on the exposed metal. For example, wrought iron will form a passive film in a
solution above pH 9. Hence sodium hydroxide, sodium carbonate or ammonium
hydroxide would be suitable for this class of artefacts. The disadvantage of these
solutions is that they still take a long time to remove all the chlorides and one is
not absolutely certain that all the chlorides have been removed even after a
period of 5 years.
For artefacts recovered from marine sites, they are often covered in con-
cretions. These are hard layers of calcareous deposit derived from decaying
shells of aquatic animals (e.g.barnacles, mussels, etc.) or hardness salts pre-
sent in seawater. The latter is present as soluble bicarbonate ions (HCO 3 ) in
sea or fresh water. At cathodic sites on the metal surface, there is a rise in the
local pH due to the production of OHduring the reduction of dissolved oxygen
gas (see Equation (10) in corrosion section). This results in the precipitation of
solid calcium carbonate (CaCO 3 ) scale on the cathodic sites according to the
following reaction:
(11)The concretions arising from the two sources mentioned above, may be up
to 15 cm thick and must be removed prior to any conservation method being
carried out. This has to be done carefully as it is very easy to cause severe dam-
age to the artefact particularly if it is in a very fragile state.
There are several techniques employed by conservators, but the most popu-
lar ones are those using mechanical methods. It is advisable to carry this out
when the artefacts are moderately dry just prior to carrying out the conservation
process. The common methods in this class are by use of chisels, dentist drills
or shot blasting using bauxite, glass beads, magnesium carbonate or just air.
Chemical methods have been utilised where the concretion is dissolved in
a suitable solvent. Solutions such as 1–2 M nitric acid, 2.5 M phosphoric acid
and 2–3 M hydrochloric acid have been used, but there is a severe danger of
the metal dissolving if the artefact is not removed at the exact moment when
all the concretions is being dissolved. Inhibitors could be added to the acids
but one is never certain of the exact composition of the metal beneath the
concretion. Each metal or alloy will have its own inhibitor so it is very easy
to add the wrong inhibitor.
Finally, thermal methods such as placing the artefact in a hydrogen furnace
at 400°C has been successfully employed on ferrous artefacts recovered from
the Mary Rose. Any remaining concretion on the surface was present as a fine
powder after conservation and was just gently brushed off the surface. An
alternative thermal method involved placing the artefact in an inert atmos-
phere at 1066°C followed by quenching. The procedure is repeated until all
the concretion has spalled from the metal.
CaHCO 3 OH→CaCO 32 H O140 Chapter 6