Influence of the Laser Wavelength on Harmful Effects on Granite Due to Biofilm Removal
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coatings Article Influence of the Laser Wavelength on Harmful Effects on Granite Due to Biofilm Removal P. Barreiro 1, A. Andreotti 2, M. P. Colombini 2, P. González 1 and J. S. Pozo-Antonio 3,* 1 Dpto. Física Aplicada, Escola de Enxeñaría Industrial, University of Vigo, 36310 Vigo, Spain; [email protected] (P.B.); [email protected] (P.G.) 2 Department of Chemistry and Industrial Chemistry, University of Pisa, 56126 Pisa, Italy; [email protected] (A.A.); [email protected] (M.P.C.) 3 Dpto. Enxeñaría dos Recursos Naturais e Medio Ambiente, Escola de Enxeñaría de Minas e Enerxía, University of Vigo, 36310 Vigo, Spain * Correspondence: [email protected]; Tel.: +34-986814077 Received: 29 January 2020; Accepted: 21 February 2020; Published: 25 February 2020 Abstract: The colonization of stone-built monuments by different organisms (algae, fungi, lichens, bacteria, and cyanobacteria) can lead to biodeterioration of the stone, negatively affecting the artistic value of the heritage. To address this issue, laser cleaning has been widely investigated in recent years, due to the advantages it offers over traditional mechanical and chemical methods: it is gradual, selective, contactless, and environmentally friendly. That said, the laser parameters should be optimized in order to avoid any by-effects on the surface as a result of overcleaning. However, as the adjustment of each parameter to clean polymineralic stones is a difficult task, it would be useful to know the effect of overcleaning on the different forming minerals depending on the wavelength used. In this paper, three different wavelengths (355 nm, 532 nm, and 1064 nm) of a Q-Switch neodymium-doped yttrium aluminum garnet (Nd:Y3Al5O12) laser, commonly known as QS Nd:YAG laser were applied to extract a naturally developed sub-aerial biofilm from Vilachán granite, commonly used in monuments in the Northwest (NW)Iberian Peninsula. In addition to the removal rate of the biofilm, the by-effects induced for fluences higher than the damage threshold of the stone were evaluated using stereomicroscopy, color spectrophotometry, and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results showed that different removal rates were obtained depending on the wavelength used and 532 nm obtained the highest removal level. In terms of by-effects, biotite melting was registered on all surfaces regardless of the wavelength. In addition, 532 nm seemed to be the most aggressive laser system, inducing the greatest change in appearance as a result of extracting the kaolinite crackled coating and the segregations rich in Fe, which are a result of natural weathering. These changes were translated into colorimetric changes visible to the human eye. The surfaces treated with 355 nm and 1064 nm showed lower surface changes. Keywords: laser; stone cleaning; Nd:YAG; granite; cleaning effectiveness; cultural heritage 1. Introduction Laser cleaning is a technique that is currently being fine-tuned in order to clean cultural heritage stones [1–3]. The advantages of this technology lie in its selectivity and graduality (precise removal of thin layers), not forgetting the fact that it does not come into contact with the surface to be cleaned and it is environmentally friendly [1,4]. Furthermore, the possibility of automation is an enticing option that is still under investigation. Since laser was first used in stone cleaning in the 1970s—to clean incrustations from Venetian marble [5]—there has been a considerable amount of scientific research focused on the Coatings 2020, 10, 196; doi:10.3390/coatings10030196 www.mdpi.com/journal/coatings Coatings 2020, 10, 196 2 of 17 optimization of lasers to maximize cleaning while minimizing damage caused to the forming minerals. A neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, a neodymium-doped yttrium orthovanadate (Nd:YVO4) laser, and an erbium-doped yttrium aluminium garnet (Er:YAG) laser have been used to extract graffiti, as well as sulphated and lichenic black crusts from different rocks with different texture and mineralogy, primarily limestone, marble, and granite [3,6–11]. In the Northwest (NW)Iberian Peninsula, cultural heritage is built with granite, which is a bioreceptive stone [12]. Considering the climate of this region (a humid climate with rainy winters), the stone’s durability would be compromised by the colonization of monuments and facades with organisms [12]. Sub-aerial biofilms (SAB), composed mainly of phototrophs (algae and cyanobacteria), are the initial colonization stage on granite. Consequently, the artistic and historical value of the monument will be negatively affected. Therefore, removal of this initial biocolonization from monuments should be carried out urgently in order to avoid greater damage from hyphae penetration and mineralogical transformations, or neoformations due to the acids generated by the organisms [13,14]. Different procedures have been implemented by professionals to extract biological patinas and crusts, mostly involving chemical methods and the mechanical scalpel [15,16]. Although scientific publications devoted to the optimization of biofilm removal from granite are scarcer than for carbonate stones, these traditional procedures have nonetheless exhibited some drawbacks in this area, making a case for the use of other less invasive techniques in granite cleaning. Pozo et al. [17] have evaluated chemical products (ethanol, benzalkonium chloride, hydrochloric acid solutions, and the commercial biocides Hyvar X® and LimpiaFachadas1®) used by professionals to extract a biofilm composed of filamentous green algae belonging to the genus Trebouxia and cyanobacteria belonging to the genera Gleocapsa and Choococcus from coarse-grained granite. Despite the satisfactory results achieved by the commercial biocides, salts were precipitated on the surface. Moreover, the same research evaluated a low-pressure (0.5–1.5 bar) air rotation system that used a mixture of air–water–silicate grains as an abrasive. Due to the formation of cracks and grain extraction, surface roughness was increased. Therefore, in order to avoid such issues that pose a risk to manually carved cultural heritage elements, laser treatment is increasingly being implemented. However, to the best of our knowledge, there is insufficient scientific information relating to the laser cleaning of SAB, with this alteration being the initial stage in the colonization of granitic monumental heritage. López et al. [18] found that by means of an Nd:YVO4 laser at 355 nm, a patina composed of filamentous green algae was successfully removed from a fine-grained granitic surface. In the cleaning of a biological colonization from marble, the third harmonic (355 nm) of the Nd:YAG system showed more advantages when compared with the fundamental wavelength (1064 nm); also, the yellowing effect observed when using the latter was avoided [19]. When treating a more complex crust composed of lichen, Sanz et al. [11] found that the optimal conditions for laser removal were highly dependent on the lichen species and, to a lesser extent, on the type of stone. They applied a Q-Switched (QS) Nd:YAG laser at 1064 nm (IR), 355 nm and 266 nm (UV), as a single wavelength and in dual sequential irradiation (IR+UV). What we learn from the research conducted so far is that every study case needs its optimization. Therefore, before to perform a real cleaning of biological colonization on a heritage stone, it is necessary to perform preliminary tests in small areas with the laser systems available, playing with the different laser parameters such as frequency, velocity, fluence, etc. According to the literature, the polymineralic composition of granite hinders the effectiveness of laser cleaning. Therefore, it is a challenge to perform a satisfactory level of cleaning without damaging some mineral grains. Biotite is the most critical granite-forming mineral [20,21]. Moreover, tests applying an Nd:YAG laser at the fundamental wavelength (1064 nm) and at the third harmonic (355 nm) on a uncoated pinkish granite have reported that the characteristic coloration became paler as a result of the physical elimination of the ZnFe2O4 particles from the feldspars due to thermal effects [22]. However, these color changes were not detected on grey granites and whitish limestones [23,24]. On carbonate stones, a yellowish effect was detected on marble when it was irradiated with the Coatings 2020, 10, 196 3 of 17 fundamental of a QS Nd:YAG laser in order to extract black crusts [7,25]. However, UV radiation using the third harmonic of a QS Nd:YAG laser induced a grey discoloration on these surfaces [7]. From a practical point of view, Nd:YAG lasers are used as portable equipment to perform cleaning in situ. Currently, these portable systems offer the possibility to experiment with different wavelengths (266 nm, 355 nm, 532 nm, and 1064 nm). Considering the findings from the references cited above, as well as the current incorporation of new wavelengths in portable laser equipment to clean cultural heritage objects in situ, it is vital to ascertain, in addition to the removal level, the effect produced by the different wavelengths on the stone when overcleaning takes place. This is because, during in situ cleaning, fluence variations could occur and lead to unintended damage in the forming minerals. Therefore, before any laser cleaning is performed, the extent of potential damage will be a decisive factor in the choice of wavelength. In this study, three wavelengths (355 nm, 532 nm, and 1064 nm) of a QS Nd:YAG laser system were used to extract a SAB from a pre-Hercynian granite commonly used in cultural heritage in the NW Iberian Peninsula. Particular attention was paid to the by-effects on the different granite-forming minerals in order to find different behaviours of the granite and each forming mineral under different wavelengths. The findings of this research will be used by conservator–restorers in order to select an appropriate wavelength to perform the cleaning of pre-Hercynian granites, characterized by their yellowish-brown coloration.