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Non-destructive XRF analysis of paintings
Nuclear Instruments and Methods in Physics Research B 226 (2004) 53–59 www.elsevier.com/locate/nimb
Non-destructive XRF analysis of paintings Z. Sz€ okefalvi-Nagy *, I. Demeter, A. Kocsonya, I. Kov acs KFKI Research Institute for Particle and Nuclear Physics, Konkoly Thege ut 29-33, H-1121 Budapest, Hungary Received 21 October 2003; received in revised form 16 March 2004
Abstract The preservation and conservation of our cultural heritage has become one of the main concerns today all over the world. In particular there is an increasing need for non-destructive investigations, as sampling from the unique and precious objects of art and archaeology. In addition to the conventional analytical procedures, techniques utilising nuclear instruments and methods play increasing role in this field. The small, portable X-ray fluorescence (XRF) spectrometers using radioisotope excitation allow in situ analysis in museums, galleries, or even on field. This paper presents illustrative applications of our XRF devices with radioisotope excitation. The detection of the presence of titanium in white spots of a painting provided scientific basis to decide that the painting in question was a fake. The difficulties caused by the simultaneous presence of Ti and Ba (a very frequent component white paints) are also discussed. 2004 Elsevier B.V. All rights reserved. PACS: 82.80.E Keywords: In situ XRF; Non-destructive analysis
1. Introduction The preservation and conservation of our cultural heritage has become one of the main concerns today all over the world. In particular there is an increasing need for non-destructive investigations, as sampling from the unique and precious objects of art and archaeology. In addition to the conventional analytical procedures, techniques utilising nuclear instruments and methods play
*
Corresponding author. Tel.: +36-1-392-2512/2513; fax: +36-1-392-2598. E-mail address: [email protected] (Z. Sz€ okefalvi-Nagy).
increasing role in this field. Energy dispersive X-ray spectrometry, where the energy and intensity of the emitted characteristic X-rays are measured, is one of the mostly accepted physical techniques for elemental analysis. Among of several possibilities for X-ray productions, those of made by energetic charged particles and X-rays are the two most widely used ones. The first case when the sample to be analysed is bombarded by a properly focused particle beam (few MeV protons in most cases) is called particle induced X-ray emission (PIXE) spectrometry, while the name of the second one is X-ray fluorescence (XRF). The energy transferred to the sample under usual measuring conditions is so small in both cases that the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.03.074
54
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
analysis can be regarded as non-destructive. This non-destructivity is especially wanted in the analysis of art and archaeological objects. So it is not surprising that both PIXE and XRF are extensively applied in these fields [1]. PIXE, for obvious reasons the special external beam version, where the focused particle beam is extracted from the vacuum chamber into the atmosphere across a thin foil ‘‘window’’, is certainly preferable when the elemental composition of small, well defined regions are important. But the necessity of transporting the objects into the accelerator laboratory causes serious limitations in its use. An XRF spectrometer, especially its version with radioisotope excitation, on the other hand, can more or less easily assembled in galleries, so in situ analyses can be carried out. The biggest difficulty in this respect could be assigned to the clumsy high energy resolution X-ray detectors; usually liquid nitrogen cooled Si(Li) detectors. In turn, by the advent of the thermoelectrically cooled, tiny portable detectors, the possibilities for in situ application in museums and galleries, or private collections, are drastically widened. In the KFKI Research Institute for Particle and Nuclear Physics (KFKI RMKI) there is long term experience both in external beam PIXE and radioisotope excited XRF analyses. In this paper the results obtained and the associated difficulties in one of the very rare analytical situations is described, where definite conclusion can be made on the authenticity of paintings using XRF spectrometry.
2. XRF detection of Ti in white spots of paintings The most straightforward and frequent application of XRF is to take the elemental map of regions of different colours. On the basis of the map obtained conclusions on the composition and kind of the paints used by the artists can be drawn. The detailed knowledge of the ‘‘palette’’ of a painter could certainly be very important and useful for artists and art historians, but the most frequent question of the public, that is whether a particular painting is a forgery, or an authentic one, can not be answered in general on the basis of the elemental compositions. There are, however, a
few special cases, when the presence of a particular paint provides unanimous evidence for the age of the painted spot. The most known example is the identification of the presence of titanium at white coloured spots. Taking into account that titanium white (TiO2 ) is available since about 1920, only, its presence provides an indisputable indication for either forging or later repainting. The decision, however, is not always easy even is this ‘‘simple’’ case. Frequently used white paints, like baryte or heavy spar (BaSO4 ), or lithopone (ZnS + BaSO4 ) contain barium [2], and the almost complete overlap of the Ti K and the Ba L lines could be difficult to resolve. In the next sections the application of this ‘‘Ti method’’ to screen off two forgeries assigned before original paintings of Geza Mesz€ oly (1844– 1887) is described first. The analysis of removed little paint pieces from a disputed painting of Tivadar Csontvary Kosztka (1853–1919) provides an example, how difficult could it be to exclude the presence of Ti when Ba is also present. In order to obtain more information in situ Ti mapping in selected white regions using a portable XRF instrument was also performed. XRF spectra were taken from test samples composed from Ti and Ba containing paint layers to estimate the detection limits of Ti in various conditions when historical white paints are simultaneously present. 2.1. Experimental In the case of the Mesz€ oly’s study a standard Canberra vertical dipstick Si(Li) detector of 175 eV energy resolution was equipped with a New England Nuclear NEC XRF excitation head containing 55 Fe ring source was used. The paintings were horizontally laid over this at a distance of 10 mm. The spots for analysis were positioned by help of a removable ‘‘aiming pin’’. The analysed area was limited by an Al collimator of 3 mm diameter inserted into central hole of the ring source. The signals from the detector were amplified and processed by Standard Canberra NIM electronics and X-ray spectra were collected in a Canberra 8100 multichannel analyser. The XRF analysis of the removed paint granules and the in situ elemental mapping of a de-
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
bated Csontv ary painting were performed by a thermoelectrically cooled, portable AMPTEK XR-100CR detector connected to the matching PXT/CR power supply and amplifier. The sensitive area, the thickness of the Be detector window, and the energy resolution of the detector were 8 mm2 , 8.5 lm and 190 eV, respectively. In this case a more compact custom designed brass excitation head with a RITVERC 55 Fe ring source of much smaller size was used. The X-ray detector was mounted on a tripod. Spectra were collected by a smaller and rather mobile Canberra 35+ multichannel analyser and transferred to a notebook computer for off-line evaluation. In Fig. 1 the picture of this XRF set-up is shown in front of the controversial painting. 2.2. Measurements and results 2.2.1. The Mesz€ oly case Three small sized paintings were brought from the Hungarian National Gallery to our laboratory for XRF analysis. One of them (‘‘Autumn Sunshine on Lake Balaton’’, 1875) was undoubtedly attributed to Mesz€ oly, therefore it was planned to use as reference. In the case of the other two pic-
55
tures there was a rather strong consensus among the restorers that they were fakes. Their quality in general was considered to be very poor, the structure of the paint layer did not resemble that of the Mesz€ oly’s paintings, there were no characteristic cracks, etc. According to their report their suspicion seemed to be proved by these observations, but a decisive argument was still wanted. From the XRF measurements such a scientific proof was expected. In Fig. 2 one of the disputed pictures entitled ‘‘Washerwomen at the Lake Balaton’’ is displayed and crosses indicate the points where XRF spectra were taken. X-ray spectra taken at five selected spots suggested by the co-workers of the Hungarian National Gallery are shown in Fig. 3 together with the spectrum measured on the original Mesz€ oly painting used as reference. In all points of the suspicious picture the strong Ti-Ka Ti-Kb doublet dominates the spectrum, while no traces of Ti can be observed in the spectrum of the original painting. In order to test what kind of white paint was used by the painter, an additional measurement was also made on the original painting using a very weak 109 Cd NEN ring source, and the detected strong Pb L lines have supported the
Fig. 1. The portable XRF set-up composed from an AMPTEK X-ray detector equipped with the excitation head containing the 55 Fe ring source in front of the debated Csontvary painting.
56
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 2. The selected white spots (·) where XRF measurements were performed on the suspicious painting entitled ‘‘Washerwomen at the Lake Balaton’’.
assumption that Geza Mesz€ oly used lead or Chremnitz white (2PbCO3 Pb(OH)2 ). Some lead were also detected in similar inquiring measurements on the controversial picture, but the presence of Pb in paint layers is very common, apart from lead white, for instance, massicot, Naples yellow or Saturn lead are also Pb compounds [2]. The same result was obtained for the second suspicious painting: all selected white spots were full with titanium. On the basis of these results the final verdict was declared: both pictures were fakes. 2.2.2. The Csontv ary–Kosztka case Tivadar Csontv ary–Kosztka was a very odd figure of the Hungarian art of painting; his decision to become a painter was influenced by his schizophrenia. His oeuvre consists of about one hundred paintings and twenty drawings – not too
many, but his paintings were the first great summary of modern art in Hungary. His creative period was very short. He painted his major pictures between 1903 and 1909. Csontvary first showed his works in Paris in 1907, after that he travelled to Lebanon. His symbolic paintings of mysterious atmosphere were painted there: ‘‘Lonely Cedar’’, ‘‘Pilgrimage to the Cedars in Lebanon’’ and ‘‘Mary’s Journey in Nazareth’’. His next exhibitions were in 1908 an in 1910, but they did not bring him the recognition he had so earnestly hoped for. After this year he hardly painted, loneliness and the lack of understanding caused in him such a severe mental condition, that he was able to create nothing else, but sketches of surrealistic visions [3]. The painting displayed in Fig. 4 belongs to a private collection and its originality has been seriously doubted, seriously, but not unanimously. One of the unfavourable expert
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 3. X-ray spectra measured at five different points of the picture ‘‘Washerwomen at the Lake Balaton’’. The location of the points is indicated and numbered in Fig. 2. The uppermost spectrum labelled as Ref. was measured on a certified work of Mesz€ oly.
opinions has referred to the detection of Ti in the paints, but no document of this observation was presented. Just the repute of the above-described XRF results prompted the owner of the painting to justify or to shake this argument. First, instead of moving the rather large painting into the laboratory, the owner offered to remove small pigment granules from white coloured regions. Altogether 7 mg paint material was carefully collected by scalpel from two white spots. Open circles in Fig. 4 indicate these regions. The XRF spectrum of the granules induced by the Ritverc 55 Fe ring source and taken with the thermoelectrically cooled AMPTEK detector is plotted in the bottom of Fig. 5 (curve e). At superficial first sight it could be
57
Fig. 4. The strongly disputed ‘‘Csontvary’’ painting. Crosses and an arrow indicate points where in situ XRF measurements were made. Open circles mark the regions where paint samples were removed from.
appeared that Ti was present in the sample. (See the rather good coincidence of the positions of the two peaks in the region from 350 to 400 channels with the marks on the top of Fig. 5 for the Ti-Ka Ti-Kb doublet!) Only the weak third peak and the unusually large value of the ‘‘Ti-Kb/Ka’’ ratio should worry. The careful analysis of the spectrum using the dedicated decomposition program package AXIL [4], however, clearly showed that these peaks did not come from Ti, but they belonged to the L X-ray multiplet of Ba. The very close positions of the three strongest Ba L lines (La, Lb and Lc lines in increasing energy) are also indicated on the top of Fig. 5. Neither the 55 Fe, nor the 109 Cd source can induce Ba K lines, but their huge intensity in the spectrum (not shown) measured with a NEN 241 Am ring source has confirmed the result of the AXIL fits.
58
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 5. In situ XRF spectra from 3 points of the disputed ‘‘Csontv ary’’ painting (curves b, c and d) together with the spectrum of the Ti white of the reference Koszta painting (curve a) and with the spectrum of the removed paint granules (curve e).
All attempts to obtain detectable amount of Ti involving its K lines into the input file for the spectrum evaluation have failed. Therefore it has been concluded that the former report of the presence of Ti in the paint could not be confirmed by XRF analysis of the removed small paint granules. More detailed measurements were obviously needed. The way of scraping off paints from more points was impossible, therefore the spectrometer was carried to the painting and in situ measurements were taken at 12 points of white and coloured points. For illustration, three of the
spectra obtained are also plotted in Fig. 5 curve ‘b’ corresponds to a white spot, curve ‘c’ to a dark brown one and the curve ‘d’ was measured at a light blue region. (See also in Fig. 4!) As far as the Ti–Ba region is concerned all spectra are rather similar to each other. The presence of barium can be easily recognised. The goodness of the AXIL fits, on the other hand, was always slightly better, when Ti was also included. The Ti-Ka/Ba-La peak area ratio varied in the range of 0.01–0.15. Strange enough, the greatest amount of Ti relative to the Ba peak was obtained at the dark brown spot marked by ‘c’ in Fig. 2, at least twice more compared to that of measured at a white point (point ‘b’). In the collection a later picture of the Hungarian painter J ozsef Koszta (1861–1949) entitled ‘‘Drying clothes’’ with large white areas was also on display. According to the owner of the paintings, those areas were painted by titanium white. The spectrum ‘a’ on the top of Fig. 5 was measured on this picture and the clear Ti peaks have confirmed this information. So this painting seemed to be a suitable in situ reference. But trials have shown that the inclusion of Ba provided better fits, but now the relation was reversed, the Ti peak was about four times more intense. The abundance and the rather uniform distribution of barium in paintings is not a surprise. The highly resistant barium sulphate (baryte or permanent white) is consistent with all pigments and therefore very frequently is used as substrate. Its combination with ZnS (lithopone or Griffith’s white, patented in 1850) is used all above in priming or as filling material of putty [2]. Art historians and restorers probably have ideas whether a particular master did or did not use these paints simultaneously with titanium white, but the analytical problem certainly exists: Ti should be detected when Ba is also present. Due to the almost complete overlap of the Ti K and the strongest Ba L lines, their accurate separation is a real challenge for the decomposing computer programs. Reduction of the number of the free parameters and introduction of experimentally determined constrains in the line intensity ratios can certainly help in fitting. Preliminary steps in this direction were already made. Layers of white paints (titanium white, Zn
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
59
layers of accurately known composition have to be analysed.
3. Conclusions An XRF spectrometer with 55 Fe excitation can be very effective in the separation of original and faked paintings if the presence or absence of Ti is decisive, and the paint layer does not contain significant amount of Ba. In these cases portable X-ray spectrometers with rather moderate energy resolution can be used in situ. By this way the administrative and safety difficulties can be reduced to a great extent. In the presence of Ba detailed determination of relative line intensities in different model paint layers of known composition is needed where self-absorption and internal enhancements are carefully taken into account. Results of auxiliary measurements of the Ba K lines with 241 Am source can help a lot in this respect.
Acknowledgements
Fig. 6. X-ray spectra of white paint layers painted on chipboard.
white and lithopone) and a composite one where the Zn white was painted over Ti white were also measured by 55 Fe excitation. The spectra are shown in Fig. 6. It can be seen from the figure and fits are also confirming that to the contrary of their specification both the ‘‘pure’’ Ti white and Zn white contain Ba. In the case of the over-painted TiO2 layer the relative intensity of the two major peaks was simultaneously influenced by the increased absorption in the ZnO layer and the additional peaks from the barium. Although these spectra can certainly be used for fitting exercises, for obtaining usable quantitative information on line shapes and relative intensities model paint
This work was partially carried out in the scope of the COST 8 action and supported by the Hungarian National Science Foundation (OTKA) under Research Contract no. T037825 and by IAEA CRP ‘‘In situ application of XRF techniques’’ under Contract no. 11307. Thanks to the director general and the co-workers of the Hungarian National Gallery for their interest in the XRF analysis and for placing the pictures at our disposal, and to the owner of the controversial ‘‘Csontvary’’ painting for his patience and assistance in the in situ measurements.
References [1] G. Demortier, A. Adriaens (Eds.), COST, EUR 19218, European Communities, 2000. [2] H.-P. Schramm, B. Hering, Historische Malmaterialen und ihre Identifizierung, Ferdinand Enke Verlag, Stuttgart, 1995. [3] Fine Arts in Hungary. http://www.kfki.hu/keptar/. [4] P. Van Espen, K. Janssens, J. Nobels, Chemom. Intell. Lab. Syst. 1 (1985) 109.
Non-destructive XRF analysis of paintings Z. Sz€ okefalvi-Nagy *, I. Demeter, A. Kocsonya, I. Kov acs KFKI Research Institute for Particle and Nuclear Physics, Konkoly Thege ut 29-33, H-1121 Budapest, Hungary Received 21 October 2003; received in revised form 16 March 2004
Abstract The preservation and conservation of our cultural heritage has become one of the main concerns today all over the world. In particular there is an increasing need for non-destructive investigations, as sampling from the unique and precious objects of art and archaeology. In addition to the conventional analytical procedures, techniques utilising nuclear instruments and methods play increasing role in this field. The small, portable X-ray fluorescence (XRF) spectrometers using radioisotope excitation allow in situ analysis in museums, galleries, or even on field. This paper presents illustrative applications of our XRF devices with radioisotope excitation. The detection of the presence of titanium in white spots of a painting provided scientific basis to decide that the painting in question was a fake. The difficulties caused by the simultaneous presence of Ti and Ba (a very frequent component white paints) are also discussed. 2004 Elsevier B.V. All rights reserved. PACS: 82.80.E Keywords: In situ XRF; Non-destructive analysis
1. Introduction The preservation and conservation of our cultural heritage has become one of the main concerns today all over the world. In particular there is an increasing need for non-destructive investigations, as sampling from the unique and precious objects of art and archaeology. In addition to the conventional analytical procedures, techniques utilising nuclear instruments and methods play
*
Corresponding author. Tel.: +36-1-392-2512/2513; fax: +36-1-392-2598. E-mail address: [email protected] (Z. Sz€ okefalvi-Nagy).
increasing role in this field. Energy dispersive X-ray spectrometry, where the energy and intensity of the emitted characteristic X-rays are measured, is one of the mostly accepted physical techniques for elemental analysis. Among of several possibilities for X-ray productions, those of made by energetic charged particles and X-rays are the two most widely used ones. The first case when the sample to be analysed is bombarded by a properly focused particle beam (few MeV protons in most cases) is called particle induced X-ray emission (PIXE) spectrometry, while the name of the second one is X-ray fluorescence (XRF). The energy transferred to the sample under usual measuring conditions is so small in both cases that the
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.03.074
54
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
analysis can be regarded as non-destructive. This non-destructivity is especially wanted in the analysis of art and archaeological objects. So it is not surprising that both PIXE and XRF are extensively applied in these fields [1]. PIXE, for obvious reasons the special external beam version, where the focused particle beam is extracted from the vacuum chamber into the atmosphere across a thin foil ‘‘window’’, is certainly preferable when the elemental composition of small, well defined regions are important. But the necessity of transporting the objects into the accelerator laboratory causes serious limitations in its use. An XRF spectrometer, especially its version with radioisotope excitation, on the other hand, can more or less easily assembled in galleries, so in situ analyses can be carried out. The biggest difficulty in this respect could be assigned to the clumsy high energy resolution X-ray detectors; usually liquid nitrogen cooled Si(Li) detectors. In turn, by the advent of the thermoelectrically cooled, tiny portable detectors, the possibilities for in situ application in museums and galleries, or private collections, are drastically widened. In the KFKI Research Institute for Particle and Nuclear Physics (KFKI RMKI) there is long term experience both in external beam PIXE and radioisotope excited XRF analyses. In this paper the results obtained and the associated difficulties in one of the very rare analytical situations is described, where definite conclusion can be made on the authenticity of paintings using XRF spectrometry.
2. XRF detection of Ti in white spots of paintings The most straightforward and frequent application of XRF is to take the elemental map of regions of different colours. On the basis of the map obtained conclusions on the composition and kind of the paints used by the artists can be drawn. The detailed knowledge of the ‘‘palette’’ of a painter could certainly be very important and useful for artists and art historians, but the most frequent question of the public, that is whether a particular painting is a forgery, or an authentic one, can not be answered in general on the basis of the elemental compositions. There are, however, a
few special cases, when the presence of a particular paint provides unanimous evidence for the age of the painted spot. The most known example is the identification of the presence of titanium at white coloured spots. Taking into account that titanium white (TiO2 ) is available since about 1920, only, its presence provides an indisputable indication for either forging or later repainting. The decision, however, is not always easy even is this ‘‘simple’’ case. Frequently used white paints, like baryte or heavy spar (BaSO4 ), or lithopone (ZnS + BaSO4 ) contain barium [2], and the almost complete overlap of the Ti K and the Ba L lines could be difficult to resolve. In the next sections the application of this ‘‘Ti method’’ to screen off two forgeries assigned before original paintings of Geza Mesz€ oly (1844– 1887) is described first. The analysis of removed little paint pieces from a disputed painting of Tivadar Csontvary Kosztka (1853–1919) provides an example, how difficult could it be to exclude the presence of Ti when Ba is also present. In order to obtain more information in situ Ti mapping in selected white regions using a portable XRF instrument was also performed. XRF spectra were taken from test samples composed from Ti and Ba containing paint layers to estimate the detection limits of Ti in various conditions when historical white paints are simultaneously present. 2.1. Experimental In the case of the Mesz€ oly’s study a standard Canberra vertical dipstick Si(Li) detector of 175 eV energy resolution was equipped with a New England Nuclear NEC XRF excitation head containing 55 Fe ring source was used. The paintings were horizontally laid over this at a distance of 10 mm. The spots for analysis were positioned by help of a removable ‘‘aiming pin’’. The analysed area was limited by an Al collimator of 3 mm diameter inserted into central hole of the ring source. The signals from the detector were amplified and processed by Standard Canberra NIM electronics and X-ray spectra were collected in a Canberra 8100 multichannel analyser. The XRF analysis of the removed paint granules and the in situ elemental mapping of a de-
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
bated Csontv ary painting were performed by a thermoelectrically cooled, portable AMPTEK XR-100CR detector connected to the matching PXT/CR power supply and amplifier. The sensitive area, the thickness of the Be detector window, and the energy resolution of the detector were 8 mm2 , 8.5 lm and 190 eV, respectively. In this case a more compact custom designed brass excitation head with a RITVERC 55 Fe ring source of much smaller size was used. The X-ray detector was mounted on a tripod. Spectra were collected by a smaller and rather mobile Canberra 35+ multichannel analyser and transferred to a notebook computer for off-line evaluation. In Fig. 1 the picture of this XRF set-up is shown in front of the controversial painting. 2.2. Measurements and results 2.2.1. The Mesz€ oly case Three small sized paintings were brought from the Hungarian National Gallery to our laboratory for XRF analysis. One of them (‘‘Autumn Sunshine on Lake Balaton’’, 1875) was undoubtedly attributed to Mesz€ oly, therefore it was planned to use as reference. In the case of the other two pic-
55
tures there was a rather strong consensus among the restorers that they were fakes. Their quality in general was considered to be very poor, the structure of the paint layer did not resemble that of the Mesz€ oly’s paintings, there were no characteristic cracks, etc. According to their report their suspicion seemed to be proved by these observations, but a decisive argument was still wanted. From the XRF measurements such a scientific proof was expected. In Fig. 2 one of the disputed pictures entitled ‘‘Washerwomen at the Lake Balaton’’ is displayed and crosses indicate the points where XRF spectra were taken. X-ray spectra taken at five selected spots suggested by the co-workers of the Hungarian National Gallery are shown in Fig. 3 together with the spectrum measured on the original Mesz€ oly painting used as reference. In all points of the suspicious picture the strong Ti-Ka Ti-Kb doublet dominates the spectrum, while no traces of Ti can be observed in the spectrum of the original painting. In order to test what kind of white paint was used by the painter, an additional measurement was also made on the original painting using a very weak 109 Cd NEN ring source, and the detected strong Pb L lines have supported the
Fig. 1. The portable XRF set-up composed from an AMPTEK X-ray detector equipped with the excitation head containing the 55 Fe ring source in front of the debated Csontvary painting.
56
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 2. The selected white spots (·) where XRF measurements were performed on the suspicious painting entitled ‘‘Washerwomen at the Lake Balaton’’.
assumption that Geza Mesz€ oly used lead or Chremnitz white (2PbCO3 Pb(OH)2 ). Some lead were also detected in similar inquiring measurements on the controversial picture, but the presence of Pb in paint layers is very common, apart from lead white, for instance, massicot, Naples yellow or Saturn lead are also Pb compounds [2]. The same result was obtained for the second suspicious painting: all selected white spots were full with titanium. On the basis of these results the final verdict was declared: both pictures were fakes. 2.2.2. The Csontv ary–Kosztka case Tivadar Csontv ary–Kosztka was a very odd figure of the Hungarian art of painting; his decision to become a painter was influenced by his schizophrenia. His oeuvre consists of about one hundred paintings and twenty drawings – not too
many, but his paintings were the first great summary of modern art in Hungary. His creative period was very short. He painted his major pictures between 1903 and 1909. Csontvary first showed his works in Paris in 1907, after that he travelled to Lebanon. His symbolic paintings of mysterious atmosphere were painted there: ‘‘Lonely Cedar’’, ‘‘Pilgrimage to the Cedars in Lebanon’’ and ‘‘Mary’s Journey in Nazareth’’. His next exhibitions were in 1908 an in 1910, but they did not bring him the recognition he had so earnestly hoped for. After this year he hardly painted, loneliness and the lack of understanding caused in him such a severe mental condition, that he was able to create nothing else, but sketches of surrealistic visions [3]. The painting displayed in Fig. 4 belongs to a private collection and its originality has been seriously doubted, seriously, but not unanimously. One of the unfavourable expert
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 3. X-ray spectra measured at five different points of the picture ‘‘Washerwomen at the Lake Balaton’’. The location of the points is indicated and numbered in Fig. 2. The uppermost spectrum labelled as Ref. was measured on a certified work of Mesz€ oly.
opinions has referred to the detection of Ti in the paints, but no document of this observation was presented. Just the repute of the above-described XRF results prompted the owner of the painting to justify or to shake this argument. First, instead of moving the rather large painting into the laboratory, the owner offered to remove small pigment granules from white coloured regions. Altogether 7 mg paint material was carefully collected by scalpel from two white spots. Open circles in Fig. 4 indicate these regions. The XRF spectrum of the granules induced by the Ritverc 55 Fe ring source and taken with the thermoelectrically cooled AMPTEK detector is plotted in the bottom of Fig. 5 (curve e). At superficial first sight it could be
57
Fig. 4. The strongly disputed ‘‘Csontvary’’ painting. Crosses and an arrow indicate points where in situ XRF measurements were made. Open circles mark the regions where paint samples were removed from.
appeared that Ti was present in the sample. (See the rather good coincidence of the positions of the two peaks in the region from 350 to 400 channels with the marks on the top of Fig. 5 for the Ti-Ka Ti-Kb doublet!) Only the weak third peak and the unusually large value of the ‘‘Ti-Kb/Ka’’ ratio should worry. The careful analysis of the spectrum using the dedicated decomposition program package AXIL [4], however, clearly showed that these peaks did not come from Ti, but they belonged to the L X-ray multiplet of Ba. The very close positions of the three strongest Ba L lines (La, Lb and Lc lines in increasing energy) are also indicated on the top of Fig. 5. Neither the 55 Fe, nor the 109 Cd source can induce Ba K lines, but their huge intensity in the spectrum (not shown) measured with a NEN 241 Am ring source has confirmed the result of the AXIL fits.
58
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
Fig. 5. In situ XRF spectra from 3 points of the disputed ‘‘Csontv ary’’ painting (curves b, c and d) together with the spectrum of the Ti white of the reference Koszta painting (curve a) and with the spectrum of the removed paint granules (curve e).
All attempts to obtain detectable amount of Ti involving its K lines into the input file for the spectrum evaluation have failed. Therefore it has been concluded that the former report of the presence of Ti in the paint could not be confirmed by XRF analysis of the removed small paint granules. More detailed measurements were obviously needed. The way of scraping off paints from more points was impossible, therefore the spectrometer was carried to the painting and in situ measurements were taken at 12 points of white and coloured points. For illustration, three of the
spectra obtained are also plotted in Fig. 5 curve ‘b’ corresponds to a white spot, curve ‘c’ to a dark brown one and the curve ‘d’ was measured at a light blue region. (See also in Fig. 4!) As far as the Ti–Ba region is concerned all spectra are rather similar to each other. The presence of barium can be easily recognised. The goodness of the AXIL fits, on the other hand, was always slightly better, when Ti was also included. The Ti-Ka/Ba-La peak area ratio varied in the range of 0.01–0.15. Strange enough, the greatest amount of Ti relative to the Ba peak was obtained at the dark brown spot marked by ‘c’ in Fig. 2, at least twice more compared to that of measured at a white point (point ‘b’). In the collection a later picture of the Hungarian painter J ozsef Koszta (1861–1949) entitled ‘‘Drying clothes’’ with large white areas was also on display. According to the owner of the paintings, those areas were painted by titanium white. The spectrum ‘a’ on the top of Fig. 5 was measured on this picture and the clear Ti peaks have confirmed this information. So this painting seemed to be a suitable in situ reference. But trials have shown that the inclusion of Ba provided better fits, but now the relation was reversed, the Ti peak was about four times more intense. The abundance and the rather uniform distribution of barium in paintings is not a surprise. The highly resistant barium sulphate (baryte or permanent white) is consistent with all pigments and therefore very frequently is used as substrate. Its combination with ZnS (lithopone or Griffith’s white, patented in 1850) is used all above in priming or as filling material of putty [2]. Art historians and restorers probably have ideas whether a particular master did or did not use these paints simultaneously with titanium white, but the analytical problem certainly exists: Ti should be detected when Ba is also present. Due to the almost complete overlap of the Ti K and the strongest Ba L lines, their accurate separation is a real challenge for the decomposing computer programs. Reduction of the number of the free parameters and introduction of experimentally determined constrains in the line intensity ratios can certainly help in fitting. Preliminary steps in this direction were already made. Layers of white paints (titanium white, Zn
Z. Sz€okefalvi-Nagy et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 53–59
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layers of accurately known composition have to be analysed.
3. Conclusions An XRF spectrometer with 55 Fe excitation can be very effective in the separation of original and faked paintings if the presence or absence of Ti is decisive, and the paint layer does not contain significant amount of Ba. In these cases portable X-ray spectrometers with rather moderate energy resolution can be used in situ. By this way the administrative and safety difficulties can be reduced to a great extent. In the presence of Ba detailed determination of relative line intensities in different model paint layers of known composition is needed where self-absorption and internal enhancements are carefully taken into account. Results of auxiliary measurements of the Ba K lines with 241 Am source can help a lot in this respect.
Acknowledgements
Fig. 6. X-ray spectra of white paint layers painted on chipboard.
white and lithopone) and a composite one where the Zn white was painted over Ti white were also measured by 55 Fe excitation. The spectra are shown in Fig. 6. It can be seen from the figure and fits are also confirming that to the contrary of their specification both the ‘‘pure’’ Ti white and Zn white contain Ba. In the case of the over-painted TiO2 layer the relative intensity of the two major peaks was simultaneously influenced by the increased absorption in the ZnO layer and the additional peaks from the barium. Although these spectra can certainly be used for fitting exercises, for obtaining usable quantitative information on line shapes and relative intensities model paint
This work was partially carried out in the scope of the COST 8 action and supported by the Hungarian National Science Foundation (OTKA) under Research Contract no. T037825 and by IAEA CRP ‘‘In situ application of XRF techniques’’ under Contract no. 11307. Thanks to the director general and the co-workers of the Hungarian National Gallery for their interest in the XRF analysis and for placing the pictures at our disposal, and to the owner of the controversial ‘‘Csontvary’’ painting for his patience and assistance in the in situ measurements.
References [1] G. Demortier, A. Adriaens (Eds.), COST, EUR 19218, European Communities, 2000. [2] H.-P. Schramm, B. Hering, Historische Malmaterialen und ihre Identifizierung, Ferdinand Enke Verlag, Stuttgart, 1995. [3] Fine Arts in Hungary. http://www.kfki.hu/keptar/. [4] P. Van Espen, K. Janssens, J. Nobels, Chemom. Intell. Lab. Syst. 1 (1985) 109.