Analysis of Paint Layers by x-ray micro-diffraction · Analysis of Paint Layers by Light Microscopy, Scanning Electron ... constituents within the diverse paint layers. By this, it

1 INTRODUCTION

Knowledge of the composition of the originally used pigments in works of art is of great importance for the restoration and conservation of such objects (Fuller & Lewis 1988, Kittel 1960). Various analytical techniques can be applied in order to gain information concerning their material composition, i.e. about the pigments used in paintings, polychromed sculptures or illuminated manuscripts (Schreiner 1995, Gettens & Stout 1966, Matteini & Moles 1990). This gained information allows in most cases the classification of this work or the correlation with a special period or a certain artist (Feller 1986, Roy 1993, West FitzHugh 1997).

The most common way to analyze a work of art is the preparation of a cross-section on properly sampled specimens, where the sequence of all layers is preserved (Kühn 1988). The examination of the cross-section by light microscopy and UV-fluorescence microscopy provides frequently sufficient information about the structure of the paint layers, grain size and grain size distribution of the various pigments as well as varnish layers or organic binding media (Banik et al. 1982). However, for the identification of individual pigments present in the various paint layers additional investigations of the cross-section are necessary. Scanning electron microscopy (SEM) combined with energy dispersive x-ray microanalysis (EDX) has been used widely to obtain information about the elements present in the pigments as well as their distribution in the different layers (Hanlan 1975, Mantler et al. 2000). Single pigments can be identified by comparing their color and elemental composition with standard materials known to be used in the present or past for painting artifacts.

However, many inorganic materials and some of the most interesting pigments can occur in different crystalline structures (Kirk & Raymond 1978, Ullmann 1979, PDF 2000). CaCO3 – chalk, which has been often used as filler in the ground layer, e. g. can occur in the modification of calcite as well as aragonite (Schramm & Hering 1988). Therefore, x-ray diffraction analysis has been proved to be a valuable tool for the examination of the paint layers too. With common

Presented at the Conference: Art 2002, June 2003, Antwerp, Belgium Manuscript for publication (Balkema)

Analysis of Paint Layers by Light Microscopy, Scanning Electron Microscopy and Synchrotron induced X-Ray Micro-Diffraction

B. Hochleitner, M. Schreiner Institute of Humanities, Sciences and Technologies in Art, Academy of Fine Arts, Vienna, Austria

M. Drakopoulos, I. Snigireva, A. Snigirev ESRF - European Synchrotron Radiation Facility, Grenoble, France

ABSTRACT: Light microscopy (LM) and scanning electron microscopy (SEM) in combination with energy dispersive x-ray microanalysis (EDX) were used for the characterization of the structure of the paint layers of a specimen taken from a mural painting. The sample consisted of 7 layers in total, whereby a thin layer of pure gold was suspected to be the uppermost layer. The sequence of the various paint layers as well as the distribution of the elements present in the pigments could be obtained from the cross-sectioned specimen. Additionally, synchrotron induced x-ray micro-diffraction analysis (XRD) enabled the identification of the crystalline structure of the pigments used for the painting. Traversing the sectioned sample through a focused x-ray beam with a size of 2 µm allows microscopic resolved analysis of the crystalline constituents within the diverse paint layers. By this, it is possible to attribute the usage of various pigment minerals within the paint layers, even including a 2 µm thick gold layer at the surface.

XRD it is rather difficult to carry out those investigations of specific pigments, as the thickness of the paint layers is in the range of several tens micrometers or even below. Therefore, synchrotron induced x-ray micro-diffraction was used in the present work, where the step scan resolution can be much smaller than the thickness of the paint layers. The combination of SEM/EDX analysis and x-ray micro-diffraction has proved to be suitable for the identification and characterization of the composition of this cross-section.

2 EXPERIMENTAL METHODS

A specimen taken from a mural painting of the Baroque periods could be employed for the investigations. The specimen was embedded with a particular orientation in a transparent resin, ground and polished with SiC-paper up to 4000 mesh perpendicular to the paint layers. Highly polished layers are required for an informative microscopic investigation and flawless photographs. For analyzing this sample three different methods were applied.

2.1 Light Microscopy

Analysis with the light microscope (Leitz, Orthoplan) was performed by using polarized light and the dark field reflectance technique. With this method it is possible to gain information about the thickness of the cross-section and the structure of the paint layers. Furthermore, this method yields information about the grain size and grain size distribution of the used pigments. This information is a useful indication for the way of manufacturing the painting, since modern pigments have uniform distribution of their grain size whereas hand ground colouring matters are strongly heterodisperse.

2.2 SEM/EDX Analysis

In addition to the light microscopic images SEM analysis of the same cross-section was carried out with a Philips XL 30 ESEM microscope without coating the specimen, although the materials in the paint layers as well as the embedding material are electrically non-conducting. Energy dispersive x-ray microanalysis in the SEM (EDAX Phoenix) was used for qualitative analysis of the elements present in the pigments as well as for x-ray mappings yielding to the distribution of the elements in the different paint layers.

2.3 X-Ray Micro-Diffraction

For this purpose a micro-diffraction facility installed at a beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France was used. The central quality of the synchrotron induced x-ray radiation is the minute source from which the radiation is emitted, and the small solid angle into which the radiation is confined. At a distance of several ten meters from the source an intensive x-ray beam displaying a cross-section of around 1 mm2 is attained. This gives the possibility to collect the majority of the beam with the x-ray optical elements, which typically have apertures of a few hundred micrometer diameter.

In particular, compound refractive x-ray lenses (CRL) (Snigirev 1996, Lengeler 1999) are used, which focus x-rays in a similar manner as glass-lenses would do with visible light. The focal spot is a demagnified image of the x-ray source, imaged by the CRL. The value of demagnification can be directly derived from the geometrical properties of the imaging set-up, by ray-tracing methods usually applied in geometrical optics. The demagnification value is the ratio between the source-lens distance and the lens-sample distance. This setup benefits from the large distance of the experimental station from the storage ring, being 42 m, which is made possible because of the slowly diverging x-ray beam. With a typical lens-sample distance of one meter it is possible to get a demagnification ratio in the order of 40. The effective source size is of 50 µm vertical and 600 µm horizontal, and consequently a focal spot of 1-2 microns in the vertical direction and 15 microns in the horizontal direction is achieved at sample location. Thus, it is possible to perform x-ray scanning microscopy with micrometer resolution, if the scanning direction is chosen to be vertical. The use of a monochromator device located in the

beam path between source and sample helps to select the desired x-ray wavelength by means of x-ray diffraction on Si single-crystals. Here a wavelength of 0.620 Å is selected, with a relative bandwidth of 1.4 ·10-4.

The focusing of x-rays with CRL is particularly advantageous for the recording of diffraction signals. Diffraction is measured by observing the angular distribution of x-ray radiation scattered by the sample. Thus, the angular divergence of the incoming beam will account for the resolution of the experiment. Because the aperture of the CRL is small in comparison to the focal length, the lens provides a small divergence in the incoming beam of only 0.2 mrad. This value translates into a relative accuracy in the determination of crystalline lattice spacing in the order of 10-3. This value is exceedingly sufficient to distinguish diffraction patterns arising from different mineral species which are envisioned for analysis.

Figure 1. Experimental set-up for micro-diffraction, at beamline ID22, ESRF, Grenoble. The monochromatic x-ray beam enters from the left. (a) is the set-up in diffraction mode, (b) shows the alignment mode.

A scheme of the instrumental set-up used is shown in Figure 1. Basically, two modes come

into operation. The micro-diffraction mode (Fig. 1a) has the CRL inserted, and the diffraction pattern is then collected by means of a two-dimensional x-ray camera behind the sample. At the chosen wavelength the diffracted intensities from the relevant crystalline lattice spacing values are confined within an angular cone of 90° opening angle. For recording the diffraction patterns, the diffraction-camera with an active diameter of 100 mm is located at a few centimeter distance from the sample (Fig. 2).

The diffraction spectrum analyzed at a later stage is obtained by summing up all intensities belonging to the same diffraction angle and plotting the so obtained intensity distribution versus diffraction angle.

Figure 2. X-ray micro-diffraction. Experimentally obtained powder diffraction patterns from different positions across the paint layers. Position number 20, 60, and 90 (from left to right, referring to numeration scheme of Figure 7).

For the second operation mode the lens and diffraction camera are removed and instead a high-resolution x-ray camera is placed directly behind the sample (Fig. 1b). Illuminated by the plane x-ray beam, the radiogram of the paint layers is obtained (Fig. 7b). Primarily, this high resolution imaging mode is used to align the sample in respect to the focused beam position, which can be made with an accuracy of 2 microns. However, the different attenuation within diverse paint layers can yield some approximate knowledge about their mean density.

As both modes are performed in transmission geometry, the sample was sectioned into a 300 µm thick slice, prior the x-ray experiment.

The complete micro-diffraction analysis was accomplished by step-wise moving the section across the focused beam in steps of 4 µm. The paint layers were oriented horizontally, parallel to the larger beam-dimension, thus allowing to traverse the layers with a beam-size limited resolution of around 2 µm. In-between each step a diffraction-pattern is recorded, with an acquisition time of 2s. The diffraction signal arises from a sample volume which is defined by the lateral beam-dimensions, i.e. 2 x 15µm², and the sample thickness, which was 300 µm. After 160 steps, the beam passed through all seven layers and 160 diffraction spectra were collected, which are representative for the crystalline composition of the sample. Finally, the diffraction spectra were processed in order to eliminate instrumental parameters like wavelength λ or diffraction angle 2Θ. By applying Bragg’s law for 1st order diffraction ( θλ sin2 ⋅= d ) the abscissa was converted into the lattice spacing d. This makes the data comparable to the tabulated values from similar compounds.

In practical terms, the measurements were evaluated by analyzing crystalline phases by means of a search/match program. In order to identify unknown sample constituents, it was necessary to compare the measured diffraction spectra with a collection of tabulated spectra of known compounds. For this purpose, the collection by the International Centre for Diffraction Data (ICDD 2000), the Powder Diffraction File (PDF 2000), was available.

Using this method it is possible to identify pigments with a crystalline structure. However, amorphous dyes and binding media can not be identified.

3 RESULTS AND DISCUSSION

In general paintings are made by mixing pigments with a liquid binding medium (e.g. animal glue or linseed oil). After applying a layer of paint, it is dried and other layers are added as needed (Mantler & Schreiner 2000). A final coating mainly made of natural resin normally protects the painting and is responsible for the visual quality of the optical depth.

3.1 Light Microscopic Examination

The gained information by light microscopy and UV-fluorescence microscopy concerns the sequence and the thickness of paint layers and distribution of the pigments (Wülfert 1999). Figure 3 depicts a light-optical micrograph of the cross-section of the specimen investigated. The thickness of all paint layers is about 500 µm. The different layers can be seen clearly, although the gold layer, which should be on top (Fig. 3), can hardly be detected at this magnification (approximately 100x).

With a higher magnification of about 400x the gold layer (layer 1) can be seen in Figure 4 as a thin dark layer on the left side of the layer 2 (red/orange layer). Both, the light microscopic (Fig. 4a) as well as the UV-microscopic image (Fig. 4b) show the surface layer as a non-continuous thin layer clearly due to its high metallic reflectance of the visible light and the total absorption of the UV radiation.

The light microscopic images in Figure 3 and Figure 4a yield to the structure of the different paint layers, where 7 layers can be determined: The surface layer (layer 1) is the gold layer followed by a red/orange layer (layer 2) clearly visible in Figure 4a. Layer 3 is a thick layer transparent in the visible light (Fig. 4a) and showing a bright bluish fluorescence in the UV-fluorescence microscope, which indicates the presence of natural resin. Underneath layer 3 is a thick yellow paint layer (layer 4), followed by a thin white layer (layer 5) as shown in Figure 3. Layer six and seven are of white color with a light touch to yellow and gray, where the transition between these two layers seems to be very unsteady.

Figure 3. Cross-section of the paint layers of a specimen taken from a golden part in a mural painting.

(a) light microscopic image Figure 4. Detail of the paint layers in Figure 3.

(b) UV-fluorescence microscopic image

3.2 SEM Analysis

The backscattered electron image in Figure 5 clearly reveals the structure of the paint layers due to the atomic numbers of the elements present in the pigments. Layers containing pigments of heavy elements such as lead present in lead white, appear bright; those containing primarily elements with medium or low atomic numbers are dark. This information may already serve as a hint for the identification of the elements present and hence the pigments used for the various layers. Additionally, in the various layers also single pigment grains are clearly visible surrounded by the binding medium, which consists mainly of an organic material with C and H (low atomic numbers) as main constituents.

Figure 5 shows a backscattered electron image at a magnification of 150x with all the 7 layers determined by light microscopy. The gold layer is visible as a thin bright line due to the high amount of backscattered electrons. Layer 2, identified with light microscopy as a red/orange layer, shows bright pigment grains between darker areas, which point to heavy elements in this

layer. Layer 3 is dark followed by layer 4 in light gray. Layer 5 appears very bright, which indicates elements with high atomic numbers. Layer 6 and 7 can be separated in this image too.

Figure 5. Back scattered electron (BSE) image of the cross-section in Figure 3.

3.3 EDX Analysis

The second step in the characterization of the individual paint layers is qualitative x-ray microanalysis usually carried out by means of energy dispersive analysis yielding the simultaneous display of all elements except H and He. Additionally, a more detailed information about the distribution of the main elements can be achieved by x-ray mappings. As an example, in Figure 6a the BE-image of a selected domain of the cross-section is depicted with the corresponding distribution of the elements Au (b), Ba (c), Ca (d), Pb (e) and C (f).

With these element distribution images the main elements in the different layers could be correlated with the pigments used for the paint layers. Table 1 shows the results of the element distribution in combination with the analysis by optical microscopy.

Table 1. Results of the element distribution images Layer Appearance of the layer in

light microscopy Elements determined with EDX analysis

1 yellow Au 2 red/orange Ca, Ba, S and/or Pb 3 transparent C 4 yellow C, Ca, S and/or Pb 5 white Ca, S and/or Pb 6 white (yellowish) Ca 7 white (gray) Ca Figures 6b-f show that the first layer consists of Au, which confirms the surface layer to be of

gold. In layer 2 the main elements are Ba and Ca, with some S and/or Pb. In this case it is not possible to distinguish between these two elements due to the coincidence of the S K-lines (2.31 keV) with the Pb Mα-line (2.35 keV), which could not be seperated with the energy dispersive system with a resolution of approximately 160 eV. Ca as well as S and/or Pb are also the main constituents of the layers 4 and 5, whereas in layer 3, which appears transparent in the light microscope, only C could be detected. In layer 7 only Ca could be determined.

With the information of the elements present in the pigments of the various layers, it is possible to suppose the materials used for these layers. In order to gain structural information to identify the crystalline composition of the pigments, x-ray micro-diffraction is performed.

(a) (b)

(c) (d)

(e) (f) Figure 6. BE image of a detail of the cross-section of the specimen in Figure 5 (a), with the corresponding elemental distributions of Au M-line (b), Ba L-line (c), Ca K-line (d), Pb M-line (e) and C K-line (f).

3.4 X-Ray Micro-Diffraction

For the positioning of the individual paint layers during the x-ray diffraction a radiogram of the specimen illuminated with the plane beam was recorded as shown in Figure 7b. The complete scan, as indicated by the vertical line, consisted of 160 single points, separated by 4 µm. The lead containing and thus strong absorbing paint layer turn up dark against the less absorbing Ca containing layers (Fig. 7b).

Figure 7. Optical image (a) and corresponding x-ray radiogram (b) of the specimen with the paint layers used for the synchrotron x-ray micro-diffraction. The vertical line in Figure (b) indicates the alignment of the micro-diffraction scan, with 160 individual points.

All 160 diffraction spectra are displayed in a contour plot in Figure 8. Although this way of

displaying the data is not appropriate to accurately identify individual crystalline phases, it brings onto view the continuous assembly of all subsequent paint layers by crystalline matter.

Figure 8. X-ray micro-diffraction. Contour plot of all 160 diffraction spectra subsequently measured across the paint layers. Diffraction angles are replaced by the according crystalline lattice spacing values. Bright gray values indicate high intensity.

Because of the small scan interval of 4 µm enough spectra of each layer are available for the crystalline identifications of the pigments in the paint layers. This line scan also shows the continuous transition between the various layers, which could be clearly separated by this analytical technique. Although the surface layer is a very thin gold layer, it could be found with the micro-XRD and identified by the ICDD-database, as shown in Figure 9.

Figure 9. X-ray micro-diffractogram of the gold layer, layer 1, identified by the ICDD-database as Au.

The results of the x-ray micro-diffraction of the other layers are summarized in Table 2 as

well as in the Figures 10 and 11. In layer 2, which contains the elements Ca and Ba with some S and/or Pb (Tab. 1) a mixture of BaSO4 and chalk could be identified. In the layers 4 – 6 mainly gypsum was identified by x-ray micro-diffraction, whereby in layer 4 and 5 also some lead white could be found (Fig. 10). In the SEM/EDX only C was registered for layer 3, which shows the bright fluorescence in the UV-fluorescence micrograph (Fig. 4b). The result of gypsum in this layer obtained by x-ray micro-diffraction can only be explained by the presence of gypsum in the neighboring layer 4. Figure 11 depicts that calcite (CaCO3) was used as ground material (layer 7).

Table 2. Results of x-ray micro-diffraction Layer (color) Phases identified 1 (yellow) Au 2 (red/orange) BaSO4, CaCO3 3 (transparent) 4 (yellow) 2PbCO3.Pb(OH)2, CaSO4.2H2O 5 (white) 2PbCO3.Pb(OH)2, CaSO4.2H2O 6 (white/yellowish) CaSO4.2H2O 7 (white/gray) CaCO3

Figure 10. Diffractogram of layer 4, identified by the ICDD-database as lead white (2PbCO3.Pb(OH)2) and gypsum (CaSO4.2H2O).

Figure 11. Diffractogram of layer 7, identified by the ICDD-database as calcite (CaCO3).

4 CONCLUSIONS

With synchrotron induced x-ray micro-diffraction new perspectives for analyzing paint layers could be demonstrated. With this method it is possible to identify the pigments used exactly by their crystallographic structure and not only by their main elements, as the other methods (e.g. SEM/EDX) work with. Because of the great variety of inorganic pigments containing similar main elements it is now much easier to find the appropriate compound. However, for a clear interpretation of the materials used in paint layers a combination of light and UV-fluorescence microscopy, scanning electron microscopy combined with energy dispersive x-ray microanalysis and x-ray micro-diffraction yields to a high degree of information.

5 ACKNOWLEDGEMENTS

The authors express their sincere thanks to Dipl. Ing. Michael Melcher for carrying out the SEM/EDX analysis.

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