Impedance analysis of varnish-modified crystal quartz resonators coupled with FT-IR and FT-Raman: Assessment of the environmental impact on artistic materials in conservation sites

There is an increasing interest in analytical sensing systems for assessing the impact of the environment on the materials of the cultural heritage stored in indoor conservation areas. In this paper, the response of sensors obtained by coating quartz resonators with thin films of artistic varnishes (mastic and dammar), artificially aged under visible or UV radiation, is discussed in terms of their impedance curves around the resonating frequency. Electrodeless quartz crystals are used on account of their enhanced recycling simplicity and lower cost. The ageing under visible radiation induces a systematic negative shift (of the order of hundreds of Hz) and a damping of the impedance curve, with an increase in the impedance minima, which seems to indicate both gravimetric and visco-elastic effects. Controls exhibit only a minor shift, but again in the negative direction. On the other hand, under UV artificial ageing the frequency change is towards more positive values and is more substantial than in the case of visible light ageing. In these impedance spectra, although the overall frequency change appears to be more directly associated with the degree of ageing and/or degradation (thus being a potentially suitable parameter for continuous monitoring in field applications), the alterations in the shape of the curves are important to distinguish different types of ageing. The impedance analysis data is combined with the information retrieved from FT-Raman and FT-IR, also non-destructive techniques. A different rate of ageing, inferred by impedance analysis, is observed in the changes of specific band markers identified in the FT-Raman and FT-IR spectra. In particular, the higher reactivity of mastic versus dammar was highlighted by both methods.