Long term assessment of atmospheric decay of stained glass windows T. Lombardo* 1 , C. Loisel 2 , L. Gentaz 1 , A. Chabas 1 , M. Verita 3 and I. Pallot- Frossard 2 Several studies indicate that the decay of medieval stained glass windows is related to both the glass composition and the characteristics of the environment. The kinetics of the decay processes has been always described through experiences performed in aqueous confined conditions, which are obviously not encountered in real condition. A research programme has been set up in order to assess the kinetics of atmospheric weathering. The long and short term weathering has been studied through respectively ancient glass fragments and glass analogues exposed in the field. This paper presents the preliminary results of the long term. A characterisation of the morphology and chemical modification induced by weathering has been performed. Results will be used in the next step to build up a model of long term atmospheric weathering kinetics. Keywords: Stained glass windows, Atmospheric environment, Weathering This paper is part of a special issue on corrosion of archaeological and heritage artefacts Introduction As any other kind of material glasses constituting stained glass windows undergo a more or less severe damage, which results from the interaction between the glass matrix (intrinsic factors) and the environment (extrinsic factors). Several researches have been performed to understand the weathering of silicate glasses in aqueous solutions, especially in confined conditions 1–14 demonstrating that glasses undergo two kinds of deterioration processes: leaching (selective dissolution) and corrosion (congruent dissolution). In the case of leaching, the cations (glass modifiers) present in the glass matrix are gradually extracted from the network and substituted by hydro- genated species (H z ,H 3 O z ,H 2 O) according to a diffusion process 1,2 and obeying to a square root kinetics. 1,3–5 Leaching leads to the formation of a hydrated layer, enriched in silanol groups (;Si–OH) and depleted in cations, at the upper most surface of the glass. 1,6 The corrosion process takes place in basic condition, leading to the destruction of the siloxane groups (Si–O– Si) forming the glass network 3–5,7 and to the precipitation of corrosion products (formation of a gel layer in which ion diffusion is quite active). The kinetics of corrosion in confined aqueous solution is linear. 5 While corrosion acts on all glasses no matter their composition, 8 leaching is strongly dependent on it. In general, pure silica glasses are considered inert; the progressive addition of cations results in durable (low proportion of cations) and low durable ones (higher proportion of cations). 7,9–11 These two processes are strongly influenced by the pH of the solution. 3–5,7,8,10,12,13 Glasses used in the medieval age to build up stained glass windows, were usually made of silica and high amount of alkali and alkaline-earth elements (mainly K, Ca, Mg, Na). The result is that most of these glasses, considered as low durable ones, underwent weathering processes because of their interaction with the atmo- sphere. Evidence of both leaching and corrosion are largely found in ancient medieval stained glass windows. 14–23 Studies have shown that a hydrated leached layer is formed and progressively a fissure network is formed into it, allowing the water to diffuse and the weathering to progress toward the bulk glass in a finger-like manner. 8,18 In general, microcracks are area favourable to the development of weathering. 8 Corrosion attack results in a more or less uniform pitting of the glass surface. Pits can also form inside the uppermost modified layer. Beside leaching and corrosion, new deterioration forms were observed on ancient stained glass windows; indeed gypsum crusts developed at their outdoor sur- face. 15–19 Crusts are formed by a complex interaction between the cations extracted from the glass, atmospheric gases (CO 2 , SO 2 ) 18,20–22 and particulate matter. 17 Finally, several researches have demonstrated that atmospheric pollutants, climate parameters (temperature, relative humidity and rain), are involved in all weathering processes affecting medieval glasses. 20–25 Although a lot of researches have been performed in the domain of the deterioration of stained glass window glasses, several issues need to be deeply investigated. In particular, the kinetics of the glass atmospheric weath- ering has to be determined, as up to now all the studies are based on the results obtained in aqueous solutions. 1 LISA, Universite ´s Paris 12 et Paris 7, CNRS, France 2 Laboratoire de Recherche des Monuments Historiques, LRMH, France 3 Laboratorio di Analisi dei Materiali Antichi, LAMA, Italy *Corresponding author, email tiziana.lombardo@lisa.univ-paris12.fr ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 15 December 2009; accepted 5 April 2010 420 Corrosion Engineering, Science and Technology 2010 VOL 45 NO 5 DOI 10.1179/147842210X12710800383800