# Varnishes with Biocidal Activity: A New Approach to Protecting Artworks
###### Abstract  
Keeping agents that cause biodeterioration under control is a common task in restoring and conserving artworks. However, there are very few studies, specifically those concentrating on pictorial works, that provide solutions compatible with the complex mix of organic and inorganic materials to be found in such cultural assets. This study aims to use biocide products that are commonly used in this field (benzalkonium chloride, o-phenylphenol, and tributyltin napthenate) and which are associated with natural varnishes with a long tradition in artistic practices, in order to protect paintings and polychrome sculptures from environmental fungi and bacteria without altering the original materials or the visual appearance of the artworks. The effectiveness or innocuousness of the treatments was tested chemically and visually via optical microscopy, gas chromatography-mass spectrometry (GC-MS), and a multidimensional scaling analysis (MDS); the treatments produced good results as regards the inhibition of microbial growth and scarce interaction with the artistic materials being studied. biocides; terpenoid varnishes; painting; fungi; bacteria; biodeterioration; GC-MS; MDS 2020 10 7319 10.3390/app10207319 10.  
alternative that has been explored very little is to use materials with biocide incorporated. Restorers often carry out these tasks empirically, mixing them with water repellents or consolidants, but only in the area of mortars for restoration have serious attempts been made to obtain products with biocidal activity [22; 23; 24]. This study raises the possibility of using varnishes with incorporated biocide to give artworks protection against environmental microorganisms. Terpenic varnishes have traditionally been used as the final layer of protection so that they can act as a barrier between the pictorial layer and the external environment. It should be noted that they are susceptible to microbial colonization, although they are a restrictive substrate only suitable for microorganisms with certain metabolic and physiological capabilities (production of spores, resistance to a shortage of nutrients, resistance to xeric environments, etc.). _Penicillium_ and _Aspergillus_ are the most commonly described fungi associated with biodeterioration in polychrome artworks, and the genus _Bacillus_ and _Arthobacter_ among bacteria [25; 26; 27; 28; 29]. Biodeterioration of these materials has been studied in past publications, which have described different analytical markers for these processes [30; 31; 32; 33]. Nevertheless, terpenic varnishes naturally suffer from ageing processes that lead to phenomena such as yellowing and a loss of transparency. A common restoration task is to completely or partially eliminate them and revarnish the artworks. This can be done with products with incorporated biocide, which can add extra protection to the pictorial layer against microbial biodeterioration of paintings and polychrome sculptures. The use of varnishes with biocidal activity has been studied very little until now. [PERSON] and [PERSON] [34] evaluated the colonization of pinewood varnished with an unspecified alkyl resin with an incorporated biocide based on 2,3,5,6-tetrachloro-4 (methyl sulphonyl)-pyridine at a concentration of 0.3%. They concluded that the effects on the varnish layer, measured using the changes on the surface observed microscopically, were significantly lower than in the same varnish not treated with biocide. Our research group also contributed some results in this area [35], which we shall look at in greater detail in this work. We propose using biocides associated with natural diterpene resins such as colophony (rosin), sandarac, and Manila copal, which are widely used in fine arts as the final layer of protection against biodeterioration in artworks (mainly paintings and polychrome sculptures). Their effectiveness against selected bacteria and fungi is tested in this study using optical microscopy, gas chromatography-mass spectroscopy (GC-MS), and a multidimensional scaling analysis (MDS).