Towards in-situ monitoring of tapestry degradation using strain-based engineering techniques

Lead Research Organisation: University of Southampton
Department Name: Winchester School of Art

Abstract

Tapestries are large and heavy and damage may be caused while they are on continuous display, not only because of their sensitivity to the environment but also through the stress imposed by their own weight. This load becomes more significant as the tapestry ages and its component materials degrade. The effects of deformation under a constant load are well understood within engineering and procedures exist for monitoring deformation. Applying engineering techniques to the monitoring of damage caused by hanging tapestries would enable areas of degradation to be identified before it is apparent that damage is occurring. Such monitoring would allow the optimum time for conservation treatment to be predicted and different methods of treatment to be quantitatively evaluated. It would help to ensure that funding available for conservation is used most effectively. The work would have wider applications for the treatment of other textiles. Although such techniques have recently been applied in the cultural heritage sector, this remains a small and pioneering field and these techniques have not been applied to tapestry conservation.

The interdisciplinary research team at the University of Southampton, consisting of researchers from the Textile Conservation Centre (TCC) and the School of Engineering Sciences, has completed a pilot study investigating a number of possible techniques for monitoring tapestries. The work examined contact and non-contact techniques, including optical fibre strain gauges, electronic speckle pattern shearing interferometry (ESPSI), thermography and photoelasticity.

The results of the pilot study indicated that optical fibres can be attached directly to a tapestry-like material and that the strain is transferred to the sensors. Optical fibres are the most suitable permanent monitoring device for monitoring the strain locally. The study also showed that 3-D photogrammetry using digital image correlation is the most suitable technique for whole-field monitoring, allowing the entire tapestry to be monitored quickly and without direct contact. The two techniques would be used in tandem, the fibre-optic sensors acting as a reference to validate the data from the photogrammetry technique.

The current proposal is aimed at developing these techniques into a hybrid system which can be used to monitor tapestries in-situ. A post-doctoral researcher will take responsibility for developing the application of the optical fibres, while a PhD student will develop the photogrammetry technique. The major challenge will be to apply the two techniques together so that strain readings from the optical fibres provide calibration for the photogrammetry. The final stage of the project will be to apply the methodology to both a custom-woven tapestry incorporating optical fibres in its structure and to a historic tapestry belonging to one of the project partners, Historic Royal Palaces, English Heritage or the National Trust. The successful result will be a fully operational monitoring system that can be installed unobtrusively in a historic house environment. The optical fibre component will be hidden from view. The photogrammetry equipment will be portable so that it can be introduced for periodic testing and then removed.

The three investigators, Frances Lennard, Dr Janice Barton and Dr Alan Chambers, will manage the project with Dinah Eastop, Senior Lecturer TCC, and Associate Director of the AHRC Research Centre for Textile Conservation and Textile Studies, the fourth member of the current project team. An advisory panel made up of representatives from the heritage field and other researchers will provide additional support.
 
Title Tapestry commissioned from West Dean Tapestry Studio 
Description The one metre-square tapestry was designed by two students from the University of Southampton's Winchester School of Art and woven at West Dean Tapestry Studio. The design was inspired by the colours of medieval tapestries and it was constructed using traditional materials and techniques. The design incorporated the formula for stress and the words diffuses and reflects, in reference to the optical techniques used for the monitoring. A silica optical fibre containing two FBG sensors was woven into the tapestry and two arrays of five FBGs each were bonded to the back. 
Type Of Art Artistic/Creative Exhibition 
Year Produced 2007 
Impact The new tapestry was displayed at Intech Science Centre near Winchester for 14 months from May 2009; monitoring was carried out in situ during the initial period. Information about the project was displayed next to it and it proved a popular exhibit; local newspapers referred to the 'technological tapestry'. The display was funded by a grant from the Institute of Physics Public Engagement Grant Scheme. 
URL http://tapestry-strain.org.uk
 
Description Tapestries are large and heavy and damage may be caused while they are on continuous display through the stress imposed by their own weight. This load becomes more significant as the tapestry ages and its component materials degrade. The effects of deformation under a constant load are well understood within engineering and procedures exist for monitoring deformation. This project aimed to enhance the long-term preservation of tapestries by developing well-established engineering techniques to monitor the mechanical behaviour of tapestries in terms of the strain developed within them, simply and unobtrusively, in situ. Strain, a measure of the percentage deformation of a material, is assumed to lead to damage in the long term.

The research questioned if the strain imposed on different areas of a tapestry can be quantified. The project objectives were to develop two techniques which an earlier pilot project had identified as the most promising for use on historic tapestries. These were firstly, a full-field, strain measurement technique based on a 3D photogrammetry technique, known as digital image correlation (DIC); and secondly, optical fibre sensors to obtain point measurements of strain. The final objective was to combine the two techniques into a hybrid system and use it to monitor tapestry samples in the laboratory, a newly woven tapestry and a historic tapestry in situ in a historic house.

The project developed the DIC technique for this new application, pioneering the use of 5-megapixel cameras to improve strain resolution. This aided the development of an algorithm to allow the camera set-up to be moved between pairs of images, enabling the equipment to be moved, and so to be used for periodic, in situ measurements. Although there were challenges in attaching optical fibre sensors to a tapestry, polymer optical fibre sensors appear to have long-term potential and showed a good strain transfer coefficient when bonded to the tapestry. The techniques were tested on tapestry-woven strips and on a new tapestry commissioned from West Dean Tapestry Studio. The final stage of monitoring took place on a historic tapestry in situ at Hardwick Hall, a National Trust property.

The research programme successfully achieved its objectives. It proved possible to quantify the global strain in at least an individual area of a tapestry. DIC generates colourful strain maps, clearly identifying areas of high and low strain. These can be superimposed on images of the tapestry, providing an extremely useful tool for conservators and custodians in demonstrating the changes experienced by tapestries. In addition, DIC has been shown to pick up areas of strain that are not visible to the naked eye, indicating that monitoring a tapestry in this way would identify areas at risk of imminent damage not apparent during visual assessment. Challenges were successfully overcome to be able to use the equipment in situ in a historic house. In addition the work clearly demonstrated the direct relationship between strain and humidity. Even small variations in RH can lead to considerable positive to negative strain cycling with strain increasing as RH rises.
Exploitation Route The research demonstrated that DIC is a successful, non-invasive method to monitor strain in tapestries. This technique can be applied to inform conservation decisions within textile conservation and the wider heritage sector:
• To gain a better understanding of the mechanisms causing damage to historic textiles. This information could be used to produce constitutive laws for tapestry behaviour and to provide a model to predict strain from RH data. This would contribute to our understanding of the optimum environmental parameters for displaying tapestries and other textiles.
• To quantitatively evaluate methods of treatment and display. The work has wider applications for the treatment of other textiles and other types of artefacts.
• One conclusion from the work is that optical fibre sensors are an easy-to-install strain measurement technique requiring very little equipment. It could be possible to use them routinely for continuous strain measurement of tapestries and other artefacts.
The work was also novel in engineering terms. The improved mapping function and Fibre Bragg Gratings inscribed in polymer optical fibres are novel technology and have implications for the development of textiles for engineering purposes
Funding is being sought by the PI (now at the University of Glasgow) to continue this research.
Sectors Culture, Heritage, Museums and Collections,Other

URL http://tapestry-strain.org
 
Description The project was actively supported by the conservation sector: the National Trust, English Heritage and Historic Royal Palaces. These heritage organisations own large collections of tapestries, requiring a large commitment of resources. Textiles, particularly tapestries, represent 65% of the objects needing remedial conservation within the National Trust's collection, for example. The project has enabled the monitoring of tapestries in such a way that results can inform conservation interventions. Allowing the optimum time for conservation treatment to be predicted helps to ensure that funding available for conservation is used most effectively. The research has laid the groundwork for a wider evaluation of tapestry conservation interventions: quantitatively evaluating the effects of conservation treatments using this technique is now a real possibility. In addition the research has highlighted the important relationship between strain and relative humidity. In practical terms the research appears to underline the importance of maintaining a steady RH for organic materials and shows that there may be issues of adding extra weight to textiles, in linings for example. As well as disseminating the research to professional and academic communities, the project was successful in disseminating information to a wider community through links with West Dean Tapestry Studio, Intech Interactive Science Centre and the National Trust at Hardwick Hall. The project website http://tapestry-strain.org.uk aims to disseminate information about the project to a wider audience. A one metre-square tapestry was designed by two students from the University of Southampton's Winchester School of Art and woven at West Dean Tapestry Studio. The design was inspired by the colours of medieval tapestries and was constructed using traditional materials and techniques. The tapestry was displayed at Intech Science Centre near Winchester for 14 months from May 2009; monitoring was carried out in situ during the initial period. Information about the project was displayed next to it and it proved a popular exhibit; local newspapers referred to the 'technological tapestry'. The display was funded by a grant from the Institute of Physics Public Engagement Grant Scheme. The tapestry monitoring project was also used as the basis for work with school children during National Science Week in 2009. Monitoring of a 16th century Flemish tapestry also took place in situ at Hardwick Hall, a National Trust property, for 8 weeks in the summer of 2010. Information about the project was disseminated to the public, highlighting the importance of caring for our cultural heritage, and demonstrating the wider benefits of the application of science to the arts and humanities.
First Year Of Impact 2009
Sector Culture, Heritage, Museums and Collections
Impact Types Cultural

 
Description Research Grant
Amount £204,283 (GBP)
Funding ID RGP-201 5-1 79 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 02/2016 
End 05/2019
 
Description Collaboration between the Uni of Southampton, National Trust, English Heritage & Historic Royal Palaces, West Dean Tapestry Studio & Intech Interactive Science Centre. 
Organisation Tapestry Studio - West Dean College
Country United Kingdom 
Sector Academic/University 
PI Contribution The PI and other investigators worked closely with West Dean to commission tapestry for the project.
Collaborator Contribution A one metre-square tapestry was woven at West Dean Tapestry Studio. The design was inspired by the colours of medieval tapestries and it was constructed using traditional materials and techniques. It was designed by two students from the University of Southampton's Winchester School of Art, who worked closely with designers and weavers at West Dean. The weavers also produced strips of tapestry for earlier stages of testing.
Impact Active engagement with tapestry weaving being undertaken at Stirling Castle. The new set of Unicorn tapestries will be used for new research project to monitor tapestries from new.
Start Year 2006