How does FTIR help in residue analysis?
Fourier transform infrared spectroscopy (FTIR) is a form of infrared spectroscopy with a mathematical transformation of the resulting data to produce a spectra. This analysis measures the energy absorbed in a bond or functional group within chemical compounds. It is traditionally applied to purified chemical compounds but more recently has been applied to mixtures of chemical compounds where it measures the absorbance in bonds or functional groups present in the mixture rather than the chemical characterization of one compound. In residue analysis which is almost always comprised of a mixture of compounds FTIR can be used in two ways, 1) a chemical fingerprint of the mixture and 2) to characterize the functional groups of the compounds present in the mixture. The first approach will sometimes identify the origins of the residue but should be combined with other analytical techniques to confirm its identity. The second approach will allow for an interpretation of the components within the residue but not identify the compounds in the mixture thus characterization of the residue can initiate a deductive approach for residue identification.
Can archaeological residues really be accurately identified?
Archaeological residue analysis has received its fair share of criticisms along with literature that challenges its reliability (eg. Barnard et al 2007). However archaeological residue analysis can be reliably identified when the residue has been preserved enough for identification and when multiple analytical methods are employed.
What if I do not want anything removed from the surface of the artefact?
At LUMARS the artefact is first screened to determine if a residue is present. This is performed on the artefact without removal so it is non-destructive. The in situ characterization is also performed on the artefact without any residue being taken off. Depending on the results that are generated will depend on the degree of characterization that may be able to be made of the residue without removal. The analysis will usually be able to identify that the residue is organic or inorganic, that it is anthropogenic or environmental and can indicate if it is contamination or not. It usually will be able to identify if the residue contains plant and/or animal origin but it will not usually be able to narrow it down any further unless there are some diagnostic features present in the residue that will allow this level of interpretation (ie. feathers, scales, hair, pollen, starch, phytoliths, tissue, fibres). The report would include any recommendations for further testing if permission was granted in the future or before the material is returned to the contributor.
How much residue is required for testing?
This depends on the composition of the residue. Some residue has been able to be identified to species and composition from the size of a period (.) on the surface of the artefact. Some residue has been removed from museum materials analyzed and identified to species without any observable evidence of its removal. In these cases it has been predominantly plant resins as part of manufacturing residues on artefacts. However in other cases the residue has been identified as plant or animal residue and no greater quantity would be able to improve that interpretation.