Chemical or biological foxing on historical papers

The term foxing defines one of the most widespread damages on works of art on paper which appears as red-brownish spots more or less diffuse on the substrate. The main hypothesis on the origin of this phenomenon distinguishes between abiotic and biotic foxing. The first is due to chemical-physical phenomena, as oxidation and/or degradation catalysed by heavy metals, while the second is the result of the action of micro-organisms, included fungi. The preliminary analysis of morphochromatic aspect of stains together with a rough characterisation of paper substrate does not always allow a clear-cut differentiation between the biotic or abiotic attack. To this end various spots on different types of paper have been selected and subjected to micromycological and instrumental analyses. The FTIR (Fourier Transform Infrared Spectroscopy) analysis has been performed over foxed and non foxed areas: this technique revealed the presence of fungal metabolites and/or paper degradation products in correspondence of the spots. The fungi isolation method allowed the identification of several fungal strains. Our results, obtained with non-invasive techniques, revealed the distinctiveness of biotic deterioration, where both alive or dead fungal bodies brought about the foxed spots. The multidisciplinary approach allows to distinguish among the biological or chemical origin of the foxing spots and enables a proper evaluation for the subsequent conservation treatments.