Paper Conservation: Decisions & Compromises

Fig. 3
similarity maps. Similarity maps between the
mean of the spectrum of the marked area and
the other points in the data are computed by using
the entire spectral bands, visible bands and
selected bands in the near-IR. The central and
right images in Fig. 2 show similarity maps in
greyscale and jet colour respectively.
Fig. 3 shows the spectral plot of the points
marked 1-8 in the colour image of Fig. 2. The
points were chosen based on colour differences
observed visually, and were intended to capture a
wide range of pigments used in the illumination.
A plot of this data allows the user to compare
spectra of different image points to determine
their similarity (or dissimilarity). This has applications
for measuring the corrosion or ink-bleed
severity and separating foreground artifacts from
the background of the image or document under
analysis.
Photographic Degradation: Historical photographs
form an important part of cultural heritage
collections as their examination allows for
the improved understanding of most subjects
of interest. They capture moments in time and
allow observers to connect with characters and
places in the past. Photographs are damaged by
direct sunlight, insects, degrading adhesives,
nearby sulphur compounds and high humidity
which encourages mould growth.
Multispectral analysis has been used recently
on daguerreotypes (Goltz, Hill 2012). Degradation
of daguerreotypes results in the formation of tarnish
on the highly polished silver surface which
can obscure the graphic content of the image. It
was found that the light absorption properties of
a photograph with tarnished and untarnished areas
had significant differences. These differences
allowed for the near-IR camera to image through
dirt and heavily tarnished areas. Multispectral
analysis can therefore be used as a means of visually
showing the conservator how much and
which parts of the photograph have the potential
to be recovered. Spectral analysis could be utilised
as a means of monitoring changes to tarnish
and other photographic artifacts allowing
preventative measures to intervene immediately.
Fig. 4 shows a 20 th century photograph of a girl
making her Holy Communion. A written inscription
in ink along the top records the date and
event. Three bands (420 nm, 620 nm, 1,000 nm)
of the 32 which are produced from the MuSIS
instrument highlight the differences observed at
different parts of the spectrum. Dirt and impurities
visible at 420 nm can be eliminated at 1,000
nm. The ink inscription along the top, which is
faded to observers in daylight, appears enhanced
and more legible at 620 nm.
From Fig. 4 it is evident that each waveband
produced by the multispectral instrument
contains different information. While it is
sometimes useful to isolate these bands it is
also advantageous to combine bands with and
provide contextual details in the entire data volume
in a process known as fusion analysis. This
technique aids in enhancing the legibility of the
data. The fusion technique is also useful in that
it can remove artifacts on the document such as
ink-bleed, ink corrosion, and foxing for research
purposes.
Conclusion
Multispectral images are useful for analysing
deterioration artifacts in archival material and
where possible should be considered as part of
the standard condition assessment process. Multispectral
data can be used to analyse the effects
of environmental aging. It is known that the
effects of changes in humidity and temperature
and exposure to light induce damage to documents
over time. These changes can be systematically
monitored with visualisation tools such
as HSI Labs to track the exact process of aging.
Parchment reflectance can be monitored to detect
degradation before it is visually observed.
Image processing is as important as image
capture. Processing and analysing digital images
ICOM-CC Graphic Documents Working Group Interim Meeting | Vienna 17 – 19 April 2013
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