5. Identifying archaeological jet and jet-like artifacts using FTIR

Recent work has illustrated the problems with identifying the various materials that were used to make black lithic ornaments in the past. The different materials that were worked include jet, cannel coal, oil shale, lignite and solidified bitumens- all of these originate from sedimentary deposits rich in organic material. Fourier Transform Infrared Spectroscopy (FTIR) has shown potential as a technique for identifying the materials; the analysis of geological samples from different material types produces distinctive spectra which can be related to their geological depositional environment, based on their chemical composition. This paper will discuss the problems encountered when the method is applied to archaeological material; the artifacts are often small and intricately carved, so that they are difficult to sample. Where artifacts have been conserved, the resins used for conservation produce an infrared spectrum which can mask the spectrum from the original material. This problem is lessened when the resin is identified and spectral subtraction methods are used to remove its IR spectrum from that of the jet or jet-like material. Oxidation of jet and coals in the archaeological burial environment can also alter their characteristic infrared spectra, and affect the success of FTIR as a discriminating technique. The results of FTIR analyses from two archaeological assemblages-from York and Verulamium- will be discussed to illustrate these points