The purpose of the present research was to compare the effect of different embedding and polishing procedures on the performances achievable with Fourier Transformed Infrared (FTIR) microscopy in Attenuated Total Reflection (ATR) mode. The attention was specifically focused on this technique since it is generally considered one of the most appropriate methods to analyze cross sections, thanks to the fact that the recorded spectra are similar to those collected in transmission mode. The research work was structured as a round robin among six different conservation and research laboratories. They were asked to embed fragments derived from the same area of a real sample following each one a different procedure. The performances of FTIR microscopy in ATR mode were evaluated on the different cross sections, considering the cross section morphology and the possible source of contamination. The cross sections were previously observed with optical microscopy, Environmental Scanning Electron Microscope (ESEM) and confocal microscopy. Three procedures were eventually selected, because they provided the best results in terms of both limiting the contamination of the embedding medium and achieving good contact with the ATR crystal. They were: a) embedding in KBr; b) cyclododecane pre-treatment before embedding in an organic resin and cutting of the sample; c) embedding in an organic resin and polishing with argon ion milling. These three procedures were finally compared and the weak points outlined, indicating where improvements could be made for further researches.

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Optical photothermal infrared (O-PTIR) is a recently developed molecular spectroscopy technique that allows to noninvasively obtain chemical information on organic and inorganic samples at a submicrometric scale. The high spatial resolution (≈450 nm), lack of sample preparation, and comparability of the spectral results to traditional Fourier transform infrared spectroscopy make it a promising candidate for the analysis of cultural heritage. In this work, the potential of O-PTIR for the noninvasive characterization of small heritage objects (few cubic centimeters) is demonstrated on a series of degraded 16th century brass and glass decorative elements. These small and challenging samples, typically encountering limitations with existing noninvasive methods such as macroscopic x-ray powder diffraction and μRaman, were successfully characterized by O-PTIR, ultimately identifying the markers of glass-induced metal corrosion processes. The results clearly demonstrate how O-PTIR can be easily implemented in a noninvasive multianalytical strategy for the study of heritage materials, making it a fundamental tool for cultural heritage analyses.

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