Biocides Based on Essential Oils for Sustainable Conservation and Restoration of Mural Paintings in Built Cultural Heritage

Microbial deterioration poses a significant threat to built heritage, particularly mural paintings, where traditional synthetic biocides can have adverse environmental and material impacts. This study evaluates the effectiveness of essential oils derived from four aromatic plants—thyme (Thymus mastichina L.), fennel (Foeniculum vulgare Mill.), pennyroyal (Mentha pulegium L.), and green lavender (Lavandula viridis L’Hér.)—as natural biocides against microorganisms isolated from mural paintings in the House of Moscadim, an 18th-century manor house in Portugal. The antimicrobial activity of the essential oils was assessed using both direct contact and micro-atmosphere methods. Four microorganisms were isolated, including two bacteria, Bacillus wiedmannii and Bacillus mobilis, and two fungi, Penicillium brevicompactum and Cladosporium cladosporioides. Fennel essential oil exhibited the strongest antifungal activity against both fungal species, surpassing the efficacy of the commercial biocide Biotin T® in some cases. Pennyroyal and lavender essential oils also showed significant inhibitory effects. The micro-atmosphere method demonstrated the potential for noninvasive application of essential oils while preserving the integrity of delicate mural surfaces. These findings suggest that essential oils, particularly fennel oil, constitute a promising natural alternative to synthetic biocides for the sustainable conservation of cultural heritage. Further research is recommended to explore their long-term effects and to optimize application techniques.