Small items from the furnace were impregnated with the same resins in a
vacuum chamber.
The major criticism of this method of conservation is that the metallurgical
structure of the artefact has been altered by heating the artefact to elevated
temperatures. For this reason, relatively few artefacts with derived microstruc-
tures such as knives, axes, etc., were treated in the furnace at Portsmouth. If
hydrogen treatment was the method selected for these, the operating temper-
ature was reduced to 350°C and the process time extended in order to min-
imise the alteration to the microstructure. Nevertheless, this method is one of
the best and quickest methods for the removal of the chloride ions. Guns from
the Mary Rosewere fully conserved within 2–3 weeks and even after expo-
sure to the atmospheric conditions for nearly 30 years, they still show no sign
of breakdown.
5 Stabilisation and Consolidation
Almost all washing and electrolytic conservation treatments leave the artefact in a
wet condition. This moisture has to be removed if further corrosion is to be pre-
vented. For small artefacts, drying in an oven is very common, while for large
ones such as cannons, hot air blasts or infrared lamps are employed.
In order to minimise the surface corrosion after conservation, dewatering
fluids can be used. These consist of compounds which displace water from the
surface, e.g.butanol, plus a corrosion inhibitor. With copper and its alloys,
problems arise if any chlorides are still present in the corrosion products after
conservation and the humidity in storage or on display rises above approxi-
mately 40% RH. Serious deterioration of the artefact can take place by the
hydrolysis of the chloride compound, cuprous chloride. These hydrated com-
pounds occupy a far larger volume than the original ones and erupt through the
corrosion layers often as light green spots. This is given the name bronze disease
and can lead to complete destruction of the artefact if left unchecked. In order
to counteract this problem, the majority of copper-based artefacts from the
Mary Rosewere soaked after washing in a 1–3% alcoholic solution of benzo-
triazole (BTA) for up to 2 weeks. BTA is a well-known corrosion inhibitor for
copper and its alloys. This functions by forming a linear polymer of copper and
BTA on the metal surface. It is not perfectly clear, however, why this inhibitor
stops hydrolysis of the cuprous chloride in the corrosion products. It is possi-
ble that the BTA reacts with the chloride compound to form a protective poly-
mer film. Alternatively, a complex could be formed between the cuprous
chloride and BTA, which renders the chloride unaffected by humid conditions.
Many artefacts are in a fragile and delicate state following conservation. The
highly friable and porous surface layers are impregnated with a suitable polymer
to achieve an acceptable mechanical strength. Thermosetting resins have been
successfully used on the wrought iron guns from the Mary Rose. These polymers
158 Chapter 6