Raman microscopy allows a non-destructive characterisation of inorganic and organic painting materials such as pigments and organic dyestuffs. The objectives of this study are the more recent organic pigments typically present in paintings and other art works from the 20th century. More than 20 organic synthetic pigments from different chemical classes could be identified by Raman spectroscopy using different excitation wavelengths (457.9, 476.5, 487.9, 514.5, 632.8, and 1064 nm). To evaluate the performance for real paint samples, varying paint mixtures of the Hansa Yellow pigment PY 3 and the binding medium Mowilith, a polyvinyl acetate (PVAC) compound, were characterised; PY 3 was determined at a 1 wt% level in the binder. In addition, commercial tube paints containing the quinacridone violet PV 19 were studied. The pigment was clearly identified in all of these more complex oil and acrylic paints. Finally, alizarin (PR 83) and a green copper phthalocyanine pigment (PG 7) could unambiguously be identified by Raman microscopy in the painting 'Woman with mandolin in yellow and red' of Max Beckmann dating 1950. The discovery of a red naphthol AS pigment by Raman spectroscopy in a sample from the 'Three field workers' by Georg Baselitz (1964/1965) demonstrated that in some cases complementary chromatographic methods are needed for a comprehensive identification of the organic pigments.
The novel and-for archaeometrical applications-still rarely used Laser-Induced Breakdown Spectrometry (LIBS) was tested on various standard glasses for its spectrometric performance. LIBS is an almost non-destructive method based on the time-resolved detection of optical emission following direct laser ablation. A frequency-quadrupoled Nd:YAG laser at 266 nm was found to cause a hardly visible crater of c. 200 µ m in diameter. The application of an innovative high-resolution Echelle spectrograph allows the fast and simultaneous determination of up to 90 elements in the spectral range from 200 to 780 nm, with a resolution of 10 -30 pm under ambient conditions. In principle, LIBS has a high analytical potential for archaeometry; in particular, for the determination of light elements (Li, Be, B and others). Because the measurement parameters (laser energy, delay time and number of pulses) do not independently influence the emission signal, their optimization proved to be complex and is rather a compromise for the range of elements. The effects of sample distance, surface roughness and the accumulation of depth profiles were studied. Problems arose for the determination of sodium and potassium in higher contents due to self-absorption effects. Quantification tests using a silicon line as internal standard yielded a precision range between 3% and 20% relative, but in general no satisfying accuracy for a number of elements. However, these problems might be overcome in the near future by improved spectrometer set-ups and matrix correction approaches.
The effect of flavour absorption on the oxygen permeability of low-density polyethylene (LDPE), polypropylene (PP), polycarbonate (PC) and polyethylene terephthalate (PET) was studied using an isostatic continuous flow system. Polymer samples were exposed to a model solution containing limonene, hexyl acetate, nonanone and decanal at 40 degrees C. After exposure, one part of each sample was analysed for absorbed flavour compounds using a Large Volume Injection GC Ultrasonic 'in vial' extraction method, and from the other part, oxygen permeability was measured in a permeation cell at 25 degrees C. After 8 h of exposure, LDPE and PP samples showed a significant linear (R2 = 0.82 and 0.99) increase in oxygen permeability of 21 and 130%, respectively. Owing to swelling of the polymer samples resulting from flavour absorption, the structure of the polymeric network changed (i.e. opened) and consequently increased oxygen permeability. The oxygen permeability of exposed PC showed a significant linear (R2 = 0.78) decrease of 11% after 21 days. PC obviously did not swell like LDPE or PP. Therefore, it was suggested that absorbed flavour compounds occupied or blocked 'microcavities' through which normally oxygen is transported. Absorption of flavour compounds by PET did not affect the oxygen permeability of PET significantly.
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