Characterization of Pigment-Binding Media Stsyems - Comparison of Non-Invasive In-Situ Reflection FTIR with Transmission FTIR Microscopy

A novel external reflection FTIR device, which enables nondestructive analyses in the region from 4000-450 cm-1 with a measuring point diameter of approximately 5 mm, was tested for the identification of pigments and binding media in two modern paintings and several mockups with traditional pigment-binding media combinations. Comparative measurements were carried out with minute samples using a diamond cell for transmission FTIR microscopy. The results were interpreted by comparison to database spectra measured in transmission mode. The appearance of distorted peaks in raw reflection spectra impedes a direct comparison with the collected spectra in the transmission mode. Best matches with database spectra were obtained applying the Kramers-Kronig transformation to the reflection spectra of substances showing narrow bands, particularly in the region of about 1900 to 580 cm-1. In the case of modern paintings a good match between transmission and reflection data could be achieved in the fingerprint region. It was possible to identify organic pigments PY 3 (Pigment Yellow 3) and PR 4 (Pigment Red 4) as well as calcite and PVAc (polyvinyl acetate). The identification of materials used for preparation of the mockups was successful for lead white, calcite, Prussian blue, indigo, ultramarine, azurite, malachite, Arabic gum and linseed oil, whereas the transformed spectra of cobalt blue, Naples yellow and especially of smalt and the proteinaceous binding media could not be identified using database spectra.