Cleaning of gypsum-rich black crusts on granite using a dual wavelength Q-Switched Nd:YAG laser

Abstract

Gypsum black crust is one of the most severe deterioration forms affecting the built Cultural Heritage (CH) worldwide. Its removal from granitic substrates is necessary since gypsum layer favours the deterioration of the rock. This study investigates the laser assisted removal of gypsum black crust, from a Hercynian granite from the North-West (NW) Iberian Peninsula. The suggested laser cleaning methodology involves the combination of two wavelengths (1064 nm and 355 nm) from a QS Nd:YAG laser system and is based on their simultaneous interaction with the involved materials at various fluence values and FIR/FUV ratios (1/1, 1/3 and 3/1). This ablative scheme was initially introduced in order to overcome undesirable discoloration issues recorded upon single wavelength laser cleaning of black crusts but its role to effectively remove different undesired over-layers was also acknowledged.The results of this study were comparatively evaluated with surfaces irradiated using one of the two specific wavelengths, while the presence of water as an enhancing cleaning agent was also considered. A multi-analytical evaluation methodology based on stereomicroscopy, Raman spectroscopy, Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), was carried out to determine the optimum cleaning strategy in terms of crust removal effectiveness and minimisation of damages to the substrate.The optimum irradiation condition determined in this study is the simultaneous application of the two wavelengths in FIR/FUV = 1/1 ratio (with FIR = FUV = 0.3 J·cm−2) on a pre-wetted surface. Under this irradiation condition a high removal level of carbonaceous pollution particles and gypsum was achieved, without any damage to the granitic minerals in the form of melting and spallation and/or exfoliation of planes of biotite and K-feldspar and cleavage fracture of quartz.