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9.6 Shipwrecks, Corrosion and Conservation: 7. Conservation and Restoration
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7. Salvage, conservation and restoration of objects
from wrecks requires careful planning and
understanding of the behaviour of chemicals
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Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 26 Jun 08]
Background: Salvage, conservation and restoration of objects from wrecks require careful planning. Artefacts brought to the surface and not treated correctly may continued to corrode, some at an accelerated rate. Others are lost because of the inappropriate procedures used to clean and preserve them. Understanding chemistry is important in the conservation of marine archaeological objects.
perform investigations and gather information from secondary sources to compare conservation and restoration techniques applied in two Australian maritime archaeological projects
Definitions:
Conservation is saving or preserving to prevent further deterioration. It involves cleaning, preserving and stabilizing an artefact.
Full restoration is restoring an artefact as close as possible to its original condition. This can be a much more challenging technique than conservation.
- Gather information to select two
Australian maritime archaeological projects and
compare conservation and restoration
techniques applied. The easiest way to find this
information is by going to the Internet.
The Western Australian Maritme Museum in Fremantle has information on dutch shipwrecks. One shipwreck is the Batavia.
Scroll down and click on this to obtain more
information. - You now need to find one other archaeological project in Australia to compare
them. A good place to look is the Australasian
Institute for Maritime Archaeology. What you are comparing are the conservation
and restoration techniques of the projects you have chosen.
explain that artefacts from long submerged wrecks will be saturated with dissolved chlorides and sulfates.
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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.
describe 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.
- 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.
identify the use of electrolysis as a means of 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.
identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
- Artefacts made from metals such as iron, copper and lead (and their alloys) can be cleaned and stabilised by electrolytic reduction.
More detail
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.
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.
Because lead oxide and lead hydroxide are soluble in alkaline solutions the current should be turned on before immersing a lead artefact. Care should be taken that current flow continues while the lead artefact is immersed.
Depending on the size of the object and the extent of corrosion, electrolytic reduction may take days or weeks to complete
discuss the range of chemical procedures which can be used to clean, preserve and stabilise artefacts from wrecks and, where possible, provide an example of the use of each procedure
- 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).
- 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.
Detailed descriptions of the methods used to conserve archaeological material from underwater sites, Nautical Archaeology Program, Texas
A&M University. Texas, USA.
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