occur but none of the problems mentioned above will commence. Very large
artefacts such as the cannons from the Mary Rose, could be covered with wet
sawdust, damp cloths or plastic wrapping in an attempt to maintain water levels
around the metal surface.
If there is a conservation laboratory nearby, immerse the artefact straight
away in an electrolyte, which passivates any exposed metal. The pH can be
ascertained from the relevant E-pH diagram for the metal concerned and
examples for ferrous artefacts would be 0.5 M sodium hydroxide, 0.2 M sodium
sesquicarbonate or 0.5 M sodium carbonate.
For transportation of the artefact it is essential to maintain treatment initiated
in the field. If the items cannot be moved in their storage tanks, they must be
packed as mentioned for the large artefacts above, i.e.use sawdust, etc. If by the
smell of the corrosion products (presence of hydrogen sulfide gas), microbial
corrosion of the artefact is suspected, it would be advisable to add a biocide in
order to minimise this type of attack until it can be treated in the laboratory.
4 Conservation
The main aims for the conservation of metals are as follows:
- Arrest the corrosion process;
- Remove chloride ions from the corrosion products;
- Leave the shape of the artefact unaltered; and
- The metallurgical structure of the artefact remains unaltered.
The removal of the chloride ion is the most essential, but the conservator
must consider the other three aims when considering what method to use. For
example, there are several proprietary solutions on the market or ones that can
be made up in a laboratory, which will dissolve all the corrosion products but
not the underlying metal. This method is not suitable if the artefact has thick
layers of corrosion products on the surface as the shape will be lost. Even
worse, if the artefact was just composed of solid-corrosion products (com-
pletely mineralised), there would be nothing left after immersion in these
solutions! This method is only suitable for those artefacts recovered with thin
layers of corrosion products on their surface.
A relatively simple method is to dissolve out the chloride ions by immersion
in a suitable solvent. Water has been used with the water being changed every
month until no further chlorides are detected. This can take up to 5 years for
marine artefacts with high levels of chloride buried within deep rust layers.
Moreover, the metal will continue to corrode, while the artefact is immersed in
the water for this length of time. By altering the pH of the solution it may be
possible to dissolve out the chlorides without corroding the metal. This is
achieved by forming a thin, passive film approximately 10 nm ( 10 ^9 m) thick
Metals 139