Plasma

The use of plasmas for various  operations with metals  is well
worked out and actually, highly controllable. However, when working
with objects that are made of different metals or are variously
corroded and not corroded you need to carefully think out your
sputtering parameters. For instance silver has an extremely high
sputtering rate, while silver oxide has a very low rate. So, if you
are sputtering corroded silver, you have to figure out how to
prevent the "already-cleaned" metal surface from being sacrificed as
areas with heavier oxidation are "cleaned." When I was working on
daguerreotype cleaning using plasmas, we even tried poisoning the
"cleaned" surface by leaking hydrogen sulfide into the sputtering
chamber to form a protective silver sulfide monofilm on the cleaned
portion of daguerreotypes. This worked, but I knew I would be hard
pressed to encourage conservators to deliberately expose a
daguerreotype to hydrogen sulfide gas as a protective measure.

As my group and others reported, reported daguerreotypes "cleaned"
in plasmas develop "White films" which is actually due to the
microetching of the silver surface which gives rise to scatter and
appears like a white film. Daguerreotypes are a particular case
because they have very peculiar requirements for the preservation of
surface quality in order to maintain their proper appearance.
Objects that do not have such rigorous parameters for their surface
quality may not have the same problem.

Another concern of ours at the time was that the plasmas might make
the daguerreotype surface more reactive for future corrosion. I have
daguerreotypes that were sputtered 18 or 19 years ago and it is hard
to say if they have begun to retarnish because they were sputtered.
Some show slightly more tarnish than others. However, in comparison
to daguerreotypes that were electrocleaned in the same time period,
the sputtered daguerreotypes all show more tarnish.

My final observation about using plasmas for routine conservation
treatments is that this is a process that requires fairly expensive
equipment and skilled workers who can be thoughtful about the
requirements imposed by working on works of art or artifacts. This
might be a process that can be done on a for-fee basis by one or two
centers who have set themselves up to fulfill a specialized niche.

M. Susan Barger, PhD
Department of Earth and Planetary Sciences
University of New Mexico

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                  Conservation DistList Instance 12:82
                 Distributed: Thursday, April 22, 1999
                       Message Id: cdl-12-82-001
                                  ***