Raman and ATR-FTIR spectroscopies applied to the conservation of archaeological Baltic amber

A recent survey suggested that the National Museum of Denmark had more than 17 000 pieces of amber in its collections. It also concluded that 45% of the amber in the collections exhibit deterioration, manifested by surface crazing, powdering and discolouration. Although the chemical composition of Baltic amber has been investigated, and it is known to comprise an alkyd polymer containing 8% succinic acids and terpenoid components, degradation pathways have not been defined. It is important that the amber collections are preserved for future study and enjoyment, and a preventive conservation strategy is under development. Such an approach involves controlling the environment in which the objects are stored to slow the rate of deterioration. Experience at the Natural History Museum, London, suggested that the rate of degradation was related to relative humidity (RH) of the storage environment. In this study, Raman and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopies were used to quantify and compare the extent of deterioration of amber after thermal ageing at various levels of RH. ATR-FTIR spectra provided information about surfaces, because the electromagnetic radiation penetrated only the upper few microns of the amber, whereas Raman spectra reflected chemical changes at the millimetre level. Raman spectra showed a loss in C=C groups on ageing of the amber samples at all levels of RH, while ATR-FTIR spectra indicated an increase in the concentration of carbonyl for the same samples. Bands corresponding to unsaturated carbon bonds (1646 cm(-1)) were strong in Raman spectra and weak in FTIR spectra, while the opposite was true for bands representing carbonyl groups (1730 cm(-1)). The findings were attributed to reaction of oxygen with unsaturated carbon-carbon bonds in the terpenoid components of amber that formed acids containing carbonyl groups (dehydroabietic and 7-oxodehydroabietic acids). ATR-FTIR spectroscopy suggested that the rate of deterioration was slowed by high RH, although further investigations are required. Copyright (C) 2006 John Wiley & Sons, Ltd.