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Cleaning silver with atmosphericpressure plasma
S i l v e r o r s i l v e r a l l o y s u r f a c e s d i s c o l o r o ver time in air: they become tar nished. The reason why tar nishing
o c c u r s i s t o b e f o u n d i n t h e s e n s i t i v i t y o f the silver surface to sulfurous gases, such as are present in small
q u a n t i t i e s i n t h e a m b i e n t a i r. E v e n a t room temperature hydrogen sulfide reacts with silver to form silver
s u lf i d e , w i t h b ro w n i s h - b l a c k d i s c o l o r a t i ons appearing on the surface of the silver.
Preservation of cultural heritage
silver-plated objects and also for items which do not have a
Works of art and other cultural objects are valuable witnesses
smooth and even surface. Every time the item is polished,
in providing information about cultural movements, past
some of its original substance is lost and thereby a part of the
customs, materials and technologies. Frequently, however,
cultural heritage.
these objects are put at risk due to weathering, environmental
influences, corrosion or microbial attack. In conjunction with
Silver reduction in atmospheric-pressure plasma
five other Fraunhofer institutes the Fraunhofer IST, within the
A discharge is ignited in a plasma nozzle working at
Cultural Heritage Research Alliance, is developing ways of
atmos­pheric pressure, carried outwards by the gas flow of
using plasma technologies for preserving our cultural heritage
a nitrogen-hydrogen mixture and brought into contact with
and thus expanding the range of restoration methods.
the tarnished material. Due to the action of the reductive gas
Scientists at the Fraunhofer IST have been researching for
mixture the tarnished surface deoxidizes and the black discolo-
some time into the careful cleaning of silver by means of
ration disappears. The major advantages of plasma treatment:
atmospheric-pressure plasma.
Problems in conventional silver polishing
The silver sulfide must be removed from time to time for
the silver surfaces to gleam as they should. Conventionally,
restorers work with an abrasive mixture of calcium carbonate
and water, known as whiting. This method does, however,
present problems, particularly for thin-walled, delicate and
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No loss of material
Cleaning of highly sensitive exhibits possible
Targeted local treatment
Simple to use: the plasma nozzle is guided over the item
like a pen
Easy to monitor – unlike low-pressure processes
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1 - 2 Silver-plated tea pot
before and after a reductive
treatment with atmosphericpressure plasma.
3 Stereomicrograph in
6.5 x magnification of a
textile with integrated silver
threads.
Application examples
This method is of interest for historical textiles in which textile
and metal are present in a fixed, inseparable association –
such as, for example, tapestries of silk and silver. Conventional
cleaning methods not only result in loss of substance in the
case of the silver but also have a damaging effect on the silk
proteins. Plasma treatment can be a protective alternative
here. Another area of application is very fragile objects which
deform plastically under mechanical polishing. Since the
atmospheric-pressure plasma process is a contactless method,
silver sulfide can be removed without deforming the object.
Outlook
To what extent other compounds occurring on silver objects,
such as, for example, silver chlorides and sulfates, and even
copper compounds, can also be reduced in the plasma is
currently still under investigation.
Contact
Dr. Michael Thomas
Phone +49 531 2155-525
[email protected]
Dipl.-Ing. Margret von Hausen
Phone +49 531 2155-622
[email protected]