Historical Glass Surfaces: an Inorganic Chemical Approach to Arrest Decay

Unweathered inorganic glasses are smooth dense substances. Their surfaces are covered with layers of adsorbed water so that few materials will adhere satisfactorily to them. Early painted glass, expecially of the XII and XIII centuries in central Europe, has the high potash, high lime composition which deteriorates with the formation of opaque crusts. As a consequence of this, most of medieval cathedrals with painted glass windows are rather dark inside. To prevent this effect a chemical approach comes from the inorganic basin through an inorganic polymeric thin layer with properties of clarity, transparency, no discoloration with aging and an improved ability to adhere to glass and to polymerize in situ at room temperature with little or no shrinkage occurring. Its name is vitreous silica and it can be obtained through the sol-gel technique (polymerization of silicon alkoxide monomers). We studied the chemical and morphological modifications induced on silica samples by the immersions in basic Ca(OH)2 solutions at different temperatures (room temperature, 60°C and 80°C) and for different time intervals (1 hour and 21 hours). Treatments with Ca(OH)2 solutions were carried out on two different kinds of pure silica: commercial fused-silica slides and silica films obtained via sol-gel process. The experimental results show that calcium hydroxide can be chemisorbed by vitreous silica structure and that it can diffuse through the network forming a surface layer of calcium silicate, that is, a layer probably containing many Si-O-Ca-O-Si bridges. Furthermore it has been clearly noted a different behaviour of sol-gel films with respect to fused-silica slides after immersion, probably because of their different tridimensional structures: sol-gel coatings present a very low-density structure with holes and voids through which calcium ions can migrate and diffuse to the inner atomic layers. On the other hand, fused silica is able to keep a very little amount of calcium ions segregated to the few outermost atomic layers for its greater density; on more the SIMS analysis shows a leaching of the fused silica substrate.