Claudine Loisel

Early stage of weathering of medieval-like potash–lime model glass: evaluation of key factors

PurposeThroughout history, a consequent part of the medieval stained glass windows have been lost, mostly because of deliberate or accidental mechanic destruction during war or revolution, but, in some cases, did not withstand the test of time simply because of their low durability. Indeed, the glasses that remain nowadays are for many in a poor state of conservation and are heavily deteriorated. Under general exposure conditions, stained glass windows undergo different kinds of weathering processes that modify their optical properties, chemistry, and structure: congruent dissolution, leaching, and particle deposition (the combination of those two leading together to the formation of neocrystallisations and eventually crusts). Previous research has studied the weathering forms and the mechanisms from which they are originated, some others identified the main environmental parameters responsible for the deterioration and highlighted that both intrinsic (glass composition) and extrinsic (environmental parameters) factors influence glass degradation. Nevertheless, a clear quantification of the impact of the different deterioration extrinsic factors has not been performed.MethodsBy analysing the results obtained with model glass (durable and nondurable) exposed in the field, this paper proposes a simple mathematical computation evaluating the contribution of the different weathering factors for the early stages of exposure of the stained glasses.ResultsIn the case of non durable glass, water runoff was identified as the main factor inducing the leaching (83.4 ± 2.6% contribution), followed by gas (6.4 ± 1.5%) and particle deposition (6.8 ± 2.2%) and adsorbed water (3.4 ± 0.6%). Moreover, it was shown that the extrinsic stimuli superimposes with the impact of glass composition to the weathering.ConclusionsThose results show that the role played by dry deposition, even if less important than that of the wet deposition, cannot be neglected.

Browning phenomenon of medieval stained glass windows.

In this work, three pieces of historical on-site glass windows dated from the 13th to 16th century and one archeological sample (8th century) showing Mn-rich brown spots at their surface or subsurface have been characterized by optical microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy. The oxidation state of Mn as well as the Mn environment in the alteration phase have been characterized by X-ray absorption spectroscopy at the Mn K-edge. Results show that the oxidation state of Mn and therefore the nature of the alteration phase varies according to the sample considered and is correlated with the extent of the brown alteration. The larger the brown areas the more oxidized the Mn. However, by contrast with literature, the samples presenting the more extended brown areas are not similar to pyrolusite and contain Mn mainly under a (+III) oxidation state.

Assessment of Transition Element Speciation in Glasses Using a Portable Transmission Ultraviolet–Visible–Near-Infrared (UV-Vis-NIR) Spectrometer

A new low-cost experimental setup based on two compact dispersive optical spectrometers has been developed to measure optical absorption transmission spectra over the 350–2500 nm energy range. We demonstrate how near-infrared (NIR) data are essential to identify the coloring species in addition to ultraviolet visible data. After calibration with reference glasses, the use of an original sample stage that maintains the window panel in the vertical position enables the comparison of ancient and modern glasses embedded in a panel from the Sainte-Chapelle of Paris, without any sampling. The spectral resolution enables to observe fine resonances arising in the absorption bands of Cr3+, and the complementary information obtained in the NIR enables to determine the contribution of Fe2+, a key indicator of glassmaking conditions.

Ubiquitous presence of laminae in altered layers of glass artefacts

Whatever the chemical composition and the origin (natural or man-made) or the surrounding environment is, glass materials undergo alteration processes leading to the modification of their structure and chemical composition. Similar alteration patterns can be observed in different historical glass types, especially alteration layers characterized by a laminated structure. The study of medieval stained glass windows (14th century AD, from Northern France) and Roman glass blocks (2nd century AD, from a shipwreck in the Mediterranean Sea) with several centuries of exposure in atmospheric and marine conditions, respectively, show that laminated features, commonly described at micro-scale (e.g. lamination), can also be found at the nano-scale (laminae) using TEM analysis on FIB ultra-thin section. These features develop on different alteration layers - in the gel layer for medieval glass and in crystalline secondary phases (smectites) for Roman glass - showing that the formation mechanisms vary according to the exposure environment and the chemical composition of the glass.

Long-term weathering rate of stained-glass windows using H and O isotopes

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. H2O 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 (D218O) 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 m2 s-1 at 70% RH and 4.9 × 10-20 m2 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.Glass alteration: Understanding unsaturated conditionsIsotopic studies of the glass from stained-glass windows have determined the effects of water vapor on their weathering. Understanding how glasses react to their environment over time is important in contexts such as nuclear-waste storage and artifact conservation. They are known to form an amorphous ‘gel’ layer on their surface and many studies aimed at improving its understanding are carried out in aqueous solution, however, stained-glass windows are exposed to alternating saturated and unsaturated conditions, characterized by liquid water and water vapor respectively. Now a team of scientists based in France, led by Loryelle Sessegolo of the Université Paris-Est Créteil and Université Paris Diderot, have studied how the gel layer present on the stained-glass windows affects its on-going alteration in unsaturated conditions; they find that the layer is not protective against vapor transport or interdiffusion.

Nondestructive Redox Quantification Reveals Glassmaking of Rare French Gothic Stained Glasses

The sophisticated colors of medieval glasses arise from their transition metal (TM) impurities and capture information about ancient glassmaking techniques. Beyond the glass chemical composition, the TM redox is also a key factor in the glass color, but its quantification without any sampling is a challenge. We report a combination of nondestructive and noninvasive quantitative analyses of the chemical composition by particle-induced X-ray emission–particle-induced γ-ray emission mappings and of the color and TM element speciation by optical absorption spectroscopy performed on a red-blue-purple striped glass from the stained glass windows of the Sainte-Chapelle in Paris, France, during its restoration. These particular glass pieces must have been produced as a single shot, which guarantees that the chemical variations reflect the recipe in use in a specific medieval workshop. The quantitative elemental mappings demonstrate that the colored glass parts are derived from the same base glass, to which TMs were deliberately added. Optical absorption spectra reveal the origin of the colors: blue from CoII, red from copper nanoparticles, and purple from MnIII. Furthermore, the derivation of the quantitative redox state of each TM in each color shows that the contents of Fe, Cu, and Mn were adjusted to ensure a reducing glass matrix in the red stripe or a metastable overoxidized glass in the purple stripe. We infer that the agility of the medieval glassmaker allowed him to master the redox kinetics in the glass by rapid shaping and cooling to obtain a snapshot of the thermodynamically unstable glass colors.