Download - Managing change: preserving history
ISSN:1369 7021 © Elsevier Ltd 2007APRIL 2007 | VOLUME 10 | NUMBER 450
The paintings, books, documents, and other riches found in our
museums, libraries, and archives appear for the most part to be
relatively inert and statically fixed in time and place. In reality,
this is an illusion: slowly over time the many materials used to
make these artifacts inevitably change and degrade, leading in
some cases to considerable loss of information and value. The
changes in materials in such collections can take tens, hundreds,
or even thousands of years and in some cases can be
imperceptible to the naked eye. These changes also affect
modern digital storage materials such as CDs and DVDs and
in some modern materials, such as plastics or synthetics, the
changes can be very rapid and the associated irreparable loss of
information and value rapidly becomes significant. Unfortunately,
for example, many important 20th century works of art made
from polymer-based materials have already shown significant
signs of decay. It is essential in all cases that we understand the
degradation processes at play and take steps to minimize them.
Here we will explore how science and technology are helping us
manage the changes in the materials that make up our cultural
heritage.
Much of our cultural heritage is preserved in museums, libraries, and archives and the artifacts therein give us unique insights into the lives of people long gone and methods of working with materials long forgotten. We describe a few of the riches to be found in the collection of The National Archives, UK. We identify some of the decay processes that are at play and show how modern scientific tools allow us to study the mechanisms and kinetics of these decay processes. Also, we discuss how conservation ethics and the associated constraints on sampling make the study of these artifacts particularly challenging. Ultimately, of course, the collective aim of scientists, curators, and conservators is to understand the fabrication of these objects better and to find the best conditions for their storage and display in order to preserve them for future generations.
Nancy Bell1* and David McPhail2
1*The National Archives, Kew, Richmond, Surrey, TW9 4DU, UK
2The Materials Department, Imperial College, London, SW7 2AZ, UK
1*E-mail: [email protected]; 2e-mail: [email protected]
Managing change: preserving history
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Many different words are used to describe the interventions used
in this complex and multifaceted area. The terms ‘conservation’,
‘preservation’, and ‘sustainability’ are commonly used, for example,
and represent different aspects of the process of managing change.
‘Conservation’ is usually narrowly defined and involves active
intervention with objects to ensure chemical and physical stability
through a specific treatment or repair. In contrast, ‘preservation’ is
more holistic and involves mitigating the physical and chemical risks
to artifacts in order to prevent or retard decay. Clearly, it is important
to manage the change in objects as best we can but at the same time
allow access. Achieving a balance between access and preservation
involves, on the one hand, making our cultural heritage available to
the public, for example by placing exhibits in display cases, and, on the
other hand, ensuring its preservation for future generations to study
and enjoy, i.e. maximizing both the artifacts’ survival and accessibility1.
This is a very difficult task and is made even more so because we can
seldom rely exclusively on a single technique, approach, or model.
However, in more recent years, the potential of science and technology
to help ‘manage change’2, that is to retard decay and deterioration
in cultural artifacts, has been recognized. Indeed, a wide range of
scientific disciplines including materials science, chemistry, engineering,
and environmental science now have an important role to play in
identifying and solving collection-based problems.
Science has always had an important role to play in the
interpretation and preservation of material culture. Modern
analytical tools also allow us to conduct a detailed inspection of the
microstructure of materials. This in turn reveals further dimensions to
the materials employed and the technology used to create the object
(Fig. 1). Analytical testing also has an important role to play in the
interpretation of artifacts and, rather like forensic analysis, is being
used to explore questions of authorship or authenticity.
Scientific and technological research is also helping us to decide
how best to display and store artifacts, as there is always a dynamic
interaction between the object and the environment around it. For
example, textiles and adhesives commonly used inside display cases
are routinely tested to ensure that they will not produce an adverse
reaction if they come into contact with the artifacts on display. If
objects are not kept in optimum conditions, decay mechanisms are
sometimes accelerated. Materials expansion and contraction arising
from variations in humidity and temperature will cause paint layers to
dislodge, while unchecked light levels will soon fade textiles and light-
sensitive photographic images, prints, and drawings.
Conservation scienceConservation science, termed ‘heritage science’, is a field of activity
that has emerged in the last three decades. Conservation science brings
together science, conservation, and heritage under one rubric and is,
therefore, a discipline operating in the area where the sciences, arts,
and humanities overlap. For some years, conservation science has been
viewed as an adaptive science, where scientists working in other fields
have brought expertise to this domain. As the discipline has evolved,
however, it has attracted a new breed of scientists working exclusively
in this arena who are developing tools and methods appropriate to
conservation, as well as supporting pure research, and thus developing
a distinct new brand of science3.
Working in the heritage science sector is fascinating, and while
the objects we study and problems we face are always engaging,
there are very distinct challenges. For example, we generally have
little or no information about the materials and techniques that were
used to make the object. The provenance of the materials used is
often unknown and the environmental history, including previous
conservation treatments, is sometimes undocumented. The materials
will often be a complex mixture of crystalline and glassy phases
containing defects and voids, and the surfaces will often be crazed
and cracked (Fig. 2). Often the objects are multilayer composites
Fig. 1 Detail of a 16th century illumination of Cardinal Wolsey (Magdalen
College, Oxford, Ms. 223). Illuminations were used as illustrations in
manuscripts/books before the age of printing. This exacting miniature artwork
was executed on parchment using finely ground pigments held in a binder such
as gum arabic and applied with a very fine brush. Here we see the lively hand of
the illuminator at work, with pencilled under-drawing still visible underneath
the thin blue washes applied in the sky area. In contrast, other elements of the
image are rendered with thickly applied pigment. Expensive gold pigments add
shading and richness. Understanding the process and the types of materials
used to execute works of art is central to the work of conservators. Noninvasive
analysis of the materials, e.g. pigments, informs our understanding of how
objects were made and used.
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of materials: for example, photographs contain materials based on
cellulose and proteins, while DVDs contain equally complex layers
of materials such as polycarbonates, acrylic lacquers, and dyes. The
complexity of these structures and the lack of knowledge often
surrounding them present us with a challenging detective story that we
need to unravel using all the evidence available together with state-
of-the-art scientific analysis tools (Fig. 3). This situation is in sharp
contrast to that presented by a modern silicon wafer, where we have
an almost complete knowledge of all aspects of the microstructure,
processing, and properties. And, unlike other scientific research based
on destructive sampling, conservation science is firmly situated within
an ethical framework precluding the sampling of objects. Historically,
this has limited the application, for example, of destructive analysis
techniques and those involving the application of a vacuum to samples.
Approaches to analysis in conservation scienceRecent advances have involved the development of a range of
microinvasive or noninvasive technologies and these are making a new
Fig. 2 Materials science and engineering is concerned with the relationship between microstructure, processing, and properties. The study of materials in collections
such as The National Archives is particularly challenging because of the lack of knowledge about the raw materials, the processing conditions, and the environmental
history of the artifacts, together with the consequent complexity of the microstructure.
Fig. 3 This micrograph (25x) shows the (surface) of a pigment layer as found in the drapery of the figure in the lower left corner (Fig. 3a). X-ray fluorescence
(XRF) confirmed the presence of both azurite and ultramarine pigments. Often these two pigments were used in combination for reasons of economy without
compromising the visual impact. (a) Detail from the Annunciation (Magdalen College, Oxford Ms. 223).
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(a)
Properties
Processing Microstructure
MaterialsScience
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range of analyses possible. Usually a hierarchical approach to analysis
is used, starting with noninvasive techniques such as optical and
optical interference microscopy. For example, the Zygo® white light
interferometer yields digital maps of surface morphology (Fig. 4) and
can be used to monitor the evolution of topographical features as a
function of time4. Such optical techniques are air-based, noncontact,
and have an open architecture so that whole objects may be inspected;
indeed, the apparatus can usually be taken to the object itself.
The next set of techniques is more sophisticated, but also air-based
and nondestructive. Raman spectroscopy is one technique that can
be used to identify organic pigments and Fourier transform infrared
spectroscopy attenuated total reflection (FTIR-ATR) is now routinely
used in many conservation studies for materials identification (Fig. 5).
Particle induced X-ray emission (PIXE) is another chemical analysis
technique with parts-per-million sensitivity in favorable cases. PIXE can
be operated with the object in air a few millimeters from a vacuum
window and has been used for a wide range of studies spanning
archaeometry and conservation5. PIXE can also be operated with a
sample from the object under vacuum and in this case a better lateral
resolution may be achieved. PIXE only introduces a modest level of
damage, which is seldom perceptible to the naked eye (Fig. 6). The
environmental scanning electron microscope (ESEM) embraces similar
concepts to avoid the damaging effects of high or ultra-high vacuum.
SEM energy dispersive X-ray spectroscopy (EDX) has proved a highly
useful technique to distinguish between corrosion products and
manufacturing impurities6.
There are other techniques that are only used if the information
they produce is essential, for example techniques that require a small
sample to be removed from the object being studied and/or that
consume the sample as the measurement is taken. Sometimes analysis
has to be conducted in a high or ultra-high vacuum, which is also
undesirable as the vacuum can produce irreversible reactions associated
with out-gassing and dehydration. Such analyses would be the last to
be conducted and indeed should only be initiated if the information
is essential, preferably using didactic material. While destructive
vacuum-based analytical techniques are far from ideal for cultural
heritage studies, they have the power to reveal information at very
high resolution and sensitivity. For example, using secondary ion mass
Fig. 4 The Zygo® microscope operates on the basis of white light interferometry and generates a digital map of the topography of a surface. The instrument is air-
based and has a large sample stage suitable for the imaging of a wide variety of objects. The example shown is an image of a cover glass for a container. The glass
has undergone atmospheric corrosion, moisture has penetrated the glass and alkali ions have migrated out to the surface where they have reacted with atmospheric
gases to form sulfates, carbonates, and formates.
Fig. 5 A letter written by an individual purporting to be Jack the Ripper. [The
National Archives, UK MEPO 3/142 (253).] FTIR-ATR analysis was used to
identify the smudges on this letter that were once thought to be blood but
are now thought to be an etching ground. [MEPO3-142 Pt. 2 (253), October
1888.]
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spectrometry (SIMS) depth profiling it is possible to measure ultra-
slow reaction rates proceeding at less than 1 nm per day, equivalent
to a millimeter of decay every 3000 years (Fig. 7). For sputter depth
profiling, an area typically 100 µm x 100 µm has to be analyzed to
a depth of a micron or so. SIMS also has the sensitivity to measure
changes in surface composition at the level of a part per million thus
obviating the need for accelerated aging experiments in this system,
and therefore detecting changes long before they become visible to
the human eye7.
Conservation science research at the National ArchivesThe National Archives (TNA)8 is the UK government’s official archive,
containing 900 years of history from the Domesday Book to the present
day, with records ranging from paper and parchment scrolls to recently
created digital files and archived websites. The collection is very
diverse and, while many may be familiar with national treasures such
as the Domesday Book (compiled in 1086) or Magna Carta (1215), less
familiar objects include curiosities such as the photographs of foremost
Fig 7 Ultralow-energy SIMS depth profiling can resolve the positions of atoms beneath the surface of a material with subnanometer precision. In this example,
four identical glass samples have been aged at room temperature for different periods of time under 55% relative humidity and SIMS has been used to monitor
the sodium in the glass. The near surface of the glass has lost sodium to the surface and the longer the aging process the greater the amount of sodium that has
been lost. The significance of this approach is that corrosion kinetics can be studied without the need for accelerated aging. This experimental methodology can be
combined with isotopic labeling where appropriate (e.g. D2O) and is transferable to a wide range of archive materials where ultra-slow corrosion processes are at
play. (Data courtesy of Sarah Fearn, Imperial College London.)
Fig. 6 PIXE microbeam images of the cross-section of a photograph. PIXE and Rutherford backscattering are used at the Surrey University Ion Beam Centre13 to
identify and quantify elements in photographic emulsions and backing paper. (Courtesy of Geoff Grime, Inma Gomez-Morilla, Chris Jeynes, Surrey Ion Beam Centre.)
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British 19th century photographer Eadweard Muybridge (Fig. 8), whose
seminal studies on animal locomotion and zoetrope design led to the
development of motion picture film and cinematography.
Preserving the national memoryPreserving both an analogue and digital collection the size and scale
of TNA, with some 178 km of shelving and well over 10 million items,
is, in the first instance, about mitigating risk factors – an important
first step in any preservation program. The major risk factors to any
collection will vary according to the materials from which the object is
made and the mechanisms and kinetics of the decay processes, which
are inextricably linked to heat, water, light, and the action of gases
such as oxygen and sulfur dioxide. Clearly, it is essential to conduct a
systematic assessment of the risks to the objects in the archive before
one can sensibly decide how to manage the collection. Therefore,
TNA has embarked on a three-year risk assessment program following
models first developed by Robert R. Waller of the Museum of Nature in
Canada9.
TNA has for some years taken its responsibility for preserving
the collections seriously. In recent years, however, as the potential
of science to inform preservation policies and practice has been
appreciated, it has been deemed necessary to establish conservation
research formally. Currently, the primary focus of the research
program is concerned with developing an in-depth understanding of
the physical and chemical profile of materials, including parchment,
photographs, and some modern records made of plastics and
synthetics. Understanding the deterioration mechanisms in play and,
most importantly, the rates of decay of specific groups of materials
is imperative to achieving the main objective of reducing or slowing
deterioration. Moreover, an evidence base of how and when materials
degrade informs when resources or methods for reducing deterioration
should be put in place – in other words, managing change. The second
area of TNA research is directed toward technical examination, defined
as the use of scientific analysis to determine what objects are made of,
when, and how. Technical examination can add a historical dimension
and improve our interpretation of the collection.
Unfortunately, the limited resources available in the cultural
heritage sector preclude support for large numbers of scientists working
in state-of-the-art facilities. While dedicated research facilities able
to take a long-term approach to collection stewardship do exist, this
activity in the UK is largely concentrated in national museums but
seldom in libraries or archives. The importance of in-house expertise
to deliver practical and preventive conservation science, as well as to
improve our understanding of collections, cannot be overestimated.
Nevertheless, new streams of funding from the public and private
sectors are proving a useful catalyst for collaborative partnerships
with other cultural institutions and university science departments to
further extend our problem-solving capacity. Thus, TNA is developing
collaborative opportunities to investigate fundamental materials
science questions that will help us understand mechanisms and rates of
decay better.
A closer look at the Domesday BookUsing recent advances in noninvasive and microinvasive analytical
testing, a major interdisciplinary project has been launched to study the
Domesday Book (Fig. 9). In partnership with the School of Optometry,
Cardiff University, and specialists from the School of Conservation,
Copenhagen, our goal is to learn more than previously possible about
the making and meaning of this icon using a hierarchy of techniques
from the visual to the nanoscopic. Optical microscopy techniques are,
as a first step, enabling comparative analysis of the parchment used as
the writing support and a range of complimentary techniques, including
Fig. 8 The experiments of Eadweard Muybridge (1830-1904) in rapid action
photography are landmarks in photographic history. [COPY1-389(ii) f.58a
Bucking horse, by E. Muybridge, 1887.]
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Fig. 9 Great Domesday, TNA. Great Domesday is made of 383 parchment folios
(~20 x 14.5 inches mm). The surface of the skin would have been smoothed
in preparation for writing. It is thought the text was written with a quill pen,
using ferro-gallo-tannate ink typical of the period. [E31-2-1 Great Domesday,
Wiltshire - Dorset 75.]
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amino acid testing, are helping us to determine conclusively the species
of animal skin. Using just a few fibers of parchment, simple shrinkage
temperature testing gives us an indication of the deterioration of the
collagen present10. At the other end of the analytical spectrum, X-ray
diffraction performed at the European Synchrotron Research Facility is
being used to determine the structural integrity of the collagen11. It is
envisaged that the results will help to determine the condition of the
collagen, which will help us to monitor changes and calibrate current
standards of display and storage of the Domesday Book.
Photographs – behind the image Emerging technologies are also enabling fascinating and important
studies of cultural heritage, particularly when used in conjunction with
documentary sources. The interplay of art, technological, and economic
history is opening new avenues in our understanding of the processes
and materials used. While technical analysis has often been applied to
the study of paintings, analysis of other artifacts such as photographs
has yet to be fully exploited. Building on protocols established at the
Getty Conservation Institute12 and in collaboration with Imperial
College London, research is underway to understand the complexity
of photographic processes used in the last 150 years. The structures
are extremely complex multilayers and it has proved necessary to
sacrifice some small samples to yield a more complete understanding
of the system. Focused ion beam (FIB) milling has been used on small
areas to prepare cross-sections for subsequent analysis (Fig. 10). In
recent experiments, PIXE has been used to provide chemical maps of
a cross-section through a photograph (Fig. 6). The initial results have
been produced with the sample in vacuum, but it is planned to conduct
analyses on whole objects with the external beam. Another promising
technique is time of flight (TOF) SIMS.
Future workOver the last 30 years, the evolution of conservation science has
changed significantly the way we see, understand, and preserve cultural
heritage. Inevitably, future technological advances will catalyze new
thinking, inspire new approaches, and develop new science. Speculating
what changes lie ahead, or the impact they may bring, is always risky,
nevertheless the potential for creative collaborative partnerships between
disciplines to help us do things better is enormous and welcome.
There can be no doubt that advances in micro-invasive and noninvasive
materials analysis techniques will be of enormous importance to
conservation science, helping us to understand basic degradation
processes better and allowing us to monitor changes over time.
Conservation science provides a vital link connecting the material aspects
of the artifact to the hand that created it and the context that valued it.
The more we can understand what we see, the more we will value our
heritage and appreciate the importance of managing change .
REFERENCES
1. House of Lords Science and Technology Committee, HL Paper 256, Stationary
Office, (2005), 11 (http://www.publications.parliament.uk/pa/ld200506/ldselect/
ldsctech/256/25602.htm)
2. House of Lords Science and Technology Committee, HL Paper 256, Stationary
Office, (2005), 15 (http://www.publications.parliament.uk/pa/ld200506/ldselect/
ldsctech/256/25602.htm)
3. Brimblecombe, P., et al., Managing dust in historic houses – a visitor/conservator
interface. In ICOM-CC Preprints of the 14th Triennial Meeting, The Hague, 12-16
September 2005, James & James, London (2005)
4. Hocken, R. J., et al., CIRP Annals – Manufacturing Technol. (2005) 54, 705
5. Budnar, M., et al., Nucl. Inst. Methods Phys. Res. B (2004) 219-220, 41
6. Doehne, E., Mikrochim. Acta (2006) 155, 45
7. McPhail, D. S., J. Mater. Sci. (2006) 41, 873
8. www.nationalarchives.gov.uk
9. Waller, R., A Risk Model for Collection Preservation. ICOM-CC Preprints of the
13th Triennial Meeting Rio de Janeiro, 22-27 September 2002 , James & James,
London (2002) 102, 100
10. Larsen, R., et al., (eds.), STEP Leather Project. Report No. 1. European Commission,
Director General for Science, Research and Development, The Royal Danish
Academy of Fine Arts, School of Conservation, (1994), 151
11. Kennedy, C. J., et al., Nano Lett. (2004) 4, 1373
12. www.getty.edu
13. www.surreyibc.ac.uk
Fig. 10 FIB secondary electron and ion images of a cross-section of a
photograph. The cross-section was prepared by ion beam milling but the
surface was first coated with a platinum bar to protect the material during this
procedure. The secondary electron and secondary ion (total ion) images reveal
the different contrast mechanisms in these two modes of analysis and highlight
different aspects of the multilayer structure. (Courtesy of Richard Chater,
Imperial College London.)
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