Laser-assisted removal of dark cement crusts from mineral gypsum (selenite) architectural elements of peripheral monuments at Knossos

Abstract The aim of this study was the efficient removal of dark cement crusts from selenite (mineral gypsum) surfaces using lasers. The conservation challenge is to remove a hard, insoluble, and often significantly thick encrustation without affecting the particularly sensitive mineral substrate gypsum, which is susceptible to chemical transformation. Preliminary studies were focused on the determination of removal thresholds for the cement bulk and the establishment of efficient laser cleaning operative parameters. Concurrently, analytical investigations were focused on the irradiated surfaces in order to ensure that the recommended laser operating parameters would safeguard the surface, colour, and composition of gypsum. Analytical techniques (Raman spectroscopy, X-ray powder diffraction, scanning electron microscopy with energy-dispersive X-ray analysis) were used to verify that no discolouration or chemical alteration occurred to the fragile gypsum substrate due to the dehydration of pristine gypsum (to hemihydrate and anhydrous calcium sulphate). The results of this study as well as its application are presented and discussed with emphasis on the surface integrity of the exposed fragile selenite substrates.