Research on the selection of biocides for the ‘disinfection’ of statues and masonry at the National Trust (UK)

Abstract In order to refresh guidance on garden statuary maintenance conservation, 16 biocides available on the British and European markets were selected for testing in situ at 11 properties of the National Trust (NT) located in England and Wales. For the testing, a range of decorative items, statues and architectural features were chosen, made of various types of stone, including limestone, sandstone, marble, slate and granite, as well as cast concrete, terracotta and brick. Tests were preceded by evaluation of the local microclimate and visual examination of the condition of the objects, including assessment of the presence of any micro-organisms and salt efflorescence. The history of previous conservation works and their materials was also reviewed. Microbiological analysis was carried out in many cases, which identified various algae, lichens and mosses. The effectiveness of the biocides and any potential side effects were evaluated periodically for 24 months after application. Three of the most effective biocides were selected for further assessment. An adenosine triphosphate (ATP) luminometer was used to measure whether biological growth had recurred on the stone at 7 and 12 months after application. Observed side effects included an occasional brown discolouration of Carrara marble after application of three of the biocides tested. An important outcome of the project was not only to recommend which biocides to use where treatment was necessary but also to determine when their use would not be advisable.