22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Reduction of the corrosion layers on archaeological glass from vessels: surface
analysis by SEM/EDS
L. Hlochova1, W.G. Graham2, D. Janova1 and F. Krcma1
1
Brno University of Technology, Faculty of Chemistry, Institute of Physical and Applied Chemistry, Purkynova 118,
61200 Brno, Czech Republic
2
Queen's University Belfast, School of Mathematics and Physics, Centre for Plasma Physics, Belfast, BT7 1 NN,
Northern Ireland, U.K.
Abstract: This work deals with the reduction of corrosion layers on archaeological glass
from vessels. The underwater discharge was used as the reducing agent. Objects surfaces
were treated by multi-electrode system for three minutes at applied power of about 5 W.
Visible changes and concentrations of the elements were monitored by scanning electron
microscopy on the glass surfaces before and after treatment and from the object core.
Keywords: corrosion layers, archaeological glass, discharge in liquids, multi-electrode
system, SEM/EDS analysis
1. Introduction
Plasma reduction of corrosion layers on metal samples
has been studied during the last decades. One of the first
groups studying this problem was Prof. Veprek group [1].
Plasma chemical process leading to the removal
of corrosion layers from metallic objects is still under
development [2, 3] and it is used in several technical
museums [4]. The method is based on a partial reduction
of the incrustation and corrosion layers by RF low
pressure hydrogen plasma. Unfortunately, this process
is impossible to use for fragile materials like glass due
to significant local overheating and consequent great
thermochemical stress. Thus in this work we used another
type of plasma source that significantly decreases this
stress and thus probability of objects damage is much
lower. The plasma created by underwater discharge was
used for original ancient glass samples surface treatment.
Corrosion of glass objects means alkali metal ion
exchange H+. The alkali metal ions become from the glass
surface and H+ originates in the water contained in the
objects environment. Corrosion layers on the artefacts
consist mainly of different oxides, various complex
compounds and incrustations [5]. Their concentrations
strongly depend on the object surrounding and thus each
archaeological object has its original composition
and structure of corrosion layers. This contribution
describes a new experiment with the artefacts from 17th18th century. These glass pieces from vessels were
excavated from the soil in Hutsky pond near the village
Josefov in Czech Republic.
2. Experimental
Plasma in water solution was created by a special multielectrode device which principal scheme is shown
in Fig. 1. This high-performance instrument is commonly
used in biomedicine for the treatment of diseased tissues
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so it is clear that is a soft tool for the ancient glass objects
treatment.
Fig. 1. The scheme of the multi-electrode system.
1 ̶ sample, 2 ̶ power electrode, 3 ̶ liquid, 4 ̶ insulator,
5 ̶ grounded electrode.
This system allows direct contact of plasma with
a targeted sample. The thermal effect on the sample
surroundings is minimized due to the presence
of the cooling water solution. In principle, this system
operates as a multi-electrode setup with driving circuit
producing 100 kHz RF bipolar square wave voltage
signals. The current supplying the plasma was measured
directly and it is determined via various impedances
influencing factors dependent on the solution properties
in which the plasma is formed. The main factor for
the treatment was obviously the solution conductivity
which in turn depends on the temperature
and concentration and distance between the treated
sample and electrode head. Also, the presence of other
dissolved or suspended particles or nearby physical
structures (like removed corrosion particles) had
a significant influence. The discharge chemistry in each
specific instance was also affected by the applied current.
The
optimal
experimental
conditions
for the archaeological artefacts treatment were found
as 0.9% water solution of sodium chloride (solution
conductivity of 13 mS∙cm-1). The distance of the treated
1
3. Results
An example of the treated glass vessel is given
in Fig. 2. It is clearly visible that treated surface has
completely different colour because corrosion was
removed and original surface was excavated.
The elementary surface composition before and after
treatment including the comparison with the bulk material
(obtained from the fracture) is given in Fig. 3. It can be
clearly seen that mainly aluminium and sodium as
the typical soil elements in given location were removed
from the surface. On the other hand, contents of calcium
and potassium were significantly increased closely
to their content in the bulk material. These elements are
typically presented in ancient glass and they reflect
the glass material manufacturing.
70
60
At %
surface was about 1 mm from the multi-electrode head
and applied power was set at 5 W. The head of this
system has diameter of 3.5 mm only, so it was relatively
easy to use it locally. This also allows plasma treatment
of objects
with
relatively
complicated
form.
The archaeological artefacts were treated for 3 minutes
on the selected small area with constant power and after
drying they were packed to the protective atmosphere
with oxygen and humidity absorbers. To verify
the surface structure changes as well as changes in
the surface elementary composition, the scanning electron
microscopy with EDS detector was applied on the same
areas of these objects before and after the treatment.
The analysis of the bulk material was added
for comparison of the elementary composition.
before treatment
50
after treatment
40
from the object core
30
20
10
0
O Na Mg Al Si P S Cl K Ca Ti Mn Fe
Fig. 3. Glass from vessel I. EDS analysis– concentration
of elements before treatment, after treatment and from
the object core.
A
B
Fig. 2. Glass from vessel I. Comparison of the visible
changes on the surface before and after treatment.
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C
A
Fig. 4. SEM analysis of glass from vessel I: A/ before
treatment, B/ after treatment, C/ from the object core.
B
Fig. 4 shows SEM photographs of the sample surface
from original, plasma treated, and a bulk material. This
clearly shows that more or less compact surface with
structure like to the object core material was discovered.
Similar results were obtained also for other two selected
plasma treated objects as it is shown in Figs. 5-7, 8-10.
C
Fig. 5. Glass from vessel II. Comparison of the visible
changes on the surface before and after treatment.
70
before treatment
60
Fig. 7. SEM analysis of glass from vessel II: A/ before
treatment, B/ after treatment, C/ from the object core.
after treatment
At %
50
from the object
core
40
30
20
Fig. 8. Glass from vessel III. Comparison of the visible
changes on the surface before and after treatment.
10
0
O Na Mg Al Si P
S K Ca Ti Fe
Fig. 6. Glass from vessel II. EDS analysis– concentration
of elements before treatment, after treatment and from
the object core.
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3
C
70
60
before treatment
after treatment
At %
50
from the object
core
40
30
Fig. 10. SEM analysis of glass from vessel III: A/ before
treatment, B/ after treatment, C/ from the object core.
20
10
0
O Na Mg Al Si P Cl K Ca Ti Mn Fe
Fig. 9. Glass from vessel III. EDS analysis– concentration
of elements before treatment, after treatment and from
the object core.
A
4. Conclusions
This work showed a practical applicability of the one
kind of plasma generated directly in liquid for
the cleaning of ancient archaeological objects made
of glass. The corrosion layers were successfully locally
removed within a few minutes. The SEM images showed
smooth surface structures after the plasma treatment
similar as the bulk glass material structure.
The elementary composition of plasma treated surface
was very similar to the bulk material composition, so
the corrosion layers were removed successfully. No
treated object was damaged during the plasma treatment,
so we can conclude that this technique should be applied
in the practice but further experiments will be needed
mainly with respect to the treatment condition
optimization.
5. References
[1] S. Veprek, J. Patscheider, J. Elmer, Plasma Chem.
Plasma Process. 5 (1985) 201.
B
[2] P. Fojtikova et al. Open Chemistry. Volume 13,
Issue 1, ISSN 2391-5420.
[3] L. Radkova. Romanian Reports of Phys. 67 (2015).
[4] K. Schmidt-Ott, Proceedings of Archaeological Iron
Conservation Colloquium (2010).
[5] M. Melcher, M. Schreiner, Anal. Bioanal. Chem.
(2004) 379.
6. Acknowledgement
This research was done under the project of Czech
Ministry of Culture no. DF11P01OVV004.
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