Influence of the Laser Wavelength on Harmful Effects on Granite Due to Biofilm Removal

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.

Effect of Inorganic Carbon Concentration on the Development of Subaerial Phototrophic Biofilms on Granite

“…These results may; therefore, involve negative impacts regarding the possible effects of an increase in atmospheric CO 2 on the stone colonization and biodeterioration processes. An increase in growth of cyanobacterial biofilms and in pigment concentrations may exacerbate the aesthetic impact of these biofilms on stone-made cultural heritage and; therefore, lead to treatments being necessary (e.g., biocide coating [42], laser cleaning [43] or application of natural oils [44]) and/or loss of value. An essential strategy to prevent colonization and subsequent biodeterioration is the selection of appropriate materials for building, ornamental purposes or replacement.…”

The Control of Cultural Heritage Microbial Deterioration

“…In the last decade, this technique has been amply studied on outdoor stone surfaces due to its advantages over traditional cleaning methods: it is controllable, selective, contactless, and environmentally friendly [ 67 , 68 ]. The main drawback is that every study case needs to be optimized.…”

“…Barreiro et al [ 68 ] investigated in vitro the effects of three different wavelengths (355, 532, and 1064 nm) of an Nd:YAG laser applied in removing a naturally developed sub-aerial biofilm from Vilachán granite, which is commonly used in monuments in the Northwest (NW) Iberian Peninsula. The results showed that a wavelength of 532 nm successfully removed the biofilm, but it induced the highest color modifications in the granite due to extraction of the kaolinite crackled layer and the Fe-rich segregations.…”

Towards safe and effective femtosecond laser cleaning for the preservation of historic monuments

“…This new technique seemed very promising for conservation treatments at a time where heritage objects needed cleaning more frequently [2]. Since Asmus' works, many studies on laser cleaning have been conducted and explored the differences induced by changing the laser parameters such as the emitting wavelength, the laser energy, and the pulse duration on different materials and various contaminants including biological colonization (lichens, algae) [3][4][5][6][7][8][9][10][11][12][13][14], black sulphated gypsum crusts [15][16][17][18][19][20][21][22][23][24], dirt and environmental soiling [25], corrosion products [26][27][28][29][30][31][32][33][34], or graffiti [35][36][37][38][39][40][41][42][43][44][45][46][47]…”