Identification of low-concentration tar in wood samples from archaeological contexts by ATR-FTIR

• ATR-FTIR used non-invasively and micro-destructively detected invisible tar in wood. • Acetone extraction of wood enabled unequivocal identification of tar. • ATR-FTIR was too unprecise to differentiate tar source as birch bark or pine wood. • Site information and visual analysis helped to identify tar source as pine wood. The investigation of pine tar residues in wood recovered from a waterlogged historic pine tar production site is presented. The site is a wood tar kiln constructed in a mire, a mode of production distinct to Central Norway. Due to lack of surviving visible wood tar, this study involves a non-invasive and micro-destructive approach. Samples taken from the wood were analyzed by a combination of fluorescence microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDS). Due to low concentrations of surviving wood tar, the study aimed at developing a method to reliably identify and differentiate the scarce tar residues from the wood substrate in the specimens. The results were compared against known reference materials of modern pine tar, as well as tar residue extracted from a medieval archaeological textile.