Natural and artificial ageing of protective coatings for outdoor bronzes protection

Any protective treatment for cultural heritage applications should fulfil some specific requirements. Besides being transparent and colourless to maintain the original aspect of surfaces, they should be chemically and physically resistant to the degradation agents, possibly not toxic and removable without altering the underling patina and metallic substrate. For a better efficacy, in some cases protective coatings are applied in association with corrosion inhibitors. A crucial issue in the evaluation of new products is related to their long-term performances. Typically, this aspect is investigated carrying out accelerated ageing tests. However, this type of procedures rarely fully represent the complexity of real exposures. For this reason, when possible, it is advisable to perform also natural ageing, but this is rarely done because it is too time consuming. This research was aimed to investigate and compare the long-term efficacy of traditional and innovative coatings for outdoor bronze surfaces. Coatings were applied in single, double and triple layer and in combination, through blend or pre-treatment, with non-toxic corrosion inhibitors. The different treatments were applied on both patinated and non-patinated bronze specimens. The characterisation of the different treatments immediately after application was performed and reported on a previous work. A multi-analytical approach was adopted for the study of the long-term behaviour of the different tested treatments, including colorimetric, SEM-EDX, FTIR, LPR and EIS measurements. Natural and artificial ageing have been compared in order to investigate whether the artificial ageing process could be considered representative of the real effect of outdoor exposure.