Fungal stains on paper: Melanins produced by fungi

Abstract

Books, prints, drawings, watercolours, engravings as well as all other works of art based on paper comprise a great portion of our cultural heritage. Therefore, its preservation is a matter of great concern. Paper can be deteriorated due to physical, chemical and biological agents. Fungi are among the most common biodeteriogens affecting paper-based collections, causing severe material and information losses. This work focused on fungal stains on paper which are often coloured. These interfere with the readability of the artefacts diminishing their artistic and monetary value. Up to now, there is still no definitive answer for this problem. The successful cleaning of fungal stains from paper is a mandatory conservation task, considered a priority by paper conservators. However, most of the authors refer the stain colour and patterns but they do not indicate the colourant or colourants (or chemical compound) responsible for the stain. Black stains on paper are of major concern because, not only they are very frequent, as well as its dark colour leads to a great loss of visibility. Therefore, this work focused primarily in the extraction and characterization of fungal melanins from three different species: Aspergillus niger, Chaetomium globosum and Cladosporium cladosporioides, known to be responsible for black staining on paper and melanin production. UV-Vis, μ-FTIR and μ-Raman analyses were carried out for all three fungi melanin extracts. UV-Vis, μ-FTIR and μ-Raman results show that, after extraction and purification, purified melanin samples were obtained from the three-fungal species. Moreover, SSNMR allowed to characterize A. niger’s melanin as a L-DOPA type melanin, and Cl. cladosporioides as a DHN type melanin, by comparison with the synthetic L-DOPA melanin (Sigma-Aldrich) and the literature. This will allow for a colourant-specific testing of newly developed cleaning methods, considering the base structure of the polymer (melanin) to be removed from the paper.