Making the invisible visible: the potential of X-radiography as an investigative technique for
textile conservation decision making
Sonia O’Connor*
Research Fellow in Conservation
Department of Archaeological Sciences
University of Bradford
Bradford BD7 1DP
United Kingdom
Fax: 01274 235190
E-mail: s.oconnor@bradford.ac.uk
Mary M Brooks
Senior Lecturer
Textile Conservation Centre
Park Avenue
Winchester
Hampshire SO23 8DL
United Kingdom
Fax: 023 8059 7101
E-mail: mmb1@soton.ac.uk
*Author to whom correspondence should be addressed
Abstract
X-radiography is an established nondestructive analytical technique in many areas of
conservation practice, which is rarely applied to the investigation of ancient and historic textiles.
This paper presents research into the potential of X-radiography as a tool for the characterization,
condition assessment, study and conservation decision making of such textiles, as well as
enabling greater public understanding of textiles. Techniques appropriate for the X-radiography
of textiles, including the use of digital image manipulation, are outlined.
Keywords
characterization, condition assessment, conservation decision making, digital image processing,
textiles, X-radiography
Introduction
This research aims to explore the potential of X-radiography as a tool for the characterization,
condition assessment and study of ancient, historic and contemporary textiles. X-radiography is a
non-destructive examination technique established in many areas of conservation practice.
However, it has rarely been applied to the investigation of textiles. The authors’ previous
research in this area was supported by the British Academy (Brooks et al. 1996). Funding from
the Arts and Humanities Research Board (now Council) Research Centre for Textile
Conservation and Textile Studies (Textile Conservation Centre, University of Southampton) has
enabled this collaborative and interdisciplinary research to be continued. Appropriate Xradiography can provide information for textile conservation research as a tool for understanding
and communicating knowledge about artifacts and for enabling appropriate treatment strategies to
be evolved. Previously unseen features such as internal stitching, seams and fillings as well as
unsuspected repairs and degradation may be revealed. Technological features such as changes in
weave structure or differential metal salt weighting may be highlighted (Brooks et al. 1996).
Using X-radiography for investigating textiles requires an understanding of a range of Xradiographic techniques as well as developing skill in reading the resulting images.
Principles of X-radiography
A brief summary of the principles of X-radiography follows. For full accounts of X-radiographic
imaging theory and practice, see Halmshaw (1986) on industrial X-radiography and Lang and
Middleton (1997) for the application of X-radiography to cultural artifacts, although not to
textiles. X-rays, like infrared (IR), ultraviolet (UV) and visible light, are a form of electromagnetic energy. However, they have much greater energy and shorter wavelengths than light.
Consequently, rather than just being reflected or absorbed, they are able to penetrate deeply into,
or through, materials that would otherwise be considered opaque. The X-radiographic image is
formed on the film in the same way as a photographic negative. Differences in the capacity of
materials to absorb X-rays depend on their thickness, density and atomic weight. These variations
in the X-ray flux reaching the film form the image.
Techniques for the X-radiography of textiles
Modifying established industrial X-radiography techniques enables the production of highdefinition images of textiles with good contrast and resolution of detail. The wider project is also
reviewing the use of specialist approaches such as filmless, stereo, micro-focus and real-time
radiography as well as computer tomography. There are several reasons why textiles are hard to
radiograph successfully and thus require specific X-radiographic techniques. The organic
molecules from which textiles are often formed are fairly radio-lucent. Their structure may also
be thin and can often be relatively open: a single thread or two threads crossing each other may
do little to attenuate an X-ray beam on its path through the textile to the film. As a result, the
images are often of very low contrast. An image may appear to be a featureless grey shadow to
the naked eye but a densitometer will be able to measure variations of density within it. Such
apparent lack of information may be made worse by problems with X-ray beam scatter,
geometric ‘unsharpness’,1 incorrect film selection, poor quality film processing and unsuitable
viewing conditions. Problems encountered when attempting to record variations in ceramic
composition and structures are not dissimilar, and Carr and Riddick (1990) provide a detailed
account of essential concepts and procedures to optimize image quality, even though there have
been many technical developments during the past decades.
X-radiography machines
This paper focuses on film-based X-radiography using equipment commonly available to the
conservator. Heavily filtered X-ray sources, such as those used for medical X-radiography, are
generally not suitable for this work. To protect the patient, such medical units are filtered to
remove all X-rays with energies up to 40 kilovolts (kV): just the energy range that is particularly
useful for X-raying textile materials. Industrial units with a molybdenum or tungsten target and a
thin beryllium window to the X-ray tube produce high levels of low-energy X-rays required for
such imaging. Small effective focal-spot size will help minimize ‘unsharpness’ as does a long
source-to-film distance. The latter is not so critical when imaging thin, flat objects that lie in
close contact with the film.
Film
Fine-grain high-contrast industrial radiographic film should be used in preference to general
medical film as it produces a bigger difference in optical density for a small change in X-ray flux.
When imaging at very low energies, the beam will be attenuated by light-tight envelopes and
paper or plastic film wrappers. Their image will be superimposed over that of the textile, thus
reducing image definition. Ideally, film needs to be used ‘naked’ although this requires an X-ray
facility that can be blacked-out to the same standard as a processing darkroom.
Low-energy X-ray imaging
Low-energy X-rays produce a higher contrast image than high-energy X-rays as they are more
readily absorbed by the material through which they pass. Very low-energy X-rays, around 15
kV, have proved best for creating images of singlelayered textiles such as fine silk fabrics
(Brooks et al. 1996). Similar energy levels may also be appropriate for objects such as coverlets,
where the thickness and layers are relatively consistent. However, X-radiographic images taken
at low energies have a narrow exposure latitude. If the radio-opacity of an object varies because
of diverse materials or thicknesses some areas may be black on the film owing to over-exposure
whereas other areas, which have not been penetrated, may be white. Such images contain very
little information. Increased beam energy increases exposure latitude but will result in lower
image contrast and the information can be lost in close shades of grey. The final choice of
kilovoltage has to be a compromise depending on the nature of the object, the materials involved
and the number of exposures that can be taken. Raising the voltage to 20 or 30 kV may produce
more useful images of objects made from a greater diversity of materials such as textiles with
leather, cardboard, plastic, and so on. For textiles incorporating materials that attenuate the X-ray
beam to very different extents, such as metal threads, it may sometimes be necessary to take
exposures at quite different voltages or to use filters or lead screens (Brooks and O’Connor
2005).
Minimizing ‘unsharpness’
Optimizing image contrast may have little effect on improving the discernibility of information in
the image if the edges of features are blurred. Geometrical sources of ‘unsharpness’ can be
minimized by maximizing X-ray source-to-object distance and minimizing object-to-film
distance. Another cause of blurring is scattered radiation resulting from interaction of the X-ray
beam with all the materials through which it passes. These scattered rays are generally of very
low energy and can be reduced by careful filtration. Aluminium cooking foil has proved a very
useful filter for this purpose.
Digitized images
Although the analogue film image can capture every shade of grey between black and white, the
human eye can only distinguish about 30 shades of grey and is particularly poor at distinguishing
shades of darker grey to black. Digital images can record hundreds of shades of grey and can be
manipulated to emphasize specific information captured in them. Digital radiography also offers
fast, filmand chemical-free imaging with improved exposure latitude and image contrast.
Unfortunately, suitably high-resolution filmless capture is not readily available in our field but it
is possible to digitize film images and manipulate them using readily available graphics software
(O’Connor and Maher 2001, O’Connor et al. 2002). A dedicated X-ray film scanner is needed to
maintain high image quality. Images can be sharpened, edges of features enhanced and, by reassigning greyscale values, information previously invisible in both under- and over-exposed
areas can largely be retrieved and made visible and comprehensible. It is easy to turn these
negative images into positives without any loss of data. This often makes features more obvious
because the brain is more accustomed to thinking of denser materials as producing darker images.
It is important to remember that image manipulation does not create information but makes
existing information more discernible, so the X-ray to be digitized must be of the highest quality
obtainable; information cannot be enhanced if it is not present.
Test subject
This mid-20th century plastic doll (private collection; Figure 1a) is dressed in synthetic fabrics,
with an underskirt fashioned into a bag to contain dried lavender. The X-radiographic image
(Figure 1b) was taken using a Hewlett Packard Faxitron at 20 kV and a tube current of
approximately 3 mA for 1 minute, with an aluminium foil filter and recorded on Agfa D4 film.
The Xradiograph revealed detailed information of the clothing construction, weave structure of
the thin fabrics, synthetic beads and sequins and a totally unsuspected dressmaker’s pin.
Information about the plastic moulding and its mineral filler, shut-eye mechanism, detached legs
secured in the ‘bag’ skirt, the remains of the elastic band securing the arms to the torso – even the
dried lavender flower heads were evident. This object is now used as a reference object when
different facilities and techniques are being evaluated.
Insights from X-radiography
The following case histories result from collaborations with several museums and collections.
They have been selected to demonstrate the potential of these X
(a)
(b)
Figure 1. The plastic doll test subject (a) and its X-radiograph (b) showing the doll’s
construction, textile structures and lavender flowers
radiography techniques on a diverse range of textiles. All the images reproduced here were
captured on high-definition, high-contrast film, digitized using a dedicated X-radiographic film
digitizer and explored and enhanced using proprietary image processing software.2
‘Object biography’: alterations and degradation
X-radiography provides a means of assessing and recording the condition of internal components
of multi-layered textile objects. This much-altered stomacher, the detachable upper front section
of 17th and 18th century women’s gowns shown in Figure 2a, was found concealed in a building
and has been the subject of intensive study.3 It is constructed from layers of silk, linen, canvas
and paper stiffened by baleen (‘whalebone’) (Eastop and McEwing due 2005). X-radiography
contributed insights into the complex ‘object biography’ (Kopytoff 1986) of this garment, which
has apparently undergone extensive modifications as it moved down the social scale. The Xradiographic images made it possible to distinguish deliberate alterations, including later
stitching, and to understand the progress and full extent of insect damage (Figure 2b). Areas
previously deteriorated by sweat were attacked first; then the insects ate their way down the
baleen strips, leaving the surrounding silk and linen untouched.
Figure 2. The stomacher (a) and details from its X-radiograph showing extensive insect damage
to baleen strips ©Textile Conservation Centre (b) and the peeled back edge of the baleen (c).
Reproduced by permission of the owner
Construction and repair
The construction of the stomacher was also dramatically revealed by the X-radiography, showing
how the narrow baleen strips had split and peeled back when they were inserted into the closely
stitched channels (Figure 2c). Studies of two quilts show how even the most subtle constructional
information can be revealed by X-radiography. The earliest known English patchwork coverlet,
dated 1718, is constructed from silk fabrics folded and tacked over paper templates (The Quilters’
Guild of the British Isles, Fenwick Smith and Osler 2003). X-radiographs provide insights into
the construction of the coverlet that cannot not be gained from visual examination (Figure 3a, b).
Seams and seam allowances became visible
(a)
(b)
Figure 3. The 1718 coverlet (a) with the X-radiograph of the same area (b) showing the
sensitivity of the technique. Reproduced by permission of The Quilters’ Guild of the British Isles
(a)
(c)
(b)
(d)
(e)
Figure 4. A corner (430 mm × 350 mm) of Mary Burnett’s quilt (a) and its X-radiograph (b). The
black lines overlaid on the X-radiograph highlight the inner quilt seams (c), the quilting through
the top covers (d) and the quilting on inner quilt (e). Reproduced by permission of York Museum
Trust, York Castle Museum
as well as the weave structures of the silk and the linen backings together with tears and seams in
the paper templates. Even tacking stitches, concealed by the construction, became clear.
Comparing the different stitching techniques suggested that more than one person had been
involved in making the coverlet (Brooks and O’Connor 2005). The condition of the 20th century
quilt made by Mary Burnett indicates both austerity and long use (York Castle Museum BA
3999/1; Figure 4a). Now cut in two, it has an earlier quilt as one of its many layers of fillings.
Tears in the top covers reveal a white fabric, a floral print and a blue checked fabric. Xradiography enables the inside of this multi-layered and padded textile to be viewed but produces
an image that is complex and difficult to interpret, crammed with details about construction
features, fillings and printing techniques (Figure 4b). Digital manipulation was used to extract
information from this X-radiograph to produce a series of images, each emphasizing a different
element. Mapping out related features highlighted hidden seams (Figure 4c), the quilting pattern
worked through the top covers (Figure 4d) and the earlier quilting on the inner re-used quilt
(Figure 4e).
Fillings and stuffings
The potential for X-radiography to enable the identification of loose fillings, such as the lavender
in the plastic doll’s underskirt, and stuffings which give form and structure, without having to
extract samples became clear during the course of the research. Inspection of the X-radiograph of
Mary Burnett’s quilt revealed fractured and curled fragments of what appeared to be vegetable
material within the stuffing (Figure 5a). These features are characteristic of cotton wadding and
are probably mostly fragments of seed coat. Figure 5a–d shows images from a growing database
recording fillings from a variety of three-dimensional artifacts, including toys, dolls, garments,
quilts and upholstery, which will aid image interpretation and, ultimately, conservation decision
making.
(a)
(b)
(c)
(d)
Figure 5. X-radiographs of various fillings and stuffings at approximately the same
magnification: cotton wadding and associated debris in a quilt (a), wood wool in a teddy bear
(b), straw in a doll (c) and eider down and feathers in a quilt (d)
Structures and supports
It was no surprise that the stomacher (Figure 2a) should be stiffened by strips of baleen as this
was clearly visible in the damaged areas. Adding an X-radiograph of a characteristic image of
this material to the database will allow it and other support materials, such as reed, cane, feather
quill or spring steel, to be identified without deconstructing artifacts. The structures and supports
of a late 19th century toy were far more varied and unsuspected (York Castle Museum BA 2578;
Figure 6a). This is a silk-covered shoe, re-used to illustrate the children’s nursery rhyme ‘There
was an old woman who lived in a shoe, she had so many children she didn’t know what to do’.4
X-radiographic images provided information about the iron wire armature but also many other
components including textiles, leather, wood, paint, fillers and stitching (Figure 6b). The mother
doll’s head and hands appear to be sculpted from a very radio-opaque material, possibly
pigmented lead putty, and the dress is supported by a fan-folded paper petticoat. But the surprise
was the supporting structure of the doll’s body: a chicken furcula (wishbone).
Aiding conservation decision making
X-radiography of this shoe was initially requested to investigate the source of rust stains in the
textile around the mother doll’s arms and so formulate an appropriate conservation strategy
(Figure 6a). Once the X-radiograph revealed that metal was only present as the doll’s arms rather
than the whole of the body, it was possible to implement a localized treatment which was less
interventive than initially considered. Previously hidden stitched repairs to the shoe itself also
became evident and could be factored into the chosen method for stabilizing the worn edges of
the shoe. Appropriate decisions could be made about display and storage conditions. The
unusually bright image of a repair thread led to a new area of conservation research exploring the
use of X-radiography to identify weighted silks (Brooks et al. 1996).
Conclusion
X-radiography, suitably adapted and combined with digital manipulation, adds value to textile
conservation research and practice as a tool for understanding artifacts and developing
appropriate conservation strategies. It should become a valuable tool in the analytical repertoire
of the textile conservator. X-radiography can be used as a vivid means of demonstrating a greater
understanding of textiles, whether as dress, upholstered furniture, toys, quilts or motor vehicles,
to the public by making the invisible literally visible.
(a)
(b)
Figure 6. Detail of the toy ‘The Old Woman who Lived in a Shoe’ (a) and its X-radiograph (b).
The arrow indicates the unusually bright thread. Reproduced by permission of York Museum
Trust, York Castle Museum
Acknowledgements
Thanks are due to private owners and colleagues for generously sharing expertise and enabling
access to equipment and collections: Conservation Centre, National Museums Liverpool;
Quilters’ Guild of the British Isles; Textile Conservation Centre, University of Southampton;
University of Bradford; York Museum Trust, especially Josie Sheppard, Curator of Costume and
Textiles. We also thank: Arts and Humanities Research Board Research Centre for Textile
Conservation and Textile Studies (Textile Conservation Centre, University of Southampton, UK)
for funding this research, especially Dr Maria Hayward, Director, and Dinah Eastop, Associate
Director; British Academy for previous support; Nell Hoare, OBE, Director, Textile
Conservation Centre, University of Southampton, for permission to publish.
Notes
1 ‘Geometric unsharpness’ is blurring of the image caused by several factors inherent to any Xray unit, such as the dimensions of the X-ray source or focal spot.
2 Films were initially digitized using a medical film scanner but an Agfa FS 50B industrial film
digitizer is now being used for its superior image quality. Digitized images were prepared using
Paint Shop Pro, a JASC graphics software package. Unless otherwise noted, all images were
taken by Sonia O’Connor.
3 Gabriella Barbieri (2003) researched the stomacher during her MA Textile Conservation,
Textile Conservation Centre, University of Southampton, under the supervision of Dinah Eastop
(Senior Lecturer, Textile Conservation Centre, University of Southampton) who initiated and
leads the Arts and Humanities Research Board research project into deliberately concealed
garments (Eastop and Dew 2003). See www.concealedgarments.org.
4 This rhyme, first published in 1784, might refer to several women renowned for their many
children, including Queen Caroline, George II’s wife (Opie and Opie 1951).
References
Barbieri, G, 2003, ‘Memoirs of an 18th century stomacher. A strategy for documenting
the multiple object biographies of a once concealed garment’, MA dissertation,
Textile Conservation Centre, University of Southampton.
Brooks, M M and O’Connor, S A, due 2005, ‘New insights into textiles: the potential of
X-radiography as an investigative technique’ in Wyeth, P and Janaway, R (eds.),
Scientific Analysis of Ancient and Historic Textiles: Informing Preservation, Display and
Interpretation. Postprints of the First Conference of the AHRB Research Centre for Textile
Conservation and Textile Studies, 13–15 July 2004, London, Archetype.
Brooks, M M, O’Connor, S and McDonnell, J G, 1996, ‘The application of low energy
X-radiography in the examination and investigation of degraded historic silk textiles:
a preliminary report on work in progress’ in Bridgland, J (ed.), Preprints, 11th ICOM
Triennial Meeting, Committee for Conservation, London, James and James, 670–679.
Carr, C and Riddick, E B, 1990, ‘Advances in ceramic radiography and analysis:
laboratory methods’, Journal of Archaeological Science 17, 35–66.
Eastop, D and Dew, C, 2003, ‘Secret agents: deliberately concealed garments as symbolic
textiles’ in Vuori, J (ed.), North American Textile Conservation Conference Biannual
Conference 2003: The Conservation of Flags and Other Symbolic Textiles, Albany, USA,
North American Textile Conservation Conference, 5–15.
Eastop, D and McEwing, R, due 2005, ‘Informing textile and wildlife conservation:
DNA analysis of baleen from an eighteenth century garment found deliberately
concealed in a building’ in Wyeth, P and Janaway, R (eds.), Scientific Analysis of Ancient
and Historic Textiles: Informing Preservation, Display and Interpretation. Postprints of the First
Conference of the AHRB Research Centre for Textile Conservation and Textile Studies,
13–15 July 2004, London, Archetype.
Fenwick Smith, T and Osler, D, 2003, ‘The 1718 silk patchwork coverlet: introduction’,
Quilt Studies 5 (special issue: The 1718 Silk Patchwork Coverlet), 24–30.
Halmshaw, R, 1986, Industrial Radiography, Mortsel, Agfa-Gevaert.
Kopytoff, I, 1986, ‘The cultural biography of things: commoditization as process’ in
Appadurai, A (ed.), The Social Life of Things. Commodities in Cultural Perspective,
Cambridge, Cambridge University Press, 64–68.
Lang, J and Middleton, A, 1997, Radiography of Cultural Material, Oxford, Butterworth
Heinemann.
O’Connor, S and Maher, J, 2001, ‘The digitisation of X-radiographs for dissemination,
archiving and improved image interpretation’, The Conservator 25, 3–15.
O’Connor, S, Maher, J and Janaway, R, 2002, ‘Towards a replacement for
xeroradiography’, The Conservator 26, 100–114.
Opie, I and Opie, P, 1951 (1985), The Oxford Dictionary of Nursery Rhymes, Oxford,
Oxford University Press, 434–435.