[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

5 mm for paintings and 01005 mm for filigree objects

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als, stones and ceramics, about 0.5 mm for paintings and 0.1–0.05 mm for filigree objects like, e.g., miniature paintings, jewellery, or painted enamels are appropriate. Some special applications require a particular lateral res- olution of 1–5 µ m to analyse crystallite inclusions in gemstones or biogenic materials like hair, bone and dentine and to carry out element mappings or line scans for the determination of concentration profiles. Recent develop- ments of total-reflection glass-capillary optics in combination with air-cooled X-ray tubes [569] and silicon drift detectors without need for liquid nitro- gen cooling provide a high lateral resolution associated with a maximum of mobility [571, 572]. A very promising new micro-XRF development consists in the design of a confocal XRF set-up by using two focusing devices as for instance capil- laries, one in the excitation and another one in the detection channel. The set-up permits to perform non-invasive depth-sensitive analyses with a reso- lution of about 10 µ m. This new 3D micro-XRF method opens up the way for many non-destructive archaeometric investigations of complex layered or inhomogeneous materials [573]. The nature of art and archaeological artefacts raises particular problems which may affect the interpretation of the analyses. The objects are usually not “ideal” but of complex shape, irregular morphology, heterogeneous com- position and may show surface alteration or several layers. However, it should be kept in mind that because of the uniqueness of art and archaeological artefacts most analyses have to be regarded as single case studies where rou- tine procedures are not easily applicable and require experienced scientists for measurement and data interpretation. The following section aims at summarizing relevant applications (1998- 2003) of XRF in the field of archaeometry. The selection was made to demon- strate the diversity of material groups and problems involved and does not claim to completely cover all published work. The order, in which the ma- terial groups are discussed, roughly corresponds to the frequency of XRF applications. 7.7.2 Materials Groups Pigments and Inks A substantial formal principle of art objects is their wealth of colours. Coloured surfaces are found in wall or panel paintings, in illuminated manu- scripts, but also in wall papers and painted sculptures. Usually paints are made by dispersing pigments in a liquid binding media (e.g., drying oil, gum Arabic, or egg white). After applying a layer of paint, it is generally dried and
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690 O. Hahn et al eventually other layers are added. In several cases, a final coating of varnish protects paintings. The thickness for the whole system ranges from a few mi- crometres up to 1 mm and more. A list of some commonly used pigments and their chemical composition is given in Table 7.20 [574, 575].
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  • Spring '14
  • MichaelDudley

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