Micro-XRF Investigations of Chlorine-Containing Wood Preservatives in Art Object
Jens Bartoll, Achim Unger, Karsten Püschner and Heike Stege
The applicability of energy-dispersive micro X-ray fluorescence (micro-XRF) spectromctry for the
analysis of chlorine-containing wood preservatives in art objects was tested. At the present stage of
development of the method, information about the relative degree of contamination and the efficacy
of decontamination measures can be gained by using the intensity of the chlorine signals. The
micro-XRF technique combines the advantages of high spatial resolution (about 100 µm spot size),
needed in studies of penetration profiles of cut sections, and the ability to analyse larger areas in a
relatively short time. The transportable set-up applied in these investigations was also used outside
the laboratory. 'The degree of surface decontamination was determined non-destructively for the
wooden scenery parts of a 'Theatrum Sacrum' from the cloister church of Neuzcllc (Brandenburg,
Germany).
INTRODUCTION
During the second half of the twentieth century, wooden art objects such as sculptures, altars,
organs, pews and furniture, as well as wooden structural members like framework or roof timbers,
were extensively treated with oil-borne wood preservatives. Most preservatives used in Germany,
especially in the former German Democratic Republic between 1950 and 1980, contained the
biocides dichlorodiphenyltrichloro-ethane (DDT), γ-hexachlorocyclohexane (lindane) and
pentachlorophenol (PCP). As a result of this treatment, the surfaces of the objects often show a
white coating consisting of DDT crystals (Figure 1). Not only is this a handicap for restoration
work, but the presence of these substances causes potential health hazards arising from the toxicity
of DDT, lindane and PCP [1]. Therefore, methods are under development which are aimed at
removing these wood preservatives from the art objects or reducing their concentration. Possible
approaches used in this
Figure 1 DDT crystals on a wooden figure from Günterberg, Germany.
context include treatment of the contaminated objects with water, special washing agents or
supercritical fluids [2-4].
An important prerequisite for the development of such decontamination procedures is to have
suitable techniques for the qualitative and quantitative analysis of the wood preservatives. Usually,
gas chromatography/mass spectrometry (GC/MS) is used to analyse DDT, lindane and PCP. This
method is relatively expensive and time-consuming. Even more important, it is destructive and,
depending on the way samples are taken, has a relatively low spatial resolution.
X-ray fluorescence (XRF) spectroscopy offers an alternative. This method has already been
used to study trace elements in wood [5]. In principle, it is possible to detect the biocides
DDT, lindane and PCP by means of XRF using the fluorescence of the element chlorine that is
present in all three compounds. Naturally, it will not be possible to distinguish between
chlorine in DDT, lindane, PCP, or in inorganic compounds (e.g., sodium chloride) by XRF.
However, in many cases it is documented which wood preservatives were used, or small samples
can be taken in order to get information about the kind of wood preservatives present using other
analytical methods. It is also of interest in this context that lindane has a relatively high vapour
pressure and is for this reason usually left only in minor concentrations in wooden objects treated
with wood preservatives some decades ago [2]. Therefore, the analytical problem is often reduced
to detecting DDT alone, in some cases with PCP.
XRF analysis has the advantage of being virtually non-destructive. There are mobile energydispersive XRF devices that can be moved directly to the art objects. A single analysis usually takes
only a few seconds and gives simultaneous information on a large number of different chemical
elements. Special micro-XRF optics allow spatial resolutions in the micrometre range.
This paper aims to test the general applicability of a mobile micro-XRF spectrometer for the
analysis of chlorine-containing wood preservatives in art objects. Fresh wood samples and original
samples taken from historic timber structures were used as models to study the local distribution of
wood preservatives on the surfaces and in the depth of wooden objects. The ability to evaluate the
effectiveness of decontamination methods by means of XRF was also tested. It should be noted that
it was necessary to cut samples at the present stage of the development of the
method. This is because the mechanisms of the penetration of wood preservatives into the wood and
decontamination methods have to be studied in detail before non-destructive routine measurements
can be applied.
The efficiency of a cleaning treatment of contaminated art objects was determined in a first field
measurement campaign. For this, the intensity of the chlorine signals of selected areas of the objects
was compared before and after a washing procedure was carried out.
EXPERIMENTAL
All measurements were performed using the ArtTAX mobile energy-dispersive micro X-ray
fluorescence spectrometer (INTAX GmbH, Berlin, Germany) [6]. This device contains an aircooled, low-power molybdenum tube as the excitation source. The X-rays emitted by the tube are
guided to the sample using polycapillary X-ray optics. The spot size of the primary X-ray beam that
hits the surface of the sample is in the range of 100 µm in diameter depending on the X-ray
energy.'The fluorescence radiation is detected by means of an electro-thermally-cooled X-Flash
detector (Rontec, Berlin, Germany). The motor-driven measurement head can be moved in x, y- and z-directions. The movement can be observed using a CCD camera. The measurement
head can be tilted to angles from -10 to +100°. This allows the measurement of objects in various
positions. A special feature of the ArtTAX spectrometer is its open helium purging equipment.
The purging with helium in the excitation and detection paths enables the detection of elements
down to sodium. Vacuum conditions and size-limiting sample chambers can thus be avoided
(Figure 2).
For this work, all spectra were measured under helium atmosphere (0.8 l.min-1), using a voltage of
45 kV and a current of 600 µA for the excitation, and 100 s of live-time for the detection. The peak
area calculation was done in the 'Super Bayes' mode (experimental detector response functions) of
the ArtTAX-Ctrl software (INTAX GmbH, Berlin, Germany). All statements concerning
fluorescence intensities are related to the Ka peaks of the elements. The maximum depth at which
the element chlorine could be detected in a wooden matrix was estimated to be about 100 µm.
Figure 3 shows the different complementary approaches applied here tor the XRF
measurements.
Figure 2
The ArtTAX spectrometer.
The measurements were performed in either stationary (Figure 3a) or mobile (Figure 3b, 3c) mode,
i.e., the measurement head was either fixed at a certain position on the sample or moved in xand/or y-direction over the surface during the measurement (100 s live-time). The step-motors for
the x- and y-movements in the mobile mode were controlled using the computer keyboard.
Preferably, the measurement head was moved perpendicular to the growth rings of the wood in
order to average effects of density differences between earlywood and latewood.
The measurement of single points or points stringed in one or two dimensions (Figure 3a) provides
detailed information about the distribution of the elements on the surface of the samples.
The mobile measurement mode (Figure 3b, 3c) is suited to fast measurements of mean
concentrations of elements that are not homogeneously distributed in larger areas. The mobile mode
is not only faster compared to the stationary mode, but it also gives mean values that are satisfying
from a statistical point of view. In theory, a single measurement in the mobile mode is equal to
infinite measurements in the stationary mode. The mobile mode introduced in Figure 3b ('ladder
scan') is a quick way of getting information about depth profiles, e.g., the penetration profile of
DDT in the cross-sections of model samples.
Test specimens for laboratory measurements
All test specimens investigated here were small blocks (5x5x2 cm) sawn from larger boards. The
boards used for the production of fresh wood samples (pine,
Figure 3 Simplified models of the approaches used for XRF measurements, (a) Stationary:
measurement of single points. (b) Mobile: measurement head osciilates over a line during the
measurement, then is moved down to the next line, parallel to the previous one ('ladder scan'), (c)
Mobile: measurement head is moved in loops over the surface ('area scan').
fir, oak, lime) came from a sawmill in the south of Germany and were stored for at least four years.
The moisture content was about 12-14% (air-dried). Old pine samples (c. 1730) contaminated with
the wood preservative Hylotox IP (DDT, PCP) were taken from the old castle of Dresden, Germany
('Griines Gewolbe'). The wooden parts were treated in 1962/63. Old fir samples came from baroque
pews from a church in Zittau, Germany ('Kreuzkirche'). The wood was preserved by brushing and
spraying with Hylotox 59 (DDT, lindane) after 1980 [7]. Some of the boards from Zittau had been
painted in the past on either one or both sides.
RESULTS
XRF spectra of wood
Figure 4 shows a typical XRF spectrum of pinewood measured with the ArtTAX spectrometer. The
broad peak centred at about 9 keV, the molybdenum peaks (Mo), and all peaks at energies higher
than 10 keV are caused by the X-ray tube. The spectra of all samples investigated show
fluorescence peaks of potassium and calcium. In some cases iron and manganese can be detected.
There are spectroscopic differences linked to the different growth periods of the trees. Spectra of
latewood show higher scattering backgrounds and more intense scattering peaks of the X-ray tube
than those of earlywood. This effect is caused by differences in the density of the wood [8].
Calcium and manganese were found to be concentrated in the latewood of the pine.
Figure 4 Typical XRF spectrum of pinewood (Grünes Gewölbe, Dresden) contaminated with
wood preservatives (a. u. = arbitrary units).
Figure 5 Distribution of the XRF intensity for chlorine in a 1 x 1 cm surface area (pine, Grünes
Gewölbe, Dresden).
The chlorine fluorescence peak at 2.6 keV in Figure 4 is that used for the calculation of the chlorine
intensity values. This peak is relatively intense in this spectrum because the sample was treated in
the past with an organochlorine wood preservative.
Chlorine bound in inorganic compounds might simulate the presence of chlorine-containing
wood preservatives in the XRF measurement and cause an overestimation of the degree of
contamination with wood preservatives. An indication for the presence of chlorine in inorganic
compounds as part of a contamination (e.g., dust) is that not only chlorine appears in high
concentration but also cations not usually found in high concentrations in the wooden matrix
(e.g., sodium, mercury, zinc, iron). The XRF spectra should therefore be checked for signals of
these elements. Furthermore, unlike wood preservatives, an inorganic surface contamination
usually does not penetrate the wooden matrix very deeply. Finally, inorganic chlorine can easily be
identified by a precipitation test with silver ions if a small area of the object can be moistened to
wash out some of the ions for the test.
Fresh wood, not treated with wood preservatives, is known to contain a variety of natural trace
elements including potassium, calcium, manganese, iron and chlorine [5]. Therefore, the XRF
spectra of different kinds of fresh wood were checked for the presence of chlorine signals, using the
standard measurement approach employed in this study. Five samples of fresh pine, fir, oak and
lime wood (moisture content 12-14%) were analysed. Weak fluorescence peaks of chlorine could
only be detected for the surface areas of fir and lime. The chlorine probably originates from
some surface contamination, because chlorine signals were not found in the spectra of the crosssections of the same specimens.
Determination of chlorine distributions on surfaces
The surface distribution of chlorine in samples treated with wood preservatives was usually found
to be very inhomogeneous, as can be seen from the following example. A randomly chosen subarea (1x1 cm) of a pinewood sample (Grünes Gewölbe, Dresden) was measured in the stationary
and mobile mode. Figure 5 shows the result of the measurements in the stationary mode (see Figure
3a). The intensity values are proportional to the chlorine content. However, a precise
quantification is not a trivial problem, as will be discussed later in this article. The intensity
values of the 156 points are scattered from 1200 to 7500 counts, with a mean value of 4000 counts.
This reflects the inhomogeneous distribution of wood preservatives even within such a small area.
The same area was measured in the mobile mode (Figure 3c). The mean value of the intensity of the
chlorine fluorescence of five measurements was f ound to be 3715 ± 32 counts. It is of particular
interest to note that the measurement of this area took as much as five hours when applying the
stationary mode but only 10 minutes in the mobile mode. Therefore, the mobile mode should be
favoured for fast routine measurements of larger areas. It leads to average intensities that can
be used to estimate the degree of contamination of samples and the effectiveness of
decontamination measures.
Determination of chlonne distributions within the wood matrix
'Ladder scans' (Figure 3b) of cross-sections of boards are a suitable approach to get fast and detailed
information about the depth profile of the distribution of wood preservatives within the wood
matrix. Figure 6 introduces three typical examples of such depth profiles for wood treated with
chlorine-containing wood preservatives in the past. Some boards were treated with wood
preservatives from both the top and the underside (see Figure 6a). The wood preservatives
penetrated about 5 mm into the matrix from each side. The inner part of the matrix is more or less
free of chlorine-containing substances. In other cases (Figure 6b), the distribution of wood
preservatives was almost homogeneous over the whole profile. This means either that a relatively
large amount of preservatives was applied or that the wood was relatively easy to penetrate. The
chlorine concentration is lower close to the painted underside. This probably means that paint
components hindered the deposition of wood preservatives in those regions. The same effect was
found for a board from the church in Zittau painted on both sides (Figure 6c). The overall chlorine
concentration for this board was much lower compared to the boards free of paint or partially
painted.
Determination of the effectiveness of decontamination procedures
Three samples of contaminated wood were treated with a decontamination solution consisting of
surfactants [9]. The decontamination solution was applied by brushing on the surface of the wood
samples. This was followed by washing the samples under running water. This procedure reduced
the chlorine content on the surfaces of the samples by about two-thirds compared to the untreated
state, as determined by XRF area scans in the mobile mode of selected areas before and after the
cleaning.
Figure 7 shows that micro-XRF 'ladder scans' can give detailed information about the effect of the
cleaning procedures on the wood preservatives located within the wood matrix. The strongest
cleaning effect could be seen within the first 2-3 mm relative to the top and underside. This
corresponds to the penetration depth of the decontamination solution. Regions further inside also
show reduced chlorine fluorescence. This effect is probably caused by the intensive rinsing.
Figure 6 'Ladder scans' (see Figure 3b) of 1 cm wide strips of cross-sections of different boards,
from the top to the bottom surface. The intensity of the chlorine fluorescence is shown as a function
of the position of the measurement, (a) Pinewood from the Grunes Gewolbe, Dresden, thickness 2
cm. (b) Fir from the Kreuzkirche, Zittau, underside painted, thickness 2.5 cm. (c) Fir from the
Kreuzkirche, Zittau, top and underside painted, thickness 2.5 cm.
Figure 7 'Ladder scans' (see Figure 3b) of 1 cm wide strips of cross-sections of a board sample
(fir, Kreuzkirche, Zittau), from the top to the under surface. The intensity of the chlorine
fluorescence is shown as a function of the position of the measurement. (a) Untreated sample, (b)
Sample treated with a decontamination solution and rinsed with water.
Field measurements accompanying a cleaning treatment
A field analysis of contaminated objects was performed on a wooden 'Theatrum Sacrum' from the
cloister church of Neuzelle (Brandenburg, Germany). The Theatrum Sacrum (4 x 1.10 x 0.02 m)
was designed by Joseph Felix Seyfried in the mid eighteenth century. It consists of 166 painted
wooden scenery parts and figures as well as 66 scenery parts on canvas. The reverse sides of the
wooden panels were treated with the wood preservative Hylotox 59 between 1974 and 1976.
Micro-XRF measurements were performed on the reverse side of one panel before and after a
vacuum rinsing process (bhd-decon®) with water [4].
A statistically significant number of different measurement spots has to be chosen for the study of
the surface contamination of larger objects, in consideration of the typically inhomogeneoiis
distribution of preservatives on wooden surfaces (see Figure 5). Eleven measurement areas (each 2
X 1 cm, distance 2 cm) were chosen here. The starting corner of each measurement field was
marked in order to enable measurements at precisely the same spot before and after the cleaning
procedure.
Table 1 lists the values of the fluorescence intensities of chlorine measured before and after the
cleaning procedure. The values of the panel surface ranged from 12 234 to 45 210 counts before the
cleaning. After the cleaning, a decrease in intensity (up to 71%) was detected for all measurement
areas except one. The cleaning procedure reduced the intensity of the chlorine signal by 34.4% on
average.
Absolute concentrations
It is of interest to relate the XRF results (counts in arbitrary units) to absolute concentrations of
wood preservatives measured with an independent method, e.g., GC/MS. For this purpose, wood
standards containing known concentrations of wood preservatives in a homogeneous distribution
are needed. Five samples from the Kreuzkirche, Zittau were chosen for
Table 1 Estimation by micro-XRF of the efficiency of the surface cleaning of a wooden panel from
Neuzelle
such an investigation, because the chlorine distribution within the matrix of these samples is
homogeneous (see Figure 6b, 6c). Blocks (1x1x2 cm) were cut from an area with a homogeneous
chlorine distribution. The surfaces of these blocks were measured by means of XRF using mobile
area scans. Then the samples were analysed by GC/MS. Note that the samples also contained small
amounts of lindane and DDE (dichlorodiphenyldichloroethene). The relation between the XRF
intensity and the total chlorine content is shown in Figure 8a and the relation between XRF
intensity and DDT content in Figure 8b. A fairly linear correlation could be found 111 both cases.
CONCLUSIONS
The applicability of micro-XRF for the analysis of the contamination of wooden art objects with
chlorine-
Figure 8 Relation between the intensity of the chlorine X-ray fluorescence and (a) the total (DDT,
DDE, lindane) chlorine content of fir samples (Kreuzkirche, Zittau) or (b) the DDT content. The
concentrations of wood preservatives were measured by GC/MS.
containing wood preservatives was studied. The main advantages of the ArtTAX system used here
are its high local resolution, its mobility, and the ability to measure at normal pressure. The high
resolution (µm range) enables the user to study the local distribution of wood preservatives
precisely. This characteristic is indispensable for the investigation of new decontamination
procedures. A device with a larger measurement spot (cm range) might be of advantage for the fast
measurement of larger surface areas in a routine mode. However, commercial hand-held
spectrometers typically equipped with a radionuclide source are not appropriate in terms of
sensitivity. The relatively small measurement spot (100 µm) of ArtTAX can be used for the
measurement of larger areas as well, when a mobile measurement mode is applied. Providing both
the stationary and the mobile mode, micro-XRF combines the advantages of a high local resolution
with the ability to measure mean values for element concentrations in large areas within a short
time.
At the present stage of the development of the method it is possible to get information on the
relative degree of contamination of wooden objects with chlorine-containing wood preservatives
before and after decontamination procedures in a non-destructive manner. The maximum depth at
which the element chlorine can be detected in wood is about 100 µm. The measured depth profiles
of the chlorine distribution within the wood matrix allow statements to be made about the way the
wood preservatives were applied.
The efficiency of the decontamination measures on a wooden panel from the cloister church at
Neuzelle was determined quantitatively using micro-XRF in a first field experiment. The
heterogeneity of the distribution of the preservatives on the surfaces makes it necessary to analyse a
larger number of sub-areas. The sub-areas should not be larger than about 2x2 cm. This is because
uneven surfaces and an imperfect positioning of the measurement head will cause changes in the
distance between the surface and measurement head if larger areas are scanned. This would cause
an incorrect determination of XRF intensities. Due to the heterogeneity it is also of great importance
to perform the measurements at exactly the same measurement spots before and after cleaning.
Discrimination between different kinds of chlorine-containing substances is not possible by XRF.
Additional information gained from historical records or by other chemical analysis is needed to
specify the origin of the chlorine fluorescence detected.
The graphs shown in Figure 8 can be used as a tool for a rough estimation of DDT content for
DDT-contaminated samples measured by XRF. More experiments will be performed in the near
future with the aim of relating XRF counts and absolute concentrations of wood preservatives using
different kinds of wood and more samples.
ACKNOWLEDGEMENTS
The authors thank Dr E. Moehle, Fraunhofer Institute for Environmental, Safety and Energy
Technology UMSICHT, Oberhausen (Germany), for the GC/MS analysis and Ms S. Krug, Rathgen
Research Laboratory, Berlin, for support with the measurements in Neuzelle. This work was
financially supported by the Deutsche Bundesstiftung Umwelt.
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AUTHORS
JENS BARTOLL is a chemist specializing in interdisciplinary applications of spectroscopic methods,
such as material analysis and dating of historical, archaeological and geological objects. He gained
a PhD at Humboldt University of Berlin in 1998 studying radiation defects in glasses. After
postdoctoral studies at McMaster University, Canada, he is presently a researcher in the department
of conservation of the Prussian Palaces and Gardens Foundation, Berlin-Brandenburg. Address:
Stifiting Prcufiischc Schlosscr und Garten Berlin-Brandenburg, Abtcilung Rcitiiuriening, Postfach
601462, 14414 Potsdam, Germany. Email: jens_bartoll@gmx. De
ACHIM UNGER, PhD, is a chemist in the Rathgen Research Laboratory, State Museums of Berlin.
His research focuses on the analytical investigation of organic materials, wood consolidants, pest
control and decontamination of biocides. Address: Staatliche Muscat zu Berlin, RathgenForschungslabor, Schhssstrafic let, 14059 Berlin, Germany.
KARSTEN PUSCHNER graduated in 2002 from the University of Applied Sciences, Cologne
(department for restoration and conservation of art objects and cultural property) with a diploma
thesis on decontamination oř wood preservatives. He is currently a private wood conservator.
Address: Hauptstrajie 25, 01762 Hartmanusdorf, Germany.
HEIKE STEGE is a research chemist at the Doerner Institute, Bavarian State Painting
Collections in Munich, hi 1998 she gained a PhD from the chemistry department of the Technical
University of Berlin with a thesis on analysis and technology of post-mediaeval glasses. Over
the past three years she has undertaken research work at the TU Berlin into the development of a
mobile micro X-ray fluorescence spectrometer and, particularly, its application tor nondestructive analysis of glass and enamel art objects. Address: Bayerischc
Staatsgemaldcsammlttng, Docrner-Institiit, Barcrstrafic 29, 80799 Munchen, Germany.
Résumé — On a testé l'applicabilité de la ¡nicro-spectroinétrie de fluorescence X (micro-XRF) à
dispersion d'énergie à l'analyse des agents de conservation du bois chlorés dans les objets d'art. Au
stade actuel de mise an point de la méthode, l'information sur le degré de contamination et
l'efficacité des mesures de décontamination peut être obtenue en utilisant l'intensité des signaux du
chlore. lui technique de micro-XRF combine les avantages d'une haute résolution spatiale
(diamètre du faisceau d'environ 100 )im), comme l'exige l'étude des profils de pénétration, et la
possibilité d'analyser de plus grandes surfaces dans un laps de temps relativement court. Le degré
de décontamination de la surface des parties eu bois du 'Theatrum Sacrum' du cloître de l'église de
Ncuzellc (Brandebourg, Allemagne) a été déterminé de façon non destructive.
Zusammenfassung — Die Anwendbarkeit der energiedispersiven Mikro-RöntgcnfluorcszenzSpektroskopie (Mikro-RFA) für die Analyse chlorhaltiger Holschutzmittel in Kunstobjekten wurde
erprobt. Der gegenwärtige Stand der Methodenentwicklung läßt es zu, anhand der Intensität der
Chlor-Signale Informationen über den relativen Grad der Kontamination und über die Effektivität
von Dekontaminationsmaßnahmen zu geu'inncn. Die Mikro-RFA-Technik vereint die Vorzüge einer
hohen Ortsauflösung (etwa 100 pm Meßflcck-Durchtncsser), wie sie beim Studium von
Penetrationsprofilen an Querschnitten benötigt wird, und die Möglichkeit, größere Flächen in
relativ kurzer Zeit zu analysieren. Der hier verwendete transportable Mcßaufltau kam auch
außerhalb des Laboratoriums zum Einsatz. Der Grad der Oberflächen-Dekontamination hölzerner
Teile des Theatrum Sacrums aus dem Kloster Neuzelle (Brandenburg, Deutschland) wurde
bestimmt.
Resumen — En el presente estudio se ha probado la posibilidad de aplicar espectrometría por
microfluorescencia de energía dispersiva de rayos X (micro-XRF) para el análisis de conservantes
de madera con contenido en cloro en objetos de arte. En la actual fase de desarrollo del método
puede ser obtenida información, a través de la medición de las señales del cloro, sobre el grado
relativo de contaminación y sobre la eficacia de las medidas de decontaminación. La técnica de la
micro-XRF combina las ventajas de la alta resolución espacial (un tamaño de punto de cerca de
WO p-in), como suele precisarse en los estudios de los perfiles de penetración en secciones
transversales, y la capacidad de analizar grandes áreas en un espacio de tiempo relativamente
corto. El dispositivo es transportable por lo que pudo ser llevado fuera del laboratorio. El grado de
decontaminación superficial fue determinado de una manera no destructiva en el caso de los
componentes de madera del escenario del 'Theatrum Sacrum', en el claustro de la iglesia de
Ncuzclle (Brandemburgo, Alemania).