Infrared spectroscopic characterization of residues on archaeological pottery through different spectra acquisition modes

•Pottery shards from ancient amphorae were analyzed by FT-IR spectroscopy.•Sampling by abrasive paper and DRIFT analyses identified only inorganic compounds.•Also analyses in transmission on powdered samples mainly found inorganic materials.•Superficial residues were easily differentiated and characterized by the μ-ATR mode.•μ-ATR mode was effective in analyzing samples in a minimally invasive way.

Fourier transform infrared spectroscopy (FT-IR) is a versatile analytical method, very useful in many fields. Although a crucial step in producing good spectra is the use of the appropriate technique, the acquisition mode is sometimes not accurately selected and the results are partial or lacking.In recent years, FT-IR analysis has been proposed as a screening method for characterization of archaeological potteries and identification of the residues on these artifacts before turning to destructive, more expensive, and time-consuming techniques.In this study, a set of pottery shards, classified as fragments of amphorae, was subjected to FT-IR analyses. The results obtained from different sampling procedures and different spectra acquisition modes, were examined and compared.The as-received ceramic fragments were subjected to micro attenuated total reflectance (μ-ATR) analyses. Investigations in diffuse reflectance (DRIFT) mode were performed on samples collected by abrading the surfaces of the shards with abrasive paper. Samples scraped from either the surfaces of the pottery fragments or the interior of the ceramic body, were analyzed in transmission mode as a powder in KBr pellets or after extraction with acetone.The sampling by abrasion of the surface with an abrasive disk, and consequently the analyses in DRIFT mode, were successful only in identifying the inorganic compounds coming from the pottery and/or the environment, while materials related to the content of the jar were not detected. Also the analyses in transmission mode provided information mainly about inorganic materials, which, even where in a limited amount, masked the signals of organic compounds. Just an extraction with a solvent made it possible a more detailed, but still partial, characterization of these organic substances. On the contrary, the content of the jar was easily detected using the μ-ATR mode, even in areas where no residue was observed.