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Atmospheric CORRosion and mechAnical alteration of bronze BELLs: a sound and artistic heritage in danger ? – BellACorr

« Atmospheric CORRosion and mechAnical alteration of bronze BELLs: a sound and artistic heritage in danger? » - BellACorr

The bronze bell (22% wt Sn - a / d phases) is a musical and communication instrument, but also by its ornamental epigraphy, a work of art. It is a historical and cultural heritage to be preserved. From its implementation, the bell is subjected to a physicochemical alteration due to an aggressive environment, and mechanical to ring it. Proposing a micro-infiltrating mechanism of its alteration and quantifying the impact on its sound qualities is a major issue for the bell conservation.

Objectives

This project aims to understand and evaluate the consequences of the corrosion processes that, in its natural environment, the material undergoes, on the surface and inside the alloy itself. Under realistic atmospheric conditions, corresponding to urban and marine environments (dry/wet deposits in alteration chambers CIME/ CIME2 – alteration on site – ancient patina), this work will be conducted through a thorough study of the preferential corrosion of a/d phases and the infiltrating networks of environmental fluids at the micro/nano-metric scale. We will take into account the mechanical constraints, intrinsic to the bell and the resulting modification of the micro/nano infiltrating ways in the corrosion layer and in the alloy. For the first time, the effect of corrosion on the sound quality of the bronze bells will be investigated using samples altered in the laboratory in controlled conditions. <br />Thus, BellACorr will decline its objectives as follows:<br />• Study the preferential corrosion of a/d phases and its effect on the protection of the corrosion layer, taking into account impurities in the alloy.<br />• Determine the impact of the mechanical stresses of the material on the alteration ways and on the location of the reactive precipitation zones correlated with the characterization of the pores/cracks network in the alteration layer and in the alloy. Its effect on a/d preferential corrosion will be studied.<br />• Analyse the micro and nano-infiltrating corrosion and its effect on the modification of the mechanical properties of the material.<br />• Quantify the impact of corrosion on the acoustic signature of bronze.<br />• Propose a mechanism featuring the synergy of physico-chemical, mechanical and sound alterations of bronze bells.

The atmospheric alteration
•To study urban and marine corrosion, the old patinas will be compared to that developed in short term on site and in laboratory. This corpus will be used (1) for the isotopic tracing of the micro-infiltration networks, (2) to measure the impurities effect on the corrosion (3) to analyse the corrosion effect on mechanical properties of bronzes
•To localize the water circulation, the reaction sites, and the new products in the patinas, the corroded samples will be submitted to different impregnation/drying cycles with D2O18 and studied by nano-SIMS
•To evaluate preferential corrosion of the phases of the alloy, their galvanic coupling will be analysed out and the protection of the patina investigated

The mechanical alteration
To ring, the bell is submitted to cyclic mechanical stresses, leading to microcracks
•To measure the stresses impact on the pathways of physicochemical alteration, samples with microcracks will be produced. The microcracks will be analysed by acoustic means. Then, the samples will be corroded in laboratory and characterized by acoustic and TEM. They will undergo an isotopic alteration and characterization
•To study the evolution of microstructure and the mechanical properties due to the corrosion, the corroded samples will be analyzed. (microhardness, residual stresses, TEM)

The sound alteration
•The bell sound is a sum of modal contributions. The corrosion impact on the sound will be evaluated by estimating the damping and distortion of the modes in the audible frequencies. Non-corroded bronzes and bronzes corroded in laboratory will be compared. The corrosion measured by ultrasounds will determine the alteration threshold that generates audible noise disturbance. The corrosion state will be also qualified by acoustic emission
•The creation and propagation of microcracks will be detected by ultrasound.
•The correlation of the audible/ultrasonic informations should determine the alterations effect on the sound

The products developed by BellACorr will establish (1) according to the physicochemical and mechanical properties of the superficial corrosion layer and the underlying alloy, the preferential zones likely to accommodate the infiltrating paths of corrosion (2) the impact of the two-phase structure on the corrosion process taking into account the impurities of the alloy, (3) the corrosion threshold impacting the sound quality of the bells (4) the physicochemical and mechanical processes of alteration of the bells in urban and marine environments (dry / wet) correlated to the evolution of their sound quality.

This project will propose the demonstration of an infiltrating mechanism of the bells bronzes, taking into account the mechanical constraints undergone by the bell throughout its life, but also of its intrinsic particular a/d structure. For the first time, it will analyze and quantify the impact of the aging of the bell bronze on its sound qualities.
Thus, BellACorr will establish a data and knowledge base of the bronze bells corrosion, essential for proposing innovative, non-invasive and ecological conservation and restoration technologies (without biocide or destructive mechanical action) that will be based on preferential corrosion of the a/d phases.
By measuring the threshold of physicochemical alteration that contributes to the sound perturbation of the bells, BellACorr will enrich and diversify a sound database that the French Society of Campanology (SFC) is putting in place through an audio inventory of the bells heritage.
Moreover, scientific research at national and European level is increasingly interested in the problem of bell-tuning. In practice, the professionals regulate the frequency partials of a bell by locally removing the alloy on its internal profile. This method, applied to new bells, is not applicable to the ancient dissonant bells of the World Heritage list. Indeed, this method degrades the integrity of the work of art. To remedy this, some scientists consider innovative non-destructive methods to find the past sound of a historic bell. These old heritage instruments are heavily corroded. This project, which aims to quantify the corrosion effect on sound qualities, will provide physicochemical knowledge on the bells alteration, which are essential to participate in research into virtual non-destructive tuning practices to find again the initial sound of ancient bells.

The results of BellACorr project will be disseminated and shared with the scientific community by the means of publications issued in international peer-reviewed journals (Corrosion Science, Applied surface science…) and oral communications (Metal…). This research will contribute to the formation of the future generation and will participate in promoting the cultural heritage of bells.

To disseminate the results of the project to the largest possible audience, an international restitution symposium will be organized at the end of the project for foundry professionals, campanists, musicians and scientists.

Bronze bells (22%wt Sn- a/d phases) is part of the intangible and material world heritage. Among the most famous bell bronzes one can mention Big Ben (1858), but also the replacement in 2013 of 8 bells of “Notre Dame de Paris” which required the casting of 23 tons of bronze for 2 million euros of turnover. This fact of industrial and economic importance underlines the universality of an ancestral and modern heritage constantly enhanced by bells professionals.
The bells are omnipresent on the territory. Indeed, the French Society of Campanology (FSC) estimates that more than 450 000 are still active. The ornamentations borne by the bells make of them unique works of art. Their musicality rhythms the time and the civilizations. However, from the moment of their implementation, the bells undergo a very complex alteration, both physicochemical environment-dependent, but also mechanical by percussions. This dual process has never been studied in detail but inevitably leads to a degradation of the instruments. Therefore, through these material alterations this is not only the artistic heritage but also the intangible sound heritage that is endangered. The bell itself is an archive and a sound memory of the past that must be protected.
By evoking the need to take into account cultural heritage in climate adaptation policies, the Intergovernmental Experts on Climate Change (IECC) has echoed this concern for the first time in its 5th Assessment Report (2013-2014).

The BellACorr project aims to understand the physicochemical and mechanical processes responsible for the micro-infiltrating corrosion of two-phase a/d bells bronze and to evaluate their consequences for the sound quality of the instrument.
BellACorr will conduct in the laboratory a study of the corrosion of the bells bronze taking into account the mechanical constraints of the alloy. These simulations will be carried out under a series of ‘realistic’ atmospheric conditions representative of the variety of the urban and marine environments (dry/wet deposits). In order to describe the physicochemical and mechanical processes of alteration, the corrosion will be analyzed at different stages. Taking into account the impurities of the alloy, the impact of the two-phase structure on the corrosion process will be quantified for the first time through a study of the preferential corrosion a/d. Isotopic tracing (D, 18O, 34S, 37Cl) of the alteration of old samples and of those reproduced in the laboratory will make it possible to trace the micro-infiltrant networks of environmental fluids and to locate and quantify the reactive zones of the material. In doing so, the influence of the mechanical fatigue and internal morphology of the corrosion layer will be taken into account. In addition, a fine characterization of bronzes (multi-scale identification of alteration and micro-infiltration networks) will provide a dynamic predictive formulation of the processes. BellACorr will quantify the effect of the alteration on the sound properties of the bells by investigating the sound signature (modal and acoustic emission analysis) on bells altered in the laboratory.

This interdisciplinary project gathers scientists from different communities (cultural heritage, environmental and material sciences, acoustic, mechanical science, physical chemistry…) to propose an innovative methodology aiming at a better quantification of the effects of alteration on the physical properties and sound quality of the bells.

BellACorr opens on innovative, non-destructive technologies for the safeguarding of the bells heritage. These technologies will be respectful of the environment and contribute to the creation of a bank of ancient bell sounds (archeo-sounds) advocated today by many organizations. The results of BellACorr will contribute to the research on the non-destructive tuning of ancient bells. Finally, this JCJC project will form the basis for setting up a wider European (JPI) project.

Project coordinator

Madame ALINE PETITMANGIN (Laboratoire inter-universitaire des systèmes atmosphèriques)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LISA Laboratoire inter-universitaire des systèmes atmosphèriques

Help of the ANR 338,852 euros
Beginning and duration of the scientific project: December 2018 - 42 Months

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