Atmospheric pressure non-thermal plasma cleaning of 19th century
daguerreotypes
M Boselli2, C Chiavari2, V Colombo1,2, E Ghedini1,2, M Gherardi1, C Martini1,2, F Rotundo2, P. Sanibondi1
Alma Mater Studiorum-Università di Bologna
Department of Industrial Engineering (D.I.N.)
2
Industrial Research Centre for Advanced Mechanics and Materials (C.I.R.I.-M.A.M.)
V.le Risorgimento 2, 40136 Bologna, Italy
e-mail: [email protected]
1
Abstract: In the present work, the feasibility of using atmospheric pressure non-thermal
plasmas to clean the surface of a corroded 19th century daguerreotype has been examined and
a proof-of-principle demonstration is given. The daguerreotype was treated by means of a
commercial plasma jet source (kINPen 10, Neoplas Tools GmbH) by different
argon-hydrogen gas mixtures (up to 35% H2 content) so as to remove corrosion products,
without immersion in solvents and chemicals. The influence of the plasma treatment on the
image quality, surface composition and its effectiveness in removing tarnishing products has
been evaluated by means of scanning electron microscopy (SEM) with energy dispersive
microprobe (EDS) for localized elemental analysis, micro-Raman and ATR-FTIR
spectroscopy for phase identification. The main process parameters, critical to achieve an
effective cleaning while preventing damage to the fragile image, have also been investigated.
Keywords: non-thermal plasma cleaning, localized plasma treatment, cultural heritage.
1. Introduction
Daguerreotypes represent the first practical form of
photographs, particularly in vogue between 1840 and
1860, before becoming outdated due to the development
of alternative and faster techniques. In daguerreotypes,
the image results from the distribution of silver/mercury
amalgam microcrystals of varying size and density on a
silver-coated copper sheet.
Through years, 19th century daguerreotypes have
generally suffered several corrosion phenomena, which
have greatly reduced their historic and artistic value. The
daguerreotype surface is typically covered by an irregular
layer of tarnish, whose composition varies from piece to
piece depending on its preparation procedure,
conservation history and possible previous cleaning
attempts. The exposure to atmospheric moisture and
sulphur caused the formation of a tarnish film layer,
composed by silver compounds such as silver sulphide,
silver oxide and silver chloride [1-3]. Moreover, a layer of
silicate particles could be found as originating from
covering glass deterioration [4].
Photo-conservation requires non-invasive cleaning
methods, which allow controlled removal of non-coherent
corrosion products without damaging the fragile image
structure and appearance before conservation procedures.
Several different approaches have been applied, starting
from first cleaning and restorations attempts in the 1930s.
Immersion in chemical solvents (cyanide and thiourea
cleaners) caused fading of images and permanent spotting
due to silver loss, so that this process was discontinued in
the 1970s [1]. In the early 80’s, low-pressure non-thermal
plasma cleaning was proven to be able to remove tarnish,
causing a visible improvement in the appearance of
cleaned daguerreotype surfaces [1, 5]. Although
microstructural features were generally preserved, faint
white areas were observed in areas that were highly
corroded, caused by micro-etching of the plate surface.
Electro-cleaning with immersion in alkaline solutions was
then found to be effective in cleaning without causing any
observable alteration or damage to the image structure.
However, when the silver electrode accidentally touched
the daguerreotype, the latter was irreversibly damaged.
Moreover, the procedure was not applicable to coloured
plates and ungilded plates [1]. None of the cleaning
methods allowed selective treatments on specific areas
until laser cleaning was introduced in late 90’ [4, 6]. The
laser cleaning process is based on thermal ablation,
relying on the different material removal thresholds for
corrosion layer and silver, allowing for removal of
corrosion with “no apparent damage to the image itself”
[6] or “minimal removal of vital information” [7].
In the present work, an atmospheric pressure
non-thermal plasma treatment was applied to clean the
surface of a corroded 19th century daguerreotype by using
different argon-hydrogen gas mixtures. This cleaning
method allowed removal of corrosion products, without
using liquid solvents or chemicals. The effectiveness of
the plasma treatment on the controlled removal of tarnish
products and on image quality has been evaluated through
optical and microstructural analyses.
2. Materials and methods
19th Century daguerreotypes
The daguerreotype used in this study is a 19 th century
portrait of an unknown seated woman. The plate is about
7 cm x 8 cm (with beveled corners). Prior to experimental
work, the surface showed significant surface corrosion or
tarnish, darkening the photographic image (Fig. 1a).
Atmospheric non-thermal plasma treatment
An high frequency (1.1 Mhz, 6 kVpp) commercial
plasma jet source (kINPen, Neoplas Tools Gmbh) [8] has
been used with an electrode head for molecular gases and
a flow rate of 5 l/min of an argon-hydrogen mixture (up to
a 35% hydrogen content). The source was mounted on a 3
directions CNC pantograph equipped with a laser position
sensing detector, with the axis of the source pointing
perpendicularly to the upper surface of the daguerreotype,
with a 1 mm gap distance between them. Starting from
the centre of the bottom short side of the daguerreotype,
the surface was treated by moving the plasma jet to the
opposite side, at a constant speed of 20 cm/min on a
parallel path to the long side of the sample. Several
further passages were repeated by moving the starting and
ending point 1 mm sideway to the left at each repetition,
till reaching the end of the sample. The treatment was
performed twice.
Daguerreotype characterization
The visual observation of the daguerreotype was
documented by macro-photography (Nikon D70S). The
morphology of the surface was investigated on the
microscopic scale by Scanning Electron Microscopy
(SEM) as well as by Atomic Force Microscopy (AFM) on
the sub-microscopic scale. AFM scans were carried out in
contact mode, under ambient conditions, with contact
forces in the range 25 nN – 30 nN, by using cantilevers
with a nominal spring constant of 0.1 N/m and integrated
tips with a 10 nm radius of curvature.
The identification of corrosion products on the surface
of the daguerreotype was carried out by non-destructive
analysis in terms of both elemental composition (SEM
with Energy Dispersive Spectroscopy (EDS) microprobe)
and phase composition (micro-Raman and Attenuated
Total Reflection (ATR) Fourier Transform InfraRed
(FT-IR) spectroscopy). Raman spectra were collected with
the Ar+ laser (λ=514.5 nm), employing low laser powers
(output ≤ 5 mW) to avoid sample degradation. ATR-FTIR
spectra were recorded in the spectral range of 400 to 4000
cm-1, resolution 4 cm-1, 32 scans, using a KBr windows.
3. Results
Visual appearance
Fig 1 shows the changes in the daguerreotype visual
appearance respectively after the 1st and 2nd plasma
treatment of the image left half. The daguerreotype was
originally covered with a bluish film as shown by the Fig.
1a. The central area clearly demonstrates an improvement
in the overall visual appearance already after the 1 st
plasma cleaning treatment (Fig. 1b). In particular, the
treated portion appears brighter and sharper. The 2 nd
treatment appears to supply a limited enhancement of the
image quality, but most importantly demonstrates that
subsequent plasma treatments at the chosen operating
conditions preserve the image visual features. The
improvements can be detected even more clearly by
observing at higher magnification particulars of the
woman’s visage (e.g. woman eye on the left side, Fig. 2)
at higher magnification. Dark blue areas, mainly located
near the daguerreotype borders, seems to be only
marginally affected be the cleaning procedure.
(a)
(b)
(c)
Fig.
Fig. 1. 19th century daguerreotype portrait: (a) untreated
conditions; (b) after 1st plasma treatment (left side); (c) after
2nd plasma treatment (left side).
The improvement in image definition can be observed
in the eye on the left-hand side, before and after the 1st
plasma treatment (Fig. 2).
(a)
(b)
Fig. 2. Optical micrographies in white light of the figure eye
on the left side in (a) untreated conditions and (b) after 1st
plasma treatment.
Morphological and chemical characterization
SEM images were taken before and after plasma
treatment. As shown in Fig. 3, plasma treatment do not
seem to alter the surface morphology, with particular
regard to features characterizing the grey levels of the
images (as the shape or number of particles of Ag-Hg
amalgam). Actually, these morphological features need to
be preserved in order to maintain the contrast to the
image.
(a)
(b)
(c)
(d)
Fig. 3. BSE images captured on a bright (a, c) and on a dark (b,
d) area before (a, b) and after (c, d) plasma treatment.
the plasma cleaning treatment was effective in removing
the tarnish film. Secondly, the plasma treatment also
reduces the baseline (which is probably related with
fluorescence due to organic compounds [10]). Therefore,
the cleaning treatment might have removed organic
compounds on daguerreotypes’ surfaces. The presence of
organic compounds might be related with the use of
varnishes and coatings. However, protective coatings and
varnishes were not frequently applied for protecting
ungilded daguerreotypes, because the damaging optical
effects from the coatings (the highlights appear less white
and the black tones become dull) seemed to
counterbalance the advantages [11]. The binding media
used for hand-coloring, which was in use also for
daguerreotypes [12], might be an alternative source of
organic compounds. Even though the daguerreotype
under investigation did not show visible evidence of
coloring, it is not possible to exclude that traces of such a
procedure might still be present on the surface, due to the
bad conservation conditions of the plate.
16000
(a)
White Collar
14000
Untreated
12000
1st plasma treatment
2nd plasma treatment
Raman Intensity
On the contrary, as reported in Table 1, the plasma
treatment induced a decrease of the environmental
elements C, S, Cl and O, likely due to the reduction of
Ag sulphides and chlorides formed as a consequence of
the interaction with the indoor atmosphere (see Raman
analyses below).
10000
8000
6000
4000
Table 1. Surface composition (by large area EDS analyses) of
the daguerreotype before and after conservation treatment,
bright (particle) and dark (background) areas.
Background
Background
Particle
Particle
Treatment
Untreated
Treated
Untreated
Treated
C
2.1
0.2
1.6
0.9
Composition, % wt (bright area)
O
S
Cl
Ag
Au
2.8 0.6 0.1 85.0 9.1
1.7
85.8 12.3
2.2 0.4 0.3 78.4 15.2
1.0
85.4 11.6
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
1800
2000
Wavenumber/cm-1
4500
Hg
0.3
1.9
1.1
Raman spectroscopy was used in situ for comparing the
phase composition of the surfaces before and after plasma
treatments, because Raman can identify deterioration
products due to the ability of the daguerreotypes’ silver
surfaces to behave as SERS (Surface-enhanced Raman
scattering) substrates, thus not requiring sample removal
[2 Centeno08].
Fig. 4 reports Raman spectra recorded in areas with
different grey levels: bright area (Fig. 4a) and dark area
(Fig. 4b). After plasma treatment, two main modifications
were observed in Raman spectra: firstly, the intensity of
the broad band at about 240 cm-1 (which can be attributed
to the Ag–Cl stretch, 242 cm-1 according to [2], as well as
to the Ag-S stretch, 243 cm-1 [9]) significantly decreases,
suggesting (in agreement with EDS data in Table 1) that
Dark Background
4000
(b)
Untreated
1st plasma treatment
2nd plasma treatment
3500
Raman Intensity
Area
2000
3000
2500
2000
1500
1000
500
0
0
200
400
600
800
1000 1200
Wavenumber/cm-1
1400
1600
Fig.4. Raman spectra recorded in a bright area (white collar,
a) and in a dark area (background, b).
FTIR
analysis
showed
the
presence
of
polydimethylsiloxane [13], which is the most common of
the silicone rubbers, likely applied on the daguerrotype as
hydrophobic coating. The Si-O-Si backbone shows a
broad band with two maxima at 1092 and 1016 cm-1. The
methyl deformation band at 1261 cm-1 is strong and sharp,
as well as the contribution from the Si-C stretch at 798
cm-1. The asymmetric CH3 stretch at 2964 cm-1 is also
sharp and relatively strong. The lowering of peaks at 2964
and 1016 cm-1 after plasma treatment is possibly related
to a partial removal or structural modification of this
coating. In the next steps of the work, further IR analyses
will be carried out for a more detailed characterization of
organic substances on the surface of this daguerreotype.
0.16
1st plasma treat.
0.14
2nd plasma treat.
Untreated
0.12
Assorbance
0.1
0.08
0.06
0.04
0.02
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Wavenumber (cm-1)
Fig. 5. FTIR spectra recorded in a dark area (background).
Conclusions
The feasibility of using atmospheric pressure non-thermal
plasmas to clean the surface of a corroded 19th century
daguerreotype has been demonstrated in the present work.
Prior to experimental work, the surface showed
significant surface corrosion or tarnish, darkening the
photographic image. Overall visual appearance clearly
improved already after the 1st plasma cleaning treatment;
the treated portion being brighter and sharper. The 2nd
treatment supplied a limited enhancement of the image
quality, demonstrating that subsequent plasma treatments
at the chosen operating conditions preserve the image
visual features. SEM-EDS, Raman and FTIR
spectroscopy analysis suggested that the treatment was
effective in removing the tarnish film.
Further investigations will also involve the use of
different plasma sources, able to perform large area
treatments at atmospheric pressure with controlled gas
composition, for the cleaning of corroded daguerreotypes
and other cultural heritage applications.
References
1. M.S. Barger, A. P. Giri, W.B. White, T.M. Edmondson,
Cleaning Daguerreotypes, Studies in Conservation 31
(1986) 15-28.
2. S.A. Centeno, T.Meller, N. Kennedy, M. Wypyski,
The daguerreotype surface as a SERS substrate:
characterization of image deterioration in plates from
the 19th century studio of Southworth & Hawes, J.
Raman Spectrosc. 39 (2008) 914–921.
3. M.S.
Barger,
R.
Messier,
W.B.
White,
Non-Destructive assessment of daguerreotype image
quality by diffuse reflectance spectroscopy, Studies in
Conservation 29 (1984) 84-86.
4. I. Turovets, M. Maggen, A. Lewis, Turovets, Cleaning
of daguerreotypes with an excimer laser, Studies in
Conservation 43 (1998) 89-100.
5. V. Daniels, Plasma Reduction of Silver Tarnish on
Daguerreotypes, Studies in Conservation, 26 (2) 1981
45-49.
6. V.V. Golovlev, M.J. Gresalfi, J.C. Miller, D. Anglos, K.
Melesanaki, V. Zafiropulos, G. Romer, P. Messier,
Laser characterization and cleaning of nineteenth
century daguerreotypes, Journal of Cultural Heritage 4
(2003) 134s–139s.
7. M.J. Abere, R.D. Murphy, B. Jackson, G. Mourou, M.
Menu, J. Mansfield, B. Torralva, S.M. Yalisove,
Cleaning Daguerreotypes with a Femtosecond Laser,
Microsc. Microanal. 17 (2) (2011) 1812-1813.
8. K. D. Weltmann, E. Kindel, R. Brandenburg, C. Meyer,
R. Bussiahn, C.Wilke, and T. vonWoedtke,
Atmospheric pressure plasma jet for medical therapy:
Plasma parameters and risk estimation, Contrib.
Plasma Phys., 49 (9) 2009, 631–640
9.
rger, M.
Schreiner, Micro-Raman characterisation of silver
corrosion products: instrumental set up and reference
database, e-Preservation Science 9 (2012) 1-8.
10. K. Nakamoto, Infrared and Raman Spectra of
Inorganic and Coordination Compounds, Theory and
Applications in Inorganic Chemistry, John Wiley &
Sons, 2008.
11. M.S. Barger, A.P. Giri, W.B. White, W.S. Ginell, F.
Preusser,
Protective
surface
coatings
for
daguerreotypes, JAIC 24, 1, (1984) 40-52.
12. H.K. Henisch, B.A. Henisch, The Painted Photograph
1839-1914 Origins, Techniques, Aspirations (1996).
University Park, PA: The Pennsylvania University
Press.
13. L.J. Bellamy, The IR spectra of complex molecules,
Wiley & Sons, New York, 1964.