Raman spectroscopy of archaeological and ancient resins: problems with database construction for applications in conservation and historical provenancing.

The adoption of Raman spectroscopy as a screening technique for the presence of organic resins on diverse substrates is now being advocated for the first pass non-destructive examination of potential sites for limited sampling using other analytical techniques. The characterisation of ancient resins in art work and specimens recovered from archaeological excavations is critically dependent upon the analytical capability of Raman spectroscopy using different wavelengths of excitation from the visible to the near infrared and the interpretation of the data illustrates the advantages and limitations of the technique. Resin specimens from art works and artefacts span a period of about 7000 years of recorded history and the influence of factors such as the environmental degradation, burial deposition, reaction with associated substrates and mineral pigments on the observed Raman spectra have been assessed. The key molecular Raman spectral features that are definitive for the discrimination between contemporary resins are considered in respect of these factors and thereby illustrative of the difficulties posed for the creation of a Raman spectral database of ancient resins, in contrast with the extensive and definitive literature equivalents that are available for their mineral pigment and organic dye analogues.