Assessment and characterization of Origanum vulgare essential oil in highly viscous polymeric dispersions for cleaning cultural heritage stone materials

• HVPDs with Origanum vulgare EO provide a biocidal alternative for stone conservation. • PVA-borax dispersions enhance EO and carvacrol stability. • 1 H NMR and rheology reveal EO effects on HVPD microstructure and viscoelasticity. • Biofilm removal efficacy confirmed by ATP bioluminescence. Stone biodeterioration caused by microbial colonisation poses significant challenges to the conservation of cultural heritage. Here, we investigate highly viscous polymeric dispersions (HVPDs) using Origanum vulgare essential oil (EO) and its primary active component, carvacrol, to provide an eco-friendly alternative to conventional chemical biocides. The formulations were characterised by rheology, revealing shear-thinning behaviour that facilitates both application and subsequent peeling removal. Proton Nuclear Magnetic Resonance spectroscopy (¹H NMR) provided insights into the chemical interactions and molecular dynamics within the HVPDs. Both HVPDs formulations with EO and carvacrol were applied to remove biofilm from a sandstone surface at Castlelaw, an ancient Iron Age fort in Scotland. Biocidal efficacy was assessed through Adenosine triphosphate (ATP) bioluminescence assays, showing substantial reductions in microbial contamination, and Fourier Transformed infra-red spectroscopy (FTIR ATR) highlighted the removal of biogenic compounds. Both formulations maintained significant antimicrobial activity over two months, suggesting their potential for long-term application in conservation. Overall, this study underscores the viability of HVPDs enriched with natural essential oils as sustainable solutions for microbial biodeterioration in historic stone materials, paving the way for future advancements in conservation practices.