and powdery with a fibrous structure (Figure 4a)
- Spherule can be smooth and round
- Both round and sharp edges
- Irregular sizes
- No smooth and round structures
- Figure 3
136, 154, 179, 202, 293, 311, 354, 367 (sh), 383 cm−1
220, 235, 339, 367 (sh), 473 cm−1
234, 337, 362 (sh), 471 cm−1
218, 234, 337, 362 (sh), 471 cm−1
(Figure 5a, b)
- Strong refraction
- Isotropic and amorphous
- Isotropic and amorphous
- No interference colors
- Bright pink and green to blue interference colors
- No interference colors
- Full extinction
- Compressed corners and edges resulting from crushing spherical particles
- Roundish morphology
- Foliated, micaceous structure
- Brittle, jagged fracture behavior
- Figure 8
- Ductile when exposed to mechanical strain
- Figure 7b
- Figure 7a
Different band ratios in the As4S4 monomer units (as suggested by the relative intensities of the Raman bands at 234 and 362 cm−1) tend to show that the different g-AsxSx used in the various panels were manufactured in different batches and were quenched at different temperatures. The more intense the bands for the As4S4 monomer units are, the higher was the quenching temperature.
Now that the arsenic sulfide pigment found in the decorative panels of the Japanese tower has been identified as arsenic sulfide glass synthesized from arsenic trioxide and sulfur, further investigations will be conducted in order to investigate stability of the different forms of arsenic sulfide glass and the influence of media  and environmental factors on its degradation.
Only hypotheses can be formulated at this stage of the study. As mentioned by Grundmann and Richter , the artificial arsenic sulfide glass might have been introduced in order to replace the very light sensitive natural pigment. In that regard, the glassy nature of the studied pigment could be the origin of its unusual stability. Another factor determining the stability of a pigment can be its use in a lacquer-containing layer in which the pigment is embedded. The lacquer-containing binder might also protect the pigment from oxidation and light due to its high stability.
aThe unpublished report of this study is archived at KIK-IRPA under the number 2010.10826.
bSamples analyzed by py-GCMS (Thermo, Waltham, MA, USA) with TMAH 2.5% in MeOH and pyrolyzed at 550°C for 12 sec. Detailed procedure and results can be found in the unpublished report of this study archived at KIK-IRPA under the number 2010.10826.
We cordially thank Günter Grundmann for his precious advice regarding PLM and arsenic sulfide glass. This research is made possible with the support of the Belgian Science Policy Office (BELSPO) through the research program Science for a Sustainable Development – SDD, “Long-term role and fate of metal-sulfides in painted works of art – S2ART” (SD/RI/04A).
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