Excimer laser irradiation at 248 nm of wooden archaeological objects and polymeric consolidants used in conservation: a study of cone formation and optimum cleaning parameters

Abstract

Samples of a prehistoric consolidated vegetable basketry, coming from an important pile building in the North-East of Italy (Fiavé Carrera, Trento), had been irradiated with excimer laser pulses to remove the consolidant layer and restore the artifact. Some problems, related to the nature of the consolidant resin, were found in the preliminary laser cleaning process. The two main obstacles were related to the unknown composition of the polymeric resin and to the irregular surface morphology of the archaeological find. We observed that large numbers of laser pulses, for selected laser parameters, strongly modify the resin surface morphology with formation of a large number of cones. After cone formation, the surface was so irregular that it was almost impossible to complete the laser cleaning procedure. To find a solution to the problem, we here try to understand the mechanisms of cone formation in the present polymeric material. Scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FTIR) have been used to study surface morphology and chemical modification after selected irradiation processes. To understand the influence of laser irradiation on surface modifications and to establish a cleaning procedure for this specific application, investigations have been carried out separately on models of resin consolidants and wood (both dried and buried wood sample). The consolidants were commercial resins, typically used in archaeological conservation (Plexisol and Paraloid), deposited on a silicon substrate. Silicon was used because it is an appropriate substrate in FT-IR analysis. Finally, we chose wood because of its wide use in prehistoric artefact (Viburnum lantana).