ferrous artefacts, as has 2.5 M H 3 PO 4 , which leaves a film of ferric phosphate on
exposed iron. A 1–2% HCl solution has been used on lead artefacts. This dis-
solves concretions and corrosion products but does not attack the underlying
metal.
The main drawback of this method is that it removes surface layers and
thus alters the shape of the artefact. Indeed, if the artefact is completely miner-
alised, the complete artefact will dissolve and there will be nothing left! This
contravenes one of the important aims of conservation in that the artefact’s
shape must be unaltered. Another serious shortcoming is that traces of acid may
be trapped in crevices and cracks in the artefact, which will continue to rapidly
corrode the metal after conservation during storage or on display. Overall,
apart from lead-based artefacts, this method is not recommended in the
majority of cases.
As an alternative to the use of acids, sequestering agents have been
employed to dissolve the corrosion products without attacking the parent metal.
The most effective formulations are based on the derivative of ethylene
diamine–tetra acetic acid (EDTA). Lead artefacts from the Mary Rosewere
cleaned in a 10% solution of this compound. The use of EDTA is not recom-
mended for cast iron as the graphite flakes embedded within the corrosion
products are also dissolved. As with the use of acids, the shape of the artefact is
altered if the corrosion layers are very thick and it is also difficult to wash out all
the solutions from cracks, crevices and pores in the artefact after cleaning.
Metals 143
Figure 6Careful removal of concretion reveals the iron rudder pintle