Abstract
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.
Highlights
Manganese staining involving built heritage and cultural artefacts is a rare occurrence that has been recently reported, mainly in underground sites, like caves and historical buildings
The formation of Mn-rich coatings occurs through the oxidation of soluble Mn2+/3+ to insoluble Mn4+, like MnO2, and inherent to this process, it can be ensued by autocatalysis and/or biogenic oxidation [12,13,14,15,16,17,18,19]
The identification of the stains on the Leclanché glass jar was previously achieved by Cadeco et al [26], which verified the presence of manganese dioxide on the stained glass
Summary
Manganese staining involving built heritage and cultural artefacts is a rare occurrence that has been recently reported, mainly in underground sites, like caves and historical buildings. (red seneca sandstone) [3], Paranhos spring water tunnel-Portugal (granite) [4] or the caves of LauscaxFrance (sandstone) [5] and Pautler-Southwest Illinois (limestone) [6]. Some articles describe this type of staining as a rock varnish defined by a thin dark Mn-enriched coating on rock surfaces. The thickness of this coating could range from 10 to 1 mm and the Mn percentage from 2 to 45 wt%, approximately [7]. A type of Mn-staining in Leclanché cells is reported on glass cell jar
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