Archaeological silver objects

Lisa A. Young asked for advice on cleaning badly corroded silver
artefacts. I think that when talking about cleaning, the first
question is "what surface would be cleaned". So if you want to
recover the ancient shape and the surface details of the object, it
is important to find where the ancient surface (called the "original
surface") can be preserved amongst the corrosion products. It is
often difficult especially when dealing with archaeological silver.

The corrosion crust is often a mixture of silver sulphide and silver
chloride in that case.

The X-rays can help you to determine the extent and the conservation
state of the remaining silver core but unfortunately will not always
give you valuable information about the location of the original
surface. A mechanical cleaning on a small area can allow you to
discover this original surface. But be careful with a mechanical
cleaning on that kind of very fragile objects. The chemical way is
often the safest way here.

If you need to get rid of the silver sulphide/chloride chemically,
you can try to use reducing agent like the sodium dithionite or a
complexing agent like thiourea. With thiourea, it must be slightly
more efficient at a acidic pH than at higher pH due to the good
stability of the silver sulphide at high pH. An electrolytic
treatment can also be considered in a sodium nitrate solution (1M
for instance) at a cathodic potential of - 1.2 Volt ESS (versus a
mercurous sulphate electrode in a saturated potassium sulphate
solution).

Regis Bertholon
Archaeological metal conservator
Institut de Restauration et de Recherches Archeologiques et
Paleometallurgiques
and teacher at
MST Conservation Restauration des Biens Culturels
University Paris 1 Pantheon-Sorbonne
Fax : +33 3 44 57 48 02

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                  Conservation DistList Instance 12:23
                Distributed: Tuesday, September 1, 1998
                       Message Id: cdl-12-23-003
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