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Among metal alloys, bronze has been used for millennia to produce many kind of objects, from decorative and religious pieces to sculptures. Although many bronze objects have been well preserved to this day, copper and bronze can be affected by severe degradation involving the formation of copper chloride products, known as “bronze disease” [1]. These chloride products are considered responsible for the rapid decay of bronze, which can ultimately cause the loss of unique and historical artworks. However, copper chlorides constitute only a small fraction of the vast and complex class of corrosion products which can form on historical bronze and copper. It is essential to be able to differentiate between stable from more dangerous unstable corrosion products.
A major challenge of bronze conservation is to non-invasively identify the unstable chloride corrosion throughout the whole artefact. This information is critical for decision-making in conservation process. Different analytical approaches are available to identify these corrosion products. However, most conventional methods require sampling which is not only destructive, but also usually unrepresentative of the object as a whole.
Spectral imaging constitutes a class of non-invasive techniques which have been largely used for the identification and mapping of pigments in paintings [2]. However, it is more challenging to apply spectral imaging to 3D objects.
We have developed a hyperspectral imaging set-up in the visible and near infrared spectral range for 3D objects to rapidly identify and map the corrosion products non-invasively [3]. A preliminary application has been done to study ancient bronze artworks from the National Museum of China. These results are further supported by analysis carried out on selected samples of the same collection with Fibre Optic Reflectance Spectroscopy, Attenuated Total Reflection (ATR)-FTIR spectroscopy, X-rays diffraction, Raman spectroscopy and SEM-EDX.
The possibility of successfully performing hyperspectral imaging for the study of corrosion products would represent a completely novel approach in the analysis of ancient bronze and copper objects.
[1] Scott, A. D. A Review of Copper Chlorides and Related Salts in Bronze Corrosion and as Painting Pigments. Studies in Conservation, 45: 39-53 (2000)
[2] Liang, H. Advances in multispectral and hyperspectral imaging for archaeology and art conservation. Applied Physics A, 106 (2), 309-323 (2012)
[3] Liang, H. et al. Remote hyperspectral imaging with simultaneous 3D texture mapping. Optics for Arts, Architecture, and Archaeology, 10331-25 (2017)
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