Plasma

I am responding to Patrick Storme's posting on the plasma cleaning
of metals, corroded silver in particular.

I would express some concerns about this. First, plasma cleaning, as
I understand it, involves the use of high temperatures in a gas to
excite ions into a high energy jet. What sort of temperatures are
you anticipating to expose the corroded silver object to? Second,
the exposure of silver to elevated temperature would certainly risk
altering the microstructure and destroying the microhistory of the
material. This is the problem that was observed when hydrogen
reduction was applied to corroded ferrous metals in the 1980's.
Third, how would the elevated temperatures affect the phenomenon
known as discontinuous precipitation that corroded ancient silver is
prone to? In this phenomenon the copper in the alloy precipitates
along the grain boundaries and causes embrittlement of the metal.
Could the elevated temperatures and the ionized stream of gas lead
to grain-boundary failure?

Perhaps you have anticipated these questions. If you have not then I
would suggest that you contact Pieter Myers at the Los Angeles
Museum of Art and David Scott at the Getty Conservation Institute
who have the expertise with ancient corroded copper alloys and
silver and their microstructures to give you specific advice.

David Harvey
Metals & Arms Conservator
Williamsburg, Virginia  USA

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                  Conservation DistList Instance 12:81
                  Distributed: Tuesday, April 20, 1999
                       Message Id: cdl-12-81-003
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