Shipwrecks, Corrosion and ConservationSummary Slides PART 6 – Jack Dengate

Long submerged artefacts

• 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.

Evaporation of saturated solutions

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.

Removing Salt

• 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.

Cleaning and stabilising artefacts

• 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.