Measuring the Primary and Secondary Bioreceptivity of Stone and Their Implications for Heritage Conservation

Abstract

Bioreceptivity measures the ability of a surface to develop and maintain a permanent ecosystem of microorganisms. In the historic built environment, this is characterised visually by the greening of monuments and other outdoor heritage. Primary and secondary bioreceptivity refer to the potential for biological growth on fresh and weathered stone, respectively. Measuring bioreceptivity helps us understand biological growth rates and allows researchers to characterise the impact of stone conservation treatments on colonisation. Understanding the relative bioreceptivity of stones allows heritage professionals to make more informed selection of replacement stone types for repairs to existing structures. The primary bioreceptivity of stones from different geographical areas cannot currently be easily compared due to a lack of consistency in approaches to measurement. We propose a repeatable lab-based methodology for measuring colour and chlorophyll a levels from a standard algal culture grown on the stone surface as a standardised testing protocol for primary and secondary bioreceptivity. This protocol controls for the effects of mineralogical colour change during testing, which is something that has not been addressed in other methodologies. This method was successfully applied to case studies measuring the bioreceptivity of English granite, sandstone and limestone, determining their position in a bioreceptivity index. Our results demonstrated that for the stones tested, primary bioreceptivity was categorised as very low or low. Secondary bioreceptivity was increased compared to primary bioreceptivity. This research is the first to fully categorise any stones for secondary bioreceptivity and provides the first primary and secondary bioreceptivity data for any UK stones. We encourage others to follow this standard protocol to add additional data and build an international bioreceptivity database accessible to heritage practitioners.