4.2 Aqueous Washing
The principle of this method of conservation is to immerse the artefact in a tank
containing a suitable solution. The chloride ion dissolves from the rust film into
the solution that is changed, initially, every week and subsequently every month.
The chloride content of the solution is analyzed at the end of each changeover.
The process is continued until there is no more chloride detected. Atthis point,
the artefact is deemed to be conserved. This can take up to 5 years for marine
artefacts with high levels of chlorides buried within deep rust layers. Even after
this length of time, one is not absolutely certain that all the deleterious ions have
been removed from the rust/metal interface.
A relatively simple method is to dissolve out the chloride by immersion in
distilled or de-mineralised water. Most of the common metals and alloys will
continue to corrode in this solution although at a slower rate due to the lower
conductivity of these solutions compared to seawater. This can be serious if the
artefact is to be immersed for periods up to 5 years, but has been employed if the
immersion times are relatively short. A small copper alloy sheave recovered
from the Mary Rosewas soaked in running demineralised water for 27 days and
the chloride levels dropped from 106 to 10 ppm over this period.
By using boiled water, the dissolved oxygen is expelled and hence, there
should be no corrosion as the cathode reactant has been eliminated from the
electrolyte. Unless the boiled water is kept in sealed containers, air (oxygen)
will slowly dissolve into the water and corrosion of the metal or alloy will
re-commence. As an alternative, using hot demineralised or distilled water will
reduce the concentration of dissolved oxygen and hence corrosion, but this
must be counter-balanced by the rise in reaction rates with temperature. In open
conservation tanks, a temperature of 70°C is required to notice a significant
reduction in rates of corrosion of metals. Small copper alloy artefacts from the
Mary Rosewere treated in this way using water at 80°C for 30 days. At the end
of this period, the chloride levels in the water dropped to below 1 ppm.
Some conservators have alternated between tanks of boiled and cold water
for their artefacts. They claimed that the expansion and contraction of the
artefact, will assist in the removal of the deeply-buried chlorides from the rust
layers. One must be careful that this does not cause the rust to spall off the
underlying metal due to the difference in expansion coefficients between the two
classes of materials.
By altering the pH of the solution, it may be possible to dissolve out the
chlorides without corroding the metal. This is achieved by the formation of a
thin, passive film, approximately 10 nm thick on the exposed uncorroded metal.
The pH required to passivate any given metal or alloy can be determined by
inspection of the relevant E-pH diagram. For wrought iron, a passive film will
form above a pH value of 9.5. This would be a disaster for aluminium artefacts
144 Chapter 6