Comparison between traditional and sustainable methods for cleaning iron stains on mosaics of the Cottanello Roman villa

The aim of this work is the evaluation in situ of the effectiveness of sustainable methods, used to remove surface chromatic alterations induced by the iron corrosion products on limestone mosaic tesserae. Iron complexing or chelating agents, including "green" solutions such as cysteine, were compared with traditional methods: ammonium thioglycolate and ammonium citrate. Cleaning tests on pilot areas were performed on the mosaics of the Roman Villa of Cottanello (Rieti, Latium). This Villa is particularly relevant for its architectural plan and for the richness of its decoration (mosaic floors, marble, architectural terracotta, mural paintings); it is one of the most extensively excavated in the territory of the Sabina Tiberina. The history of the building is attested between the late republican period and the late antiquity; its rich repertoire of mosaics is dated between the first half of the I century B.C. and the first half of the I century A.D. Nowadays, the excavated rooms are covered by deteriorated sheet iron structures which have given rise to rust stains on the mosaic floors. The cleaning systems of rust stained tesserae were evaluated by analytical investigations (measurements of colorimetric variations, optical and SEM observations). All the methods tested in situ showed satisfactory results of cleaning effectiveness in comparison with the traditional ones, suggesting a replacement of the methods used so far and less sustainable for the environment and for the restorers. In addition, the low costs of the new tested products used could be an incentive to their use, maybe encouraging in the future a restoration not yet planned by the Archaeological Superintendence, due to current lack of funding.