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The understanding of glass alteration is a biogeochemical, industrial, societal (radioactive waste confinement), and cultural heritage issue. Studies have been mainly performed in aqueous conditions. However, glass reactivity under hydraulically unsaturated conditions may be more important than previously recognized. In this context, we evaluate here the role of the alteration layer formed on medieval stained-glass windows on the ongoing alteration in unsaturated conditions. H 2 O adsorption isotherms were measured to study the relation between the vapor sorption and the relative humidity inside the alteration layer. From it, the average pore radius was calculated, yielding a water vapor diffusion coefficient of 7.8 × 10 -7 m² s -1 inside the pore network. Experiments using doped water vapor (D 2 18 O) confirm the vapor transport up to the alteration front via fractures and pore network. They also demonstrate that the alteration mainly progresses via an interdiffusion mechanism. The calculated interdiffusion coefficients at 20°C are 3.6 × 10 -20 m 2 s -1 at 70% RH and 4.9 × 10 -20 m 2 s -1 at 90% RH, which is similar to the values measured on model stained-glass samples altered in short durations (1-4 years). Therefore, this study highlights that, given its morphology, the alteration layer is not protective against vapor transport and interdiffusion.

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The alteration of a four-oxide alumino-borosilicate glass close to International Simple Glass (ISG) was studied in Si saturation conditions at pH 3, pH 7, and pH 9 using solution and post mortem solid analyses. In particular, the study examined the impact of structural disorder on the glass alteration generated by swift heavy ion irradiation. It is shown that the pre-irradiated glass altered three to four times more than the non-irradiated glass, depending on the pH. This increased alteration can be attributed to two factors: an increased hydrolysis speed of the Si–O–B bonds, which control the glass dissolution under these Si saturation conditions, and a less passivating alteration layer. The latter effect suggests that the glass structural disorder is retained by the first gel formed, which means very low Si atom mobility during alteration. However, gel maturation tends to mitigate the detrimental effect of radiation damages. These results will contribute to the development of a predictive residual rate model.

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