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Shipwrecks, Corrosion and
Conservation
Summary Slides PART 6 – Jack Dengate
• Artefacts recovered from shipwrecks
are generally in poor condition
because:
• metals are corroded
• objects are encrusted with calcium
carbonate deposits
• porous objects are soaked in seawater
rich in chloride and sulfate salts. Care
is required to restore these artefacts to
their original condition.
Long submerged
artefacts
The processes that occur when a saturated solution
evaporates and relate this to the potential damage to drying
artefacts.
• The solution becomes more concentrated as water
evaporates.
• At some time a saturated solution is formed.
• Further evaporation from the saturated solution results in
the production of salt crystals.
• These salt crystals can form throughout the artefact.
Evaporation of saturated
solutions
• The slower the evaporation of
the water the larger the salt
crystals formed.
• The formation of salt crystals
can damage the artefact by
pushing it out of shape,
causing the object to crack.
Concentrated salt solution
may form in air humidity as
low as 50% and then react
chemically with the artefact.
Evaporation of
saturated
solutions
• Chloride is difficult to remove from iron objects by
simple leaching.
• Insoluble hydroxy chlorides such as Fe(OH)Cl can be
trapped in Fe(OH)2 or Fe2O3.x H2O deposits.
• Electrolysis is used to free the chloride ions from the
insoluble compounds into the solution.
Removing Salt
• Artefacts made from metals such as iron, copper and lead (and
their alloys) can be cleaned and stabilised by electrolytic
reduction.
• In this process the artefact is used as the cathode.
• The metal ions in the insoluble corrosion products are reduced to
metal atoms.
• The metal atoms formed are deposited on the surface of the
artefact.
• An inert electrode such as stainless steel is used as the anode.
Cleaning and stabilising
artefacts
• Various oxidation reactions may occur at the anode
depending on the voltage of the applied current and the
concentration of the anions.
• Alkaline solutions such as dilute sodium hydroxide or
sodium carbonate solutions are used as the electrolyte as
the high pH discourages further corrosion of the metal.
Cleaning and stabilising
artefacts
• Artefacts are subjected to a range of procedures so they
may be preserved.
• The surface deposits of calcium carbonate, including
coral, on the artefact are removed by physically chipping
them away or by dissolving them in dilute acid. (Both
methods should be applied carefully as there is the
possibility of damaging the artefact).
Cleaning and stabilising
artefacts
• The artefact is restored by electrolytic reduction to
remove chloride ions and sulfate ions and to reduce metal
oxides and sulfides.
• The artefact is preserved by coating it in a layer of clear
polyurethane polymer or microcrystalline wax.
Cleaning and stabilising
artefacts