A brief note on the elimination of dark stains of biological origin

The elimination of dark stains of biological origin is a very difficult conservation operation when no harmful side-effects are acceptable for the substrate. This situation was encountered during conservation work on the cloister of the Jerónimos Monastery in Lisbon, Portugal. The removal of the very dark coloration that had penetrated into the substrate could not be accomplished with the available cleaning methods. The dark coloration was found in places where a very adherent biological colonization was present, and it remained within the stone substrate even after attempts to remove it with several applications of biocide and soft, wet brushing. The use of a solvent specific for melanins (a mixture of toluene, dimethyl dialkyl quaternary ammonium hydroxide and methanol) proved very effective for removing the dark patina and solved that conservation problem. This paper reports this successful experience which has not, to the authors' knowledge, been applied elsewhere. The widespread occurrence of similar biological patinas can find in this method a suitable approach for their elimination.