Archaeological silver objects

I would like to follow Paul Storch's excellent advice on this query
with several points:

    *   Radiography of these materials seems to be indicated, not
        only for artifact identification, but to assess how much
        core metal survives and in what condition (i.e., dealloying
        or intergranular corrosion might be present).  If you do not
        have regular access to an industrial x-ray unit then try one
        of the conservation labs in the Washington D.C. region or
        contact either a factory that manufactures high tech
        castings, or, a airport-maintenance shop where
        non-destructive examination (NDE) is used.

    *   Verify your corrosion products.  Are you sure that you have
        a silver sulfide crust?  Have you tested samples for
        chlorides?  A tiny sample taken to an experienced
        metallurgist, such as Martha Goodway at SCMRE, the lab
        formerly know as CAL, could produce just the information and
        advise you need.  The corrosion morphology might be
        interesting given the fact that these objects came from a
        privy environment and may have biological corrosion present
        in addition to attack from organic acids from the other
        materials in that environment.

    *   I entirely agree with Paul.  Mechanical cleaning on heavily
        corroded silver probably incurs more risk to the artifact
        than other methods.  Silver, even in a pristine state, can
        either be very ductile or extremely brittle, depending on
        its alloy composition and fabrication history.
        Hammer-hardened silver and silver than has been
        over-oxidized (e.g. ":cooked" as a silversmith friend calls
        it) can have a network of macro and micro cracks and be
        extremely fragile when further stresses are applied via
        mechanical cleaning.  It is also important that you make
        sure to verify whether you are dealing with either a silver
        alloy material or silver plate.  Silver plate, especially
        early Sheffield plate,  is more problematic to deal with in
        treatment (especially electrolysis).

    *   If you have not already done so, read both "The Conservation
        of Archaeological Artifacts From Freshwater Environments" by
        Katherine Singley (Lake Michigan Maritime Museum, 1988);
        and, "The Conservation of Metal Artifacts From Underwater
        Sites: A Study in Methods" by Donny Hamilton (Texas memorial
        Museum Miscellaneous papers 4, 1976). Any other publications
        on the conservation of waterlogged materials can also help
        you sort out the pros & cons of different treatment
        approaches.  Also check out the AATA for references. Ann B.
        N'Gadi, the technical information specialist at SCMRE, can
        offer invaluable assistance in helping you with literature
        searches, call her at 301-238-3700 x134 to make an
        appointment.

Best Wishes,

David Harvey
Associate Conservator,
Metals & Arms
The Colonial Williamsburg Foundation
P.O. Box 1776
Williamsburg, Virginia  23187-1776  USA
757-220-7039

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                  Conservation DistList Instance 12:22
                  Distributed: Friday, August 28, 1998
                       Message Id: cdl-12-22-001
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