Efficacy of mechanical procedures for removal of a lichen and a gypsum black crust from granite

Abstract

This paper provides preliminary data on the use of three different mechanical procedures to remove the lichen Diploschistes scruposus and a gypsum black crust from an ornamental granite. The methods used were wet sandblasting (Hydrogommage®), dry sandblasting (IBIX®) and dry-ice blasting. In addition, lichens were also removed by scraping with a scalpel. The cleaning efficacy and undesirable effects to granite were evaluated by stereomicroscopy, colour spectrophotometry, Fourier-transform infrared spectroscopy and scanning electron microscopy. The cleaning efficacy of the mechanical methods depended on the deposit being removed: i) for lichen, dry sandblasting and dry ice blasting produced the best results, while for the black gypsum crust, although both dry and wet sandblasting removed the black particulate matter from the crust, gypsum crystals remained on the surface; ii) the dry sandblasting method induced extraction of granite grains with the subsequent appearance of fissures; iii) for the lichen, the wet sandblasting method was totally unsatisfactory and iv) for the gypsum black crust, dry ice blasting failed to remove the black particulate matter from the crust.