metal left. The formation of concretions on the surface will have hindered the
arrival of dissolved oxygen in the seawater and slow or even arrest the corro-
sion process. The heavy wrought iron guns from the Mary Rosewere prevented
from corrosion by being buried deep in the Solent silt. This prevented the
ingress of oxygen to the metal surface. Artefacts buried in soils may remain
uncorroded by similar effects.
Problems arise when these items are excavated and oxygen is again able to
reach the metal surface. Corrosion re-commences at an alarming rate if aggres-
sive ions like chlorides are present in the corroded layers on top of the metal
surface. The aim of the conservator is to prevent this from occurring and to
remove all the deleterious ions in the corrosion products. This must be carried
out without destroying the original shape of the artefact and then allows the
conserved artefact to go on display or in a store without suffering any further
corrosive attack.
In order to fully appreciate the reasons for carrying out the conservation
method selected, it is important to understand in the first instance how the metal
or alloy was manufactured. From modern theories of corrosion of metals in
marine environments, it is possible to predict the mode of corrosive attack that
the artefact may have experienced while being buried or laying on the bottom of
the ocean floor. Any adverse effect on the rate of corrosion on exposure to the
atmosphere can possibly be predicted. From this knowledge, the most effi-
cient methods of field treatments, storage conditions and conservation can be
recommended.
Methods of obtaining the metal or alloy from its ores will be briefly covered
here as with the possible shaping processes to produce the finished object.
The corrosion behaviour of the metal while buried will also be discussed and
this will be related to various methods available in the conservation of these
materials.
1.1 Extraction
Metals are rarely found in their native form with gold being the main exception.
They are present in the earth’s crust as minerals or ores and exist in chemical
combination with other elements, the most important ones being oxygen and
sulfur. Hence, the most common ores are oxides and sulfides, which are mixed
with gravel, limestone, sand and clay. This unwanted material is referred to as
gangue. The amount of metal present in the ores varies from less than 1% to
10%. These ores are very stable and have existed for many thousands of years.
It requires a lot of energy to remove the oxygen or sulfur from the metal and
the best way to achieve this is to heat the ore in the presence of a reducing
agent. This process is called smelting and the chief reducing agent is carbon
either in the form of charcoal or coal. The active reducing agent would either
Metals 123