Glass and Limestone Alteration: an innovative Methodology to study the mechanisms and kinetics – GLAM

To predict the alteration rates of stained glass and limestone in an evolving environment (climate and pollution), it is necessary to build up a geochemical model based on physic-chemical processes and kinetics parameters. To this purpose, we have set up dedicated laboratory experiments in realistic conditions of rainfall, condensation and unsaturated conditions. To evaluate the role of the alteration layer formation on the kinetics, materials at different alteration stages were analyzed: pristine, laboratory weathered, and middle and long-term weathered in a real environment. Their alteration in laboratory was combined with isotopic tracing of water transfer (D218O) and of secondary phase formation zones (18O and 29Si). This enabled to locate the reaction zones inside the materials in order to provide data on the alteration mechanisms and kinetics. Moreover, a fine characterization of the environmental conditions (weathering solution, temperature, relative humidity, composition of the atmosphere) and of the materials (multiscale identification of the alteration pattern and of the pore/crack network) provides a conceptual formalization of the mechanisms. All these data were used as inputs in a numerical geochemical alteration model. Different climatic scenarii types were tested to evaluate their impact on the durability of the materials. The robustness of the geochemical model will be tested by comparing the output data with the study of the alteration layer formed on materials from historical buildings.

The alteration mechanisms and kinetics of stained glass and limestone have been determined in different environmental conditions (water run-off, relative humidity, isotopically marked medium or not). Their implementation in a geochemical model to simulate the long-term alteration of stained glass and limestone has permitted the reconstruction of the alteration history of these materials. Moreover, the role of the alteration layer on the subsequent alteration has been assessed. These results are thus of interest for the restorers/conservators to help them determining their restoration treatments, but also researchers working on nuclear glass used for the storage of radioactive wastes in vapor phase.

The JCJC ANR GLAM project is a research project in fundamental sciences, coordinated by Mandana Saheb, research scientist at the LISA. The LRMH is also partner of the project. The project began in October 2014 and lasted 54 months. It received an ANR funding of 388 584 €.

Ten publications have been written on the results obtained during the GLAM project: 5 international articles and 5 proceedings. An article on the experimental chamber developed to simulate the alteration of the materials in controlled environments has been published and 4 articles deal with advances on the alteration mechanisms understanding of stained glass and limestone. Results on the isotopic tracing have been valorized through publications. The data obtained from the thermodynamic modelling have been presented in international congresses, and it is planned to write articles based on these results.

Materials are subject to alteration when exposed to an urban environment and are potentially sensitive to the evolution of environmental parameters (rapidly densifying urbanization and environmental changes). For this reason the sustainable building has lately become a hot topic and the preservation of ancient monuments, an environmental, economic, and cultural challenge.

To predict the alteration rates of materials from the built heritage in an evolving environment (climate and pollution), a geochemical model based on physic-chemical processes and kinetics parameters must be developed.

Thus, the GLAM project aims at modeling the alteration of two reference materials - stained glass and limestone - widely used in historical buildings exposed to an urban area. They have also been selected because they are different in terms of chemical composition and porosity so that the model could be extended to a large range of materials.

To this purpose, we propose to set up dedicated laboratory experiments in realistic conditions of rainfall, condensation and unsaturated conditions. To evaluate the role of the alteration layer formation on the kinetics, materials at different alteration stages will be analyzed: pristine, laboratory weathered, and middle and long-term weathered in a real environment. Their alteration in laboratory will be combined with isotopic tracing of water transfer (D218O) and of secondary phase formation zones (18O, 29Si, 13C and 34S). This will enable to locate and quantify the reaction zones inside the materials in order to provide data on the alteration mechanisms and kinetics. Moreover a fine characterization of the environmental conditions (weathering solution, temperature, relative humidity, composition of the atmosphere) and of the materials (multiscale identification of the alteration pattern and of the pore/crack network) will provide a conceptual formalization of the mechanisms. All these data will be used as inputs in a numerical geochemical alteration model. Two approaches for the models will be developed in parallel and compared: a chemistry-geochemical transport coupled model, and a pore network model to assess the validity of simplifying hypotheses concerning the transport and to reinforce the robustness of the geochemical model.

This interdisciplinary project gathers scientists from different communities (cultural heritage, environmental sciences, materials sciences, physics and geochemistry) in order to propose an innovative methodology aiming at understanding the alteration of built heritage materials and to develop alteration models to assess the impact of climate and pollution on their alteration.

Thus the results of the GLAM project will be of major importance in the community of the environmental sciences as well as the one of the conservation. First it will help developing more accurate strategies for the preservation of the built cultural heritage. Second it will provide efficient tools to assess the impact of pollution on built heritage that is now a criterion in the reducing and prevention pollution policies (e.g. UNECE Convention on Large-Range Transboundary Air Pollution).