Influence of Outdoor Bronze Alloying Elements on the Performances of a Biological Treatment

In the last decade, conservation research emphasized eco-friendly and sustainable approaches that could offer advantages in terms of durability, compatibility and safety. For example, a fungal treatment based on the use of a specific strain of Beauveria bassiana was proposed to convert soluble and/or active bronze corrosion products into oxalates. In the case of bronze exposed to acid rain, these oxalates would improve the long-term stabilization of the corrosion patinas due to their high insolubility. The aim of this study is to proof-test biological treatment for different bronze alloys and artificial patinas used in art and architecture. It is important to take into account that corrosion of bronze differs from that of pure copper due to the presence of additional alloying elements. In outdoor environments in particular, selective decuprification leads to a relative enrichment of tin in the corrosion layers. Understanding the influence of alloying elements on the interaction between the alloy and the protective layer is thus essential for setting up effective conservation treatments. In the present work, several pre-patinated bronze coupons were treated with a novel biological system based on Beauveria bassiana and the performance was evaluated. The results of the laboratory tests will be presented on different bronze alloys: binary 90Cu/10Sn, ternary 90Cu/8Sn/2Pb, quaternary 85Cu/Sn5/Zn5/Pb5 coated with an artificial copper sulfate patina and quaternary bronze alloy (UNS C83600, 85Cu/Sn5/Zn5/Pb5) patinated with a tin enrichment obtained by artificial ageing. In particular, this contribution will help in better understanding the response of outdoor bronze monuments in unsheltered (i.e. exposed to runoff) and sheltered areas to the biological treatment proposed, as well as to extend the application of biopatina to other metal surfaces used in contemporary art.