Archaeological silver objects

I am working on a variety of waterlogged materials from an 19th c.
privy context in Alexandria Virginia.  A majority of the silver and
copper alloys, particularly the silver artifacts, are covered in a
voluminous black sulphide corrosion product.  Careful mechanical
cleaning of these artifacts is very difficult due to the extremely
hard corrosion crusts which are present. It has taken many hours to
remove some of the crusts, if at all possible.  Although I do not
regularly practice cleaning with acids, I was wondering if anyone
who works on excavated artifacts has used a dilute acid to soften or
wear down extremely hard to remove sulphide corrosion crusts such as
these.  The majority of the artifacts are small, such as buttons
with bone/wood backings, coins, etc.  I have searched the literature
but I am drawing a blank. One cleaning method I found from early
papers suggested the use of sodium dithionite to clean silver
metals, but I am not keen to have this chemical in the lab we are
in.  Any suggestions would be appreciated.

Lisa Young
Alexandria Conservation Services, Ltd.
703-317-8400
Fax: 703-317-1148

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                  Conservation DistList Instance 12:20
                Distributed: Wednesday, August 26, 1998
                       Message Id: cdl-12-20-012
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