Abstract
The glass composition is a determining parameter that influences the glass chemical durability, particularly in atmospheric conditions (defined by the relative humidity, RH, < 100%). This is obvious in the field of the cultural heritage (CH), where some glass compositions qualified as unstable show advanced signs of degradation under atmosphere, while others seem, on the contrary, stable. This study investigates the differences between stable and unstable glass compositions regarding the phenomenology of the atmospheric glass alteration, by means of accelerated ageing of three glass replicas followed by the characterization of their alteration layers at different scales. Over the same ageing period and experimental conditions, the two glass compositions qualified as unstable develop thick hydrated layers and a thin top layer of carbonate precipitates. Their hydrated layers are depolymerized, and they remarkably retain alkalis and non-bridging oxygens in a dense network of hydrogen bonds, as demonstrated by 29Si and 1H MAS NMR. On the contrary, the stable glass composition shows a considerably thinner hydrated layer and, relatively, a higher amount of carbonates on the surface. In unstable glasses, the retention of a significant proportion of alkalis and NBOs, probably by maintaining a basic character to the hydrated layer, seems comparatively a destabilizing factor sustaining hydration by fast network hydrolysis.
Highlights
Oxide glasses have been produced since the Neolithic period, ~4000 BC for first glazes on decorated ceramics, ~2500 BC for the first beads totally made of glass
These research efforts based on the observation of ancient glasses and the artificial ageing of glass replicas have led to two essential results reaching a consensus in both the scientific and conservation communities[5]: (i) the macroscopic signs of the atmospheric glass alteration are always the same and very specific
The compositions referred to as “A” and “P” are a mixed-alkali (Na and K) lime silicate glass and a potassium lime silicate glass, respectively, both recognized as unstable compositions of the cultural heritage (CH)
Summary
Oxide glasses have been produced since the Neolithic period, ~4000 BC for first glazes on decorated ceramics, ~2500 BC for the first beads totally made of glass. Other glass art objects, dating back from the Egyptian, Celtic or Roman Antiquity for a few, and many from the end of the middle-age period up to now, have never been buried and could be conserved in the atmosphere in the collector’s homes and museums[1]. Depending on their composition and on the atmospheric conditions of their conservation, these glass objects show various degrees of alteration, from the unaltered state to the heavy deteriorated state where the object looses its mechanical integrity[1,2]. The fracturing of the hydrated glass layer is named “crizzling”, it consists of a network of cracks intersecting at right angles[6,7]
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