Assessment of plasma torches as innovative tool for cleaning of historical stone materials

Cleaning of historical stone surfaces has always been a challenging task, moreover in the last decades arose new restorations issues such as the need to remove aged conservation polymeric materials to avoid further damage. Different cleaning methodologies flourished in the past, mostly based on chemical, mechanical methods and on laser technology too. Nevertheless, these methodologies could not be so efficient in the removal of epoxy resins, acrylic polymers and hydrophobic siloxanes, because of their low solubility in solvents when aged or their high adhesion with the substrate. More recently, atmospheric plasma has been tested for such application even if it is not yet widely applied due to the lack of knowledge about possible side-effects on the artefacts. In the present work, assessment of three commercial atmospheric plasma devices (plasma torches) illustrated the potentialities and drawbacks of polymers’ removal from stone surface. Commercial epoxy resins, acrylic polymers and hydrophobic siloxanes were chosen for the removal test by plasma devices. Physical and chemical effects on the stone surface and the process efficiency were investigated by means of macro- and microscopic observations, preferring, when possible, non-invasive techniques and consolidated methodologies in the field of Stone Conservation Science. An introductory experimentation on coated Si specimen has allowed to find the proper working parameters, i.e. working distance, exposure time, to have an effective removal. The experimentation conducted on different lithic substrate, coated with the commercial protective, has showed that commercial devices are effective in the removal of epoxy and acrylic coatings via chemical and physical interactions. On the contrary, the removal of siloxane products is incomplete, because of the high stability of the bond Si–O in the back bone, which is not affected by the plasma. In general, the present trials highlighted that DBD apparatus used does not promote any macroscopic effects on the polymeric coating, while arc discharge ones guarantee satisfactory results. According to these preliminary trials, it was clearly evidenced that plasma is a potential cleaning tool, despite DBD systems need higher power or arc discharge needs treatment temperature mitigation and to avoid the deposition of metallic drops on the surface of the object due to electrode deterioration.