Eco-friendly control method of biodeteriogens growing on stone surfaces by microwave heating

Biodeterioration processes are correlated with the chemical and geological composition of the material in itself, with the exposure to environmental factors (temperature, humidity, rain, wind, and solar irradiation), and with the frequency and quality of maintaining procedures applied. These factors influence the growth of biological organisms that can determine not only aesthetical alterations (e.g. colored patches, patinas, crusts) of the surface of stone material, but may erode the material by means of both physical and/or chemical processes, leading to alteration of the material integrity. When the material is part of a structural element the consequences of the degradation can affect even the stability of the architecture, a fact particularly evident on the wood beams heavily infested by insects or caries. The control of biodeterioration on Cultural Heritage artifacts includes all the activities used to eliminate degradation/alteration forms induced by the biological activity of colonizers (microorganisms and/or macro-organisms), and, whenever possible, the necessary activities to be implemented to delay/avoid the recolonization. For disinfection of biological colonizations that may induce progressive biodeterioration and associated damages, the chemical methods are available and widely used, sometimes with detrimental effects on the environment. Chemical treatments include liquid biocides and fumigation with gases. The choice of the biocide is regulated by the European Union's Biocidal Products Directive (BPD) (http://ec.europa.eu/environment/biocides/index.htm). The authors' research activity is oriented to propose physical methods as alternatives to classical biocides, in order to avoid the drawbacks correlated with the toxicity of sole biocides which can pose risks to the environment. In this perspective, the idea of using localized microwave heating treatments can be an effective alternative for controlling the biodeteriogens development to the chemical biocides. The use of microwave heating to control biotic agents was already applied in several fields, in particular in the agri-food industry for disinfestation of grain and other seeds from insects, and for sterilization of soil. In this research, the authors present some of the results on the application of a localized microwave heating to treat the biotic agents which infest the surface of stone artifacts. The microwave heating approach has some important advantages over conventional techniques (chemical biocides and mechanical removal) and other physical methods (thermal radiation, UV, gamma rays, laser cleaning), such as: it respects the operator safety, it is environmentally friendly, and has a low-interaction with the substrate thanks to its selective action- with reference to the conventional techniques - treatment extension, colorindependence, and penetration depth - with respect to other physical methods. The evaluation of the efficiency of the method against infesting agents growing on surfaces of monumental stone was studied with relation to the characteristics of the microwave system (as operative conditions, frequency, power, time and temperature of exposure), the type of infestant organism (lichens, fungi, biofilms-associated microorganisms), and the type of stone support (sandstone, marble).