Natural biocides for the conservation of stone cultural heritage: A review

Abstract The use of traditional biocides to reduce bio-deterioration phenomena of stone monuments is increasingly deterred, due to risks for human health and the environment, as well as for potential interference with materials. Extensive research is now ongoing, aiming to find alternative and eco-friendly substances or methods to halt or reduce bio-deterioration. Here we aim to provide an assessment of the results of scientific tests, in order to evaluate the most promising substances, likewise possible deficiencies and future directions in research. We performed an extensive literature review on natural biocides for controlling bio-deterioration of stone using peer-reviewed articles between 1986 and 2018. The dataset created displays information about the experimentation of a total of 61 natural substances, mostly essential oils (23), or substances of plant origin and other compounds. The methods of application of such biocides were diverse, but the in vitro tests were the prevailing ones. The most tested organisms were fungi, followed by cyanobacteria and algae. The efficacy of the substances resulted highly variable, as did protocols and experimental doses, resulting in a lack of a robust and coherent assessment of best practices. Also, we found a few papers discussing the interference of these substances with the substrate. We believe that this area of research is very promising and necessary, although additional tests with a standardised methodology are still needed.