Role of Weathering Layers on the Alteration Kinetics of Medieval Stained Glass in an Atmospheric Medium
- PDF / 2,979,370 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 112 Downloads / 211 Views
Role of Weathering Layers on the Alteration Kinetics of Medieval Stained Glass in an Atmospheric Medium Aurélie Verney-Carron1, Anne Michelin1, Lucile Gentaz1, Tiziana Lombardo1, Anne Chabas1, Mandana Saheb1, Patrick Ausset1, Claudine Loisel2 1 2
LISA UMR7583 CNRS/UPEC/UPD, 61 avenue du Général de Gaulle, 94010 Créteil, France. LRMH, USR 3224 CNRS, 29 rue de Paris, 77420 Champs-sur-Marne, France.
ABSTRACT In order to model and predict the alteration of medieval potash-containing stained glass, it is necessary to understand the mechanisms of alteration layer formation at the glass surface and its role on the evolution of alteration kinetics. Moreover, the alteration layers observed on stained glasses are particular, as they are often fractured and heterogeneous in terms of thickness, with the appearance of pits and the detachment of scales. Contrary to silicate glasses altered in aqueous environment where the gel layer has a protective role, cracks and scales are harmful to the durability of stained glasses altered in air. In order to address these mechanistic issues, a program of experiments in the laboratory and in the field were performed. The fracturing was shown to be caused by the growth of the alteration layers and amplified by the alternation of humid and dry periods changing the density of hydrated layers. The pitting is initiated by defects at the glass surface and increased in external atmospheric medium as these defects fix the precipitated salts. However, despite fracturing and pitting, the development of an altered layer imposes a diffusive transport of the solution between the external medium and the bulk glass.
INTRODUCTION Alteration of silicate glasses has geochemical consequences as the release of elements from natural glass contributes to the composition of rivers and oceans [1]. High-level activity nuclear wastes are also vitrified, and the assessment of their durability is essential to constrain the radionucleides release in the geological disposal [2]. Likewise, archaeological glass objects or stained glass windows are subject to alteration whether it is in buried soils, in seawater (shipwrecks), in museums or in contact with the atmosphere and they are required to be preserved as cultural heritage. That is why numerous studies deal with the understanding of the glass alteration mechanisms, the measurements of the kinetics of these processes and the development of predictive models of alteration. Mechanisms of glass alteration are common to all silicate glasses in contact with aqueous solutions. Three mechanisms can be distinguished: (1) interdiffusion or ion exchange between glass alkali elements and hydrogenated species in solution (H3O+, H2O) that leads to a selective leaching and to an increase of pH of the solution [3-5], (2) dissolution of the glassy network by hydrolysis of iono-covalent bonds (Si-O-Si, Si-O-Al) [6], and (3) secondary phase precipitation. These alteration mechanisms lead to the formation of alteration
products. Interdiffusion and/or local reactions of hydrolysis/con
Data Loading...
