Frances Lennard

Deformation and strain measurement techniques for the inspection of damage in works of art

Abstract The engineering techniques used for inspecting structural damage are not widely known in the conservation sector. Techniques are available based on deformation or strain measurement that have the ability to provide quantitative data. This paper reviews currently available techniques, covering point-strain measurements using resistance strain gauges and fibre-optic sensors, as well as full-field optical measurement approaches such as holography, electronic speckle pattern interferometry, photoelastic stress analysis and photogrammetry. The underlying technology of each of the techniques is described for the non-specialist. The relevance of each technique is established from a conservation perspective through accounts of usage. The application of the techniques to a wide range of artwork, including panel paintings, statues, murals and mosaics is described and the results critically reviewed. The paper also provides an insight into possible future applications of the techniques and identifies areas for further investigation.

Applications of polymer optical fibre grating sensors to condition monitoring of textiles

Fibre Bragg gratings (FBGs) in polymer optical fibres (POFs) have been used to measure the strain in a woven textile. FBGs in both POFs and silica optical fibres were attached to a woven textile specimen, and their performance characterised. It was demonstrated that the POF FBGs provide improved strain transfer coefficients and reduce local structural reinforcement compared to silica FBGs and therefore make a more suitable proposition for textile monitoring.

Assessing the Feasibility of Monitoring the Condition of Historic Tapestries Using Engineering Techniques

The findings of a year-long programme carried out by a multidisciplinary engineering/conservation team are described. A mass-produced textile material that can be used to represent tapestries is identified and mechanical tests are detailed which demonstrate it behaves in a similar way to tapestry. The feasibility of using optical fibre sensors, full-field optical strain measurement techniques and thermography for monitoring tapestry degradation is assessed. The results of preliminary findings are presented and a rationale is developed for in-situ quantitative strain monitoring of tapestries.

Condition monitoring of textiles using optical techniques

In the paper it is proposed to use fibre Bragg grating (FBG) sensors to monitor the deformation and strain in a woven textile. Non-contact digital image correlation (DIC) is used to validate the results. The principal objective of the work in this paper is to identify a suitable adhesive for attaching the FBG sensors to tapestries and textiles. To do this, the interfacial interactions of the optical fibre, the textile material and the necessary adhesive must be considered. The performance of two types of adhesive are studied: a PVA conservation adhesive and a two-part epoxy adhesive Araldite 2015. The effect of the application of the adhesives on the mechanical response of the textile is investigated. Full-field stain maps are obtained from the DIC and are used as the basis to characterise the behaviour of the FBG sensors/adhesive system. The strain transfer coefficients and a reinforcement factor are determined under quasi-static conditions. It is shown that the local reinforcement introduced is more significant in the specimen with the FBG bonded using the Araldite adhesive than those with conservation adhesives. Nevertheless, the Araldite adhesive has a better strain transfer coefficient than the conservation adhesive, although not as high as that expect with conventional engineering materials.

Long term condition monitoring of tapestries using image correlation

Digital Image Correlation (DIC) is used to extract non-contact full-field three-dimensional displacement and inplane strains from an historic tapestries. A DIC-based approach is devised that allows the effect of RH variations on a tapestry to be quantified. A historical tapestry has been monitored in a closely controlled environment and in the natural environment. The results revealed that very small variations in RH can have significant effects on strain. An automated long term monitoring approach has been devised to allow strain data to be extracted in real time from tapestries in remote locations. The results show that DIC provides better understanding of the effect of RH fluctuations on strain which will ultimately lead to more insight into the degradation process of historical tapestries. The paper demonstrates the potential for using DIC as a condition monitoring tool

Using digital and hand printing techniques to compensate for loss: Re‐establishing colour and texture in historic textiles

Abstract Conservators use a range of ‘gap filling’ techniques to improve the structural stability and presentation of objects. Textile conservators often use fabric supports to provide reinforcement for weak areas of a textile and to provide a visual infill in missing areas. The most common technique is to use dyed fabrics of a single colour but while a plain dyed support provides good reinforcement, it can be visually obtrusive when used with patterned or textured textiles. Two recent postgraduate dissertation projects at the Textile Conservation Centre (TCC) have experimented with hand printing and digital imaging techniques to alter the appearance of support fabrics so that they are less visually obtrusive and blend well with the colour and texture of the textile being supported. Case studies demonstrate the successful use of these techniques on a painted hessian rocking horse and a knitted glove from an archaeological context.

Quantifying and visualizing change: Strain monitoring of tapestries with digital image correlation

Abstract A three-year research project at the University of Southampton (2007–2010) investigated whether monitoring techniques commonly used by engineers to assess the strength and durability of materials could be usefully applied to inform the condition assessment of historic tapestries. To date it has not been possible to obtain an objective picture of the overall condition of a tapestry; the study investigated whether it is possible to identify precursors of structural damage. The two techniques, digital image correlation (DIC) and optical fibre sensors, were used to monitor a representative wool fabric, specially woven tapestry samples, a newly woven tapestry, and historic tapestries, both in the laboratory and in situ in a historic house. This study first sets out to answer the question: can DIC be used to monitor and visualize strain in historic tapestries? It is shown that DIC can be used successfully. Secondly, it discusses the map function, a novel development which allowed the monitoring equipment to be moved, so that it could be used in situ in a historic house. Thirdly, it provides further details of the experimental work using optical fibre sensors to confirm the accuracy of the DIC technique.

A literature review of analytical techniques for materials characterisation of painted textiles—Part 2: spectroscopic and chromatographic analytical instrumentation

AbstractPart 2 of this Literature Review of analytical techniques for materials characterisation of painted textiles focusses on the application of vibrational and X-ray spectroscopic and chromatographic techniques used in the analysis of painted textiles to inform understanding of their materials, methods of making and degradation. The principles of detection methods, technique limitations and advantages, and how they complement each other, are explained and advances in techniques applicable in the study of painted textiles are discussed, such as mapping in Fourier transform infrared spectroscopy and Raman, surface-enhanced resonance Raman spectroscopy, and secondary ion mass spectrometry. Most informative work relating to painted textiles comes from close collaboration between conservators and scientists in interpreting findings and this literature review provides a useful starting point to further develop the capabilities of analytical techniques to enhance the study and conservation of painted textiles.

A literature review of analytical techniques for materials characterisation of painted textiles—Part 1: categorising painted textiles, sampling and the use of optical tools

AbstractMany types of painted textile are represented in museum collections. Their flexibility, draping qualities, and heterogeneous, layered nature make painted textiles complex objects to conserve. What do we know about their materials and making? There has been limited research into painted textiles and particularly their analysis. Whilst much valuable information can be gleaned from paintings analysis, there are many distinct differences in materials behaviour between stretched paintings and painted textiles that need to be identified and addressed. This article, together with Part 2, aims to raise the awareness of textile conservators, in particular of potential analytical techniques to identify and characterise the materials, thus enhancing understanding and conservation of painted textiles. Part 1 focuses firstly on the categorisation of different groups of painted textile providing a context for their study and secondly, it reviews sampling and optical techniques that can be used by conservators, ...

Mechanical properties of wool and cotton yarns used in twenty-first century tapestry: Preparing for the future by understanding the present

Abstract The conservation of historic tapestries is a complex and highly skilled task. Tapestries now being woven will need conservation in years to come. Can we, by understanding the properties of these contemporary works, assist the conservators of the future? The recreation of the Hunt of the Unicorn tapestries being undertaken by the West Dean Tapestry Studio offers a unique opportunity to access the materials being used and to create a body of data on their initial properties. This study uses tensile testing of the warp and weft materials to determine their maximum load at break, extension at maximum load, and specific stress (tenacity). Wool weft yarns from two different sources and of two thicknesses were examined. These wools were dyed ‘in house’ and the effect of the different dyes used was also assessed. These parameters all showed some significant (P < 0.05) differences. Cotton warp yarns of differing thickness and a gold thread were also tested. The comparison of how cotton and wool break demonstrates that when a tapestry is put under sufficient stress the cotton will snap but the wool may only stretch. However, this could often be beyond its recovery range resulting in a failure to return to shape.