Study of a surface treatment based on zinc salts to protect glasses from atmospheric alteration: Mechanisms and application to ancient glass objects in museum

Inspired by an industrial process to protect manufactured glass windows against atmospheric alteration during their storage and transport, the current research is focused on the potentiality that a surface deposit of a small amount of zinc salts may efficiently reduce the alteration kinetics of chemically unstable ancient glass (alkali (Na,K)–rich silicate). To this purpose, the protective action of zinc salts was investigated by the mean of ageing experiments (temperature and humidity control) on a relevant ancient glass composition replica. When the treatment was performed on pristine glass (about 1 μg/cm2 of deposited Zn2+ ions), treated glass plates developed a significantly thinner hydrated layer than the untreated one at 80°C or 40°C (85 RH%). Furthermore, the zinc salts treatment modifies the nature of salts on glass surface. Different Zn(II) species were identified on the glass surface prior to or after the ageing. The study of the Zn(II) speciation on glass surface by grazing incidence GI-XAS (Grazing Incidence-X-ray Absorption Spectroscopy) showed the insertion of Zn(II) into the silicate network with the ageing. Several hypotheses about the protective action of Zn(II) against glass atmospheric alteration are formulated. They highlight the predominant role of the chemisorbed zinc species in the protective action. Because these chemisorbed species are formed by thermal activation, a moderate heating step in the treatment protocol is necessary. From preliminary results, the glass surface state prior to the zinc treatment appears critical regarding its efficiency and should be investigated deeper in view of the application.