## 4 Discussion  
Among the tested EOs, oregano, clove bud, thyme, and cinnamon cassia exhibited the most potent antifungal and antibacterial activities. However, for an EO to effectively conserve the murals of ancient tombs, we also considered how the EOs functioned in the closed environment of the ancient tomb and their impacts on mural materials (e.g., pigment, medium, substrate) [29; 30; 56]. Accordingly, we analyzed the inhibition of microbial growth caused by the four most potent EOs following dilution to low concentrations. Oregano and cinnamon cassia EO showed the highest antifungal and antibacterial activities against the six microbial strains, with inhibitory effects observed from a concentration of 3%. In previous studies, the oregano EO extracted from the aerial parts of the plant was found to inhibit the growth of bacterial strains of _Bacillus_, _Achromobacter_, _Sinorhizobium_, _Paenibacillus_, _Rhodococcus_, and _Stenotrophomonas_ and fungal strains of _Penicillium_, _Clonostachys_, _Fusarium_, _Doratomyes_, and _Acremonium_ at a concentration as low as 0.1% [42]. The oregano EO was also reported to exhibit antimicrobial activities against fungal strains isolated from stone and wood materials (_Aspergillus niger_, _A. clavatus_, _Penicillium_ sp., and _Fusarium_ sp.) at a concentration of 7.5% [57] and against the wood-destroying fungal strains _Postia placenta_ and _Trametes versicolor_ at concentrations of 0.25-1% [58]. The antimicrobial activities of 7.5% oregano EO against _Fusarium_ sp. observed in that study are consistent with our findings. In light of these findings, oregano EO extracted from the dried flowering herb may inhibit ascomycetes isolated from ancient tombs. Cinnamon cassia EO at concentrations of 0.1-0.5% was reported to inhibit the growth of _B. safensis_ and _S. rochei_ isolated from limestone mounents [59]. These studies reported inhibitory effects at lower concentrations of EOs than those reported in the current study, which may be related to differences in the method of EO extraction and the plant parts used, along with the type and content of emulsifier used. The use of emulsifiers in the dilution of EOs is unavoidable [60; 61]. Although most researchers use dimethyl sulfoxide (DMSO) and ethyl alcohol [22; 56; 57], DMSO could have a deteriorating effect on archaeological materials and is hazardous to human health [61], and ethyl alcohol could serve as a nutrient source for microorganisms [26]. Thus, the selection of an appropriate emulsifier is a critical consideration in the application of EOs in ancient tomb murals. Although numerous previous studies have reported high antifungal and antibacterial effects of thyme and clove bud EOs [34; 40; 58], these EOs displayed low inhibitory effects in the present study. This discrepancy could stem from the use of undiluted EOs in previous studies or the evaluation of their inhibitory effects at concentrations of 10%. We further evaluated the impact of low-concentration oregano, thyme, clove bud, and cinnamon cassia EOs on stone and painting layers representative of the murals of Buyeo Royal Tomb No. 1. None of the EOs affected the surface property or mineral composition of the granite and gneiss specimens. Although some studies have shown that biocide treatment causes minimal changes in the weight of stone compared with distilled water treatment [34; 53], our findings suggest that clove bud EO may remain on some granite and gneiss specimens. On Lascaux cave paintings, _F. solani_ fungal invasion was caused by residues of previously treated biocides that acted as nutrient sources for the microorganisms [28]. Similarly, biocide treatment may alter the diversity of the microbial community [62]. Hence, caution must be exercised for potential new challenges that may arise as EOs remain on ancient tombs. The stability of low-concentration EOs on painting layers was primarily assessed based on the color change in the oyster shell white pigment, which can be readily detected by spectrophotometric or microscopic observation.