Optical Coherence Tomography (OCT) for the Conservation and Interpretation of Enamels
Vitreous materials are glass or glass-like substance. This project is focused on the study of medieval Limoges enamels and, with main focus on the investigation of their degradation.
The project studies the degradation of medieval enamels through application of OCT combined with other analytical techniques to understand the deterioration processes in vitreous materials in the British Museum's collection. Optical Coherence Tomography (OCT) will be the main technique employed, as well as supporting techniques such as: reflectance spectral imaging (both in VIS/NIR and SWIR regime) and X-ray radiography, as well as X-ray Fluorescence (XRF), Fourier Transform infrared (FTIR) and Raman spectroscopy.
Limoges enamel from the British Museum collection
Specific aims of the project include:
Analyse the degradation of vitreous objects, in particular Limoges Enamels, by combining non-invasive non-contact techniques to quantify degradation and suggest preventative measures to improve the conservation of the objects.
Develop depth resolved material identification using OCT
Using multimodal techniques, such as, OCT, spectral imaging and XRF to understand aspects that affect the glass deterioration in Limoges Enamels.
Addressing the Challenge
Investigation of the enamels structural composition - OCT examination
Enamelling developed and changed during the 14th, 15th and 16th centuries. During the 14th century the medieval champlevé technique was used, where channels were made in the metal layer. During the late 15th century, in Limoges, France the method for enamelling changed, this new technique meant the metal layer was treated with the enamel then painted over it. Degradation of these vitreous objects continues with conservators still trialling and testing different ways of preventative conservation. OCT gives information about the structural composition of the enamels, giving indications on how the objects are degrading.
Colour image of enamel panel 1855,0305.2 from the British Museum Limoges enamel collection (white box indicates the OCT scanned area),along with the 3D video that shows the OCT cube. The subsurface structure, including the lower enamel layers and underdrawings, as well as the surface decoration can be seen.
Degradation quantification- Hydration Mapping
Limoges enamels are composed from mostly layers of coloured glass and a metal base. Limoges enamels from the late 15th to early 16th century seem to be particularly susceptible to alteration of the glass structure. A major problem for glass is that it can be altered by the presence of water in the surrounding environment. Short-wave Infrared (SWIR) spectral imaging can non-invasively investigate glass deterioration and the alteration of the glass due to the presence of water can be mapped across each enamel. Hydration mapping is the result of mapping the level of hydration of each pixel over the entire enamel.
Colour image of 1895,1223.2 enamel from the British Museums Limoges enamel collection, along with the corresponding hydration map. The hydration map shows that across the enamel there are several areas with no hydration (deep blue), to moderate (green) and high (red) hydration.
A large survey was conducted of the Limoges enamel collection at the British Museum in Spring 2019 using OCT, Spectral Imaging and XRF.
Preliminary results were presented at the SPIE conference in Munich, Germany with an article published in the proceedings; https://spie.org/EOM/conferencedetails/optics-for-art-architecture-archaeology.
Margaret Read presented a poster about 3D imaging with OCT on Enamels at the biannual ICOM – CC Enamels 18 Conference held in Stuttgart, Germany; https://www.iiconservation.org/node/7056
Margaret Read presented a poster about 3D imaging with OCT on Enamels at the Global Heritage Research Theme Showcase Event held at Nottingham Trent University on the 18th May 2018.
A preliminary survey of a set of Limoges Enamels at the British Museum and Rangers' House (English Heritage) was conducted in May 2018 looking at the deterioration of the enamels with OCT 3D imaging complemented by XRF.
Read, M., Cheung, C.S., Ling, D., Korenberg, C., Meek, A., Kogou, S. and Liang, H., 2019. A Non-Invasive Investigation of Limoges Enamels using both Optical Coherence Tomography (OCT) and Spectral Imaging: A Pilot Study. In: H. Liang and R. Groves, eds., Optics for Arts, Architecture, and Archaeology Vii. SPIE Optical Metrology, Munich, Germany, 24-27 June 2019. SPIE Proceedings 11058, 1105803
Dr Fouzia Ouali (Nottingham Trent University)
Dr Capucine Korenberg (The British Museum)
Dr Andrew Meek (The British Museum)
Denise Ling (The British Museum)
Margaret Read (Nottingham Trent University)
AHRC Collaborative Doctoral Award
The British Museum