Portable XRF as a valuable device for preliminary in ...

Appl. Phys. A (2008)

Applied Physics A

DOI: 10.1007/s00339-008-4470-x

Materials Science & Processing

v. desnica1,2,u k. sˇ karić3 d. jembrih-simbuerger4 s. fazinić1 m. jakˇsić1 d. mudronja3 m. pavliˇcić3 i. peranić3 m. schreiner4

Portable XRF as a valuable device for preliminary in situ pigment investigation of wooden inventory in the Trski Vrh Church in Croatia 1

R. Boskovic Institute, Bijenicka 54, 10000 Zagreb, Croatia Laboratory for Science and Technology in Art, Department for Conservation and Restoration, Academy of Fine Arts, Ilica 45, 10000 Zagreb, Croatia 3 Croatian Conservation Institute, Grskoviceva 23, 10000 Zagreb, Croatia 4 Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 5, 1010 Vienna, Austria 2

Received: 20 August 2007/Accepted: 15 January 2008 © Springer-Verlag 2008

The aim of this work was the investigation of pigments from the painted wooden inventory of the pilgrimage church of Saint Mary of Jerusalem in Trski Vrh – one of the most beautiful late-baroque sacral ensembles in Croatia. Being an object of high relevance for the national cultural heritage, an extensive research on the wooden polychromy was undertaken in order to work out a proposal for a conservation treatment. It consists mainly of two painted and gilded layers (the original one from the 18th century and a later one from 1903), partly overpainted during periodic conservation treatments in the past. The approach was to carry out extensive preliminary in situ pigment investigations using a portable XRF (X-ray fluorescence) device, and only the problems not resolved by this method on site were further analyzed using sophisticated laboratory equipment. Therefore, the XRF results acted as a valuable guideline for subsequent targeted sampling actions, thus minimizing the sampling damage. Important questions not answered by XRF (identification of organic pigments, ultramarine, etc.) were subsequently resolved using additional ex situ laboratory methods, primarily µ-PIXE (particle-induced X-ray emission) at the nuclear microprobe of the Rudjer Boskovic accelerator facility as well as µ-Raman spectroscopy at the Institute of the Academy of Fine Arts in Vienna. It is shown that by the combination of these often complementary methods a thorough characterization of each pigment can be obtained, allowing for a proper strategy of the conservation treatment.

ABSTRACT

PACS 89.20.-a;

1

82.80.Ej; 82.80.Gk

Introduction

Trski Vrh is a small village near Krapina, some 40 km to the north of the Croatian capital Zagreb. The church in Trski Vrh is one of the most beautiful late-baroque sacral ensembles in Croatia. In the 18th century, when the wars against the Ottoman Empire came to an end, many sacral u Fax: +385-1-4680-239, E-mail: [email protected]

buildings were built. Two of the most famous are the complexes in Trski Vrh and Belec. This type of church with a cincture, typical for northern Croatia, was built in the 18th century in northern Croatia to receive a large number of pilgrims coming for special festivities. The interior decoration of the church dedicated to Saint Mary of Jerusalem consists of a wall painting, five altarpieces, a pulpit and an organ that were built within less than two decades, between 1758 and 1777. This explains its extraordinary unity of style that we still feel today. Such unity is unusual in this area, because a great majority of churches in northern Croatia combine gothic, baroque and neoclassical parts, due to the fact that they were built and rebuilt over centuries. In the spring of 2006, the Croatian Conservation Institute organized a research project on the painted and gilded wooden objects in the church. The research was undertaken in order to work out a proposal for conservation treatment. The scientific examination and comparative investigation of pigments is fundamental for further understanding and analyzing of historic and artistic works, and particularly useful for conservators. Since there is no one technique allowing for positive and unambiguous identification of every pigment, a combination of techniques is often required. The investigations were carried out using two spectroscopy methods in the X-ray range, X-ray fluorescence (XRF) and particle-induced X-ray emission (PIXE), as well as one in the visible/near-infrared (NIR) range, µ-Raman spectroscopy. All these methods have been used for pigment identification with success in the past [1–3], with the first two methods being specific to the elements present in the sample, and Raman spectroscopy being specific to the compounds [4]. The widespread use and number of different instruments that have been developed and employed for the application of XRF analysis illustrates well the value, advantages and usefulness of this analytical method in the field of art and archaeology [5–8]. Development of portable XRF instrumentation expands the range of use of this technique to an even wider area, by allowing in situ measurements on objects regardless of their shape, size or place where they are stored and/or displayed [5, 9–13]. Employment of this non-destructive method

Applied Physics A – Materials Science & Processing

in situ makes it a truly non-invasive technique as well, thus raising its importance for application in cultural heritage to an even higher level. The most frequent use of XRF is characterization of materials, based on their qualitative or quantitative elemental composition. PIXE is a fast, simultaneous, multi-elemental technique, and with controlled target energy deposition it is nondestructive as well. Since cultural heritage artifacts can often exhibit extremely different material composition and structure, methods allowing sensitive and multi-elemental characterization are highly required. Furthermore, since elemental distribution in artistic samples is seldom homogeneous, a good spatial resolution of the analysis is frequently required, which µ-PIXE can surely provide. When comparing XRF and PIXE it becomes apparent that the methods are quite complementary and there are many benefits regarding their analyzing capabilities when using them together [14]. XRF generally exhibits higher sensitivity for heavier elements, whereas PIXE is more sensitive for analysis of light elements. Furthermore, the penetration depths and irradiated areas, and hence analytical volumes, are rather different in PIXE and XRF. Moreover, PIXE exhibits relatively straightforward and, compared to XRF, simpler quantification calculations in unknown matrices. However, a serious problem in the use of PIXE in art and archaeology is the energy deposition that may damage the object as well as the possibility of charge buildup in the non-conducting samples and subsequent spectrum-distorting sparking. XRF does not suffer from these effects [14–16]. Due to all these differences, it was interesting to take the samples not fully characterized with XRF and additionally submit them to a laboratory µPIXE analysis. Raman spectroscopy is a non-destructive analysis method [1, 3], based on the spectral analysis of light inelastically scattered from a sample irradiated by a monochromatic light beam. Unlike element-sensitive X-ray methods, XRF and nuclear microprobes, Raman spectroscopy is a compoundsensitive optical method [4]. The spectrum (as a function of the Raman-frequency shift) yields direct information of the molecular vibrational frequencies. By comparison with standard samples, this frequency shift can be used to identify the molecular composition of the investigated sample. Furthermore, compared to the X-ray-based techniques, Raman spectroscopy is not restricted to inor-

FIGURE 1 Application of the portable XRF instrument during in situ pigment analysis

ganic materials and can successfully determine both pigments and binders, including compounds containing only light elements ( Z = 11 or less) only [17, 18]. As such, it is an important asset for characterization of an unknown sample. 2 2.1

Instrumental XRF

For in situ pigment analysis of the wooden polychromy in the Trski Vrh church a commercially available, portable XRF system (Artax µXRF spectrometer) from Bruker AXS was used. It consists of a 50-kV Rh excitation tube, a Peltier-cooled silicon drift detector (with energy resolution of 135 eV at the Mn K α excitation line) and a set of pinhole collimators ranging from 2 mm down to 200 µm. It allows determination of elements from Al to U, and with optional He purging it can detect elements down to Na. An in situ analysis of the church mural painting, using this XRF system, is shown in Fig. 1. The operating parameters for tube voltage and anode current during the measurements were set to 50 keV and 0.7 mA, respectively, and the acquisition real time was 60 s. The diameter of the beam was set to 600 µm. No He purging was used during the measurements.

FIGURE 2 View of the Rudjer Boskovic Institute nuclear microprobe focusing system (a) and a close up of the scattering chamber (b)

DESNICA et al.

2.2

Portable XRF for preliminary in situ pigment identification

PIXE

PIXE investigations were carried out using the focused proton beam at the nuclear microprobe of the Rudjer Boskovic Institute (RBI) accelerator facility (Fig. 2). For this purpose, a 2-MeV proton beam of 100-pA current was focused to about 1 × 2 µm2 spot size. Elemental distribution maps for major elements were created from the PIXE spectra with the help of the SPECTOR data acquisition and analysis software [19]. More details about the RBI microprobe facility and the data acquisition and analysis software may be found in [19–21]. The collection time for each two-dimensional (2D) elemental map was about 20 min. Altogether six samples were investigated, with the paint layer cross sections being embedded in a polyester resin. 2.3

Raman spectroscopy

The Raman spectroscopy measurements were carried out using a Horiba Jobin Yvon Raman spectrometer, model LabRAM ARAMIS Vis (400 – 1100 nm), with a confocal microscope (spatial resolution ∼ 2 µm2 ) and the He–Ne laser (633 nm), at the Institute of Science and Technology in Art of the Academy of Fine Arts in Vienna. 3

Results and discussion

The investigation of the pigments was carried out on more than 80 points of the wooden polychromy. It revealed mainly two painted and gilded layers. The lower one is attributed to the original painting and the overpaint can be attributed to the 1903 work by M. Strobach, who was signed as a gilder. The in situ pigment investigations were carried out using the portable XRF system described above, which allowed identification of practically all pigments used on the wooden polychromy. In order to deduce the layer sequence from the XRF results, measurements were carried out several times on the same spot and after each measurement a small portion of the successive, uppermost paint layer was carefully removed. The following pigments were determined using XRF. White pigments: barium sulphate (BaSO4 ), zinc white (ZnO), lead white (2PbCO3 · Pb(OH)2 ), calcium carbonate (CaCO3 ) and/or calcium sulphate (CaSO4 · nH2 O);

red pigments: cinnabar (HgS) and/or red ochre (iron oxide Fe2 O3 · nH2 O); brown: umbra (Fe2 O3 · nH2 O + MnO2). Using this method most of the questions regarding the pigment characterization were resolved in situ. The only area not unambiguously characterized was the grayish-greenish surface of the overpainting. Because of the frequency of appearance of similar gray-green surfaces at the time, this color needed to be thoroughly analyzed in order to provide information important for dating similar unsigned artifacts. One hypothesis was that a pigment so widely used on the largest surfaces must have been a cheap and widespread material at the time. The XRF analysis identified the following main elements in the problematic area: Ca, Fe, Zn, Ba, Pb and Sr. A typical XRF spectrum from that region is shown in Fig. 3. However, keeping these elements in mind, the in situ measurements with XRF could not explain the greenish (possibly a combination of yellow and blue) component of the surface appearance, thus making it necessary to take several samples and carry out further analysis ex situ using the laboratory equipment. µ-PIXE measurements on the pigment cross-section samples yielded distinct and clear elemental composition for each layer, except for the problematic, greenish layer, where, again, no components of green (or blue and yellow) could be determined. A close up of the 2D element distribution in the greenish layer, where Ba, S and Zn dominate, is depicted in Fig. 4. Direct correlation of the Ba and S intensities confirmed the presence of the white barium sulphate. No signal of chrome, indicative for chrome yellow, was detected. µ-Raman spectroscopy finally identified the source of the greenish tone to the surface layer to be chrome green, which is a mixture of two remarkable pigments, Prussian blue and chrome yellow [22]. The blue and the yellow grains of the mentioned pigments are clearly seen in Fig. 5. Prussian blue (Fe4 [Fe(CN)6 ]3 ) is a synthetically produced ferric ferrocyanide, discovered in 1704 [23–26], which could not have been unambiguously identified using the mentioned X-ray methods. The characteristic pigment bands are clearly seen in Fig. 6a, with the strongest peak at 2157 cm−1 . The figure exhibits two spectra – one is the measured Raman spectrum of the blue grain and the other is the Prussian blue standard reference spectrum. The second identified pigment is chrome yellow, a lead sulphate/lead chromate pigment, discovered in 1809 [23–26]. The vibration bands characteriz-

FIGURE 3 Typical as-measured spectrum of the ‘problematic’ gray-green surface obtained with the portable XRF system. No green pigments (or possibly a combination of blue and yellow) could be determined using this technique

Applied Physics A – Materials Science & Processing

µ-PIXE 2D elemental distributions of Ba, Zn and S in the greenish layer (left-hand part of the map), stemming from barium sulphate (BaSO4 ) and presumably zinc oxide (ZnO). No Cr signal, which would suggest the existence of chrome yellow grains, was detected FIGURE 4

FIGURE 5 The greenish tone of the surface layer was due to chrome green, a mixture of Prussian blue and chrome yellow pigments. The blue and yellow grains (size 2–5 µm) of the mentioned pigments are clearly seen in the microscopic figure. The scale bar (lower right corner) is 1 µm

ing chrome yellow, with the strongest peak at 840 cm−1 , are shown in Fig. 6b, again displayed in comparison with the chrome yellow standard reference spectrum.

Even though Cr is usually detected by XRF and PIXE, in this case using X-ray spectroscopic techniques no Cr was detected, possibly due to the following three reasons. First, there was a fair amount of Ba in the investigated pigment layer, presumably stemming from barium sulphate (BaSO4 ). Even though the characteristic lines of Ba and Cr do not exactly coincide, the resulting strong L -lines from Ba could interfere with the Cr K -lines, and possibly even completely conceal them, if the chromium content in the layer were very low. Second, the Cr K -line has the energy of 5.414 keV, while the absorption edge for Ba L -lines lies at 5.247 keV. This means that the energy from Cr will be to a certain extent absorbed by the Ba atoms; thus, the small amount of Cr in the fluorescence spectrum will be additionally reduced due to the Ba content in the sample. Finally, subsequent thorough and targeted optical microscopy revealed that a very small number of chrome yellow grains (size 2 – 5 µm) was present in the samples, making it understandable why no Cr signal was detected when X-ray spectroscopy was carried out over a larger portion of the sample (two mm2 ).

FIGURE 6 Raman spectra taken on the (a) blue and (b) yellow grains, corresponding to the Prussian blue and chrome yellow pigments, respectively. Even though the background due to the fluorescence of the binding medium was rather high in both spectra, the characteristic Raman signal was attributed without difficulty to the appropriate pigments

DESNICA et al.

4

Portable XRF for preliminary in situ pigment identification

Conclusion

For elemental analysis of the pigments during in situ investigations of the wooden polychromy, portable XRF analysis proved to be the preferred technique for routine work. By being a non-destructive and non-invasive technique, and by allowing identification of most of the pigments right on site, it helped preserve the cultural heritage objects to the highest possible level. Its valuable results helped to minimize the sampling actions, thus minimizing the overall damage. Only the questions not answered by this method were subsequently resolved using further sophisticated laboratory equipment, such as µ-PIXE and µ-Raman spectroscopy. REFERENCES 1 R.J.H. Clark, J. Mol. Struct. 480, 15 (1999) 2 B. Hochleitner, V. Desnica, M. Mantler, M. Schreiner, Spectrochim. Acta B 58, 641 (2003) 3 L. Burgio, D.A. Ciomartan, R.J.H. Clark, J. Mol. Struct. 405, 1 (1997) 4 R.J.H. Clark, Chem. Soc. Rev. 24, 187 (1995) 5 R. Cesareo, Nucl. Instrum. Methods B 211, 133 (2003) 6 K. Sugihara, K. Tamura, M. Satoh, Y. Hayakawa, Y. Hirao, S. Miura, H. Yotsutsuji, Y. Tokugawa, Adv. X-ray Anal. 44, 432 (2001) 7 J.L. Ferrero, C. Roldan, M. Ardid, E. Navarro, Nucl. Instrum. Methods A 422, 868 (1999) 8 J.L. Ferrero, C. Roldan, D. Juanes, C. Morera, E. Rollano, Adv. X-ray Anal. 44, 425 (2001) 9 V. Desnica, M. Schreiner, X-ray Spectrom. 35, 280 (2006) 10 Z. Szökefalvi-Nagy, I. Demeter, A. Kocdonya, I. Kovacs, Nucl. Instrum. Methods B 226, 53 (2004)

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