steel in the passive region. Once all the sections had been conserved, they
were drained, pressure-washed, dried and painted with a temporary coating until
the future of the ship has been determined. The coating chosen was a rust
converter Ferrozinc (HMG Paints) that converts any remaining red rust into a
protective layer, which does not allow the corrosion reactions to take place on
its surface.
4.4 Hydrogen Reduction
Several thermal methods have been used in an effort to conserve metal arte-
facts. For those that have been completely mineralised, heating in a furnace
at 800°C for 1–12 h was said to drive off all the volatile chlorides within the
corrosion products. The deeply-buried chlorides would still have been
trapped within the corrosion products and several forms of chlorides would
not have sublimed under these conditions. A modification of this technique
involved soaking the artefacts in ammonium carbamate/ammonium hydrox-
ide solution prior to heating in a furnace under vacuum. This soaking was
said to convert all the chlorides into ammonium chloride, which is volatile at
elevated temperatures. This would be true if all the chlorides were originally
in the form of sodium chloride but this is not the case for most metals and
alloys. Another variation on this theme was to pack wood charcoal around the
artefact prior to placing it in a furnace at 800°C. The wood creates a slightly
reducing atmosphere, which assists in the removal of the chloride ion. As air
was not excluded from the furnace, it is unlikely to have reduced the oxides
of iron to any great extent.
The hydrogen reduction conservation process was first employed in Sweden
in 1964 for ferrous artefacts recovered from the Swedish warship, the Vasa. The
method was further developed at Portsmouth to treat the large number of finds
recovered from the Solent and land-based archaeological sites within the
Wessex region. The principle of the process is to heat the artefact in an atmos-
phere of hydrogen in order to sublime off the volatile chlorides and at the same
time reduce the oxides, hydroxides, chlorides and eventually to the metallic
state. The volume change associated with the reduction of the iron compounds
is sufficiently high to enable the release of deeply-buried chlorides particu-
larly at the metal/corrosion product interface.
The furnace at Portsmouth was installed in the Conservation Department
of the City Museum in 1975. The design of the furnace was influenced by the
considerable number of large cannons recovered from the Solent and in par-
ticular, from the Mary Rose. It was a vertical retort furnace with a cylindrical
bell type retort 2.5 m high by 0.7 m in diameter made of Nimonic, a heat
resistant alloy. Once loaded with the artefacts, the retort was wheeled into
an electrically-heated furnace with three banks of independently-controlled
154 Chapter 6