SURFACE FINISHES AND MATERIALS: FIBER-OPTIC REFLECTANCE SPECTROSCOPY (FORS) PROBLEMS IN CULTURAL HERITAGE DIAGNOSTICS

The coupling of mid-IR spectroscopy and fiber-optic technology(chalcogenide and halide fibers) has allowed for theintroduction of a powerful non-invasive and extremely usefultool for analysis of cultural heritage materials. Thispaper critically evaluates the problems, such as opticalanomalies, observed when a surface is investigated withthis technique in order to simplify a comparison withabsorbance/transmittance reference spectra.Total IRreflectance spectra are affected by important modificationsin shape, frequency and intensity of absorption. Inthis work, mid-IR FORS spectra coming from different materials(marble, calcarenite, acr ylic resin) with differentsuperficial finishes are investigated in order to evaluate theinfluence of nature, opacity and superficial roughness onpeak shifts and intensities.Investigation was performed in the following steps: (i)establishment of the optimal measurement conditions(working distance and incidence angle) for each substrate,(ii) recording and comparison of material spectra, (iii) evaluationof the correction algorithms (Kubelka-Munk and/orKramers-Kronig), (iv) comparison of the spectra with thoseobtained with traditional ATR technique. The objective wasto evaluate the reliability of the diagnostic setup and verifythe use of the mid-IR FORS technique to different substrateseven when used in situ, where surface conditions(roughness, pollutants, treatments, position of the probeetc.) cannot be controlled.