Paper Conservation: Decisions & Compromises

Fig. 1: Germanium ATR FTIR spectra of carbon black inks on
modern printer paper.
Fig. 2: Results of the subtraction of the paper spectrum from the
spectra presented in Fig.1.
nium ATR crystal are presented (Figs. 1-2); note
that only the results for the samples on modern
printer paper are given, but those for the other
three types of substrate were similar. The results
obtained using the diamond ATR crystal were
also broadly similar, but the results of the spectral
subtraction were less well defined.
If the subtraction spectra are considered, it
can be seen that certain characteristic features
may be observed, related to the origin of the
ink: For the modern carbon black ink, a largely
featureless background curve is observed, indicating
that the ink spectrum consists almost
entirely of a simple carbon black absorption. The
two cellulose based inks (coconut and bamboo)
yield spectra containing an indication of residual
polysaccharide material (highlighted by the typical
cluster of peaks in the region 1100 - 900 cm -1 );
this is poorly defined, as the cellulosic substrate
complicates the spectral subtraction, but is sufficiently
good for the purposes of identification.
For these inks, the results were more clear when
the samples using the parchment (proteinaceous)
substrate were considered. As may be expected,
it was not possible to differentiate the carbon
sources for different types of cellulosic material.
Ivory black ink gives a characteristic sharp
peak centred on 1050 cm -1 . Lamp black contains
residual oils, indicated by the pair of sharp C-H
stretch bands in the region 2800 - 2900 cm -1 .
It is apparent that the results obtained using
the germanium ATR crystal are generally superior
to those achieved with a diamond crystal.
This may be explained if the difference in ATR
sampling depth for the diamond and germanium
crystals is considered (Fig. 3), based on the following
formula (Coates and Sanders 2000; Coombs
1998; Spectra-Tech 2000):
Where:
= Depth of sampling penetration
= Wavelength of incident radiation
= Refractive index of ATR crystal
= Refractive index of sample
= Angle of incidence
The ATR accessory employs an angle of incidence,
, of 45°; over the range that the spectra
were recorded, the refractive index of the crystal,
n c
, is approximately 2.4 for diamond and 4.0 for
germanium; the refractive index for the sample,
n s
, is taken to be 1.6 for a nominal organic material.
This allows the way that the ATR sampling
depth varies with incident wavelength and,
importantly for this experiment, at any given
wavelength is approximately four times great for
diamond than for germanium. This means that
if a germanium crystal is used, a greater proportion
of any material at the surface (e.g. ink) will
be observed in comparison to the bulk substrate,
than if a diamond crystal is employed.
When the spectra of the unknown historic ink
ICOM-CC Graphic Documents Working Group Interim Meeting | Vienna 17 – 19 April 2013
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