Abstract: In the field of FTIR spectroscopy, the far infrared (FIR) spectral region has been so far less investigated than the mid-infrared (MIR), even though it presents great advantages in the characterization of those inorganic compounds, which are inactive in the MIR, such as some art pigments, corrosion products, etc. Furthermore, FIR spectroscopy is complementary to Raman spectroscopy if the fluorescence effects caused by the latter analytical technique are considered. In this paper, ATR in the FIR region is proposed as an alternative method to transmission for the analyses of pigments. This methodology was selected in order to reduce the sample amount needed for analysis, which is a must when examining cultural heritage materials. A selection of pigments have been analyzed in both ATR and transmission mode, and the resulting spectra were compared with each other. To better perform this comparison, an evaluation of the possible effect induced by the thermal treatment needed for the preparation of the polyethylene pellets on the transmission spectra of the samples has been carried out. Therefore, pigments have been analyzed in ATR mode before and after heating them at the same temperature employed for the polyethylene pellet preparation. The results showed that while the heating treatment causes only small changes in the intensity of some bands, the ATR spectra were characterized by differences in both intensity and band shifts towards lower frequencies if compared with those recorded in transmission mode. All pigments' transmission and ATR spectra are presented and discussed, and the ATR method was validated on a real case study.

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Abstract: Few research studies from the early seventies reported the use of far infrared (FIR) spectroscopy and its analytical potential. Especially in the field of cultural heritage, particular attention has not been given to this technique because of its destructive character; however, this can be overcome by employing Raman microscopy. Provided that enough quantity of the sample is available, FIR spectroscopy may represent a useful analytical method in the case of inorganic compounds that are not active in the mid infrared region and, because of the fluorescent effect produced by the organic media, not detectable even by Raman spectroscopy. In this article, the application of FIR spectroscopy has been applied for the characterisation of several inorganic pigments. A preparation method based on the use of polyethylene has been developed that allows analyses with moderate quantity of sample (0.5-1.5 mg). The data obtained have been compared with Raman results with the aim to build a common database. Some applications on real case studies are presented and discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract: The interaction of pigments and binding media may result in the production of metal soaps on the surface of paintings which modifies their visible appearance and state of conservation. To characterise more fully the metal soaps found on paintings, several historically accurate oil and egg yolk tempera paint reconstructions made with different pigments and naturally aged for 10 years were submitted to attenuated total reflectance Fourier transform infrared (ATR FTIR) microspectroscopic analyses. Standard metal palmitates were synthesised and their ATR spectra recorded in order to help the identification of metal soaps. Among the different lead-based pigments, red lead and litharge seemed to produce a larger amount of carboxylates compared with lead white, Naples yellow and lead tin yellow paints. Oil and egg tempera litharge and red lead paints appeared to be degraded into lead carbonate, a phenomenon which has been observed for the first time. The formation of metal soaps was confirmed on both oil and egg tempera paints based on zinc, manganese and copper and in particular on azurite paints. ATR mapping analyses showed how the areas where copper carboxylates were present coincided with those in which azurite was converted into malachite. Furthermore, the key role played by manganese in the production of metals soaps on burnt and raw sienna and burnt and raw umber paints has been observed for the first time. The formation of copper, lead, manganese, cadmium and zinc metal soaps was also identified on egg tempera paint reconstructions even though, in this case, the overlapping of the spectral region of the amide II band with that of metal carboxylates made their identification difficult.

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Abstract: Architectural paint analysis is an important technique for establishing the palette of historical paint colours, and reconstructing how a room or a facade might have looked at an earlier time. In its simplest form, it has often been achieved by using paint scrapes, which involves revealing a stepped sequence of layers of paint using a blade or solvents. Cross-sections are used as an alternative to, or in combination with, scrapes. Mounted cross-sections can be examined by optical microscopy. Accumulated layers of paint, wallpaper or other applied decorative finishes can be defined from their textures and colours. Knowledge of the identity of the components in paint layers can be used to reconstruct a certain paint and also to date the paint layers. It is demonstrated that micro-Raman spectroscopy combined with optical microscopy and low-vacuum scanning electron microscopy-energy-dispersive spectrometry are versatile tools for the characterization of pigments and fillers in cross-sections from architectural decorations. The paper gives an example of the examination of an interior decoration from a historical building. Copyright © 2004 John Wiley & Sons, Ltd.

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Abstract: Most infrared analysis is performed in the Mid-IR (MIR) range (4000-400 cm-1). In the case of inorganic components, the normal frequency of an organic stretching vibration decrease as the molecular weight increase, because the frequency of a vibration is inversely proportional to the square-root of the reduced mass. Thus, for high molecular-weight species such as metallic complexes or some halides, oxide and sulphides, this relationship shifts the vibrations into the Far-IR (FIR) region, roughly defined as below 400 cm-1. FIR spectroscopy is thus an alternative way of collecting spectra of many inorganic pigments and corrosion products found on art objects, which is not normally observed in the MIR region. Most FIR spectra are traditionally collected in transmission mode but as a real novelty it is now also possible to record FIR spectra in ATR (Attenuated Total Reflectance) mode. In FIR transmission we employ polyethylene (PE) for preparation of pellets by embedding the sample in PE using much the same procedure as for preparing KBr pellets. Unfortunately, the preparation requires heating of the PE in order to produces at transparent pellet. This will affect compounds with low melting points, especially those with structurally incorporated water. Another option in FIR transmission is the use of thin films. We test the use of polyethylene thin film (PETF), both commercial and laboratory-made PETF. Here there is no heating of the sample and almost no preparation time, as the powdery sample is merely brushed onto the surface of the PETF and then the spectrum can be collected. ATR collection of samples is possible in both the MIR and FIR region on solid, powdery or liquid samples. Changing from the MIR to the FIR region is easy as it simply requires the change of detector and beamsplitter (which can be performed within a few minutes). No preparation of the sample is necessary, which is a huge advantage over the PE transmission method. The only concern with ATR is the need for good contact between the ATR crystal and the sample. This can be a problem especially with solid samples as not all solid samples have a planar surface that can come into good contact with the ATR crystal. In this case micro-sampling from the sample surface is necessary. In most cases comparison of standard spectra collected in FIR transmission and FIR ATR mode leads to only a few differences. The most obvious, when comparing transmission with ATR, is the distortion of band shape. The ATR band shape appears asymmetrical in the lower wavenumber region when compared to spectra collected in transmission mode. Also, intensity differences are noticed. However, the biggest difference can be the shift of strong absorbing bands moving to lower wavenumbers in ATR mode. In the majority of compounds, the shifts observed are small, approximately 1-10 cm-1, but for very strong absorbing compounds the shifts observed can be as big as 30-50 cm-1. These observed changes are in accordance with the established theory of ATR spectroscopy. It necessitates the collection of library standard samples in both FIR transmission and ATR modes, provided these two methods of collecting are to be employed for analyses of unknown samples. Standard samples of 150 pigment and corrosion compounds are thus collected in both FIR transmission and ATR mode in order to build up a digital library of spectra for comparison with unknown samples. XRD, XRF and Raman spectroscopy assists us in confirming the purity or impurity of our standard samples. Focus of our attention are on the oxides and sulphides of metals, as they are not normally observed in the MIR region, but are visible in the FIR region. Many of these pigments and corrosion standards, particularly those originating from minerals, are available in FIR transmission libraries and literature. However, ATR spectra of pigments and minerals, are hardly available in the literature. This can complicate ATR identification of unknown samples in the FIR region. 24 didactic test tables, with known pigment and binder painted on the surface of a limestone tablet, are used for testing the established library and different ways of collecting in ATR and transmission mode. In ATR, micro samples are scratched from the surface and examined in both the MIR and FIR region. Additionally, direct surface contact of the didactic tablets with the ATR crystal are tested together with water enhanced surface contact. A drop of water on the ATR crystal before contact with the solid sample can enhance the signal intensity when there are problems achieving contact due to planarity issues. But the question is if the sample can tolerate the exposure to water. When the binding media on our test tablet is glue, it is not advisable to use water. Oil binding medium seem to be less affected by contact with the water drop. In FIR transmission we compare the powder from our test tablet on the laboratory PETF and embedded in PE. We also compare the PE pellets collected using a 4x beam condenser, focusing the IR beam area from 8 mm to 2 mm. In theory, this should make it possible to use less sample amount in our pellets. In practice we experience an increase in noise sensitivity in the FIR region compared to spectra of PE pellets collected without the beam condenser. A few samples collected from a mural painting in a Nepalese temple, corrosion products collected from archaeological Chinese bronze objects and samples from a mural paintings in an Italian abbey, are examined by ATR or transmission spectroscopy.