According to curators, 15-30% of glass objects displayed in Northern Europe museums show visible degradation signs more or less important even with good preservation conditions, depending on their chemical compositions. 

Depending on the glass composition (relative ratio in alkalis (Na2O, K2O) and alkaline earths (CaO, MgO), and content in SiO2 and Al2O3), the glass could be more or less reactive in contact with liquid or vapour water. It leads to glass chemical deterioration according to two reactions: an acid-base reaction (reaction between non-bridging oxygen in water) and hydrolysis (network depolymerisation). 

In Cultural Heritage, the concept of unstable glasses is used to describe glasses with severe degradation signs on the surface (crizzling, flaking, deposits…) in atmospheric conditions (Relative Humidity, RH < 100%). Until now, to slow down the atmospheric alteration of ancient glass, curators preserve their glass objects in a monitored environment (temperature, relative humidity, pollutants). External adherent coatings could be applied on the glass surface as a protective barrier against moisture. However, due to the lack of reversibility, curators are reluctant to apply this treatment. Occasionally, they may rinse their glass objects with water (slightly acid or with soap) to remove the surface salts.  

Previously, a zinc salt spraying treatment was explored on two unstable culture heritage model glasses (Na, K-rich silicates). In this paper, our study aims to investigate the changes produced on the glass surface by the rinsing step, and deepen our research on the zinc salt spraying treatment. Three compositions were prepared for this study: an alkali mixed composition representative of chemically degraded Limoges enamels made in 1480-1530 (Na, K-glass) and the same composition with K2O only (K-glass) or Na2O only (Na-glass). 

Two types of samples were prepared: powders and polished plates to be able to use different characterization techniques. These glasses are artificially aged at different temperatures (35°C or 80°C), fixed humidity (80%RH) and different duration (between a few days to a few months) in a climatic chamber before being characterized. The surface state is described using optical microscopy, SEM and Raman. The thickness of the hydrated layer and its profile concentration is evaluated by SEM-EDX and ToF-SIMS. The water content of the hydrated layer is also assessed by DTA-TGA and its structure by solid-state NMR of the altered powders.  

In this paper, we will focus on the comparison of the effects of a rinsing step, of the combination of rinsing and applying the Zn salt treatment, and of the Zn salts treatment only, at 35°C and 80°C on the three compositions. The zinc salt spraying treatment shows promising results for slowing down atmospheric alteration of pristine glasses, but the deposit was not efficient on pre-hydrated glass surfaces. However, the combination of rinsing and zinc salts seems to efficiently slow down atmospheric alteration of pre-hydrated glass surfaces. 

ALLOTEAU, Fanny, MAJÉRUS, Odile, VALBI, Valentina,?et al.?Efficacy of zinc salts to protect glass against atmospheric alteration. Part I and Part II Journal of the American Ceramic Society, 2021, vol. 104, no 5, p. 2039-2051.