Black manganese crusts on Galician Romanesque buildings: first analytical data on characterization and cleaning

• Mn-rich black crusts developed on ancient granite structures. • Possible origin from ancient treatments. • Evaluation of mechanical, physical, and chemical methods for in situ removal. • Oxalic acid in carboxymethylcellulose the most effective method. • Laser and microabrasion increased rock roughness. This work presents the results of the characterization of Mn-rich black crusts developed on granite in a Galician Romanesque monastery and the initial experiments aimed at their removal. Once the crusts were characterized from a chemical, mineralogical and micromorphological point of view, several in situ cleaning methods was performed: microabrasion, a Nd:YAG laser and chemical cleaning processes (aqua regia, EDTA, oxalic acid, gallic acid and formic acid solutions). An in situ analytical methodology based on optical microscopy, colour spectrophotometry, LIBS and Raman spectroscopy was applied to characterize the cleaning effectiveness and the side effects exerted in the granite underneath. Among all the methods, saturated oxalic acid proved to be the most effective, although slight calcium contamination was detected. In contrast, less effective methods such as laser cleaning and microabrasion resulted in textural alterations in the granite.