Evaluation of cleaning procedures on the facades of the Bank of Greece historical building in the center of Athens

The facades of the Bank of Greece historic building, mainly consisted of porous stone, gray marble and white pentelic marble, are subjected to an intensive air pollution attack in the center of Athens. A diagnostic study was carried out prior to the cleaning procedures for the weathering evaluation. Weathering appears mainly as black depositions, salt crusts, and oily depositions, due to the heavily polluted urban atmosphere from the nearby traffic. Previous improper cleaning treatment by water spray under high pressure caused detachment of grains and fissuring which were recognized as mechanical abrasion of the surfaces. In this study, a pilot investigation is performed with the intention of evaluating the most appropriate cleaning treatment. Therefore, several cleaning procedures were performed in the laboratory and in situ for the evaluation of methods and products applied on the facades. Both chemical and physical cleaning procedures were applied and they were chosen for their selective action: only water and sepiolite for solvent action, ammonium bicarbonate for exchange action, EDTA for the chemical chelating action and microblasting for physical action were used. In situ non-destructive evaluation was performed by Fiber Optics Microscopy in order to assess the counteractions of each cleaning method to the original surfaces. Digital Image Processing was also used to account for the efficiency of each cleaning method.