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.

Fourier transform infrared (FTIR) spectroscopy is one of the most widely applied techniques for the investigation of cultural heritage materials. FTIR microscopy is well established as an essential tool in the microdestructive analysis of small samples, and the recent introduction of mapping and imaging equipment allows the collection of a large number of FTIR spectra on a surface, providing a distribution map of identified compounds. In this Account, we report recent advances in FTIR spectroscopy and microscopy in our research group. Our laboratory develops, tests, and refines new and less-studied IR spectroscopy and microscopy methods, with the goal of their adoption as routine analytical techniques in conservation laboratories. We discuss (i) the analysis of inorganic materials inactive in the mid-IR region by means of far-IR spectroscopy, (ii) the development of new methods for preparing cross sections, (iii) the characterization and spatial location of thin layers and small particles, and (iv) the evaluation of protective treatments. FTIR spectroscopy and microscopy have been mostly used in the mid-IR region of 4000-600 cm(-1). Some inorganic pigments, however, are inactive in this region, so other spectroscopic techniques have been applied, such as Raman spectroscopy. We suggest an alternative: harnessing the far-IR (600-50 cm(-1)). Our initial results show that far-IR spectroscopy is exceptionally useful with mural paintings or with corrosion products from which larger sample quantities can generally be collected. Moreover, the inorganic composition of a sample can be characterized by the presence of several compounds that are inactive in the mid-IR range (such as sulfides, oxides, and so forth). Stratigraphical analyses by FTIR microscopy can be hindered by the process of cross section preparation, which often involves an embedding organic polymer penetrating the sample's porous structure. Here, the polymer bands may completely cover the bands of organic compounds in the sample. However, a correct methodological approach can prevent such limitations. For example, it is always advisable to analyze the sample surface before preparing the cross section in order to characterize the preparation layers and the varnish layers, which are generally applied to the surface of a painting both to protect it and improve the color saturation. Furthermore, the innovative use of IR-transparent salts as embedding material for cross sections can prevent contamination of the embedding resin and improve detection of organic substances. Another key point in the use of FTIR microscopy in artwork analysis is spatial resolution. The high-energy output of a new integrated FTIR microscope enhances the ability to characterize and spatially locate small particles and thin layers. Moreover, the new configuration proves extremely useful in the evaluation of protective treatments, because larger areas may be analyzed in less time in comparison to traditional systems, allowing the collection of more statistical data.

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Different Fourier transform infrared microspectroscopic techniques, using attenuated total reflection (ATR) mode and single-element mercury-cadmium-telluride (MCT) detector (mapping) or multielement MCT detector (raster scanning), are compared with each other for the characterisation of inorganic compounds and organic substances in paint cross sections. All measurements have been performed on paint cross sections embedded in potassium bromide, a transparent salt in the mid-infrared region, in order to better identify the organic materials without the interference of the usual embedding resin. The limitations and advantages of the different techniques are presented in terms of spatial resolution, data quality and chemical information achieved. For all techniques, the chemical information obtained is found to be nearly identical. However, ATR mapping performed with a recently developed instrumentation shows the best results in terms of spectral quality and spatial resolution. In fact, thin organic layers (approximately 10 microm) have been not only identified but also accurately located. This paper also highlights the recent introduction of multielement detectors, which may represent a good compromise between mapping and imaging systems.