the Solent, they were in very good condition and were protected by a thin
1–3 mm of corrosion products.
Once the artefacts are brought up from the sea-bed as illustrated in Figure 3,
oxygen can reach the metal surface and corrosion can recommence due to dam-
age or removal of the protective layers. Artefacts require some form of imme-
diate treatment, therefore, to prevent deterioration while awaiting permanent
conservation. In addition, slow atmospheric drying produces a concentrated
salt solution in the pores or cracks beneath concretions, etc., which is con-
ducive to rapid corrosion. The role of the chloride ion in the corrosion
process has already been discussed and as the concentration of chloride in the
remaining electrolyte increases so does the rate of corrosion. Cast iron is very
much prone to this form of attack. A final problem is that the corrosion products
will slowly dry out and lose their water of crystallisation. This involves a reduc-
tion in volume of the corrosion products, which may lead to cracking and
exfoliation of the corroded surfaces. The shape of the artefact may be lost or
seriously damaged and dissolved oxygen can now easily reach the metal surface
with a dramatic increase in corrosion rate.
The first priority is to keep the artefact wet immediately it is brought to the
surface. If the object is very small and shows little evidence of corrosion, it may
be placed in an airtight container along with a desiccant such as silica gel. In
the absence of nearby conservation laboratories, totally immerse the artefact
in seawater or preferably freshwater. Corrosion of any remaining metal will
138 Chapter 6
Figure 3Artefacts recovered from the seabed