## 1 Introduction  
Conservation of stone surfaces, in hypogea or in confined monuments, is threatened by the addition of artificial lighting that is essential for visibility; however, when it is combined with humidity, nutrient availability and particular physical substrate characteristics, light facilitates the colonisation by biodeteriorative biofilms [1; 2; 3]. These biofilms are formed by diverse microbial communities of cyanobacteria, bacteria and, to a lesser extent, by algae and fungi [4; 5]. Different studies have demonstrated their deleterious effects (aesthetical, chemical and physical) on diverse substrates (stone and painted surfaces [6; 7], as well as other valuable cultural heritage materials [8; 9; 10]). While there is the desire to rid surfaces from these deteriogenic biofilms, the general confinement of these sites hinders the use of chemical biocides, even if they are effective, as they cause major health issues for operators. These biocides also tend to be polluting substances that can persist in the natural environment [11]. While biocides regularly used in the restoration field (e.g., benzalkonium chloride, nystatin and sodium fluoride, among others) have known bacterial, fungal and insecticidal activities [12], their effectiveness on algae and cyanobacteria is still not fully understood. The challenge is now to develop more safe and sustainable methods in the restoration and conservation of lithtic cultural heritage using products and practices that take environmental, social and economic aspects into account [13; 14; 15; 16; 17]. This challenge will involve players from multiple disciplines, i.e., not only restorers and art historians, but also scientific researchers, politicians, entrepreneurs and economists. The most notably harmful products in use are being eliminated from the market and others should follow, and the search for new bio-based and sustainable products, as replacements, could open a new potential market in the bioeconomy chain. Substitution with safer alternatives and greener technologies is strongly driven by regulation and contributes to the overarching EU objectives for a non-toxic environment and a circular economy [18]. Among the alternative treatments to be investigated, plant extracts are showing promise as biocides by having many advantages over the traditional synthetic options. They are environmentally friendly (biodegradable) and have a functional complexity, which is thought to evade the possibility of resistance, one of the main negative characteristics of synthetic biocides. Plant-based biocides are generally alcohol extracts or essential oils that have been used in medical, food and pharmaceutical industries. Their effectiveness against common human pathogens including bacteria, fungi and yeasts is well-known, but more recently tests on their antimicrobial activity against organisms associated with biodeterioration of archives, libraries, museums and stone monuments have been carried out [12; 13; 19]. In the present study, liquorice leaf extract and lavernder essential oil were chosen for two reasons: the biofilms to be treated are complex communities, and the ineffectiveness of the synthetic biocides previously used, which required ever more frequent treatments (data not published). While the antimicrobial activity (against bacteria and fungi) of liquorice (_Glycyrrhiza glabra_ L.) root extracts has been well established [20; 21; 22], less is known about the full biocidal potential of liquorice leaf extracts. Two antimicrobial flavonones were isolated from the leaves of _G. glabra_: Pinochemrina and lycoflavanone [23]. A comparative study on the antimicrobial effects of leaf and root ethanolic extracts carried out on human pathogenic microorganisms demonstrated that there was overlapping activity and that the leaf extract was more effective against some strains [24; 25]. Leaf extract has also been used as a fungicide for agricultural purposes [26].