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Removal of iron oxides from a sixteenth century ornamental fountain belonging to Quinta da Cardiga (Torres Novas, Portugal)

The ornamental fountain in marble and breccia is a 16th century Italian work integrated in the small courtyard of the Cardiga Palace (Torres Novas, Portugal) currently in restauration process. The iron spikes used to connect the constituent elements were corroded and contaminating the base stone elements surfaces with intense stains of metallic oxides. There was a suspicion that, apart the surface, these oxides could have migrated in depth through the porous structure of the material. The performance of the iron oxides removal tests was evaluated using optical measurements in the CIELAB color space after each chelant application. Sodium dithionite was found to exert a substantially higher cleaning action than ammonium citrate. After finishing the treatment with the most efficient chelant, the chelating action was verified below the surface of the marble. It was found that at the depth of 1 mm the material remained contaminated with iron oxides, with an evident color contrast with a treated surface.