sites. Lead artefacts form relatively insoluble compounds as corrosion products
in marine environments such as PbSO 4 , PbCl 2 and PbCO 3. These form adherent
films on the metal surface that insulate the metal from the electrolyte and pre-
vent further attack. This results in lead artefacts being recovered from marine
sites in very good condition even after burial for over 400 years, e.g.lead shot
and weights from the Mary Rose.
Tin and pewter artefacts with low lead contents will form a non-protective
SnO 2 layer on the surface in well-aerated seawater environments. Hence, arte-
facts recovered from the surface of the seabed, will be almost completely
mineralised. The shape of the artefact may still be maintained by the tin oxide,
but it will be very fragile and must be handled with great care. In anaerobic
conditions, however, a very protective sulfide film will form, which virtually
inhibits further corrosion. Artefacts recovered from these sites are usually in
excellent condition and require minimum conservation.
Silver items recovered from marine sites are often completely mineralised
due to the non-protective nature of the corrosion products formed in both
aerobic and anaerobic sites. The corrosion product is either silver chloride
(AgCl) or silver sulfide (Ag 2 S). All silver artefacts recovered from the Mary
Rosewere found to be in very poor condition.
As mentioned above for pewter objects, the corrosion products may maintain
the shape of the object even though no metal is left. One of the best examples of
this is observed in cast irons. The iron phase (ferrite) corrodes to form the same
corrosion products as already stated above. The graphite flakes in the cast iron
are inert and trap the corrosion products and the shape is maintained. This is
called graphitisation of cast iron and almost all cast iron artefacts recovered
from archaeological sites have graphitised layers on their surface.
Chloride-containing compounds are, in general, less protective than the
corresponding oxides, hydroxides, carbonates, etc. The chloride ions are said to
be very deleterious as they will rapidly corrode most of the metals with the pos-
sible exception of gold when the artefact is freshly exposed to the atmos-
phere. There are several reasons for this excessive rate of corrosion. The first is
that chlorides will readily dissolve into any moisture films that condense on the
artefact during display or in storage and increase the conductivity of the water.
Ionic conduction is easier through the electrolyte and results in a higher flow of
current in the corrosion cell. The second reason is that chloride-containing
compounds in the corrosion products tend to be more soluble than oxides or
hydroxides, which are the predominant compounds formed in the absence of
chlorides. Liquid corrosion products will not hinder the arrival of oxygen to the
metal surface and the cathode reaction is less impeded. The chloride-containing
compounds are also quite deliquescent and readily attract water. To maintain
completely dry surfaces on the metal artefact, it is necessary to keep the humid-
ity as low as 16% RH for some chloride-containing compounds. This is difficult
134 Chapter 6