Effectiveness and durability of chemical- and laser-based cleanings of lichen mosaics on schists at archaeological sites

Lichen mosaics on different-textured schists located in the Coa Valley (Portugal) and Siega Verde (Spain) archaeological sites were cleaned using different chemicals, namely ethanol (50% v/v in distilled water), benzalkonium chloride (3% v/v) or Biotin T® (3% v/v), and different Nd:YAG laser wavelengths (1064 nm or 266 nm). The surfaces were evaluated 24 h and 4 years after cleaning to determine its durability using colour spectrophotometry and Raman spectroscopy. Unlike lasers, chemicals achieved overall satisfactory results. Cleaning effectiveness, harmfulness and durability of chemicals were highly influenced by the orientation of the schistosity planes of the stone; in Siega Verde samples, the schistosity planes parallel to the surface contributed to a low impact of the methods on the surface colour and absence of lichen recolonization. Cleaning carried out upon the devitalization of lichens with benzalkonium chloride and Biotin T were maintained longer in both sites.