Influence of wavelength on the laser removal of lichens colonizing heritage stone

Abstract

Laser irradiation of lichen thalli on heritage stones serves for the control of epilithic and endolithic biological colonizations. In this work we investigate rock samples from two quarries traditionally used as source of monumental stone, sandstone from Valonsadero (Soria, Spain) and granite from Alpedrete (Madrid, Spain), in order to find conditions for efficient laser removal of lichen thalli that ensure preservation of the lithic substrate. The samples presented superficial areas colonized by different types of crustose lichens, i.e. Candelariella vitellina, Aspicilia viridescens, Rhizocarpon disporum and Protoparmeliopsis muralis in Valonsadero samples and P. cf. bolcana and A. cf. contorta in Alpedrete samples. A comparative laser cleaning study was carried out on the mentioned samples with ns Q-switched Nd:YAG laser pulses of 1064 nm (fundamental radiation), 355 nm (3rd harmonic) and 266 nm (4th harmonic) and sequences of IR-UV pulses. A number of techniques such as UV-Vis absorption spectroscopy, stereomicroscopy, scanning electron microscopy (SEM), SEM with backscattered electron imaging (BSE), electron dispersive spectroscopy (EDS) and FT-Raman spectroscopy were employed to determine the best laser irradiation conditions and to detect possible structural, morphological and chemical changes on the irradiated surfaces. The results show that the laser treatment does not lead to the complete removal of the studied lichen thalli, although clearly induces substantial damage, in the form of loss of the lichen upper cortex and damage to the algal layer. In the medium term these alterations could result in the destruction of the lichen thalli, thus providing a degree of

A review of the nature, role and control of lithobionts on stone cultural heritage: weighing-up and managing biodeterioration and bioprotection

“…Removal by (pressurized) water and brushing may be effective in the case of superficial, subaerial biofilms (Sanmartín et al 2020), but in other situations they may spread microbial structures, pushing them deeper within the substrate (Pinna 2017). Valuable protocols for the physical removal of lithobionts by laser ablation have been drawn up in the last decade (Sanz et al 2017), but even the combination of scalpel and laser may sometimes fail to eliminate lithobiontic structures within fissures (Rivas et al 2018). Similarly, some laser ablation can cause damage to the mineral surface whilst not completely removing the lichens (Pozo-Antonio et al 2019).…”

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

“…As the damage threshold for this kind of granite is set in the 0.7-1.5 J•cm −2 range for an Nd:YAG laser [20,29], and because the laser effect depends on the patina or coating [11], fluences of 2 J•cm −2 and 5 J•cm −2 were used in order to exert damage on the stone. In previous studies, the damage threshold for the same granite as that used in this research was 1.1 J•cm −2 for 1064 nm and 0.7 J•cm −2 for 355 nm [29,30].…”

“…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 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:YVO 4 ) 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][7][8][9][10][11].…”

“…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).…”

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“…Recientemente, se ha abordado, en colaboración con LANAPAC y PAP-IGEO, un estudio comparativo del efecto del láser sobre diferentes especies liquénicas, identificadas por el Dr. S. Pérez-Ortega (RJB-CSIC; en adelante SPO) en piedras de las canteras de Alpedrete (Madrid) y Valonsadero (Soria), ambas usadas tradicionalmente como fuente de piedra monumental. El uso de un láser Q-Switched Nd:YAG de ns a 1064, 355 y 266 nm y secuencias de pulsos IR-UV indujo daños substanciales en talos de las especies analizadas (Sanz et al 2017).…”