The tendency to employ natural products in the search for new eco-friendly solutions, according to the Green Chemistry principles, is spreading in many fields, including in the conservation of cultural heritage. In light of the above, the biocidal efficacy of three selected essential oils (C.nepeta, O.vulgare, T.vulgaris) and their respective major components (pulegone, carvacrol, thymol) has been assessed in a preliminary study that involved the application of these products, alone and/or in combination, on a biodegraded travertine wall. To have a systematic evaluation of the efficacy of this method, some travertine samples will be colonized with a phototropic multi-species culture and the products containing the phytochemical compounds, in different concentrations and compositions, will be applied. The preliminary phases of the project provide the physical and colorimetric characterization of the samples and the determination of their specific bioreceptivity. The inoculation of the samples with the biological culture will last three months and will be performed at the Dept of Edafología y Química Agrícola of the University of Santiago de Compostela, where this kind of procedure was ideated. The biocidal action of the phytochemical compounds on the biocolonized samples will be determined using i) colorimetric approach ii) quantification of photosynthetic pigments and extracellular polymeric substances iii) microscopic analysis. To better investigate the behavior and eventual changes of the physical properties of the stone in presence of the biofilm and the phytochemical compounds, portable NMR technology will be employed in an innovative way.
The employment of eco-friendly substances for the treatment of biodegraded stone artefacts would represent an important progress in the field of chemical research applied to the conservation of cultural heritage, where the theme of eco-sustainability is becoming increasingly important. In recent years the chemical products employed in this field, and not only this field, were targeted mainly on the resolution of the problems that affect the work of art, not considering possible secondary effects on human health (restorers and visitors) and, in the case of outdoor-preserved stone materials, on the surrounding environment. The selected essential oils are natural products normally used in everyday life and their toxicity, in the employed concentrations, is considerably lower than the one of classical biocidal products. However, even if the biological activity and the eco-sustainability of essential oils is well known, little is being said about their high costs and the impossibility of controlling their chemical composition. For these reasons, this research proposes a possible alternative that provides the employment of the major components of the selected essential oils, mixed together in the appropriate concentrations, to provide a better control in the elimination of the microorganisms.
Another innovative aspect concerns the application of the products on real biocolonized travertine samples, overcoming the existing "in vitro" studies by recreating, as much as possible, the real conditions, and thus evaluate eventually secondary effects depending on the interactions of the phytochemicals with the stone matrix. All the methodologies that will be used to investigate the bioreceptivity, the biofilm growth and the effect of the products on the stone, are non-invasive and non-destructive, in keep with the basic principles of the diagnostics on cultural heritage. Among these methodologies, the unilateral portable NMR stands as an innovative advanced application of the classical nuclear magnetic resonance and is particularly suitable for the monitoring of the conservation status of non-transportable artistic artefacts, that will be employed for the first time in this kind of study.