The cleaning or removal of manganese stains on Cultural Heritage has not been much tested or successful so far. The aim of this article was to assess a new green cleaning gel for Mn-rich black-blue stains on different substrates. The black-blue stains were characterized at optical and chemical level through colour-related data, optical microscope, FTIR, XRF and XPS. Mn-stained granite found on historical churches at Vila Real (North of Portugal) and glass jars of Leclanché cells, belonging to the ISEP’s Museum (Portugal) collection, were the ideal case studies to test the efficiency of chitosan: thiourea dioxide (TD) cleaning gel. TD proved to be the best candidate to reduce insoluble manganese oxides, over Hydroxylamine Hydrochloride and Hydroxymethanesulfinic Acid. Cleaning assays performed on stained granite samples collected at a historical quarry and in situ application on stained granite churches allowed removal of the stains to a satisfactory level. Similar results were obtained on stained glass jars.
This article discloses a rare and outstanding type of Mn-rich black-blue patina found on mural painting and granite ashlars located in the church of Sta. Marinha, north of Portugal, and conjectures the phenomenon associated to the appearance of such patinas in different surface materials. This Mn-patina reported on mural painting and their origin is probably assigned to manganese leaching from building materials (i.e., granite and phyllites). Stained mural painting and granite examined by XPS and SEM have showed patinas enriched with manganese (IV) oxide, potentially catalysed by a microbiota, like fungi, observed in SEM micrographs. The pigments used to depict mural painting and groundwater were also analysed by micro-Raman and ICP-MS, respectively, indicating that they are unlikely manganese sources. Unstained building materials, such as granite ashlars, historic joints, mortars and phyllite rocks, were also analysed by ICP-MS showing that historic joints and mortars present significant concentrations of manganese, possibly associated to their absorbing feature. The main materials with potential to impart manganese to Mn-rich patinas are granite ashlars and phyllites. The aim of this investigation is to reveal and ascertain the hypothetical sources and the phenomenon responsible for the Mn-rich black-blue patina appearance, both on mural painting and granite ashlars.
The cleaning or removal of manganese stains on Cultural Heritage has not been much tested or successful so far. The aim of this article was to assess a new green cleaning gel for Mn-rich black-blue stains on different substrates. The black-blue stains were characterized at optical and chemical level through RGB data, colorimetry, optical microscope, FTIR, XRF and XPS. Mn-stained granite found on historical churches at Vila Real (North of Portugal) and glass jars of Leclanché cells, belonging to the ISEP’s Museum (Portugal) collection, were the ideal case studies to test the efficiency of Chitosan:Thiourea Dioxide (TD) cleaning gel. TD proved to be the best candidate to reduce insoluble manganese oxides, over Hydroxylamine Hydrochloride and Hydroxymethanesulfinic Acid. Cleaning assays performed on stained granite samples collected at a historical quarry and in situ application on stained granite churches showed the complete removal of the stains. Similar results were obtained on stained glass jars. This article presents an innovative, feasible and green Chitosan gel embedded with TD and phosphoric acid, applied to the cleaning of Mn-stained granite and glass substrates.
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