22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Application of atmospheric plasma-jets for the conservation of cultural heritage
E. Grieten1,2, P. Storme2, J. Caen2, O. Schalm2 and D. Schryvers1
1
2
EMAT, Department of Physics, University of Antwerp, BE-2020 Antwerp, Belgium
Heritage and Sustainability, Department of Design Science, University of Antwerp, BE-2000 Antwerp, Belgium
Abstract: New technological solutions from different fields such as atmospheric plasma
can be adapted for the conservation of cultural heritage objects. One of the applications
with atmospheric pressure plasma is the selective removal of corrosion from historical
photographs. To study the influence of the plasma, the material was characterized with ex
situ TEM during several phases of the treatment.
Keywords: cultural heritage, photographs, atmospheric plasma, TEM characterization
Nowadays, atmospheric pressure plasma-jets are being
used for many industrial and medical applications [1].
They also show a high potential for the field of cultural
heritage (CH) as an alternative conservation to current not
sustainable methodologies [2]. Nevertheless, there are
several barriers that slow down the implementation of
these new technologies in the sector. Firstly CH is only a
small market but also the stakeholders itself often lag
behind with their approval of the new techniques.
Secondly, the demands that are enforced on the treatment
itself especially on its long term stability and noninvasiveness. However there is a demand for new
methodologies since today’s solutions involve toxic
substances, release VOCs and are often invasive which
results in the loss of original image material. The proof of
concept of the application for historical photographs was
given by Boselli et al. [3]. They showed that H 2
atmospheric plasma- jets selectively remove the tarnish
layers from a photograph, which results in an enhanced
readability of the image (see Fig. 1). This technique
shows several advantages compared to traditional
treatments because it is a local, selective, sustainable and
non-contact method.
types of photographs were selected. A daguerreotype
(Fig. 1a) is the first successful photographic process and
consists of a silver plated copper substrate with an image.
The glass negative (Fig. 1b) is one of the first industrially
mass-produced photographic techniques where the image
is formed by metallic silver particles suspended in a
gelatine emulsion on a glass substrate. Both techniques
are sensitive to the degradation of the image. This can be
identified as a discoloration or tarnishing of the image
altering the readability and stability of the photograph.
Fig. 2. a) Daguerreotype with corrosion; b) gelatine glass
negative with corrosion (blue discoloration and accretions
form water damage (see top part image).
Fig. 1. Detail of historical photograph (daguerreotype)
before (left) and after (right) plasma cleaning.
The next step in implementing this application in CH is
to have an in-depth characterization of the impact of the
new method for different CH objects. In this study, two
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A dielectric barrier discharge (DBD) commercial
plasma jet is used for the experiments. For the removal of
silver based corrosion a reducing gas mixture of 5% H 2 in
He is selected. The plasma-jet is mounted perpendicular
to the work surface which can be moved in the x,y,z
direction.
To evaluate the selectivity and effectiveness of the
technique, both glass negatives and coloured gold-toned
daguerreotype are characterized before and after plasma
cleaning using a multi-analytical approach including
scanning electron microscopy (SEM) and (scanning)
transmission electron microscopy ((S)TEM) coupled with
spectroscopy techniques such as energy-dispersive X-ray
spectrometry (EDS) or electron energy-loss spectroscopy
1
(EELS). By using spectral imaging we can link the
interaction or chemical and physical changes to the spatial
distribution of the corrosion phenomena. In this project
we study the plasma results by characterizing both the
surface of the photographic object as the internal structure
(via FIB sample preparation) so the changes to the
topography and chemical composition due to the plasma
treatment can be evaluated.
To optimize the application several parameters such as
power, gas flow, work distance and time were studied for
both bulk metal with corrosion as photographic materials.
Fig. 3 shows the colour change (i.e., the Euclidean
distance between 2 colour coordinates, ΔE 76) indicating
that the corrosion is more efficiently removed when the
speed of the sample movement is lowered. Similar colour
difference is seen when the gas flow is increased. These
practical experiments gave an insight in how the plasma
could be optimized for several objects with Ag based
corrosion. In the presentation an overview of the different
user parameters, the location of the object in respect to the
plasma and the influence of the environment itself will be
given.
similar results (see Fig. 5). Also the morphology and the
thickness of the corrosion layer remains the same (see top
Fig. 5).
Fig. 4. 1 & 3: EFTEM map before plasma treatment, and
2 & 4: after plasma treatment.
Fig. 3. Influence on ΔE of the colour of the corrosion
after treatment when the linear sample travel speed
(mm/s) and power (W) of the plasma are changed.
Experiments on the two types of photographs
(Daguerreotype and glass negatives) demonstrate that
under certain conditions, the low temperature plasma is
able to remove the corrosion without damaging the
original material of the artwork. When the corrosion is
pure silver corrosion (Ag 2 S) on a metallic substrate the
plasma reduces the entire layer to metallic silver which
results in an improvement of the image. However, when
the corrosion is present on a different substrate such as
gelatine in case of the glass negatives, the plasma is not
able to reduce the nanoparticles to metallic Ag but the
results the treatment in a change in topography. This can
be seen in Fig. 4 where the EFTEM maps shows that the
chemical composition of the corrosion didn’t change by
the plasma treatment but that the image quality is altered
due to the change in topography or arrangement of the
nanoparticles in the gelatine matrix. Also when the
corrosion contains other products such as Cu, we see
2
Fig. 5. The plasma is not effective at the edge corrosion
of the daguerreotype which contains Ag, Cu and S
compounds.
Acknowledgements
The authors thank Herman Maes and Elodie TexierBoulte from the CRP2 in Paris for the daguerreotypes
used in this study. E. Grieten is grateful for a BOF fund
of the University of Antwerp.
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References
[1] O. Goossens, et al. Surf. Coatings Technol.,
142-144 (2001)
[2] A. Patelli, et al. Lecture notes in computer science.
7617 (2012)
[3] M. Boselli, et al. ICOPS (2013)
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