The Oslo Opera House–Condition Analysis and Proposal for Cleaning, Protection and Maintenance of Exterior Marble

The photo-oxidative degradation of the polymers applied in the past onto the marble slabs of the Oslo Opera House (OOH) led to the yellowing of the coatings and/or to the formation of insoluble fractions of polymer. Despite the high reduction of the b* parameter (yellowness) observed after the mechanical abrasion, this cleaning treatment cannot be recommended for further application, due to the unacceptable removal of structural material during the abrasion. Cleaning of historic masonry has progressed significantly in recent years, thanks to methods that rely on nanoscience. Therefore, in this research project instead of using organic solvents exhibiting toxicity and poor performance, advanced nanostructured fluids, such as micelles solutions (MC) and microemulsions (ME), embedded in nanogels with industrial solvents were tested aiming at the efficient removal of the hydrophobic coatings from Carrara marble slabs. These nanogels are amphiphilic-based formulations using a system of water, oil and surfactant. However, lack of reproducibility characterized the application of these advanced nanogels, due to partly removal of the unevenly distributed polymer products. Therefore a three-step cleaning methodology was adopted including: the introduction of a chelating agent capable of dissolving the hard film created by the applied coatings, thus allowing any subsequent MC and ME solutions applied in the second step, to swell the polymeric chains and to facilitate their removal, which was further completed by suitable oxidants. Optical and scanning electron microscopy coupled with EDX, along with colorimetric measurements proved that the three-step cleaning tests applied to yellowed samples can successfully remove the colour discoloration