Protective silica thin coatings for historical glasses

During the second half of 20th century the amount of SO2 in the atmosphere has gradually increased, due to the emission of pollutants from different sources. Atmospheric control systems installed in the last years have allowed the diminishing of the SO2 content. The combination of this pollutant with humidity and other damaging gases, is causing continuous degradation of various types of materials and affect negatively also the surface of historical glasses. The intensity of deterioration depends on the type of glass and its chemical composition. With the aim to preserve historical glasses against future weathering, several reference glasses have been designed and prepared. The chemical compositions of reference glasses correspond to those of medieval and conventional modern glass. Two types of silica coatings have been deposited on the reference glasses: sol-gel glassy coatings and thin films obtained by sputtering. Coated glasses were tested under several simulated ageing conditions, using different concentrations of SO2. Sol-gel glassy silica coatings protected well the glass reference substrates, especially if the coating densification temperature is higher than 250 degreesC. Sputtering silica films protected even better the glass reference substrates and do not require thermal densification. (C) 2003 Elsevier B.V. All rights reserved.