Research

One project aim was to create a rigorous digital model of a tapestry system using the latest research developments in finite element analysis (FEA) and software development, as a tool for investigating conservation treatment plans and display options. Modelling was employed as a way of validating the ability of the DIC technique to measure full-field strains across historic tapestries.

Tapestries are extremely complex structures, employing different materials - wool and silk yarns - in the warp and weft and a discontinuous weaving technique which creates ‘slits’ which are closed by stitching. The degradation and damage caused by centuries of use creates additional complexity. It was not possible to create a completely accurate model of a tapestry using FEA as this would be an extremely time-consuming task, but we were able to automatically create a bespoke full-field synthetic strain field based on a photograph of the tapestry, that bears a probable relation to reality, including likely sites of strain and damage.

Dr Jafar Alsayednoor discussing tapestry with Maggie Dobbie, textile conservator at Glasgow Museums. © University of Glasgow

Using a digital image of a 15th-century tapestry from the Burrell Collection, Two Episodes from a Chivalric Romance, Perhaps the Tale of Florence of Rome, as an example, a representative model of the tapestry was created by mapping the tapestry details onto a finite element mesh. Layers of complexity were successively added to the model. Firstly, the Matlab Canny edge detection algorithm was used to identify joins in the weaving which create natural points of weakness (albeit simplified here). Secondly, areas of damage were artificially created by deleting the elements in five of these areas. Thirdly, conservation patches were modelled in these areas by adding larger ‘patches’ of a stiffer material.

The models gave inherent information on the effect of these areas of damage and the conservation interventions on the surrounding tapestry. More importantly here, the strain field generated by the simulation was used as a test case to evaluate the measurement error produced by DIC when using the tapestry image to track displacements. We wanted to establish whether the tapestry image itself (rather than the standard, applied, speckle pattern used routinely with DIC) would give accurate DIC results. DIC data were compared with the known deformations from the FEA model.

This technique demonstrated that the optimum imaging settings and the choice of subset size for DIC are strongly influenced by the tapestry image. Images with sharp colour contrasts or large homogenous areas will result in less reliable DIC data. It also demonstrated that selecting the optimum subset size depends on a trade-off between measurement accuracy and maximising resolution. For more information see our paper evaluating the use of DIC. 

As a result of this investigation, tools have been produced which allow the assessment of a tapestry for its suitability for interrogation with DIC. Three factors affecting the quality of DIC results have been investigated in more depth: the suitability of the tapestry image, the image acquisition process such as the camera settings and lighting conditions, and the DIC settings. For more information see our paper investigating the accuracy of DIC. 

Read more

• Alsayednoor, J., Harrison, P., Dobbie, M., Costantini, R. and Lennard, F. (2019) Evaluating the use of digital image correlation for strain measurement in historic tapestries using representative deformation fields. Strain, 55(2), e12308. (doi: 10.1111/str.12308)
• Alsayednoor, J., Lennard, F., Yu, W.R. and Harrison, P. (2017) Influence of specimen pre-shear and wrinkling on the accuracy of uniaxial bias extension test results. Composites Part A: Applied Science and Manufacturing, 101, pp. 81-97. (doi: 10.1016/j.compositesa.2017.06.006)
• Nwanoro, K., Harrison, P.  and Lennard, F. (2022) Investigating the accuracy of digital image correlation in monitoring strain fields across historical tapestries. Strain, 58(1), e12401 (doi: 10.1111/str.12401)

It is now common in mainland Europe to display tapestries on slanted supports – these are boards covered in fabric, tilted at a small angle (often 5° from the vertical), which aim to provide greater support to a fragile historic tapestry on display. However there had been no previous research to quantify the optimum display angle. Our investigation looked at both the degree of slope and the amount of friction provided by commonly used board-covering fabrics - these are often pile fabrics such as brushed cotton domette (also known as molton), velvet and polyester felt.

A simple mathematical calculation demonstrates that a small angle of slope gives a negligible decrease in the load experienced by the tapestry when the support is moved from vertical to the slight angle of 5° from the vertical. A tilt of 45° gives much greater support, but is generally impractical for displaying large tapestries. Tests on model fabrics suspended from a board at different angles provided a visual illustration of this, showing that strain barely decreased with a small degree of slant. However it was clear that increasing friction between the support and the tapestry had a considerable impact. 

Rosa Costantini suspending model fabric on a slanting support. © University of Glasgow

We used a simple inclined plane method to test friction, covering a board with one of the fabrics, placing a tapestry fragment on it, then tilting the board until the tapestry began to slip. Friction with all the fabrics was so high that the board could be tilted beyond vertical, until the tapestry was beginning to face the floor (at around 100° from the vertical, before it began to slip). This made it impossible to calculate a value for the coefficient of friction and so it was clear that the simple Coulomb model of friction does not apply in these cases. Instead it appears that mechanical interlocking between the fibres of the tapestry and the fabric covering generates a ‘mechanical adhesion’ between the fabric surfaces.

We believed that the effect of friction was so great that a covered board would provide good support to a tapestry if it was displayed vertically, with no slant, and this was demonstrated in a trial. A small tapestry was suspended against a vertical board, which was half-covered with a cotton molton fabric.

A small tapestry suspended against a vertical board, half-covered with a cotton molton fabric. © University of Glasgow

Monitoring with DIC showed that the strain value was low (around 0.01%) in the area with molton, compared with almost 0.05% in the area without the molton board covering. The part of the tapestry on the covered section of the board was less affected by fluctuations in humidity than the tapestry on the uncovered board, indicating that the molton board cover had a stabilising effect, reducing expansion and contraction of the tapestry. 

Read more

• Costantini, R., Lennard, F., Alsayednoor, J. and Harrison, P. (2020) Investigating mechanical damage mechanisms of tapestries displayed at different angles using 2D DIC. European Physical Journal Plus, 135, 515. (doi: 10.1140/epjp/s13360-020-00520-7)
• Lennard, F., Costantini, R. and Harrison, P. (2021) Understanding the role of friction and adhesion in the display of tapestries on slanted supports. Studies in Conservation, 66(1), pp. 32-43. (doi: 10.1080/00393630.2020.1761184)